TWI Welding

TWI WORLD CENTRE FOR MATERIALS JOINING TECHNOLOGY

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The Manual Metal Arc process

Manual metal arc welding was first invented in Russia in 1888. It involved a bare metal rod with no flux coating to give a protective gas shield. The development of coated electrodes did not occur until the early 1900s when the Kjellberg process was invented in Sweden and the Quasi-arc method was introduced in the UK. It is worth noting that coated electrodes were slow to be adopted because of their high cost. However, it was inevitable that as the demand for sound welds grew, manual metal arc became synonymous with coated electrodes. When an arc is struck between the metal rod (electrode) and the work piece, both the rod and work piece surface melt to form a weld pool. Simultaneous melting of the flux coating on the rod will form gas and slag which protects

the weld pool from the surrounding atmosphere. The slag will solidify and cool and must be chipped off the weld bead once the weld run is complete (or before the next weld pass is deposited). The process allows only short lengths of weld to be produced before a new electrode needs to be inserted in the holder. Weld penetration is low and the quality of the weld deposit is highly dependent on the skill of the welder.

Types of flux/electrodes

Arc stability, depth of penetration, metal deposition rate and positional capability are greatly influenced by the chemical composition of the flux coating on the electrode. Electrodes can be divided into three main groups:

Cellulosic Rutile Basic

Cellulosic electrodes contain a high proportion of cellulose in the coating and are characterised by a deeply penetrating arc and a rapid burn-off rate giving high welding speeds. Weld deposit can be coarse and with fluid slag, deslagging can be difficult. These electrodes are easy to use in any position and are noted for their use in the 'stovepipe' welding technique.

Features: deep penetration in all positions suitability for vertical down welding reasonably good mechanical properties high level of hydrogen generated - risk of cracking in the heat affected zone (HAZ)

Rutile electrodes contain a high proportion of titanium oxide (rutile) in the coating. Titanium oxide promotes easy arc ignition, smooth arc operation and low spatter. These electrodes are general purpose electrodes with good welding properties. They can be used with AC and DC power sources and in all positions. The electrodes are especially suitable for welding fillet joints in the horizontal/vertical (H/V) position.

Features: moderate weld metal mechanical properties good bead profile produced through the viscous slag positional welding possible with a fluid slag (containing fluoride) easily removable slag

Basic electrodes contain a high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid than rutile coatings - this is also fast-freezing which assists welding in the vertical and overhead position. These electrodes are used for welding medium and heavy section fabrications where higher weld quality, good mechanical properties and resistance to cracking (due to high restraint) are required.

Features: low weld metal produces hydrogen requires high welding currents/speeds poor bead profile (convex and coarse surface profile)


slag removal difficult Metal powder electrodes contain an addition of metal powder to the flux coating to increase the maximum permissible welding current level. Thus, for a given electrode size, the metal deposition rate and efficiency (percentage of the metal deposited) are increased compared with an electrode containing no iron powder in the coating. The slag is normally easily removed. Iron powder electrodes are mainly used in the flat and H/V positions to take advantage of the higher deposition rates. Efficiencies as high as 130 to 140% can be achieved for rutile and basic electrodes without marked deterioration of the arcing characteristics but the arc tends to be less forceful which reduces bead penetration.

Power sourceElectrodes can be operated with AC and DC power supplies. Not all DC electrodes can be operated on AC power sources, however AC electrodes are normally used on DC.

Welding currentWelding current level is determined by the size of electrode - the normal operating range and current are recommended by manufacturers. Typical operating ranges for a selection of electrode sizes are illustrated in the table. As a rule of thumb when selecting a suitable current level, an electrode will require about 40A per millimeter (diameter). Therefore, the preferred current level for a 4mm diameter electrode would be 160A, but the acceptable operating range is 140 to 180A.

What's newTransistor (inverter) technology is now enabling very small and comparatively low weight power sources to be produced. These power sources are finding increasing use for site welding where they can be readily transported from job to job. As they are electronically controlled, add-on units are available for TIG and MIG welding which increase the flexibility. Electrodes are now available in hermetically sealed containers. These vacuum packs obviate the need for baking the electrodes immediately prior to use. However, if a container has been opened or damaged, it is essential that the electrodes are redried according to the manufacturer's instructions.

Solid wire MIG welding

Metal inert gas (MIG) welding was first patented in the USA in 1949 for welding aluminium. The arc and weld pool formed using a bare wire electrode was protected by helium gas, readily available at that time. From about 1952 the process became popular in the UK for welding aluminium using argon as the shielding gas, and for carbon steels using CO2. CO2 and argon-CO2 mixtures are known as metal active gas (MAG) processes. MIG is an attractive alternative to MMA, offering high deposition rates and high productivity. Process characteristicsMIG is similar to MMA in that heat for welding is produced by forming an arc between a metal electrode and the work piece; the electrode melts to form the weld bead. The main

difference is that the metal electrode is a small diameter wire fed from a spool. As the wire is continuously fed, the process is often referred to as semi-automatic welding. Metal transfer modeThe manner, or mode, in which the metal transfers from the electrode to the weld pool largely determines the operating features of the process. There are three principal metal transfer modes:

Short circuiting Droplet / spray Pulsed

Short-circuiting and pulsed metal transfer are used for low current operation while spray metal transfer is only used with


high welding currents. In short-circuiting or’ dip' transfer, the molten metal forming on the tip of the wire is transferred by the wire dipping into the weld pool. This is achieved by setting a low voltage; for a 1.2mm diameter wire, arc voltage varies from about 17V (100A) to 22V (200A). Care in setting the voltage and the inductance in relation to the wire feed speed is essential to minimise spatter. Inductance is used to control the surge in current which occurs when the wire dips into the weld pool. For droplet or spray transfer, a much higher voltage is necessary to ensure that the wire does not make contact i.e. short-circuit, with the weld pool; for a 1.2mm diameter wire, the arc voltage varies from approximately 27V (250A) to 35V (400A). The molten metal at the tip of the wire transfers to the weld pool in the form of a spray of small droplets (about the diameter of the wire and smaller). However, there is a minimum current level, threshold, below which droplets are not forcibly projected across the arc. If an open arc technique is attempted much below the threshold current level, the low arc forces would be insufficient to prevent large droplets forming at the tip of the wire. These droplets would transfer erratically across the arc under normal gravitational forces. The pulsed mode was developed as a means of stabilising the open arc at low current levels i.e. below the threshold level, to avoid short-circuiting and spatter. Spray type metal transfer is achieved by applying pulses of current, each pulse having sufficient force to detach a droplet. Synergic pulsed MIG refers to a special type of controller which enables the power source to be tuned (pulse parameters) for the wire composition and diameter, and the pulse frequency to be set according to the wire feed speed.

Shielding gasIn addition to general shielding of the arc and the weld pool, the shielding gas performs a number of important functions:

forms the arc plasma stabilises the arc roots on the material surface ensures smooth transfer of molten droplets from the wire to the weld pool

Thus, the shielding gas will have a substantial effect on the stability of the arc and metal transfer and the behaviour of the weld pool, in particular, its penetration. General purpose shielding gases for MIG welding are mixtures of argon, oxygen and C02, and special gas mixtures may contain helium. The gases which are normally used for the various materials are:

steels o CO2 o argon +2 to 5% oxygen o argon +5 to 25% CO2

non-ferrous o argon o argon / helium

Argon based gases, compared with CO2, are generally more tolerant to parameter settings and generate lower spatter levels with the dip transfer mode. However, there is a greater risk of lack of fusion defects because these gases are colder. As CO2 cannot be used in the open arc (pulsed or spray transfer) modes due to high back-plasma forces, argon based gases containing oxygen or CO2 are normally employed.

ApplicationsMIG is widely used in most industry sectors and accounts for almost 50% of all weld metal deposited. Compared to MMA, MIG has the advantage in terms of flexibility, deposition rates and suitability for mechanisation. However, it should be noted that while MIG is ideal for 'squirting' metal, a high degree of manipulative skill is demanded of the welder.


Submerged-arc Welding

The first patent on the submerged-arc welding (SAW) process was taken out in 1935 and covered an electric arc beneath a bed of granulated flux. Developed by the E O Paton Electric Welding Institute, Russia, during the Second World War, SAW's most famous application was on the T34 tank.

Process features

Similar to MIG welding, SAW involves formation of an arc between a continuously-fed bare wire electrode and the work piece. The process uses a flux to generate protective gases and slag, and to add alloying elements to the weld pool. A shielding gas is not required. Prior to welding, a thin layer of flux powder is placed on the work piece surface. The arc moves along the joint line and as it does so, excess flux is recycled via a hopper. Remaining fused slag layers can be easily removed after welding. As the arc is completely covered by the flux layer, heat loss is extremely low. This produces a thermal efficiency as high as 60% (compared with 25% for manual metal arc). There is no visible arc light, welding is spatter-free and there is no need for fume extraction.

Operating characteristics

SAW is usually operated as a fully-mechanised or automatic process, but it can be semi-automatic. Welding parameters: current, arc voltage and travel speed all affect bead shape, depth of penetration


and chemical composition of the deposited weld metal. Because the operator cannot see the weld pool, greater reliance must be placed on parameter settings.

Process variantsAccording to material thickness, joint type and size of component, varying the following can increase deposition rate and improve bead shape.

WireSAW is normally operated with a single wire on either AC or DC current. Common variants are:

twin wire triple wire single wire with hot wire addition metal powdered flux addition

All contribute to improved productivity through a marked increase in weld metal deposition rates and/or travel speeds.

FluxFluxes used in SAW are granular fusible minerals containing oxides of manganese, silicon, titanium, aluminium, calcium, zirconium, magnesium and other compounds such as calcium fluoride. The flux is specially formulated to be compatible with a given electrode wire type so that the combination of flux and wire yields desired mechanical properties. All fluxes react with the weld pool to produce the weld metal chemical composition and mechanical properties. It is common practice to refer to fluxes as 'active' if they add manganese and silicon to the weld, the amount of manganese and silicon added is influenced by the arc voltage and the welding current level. The the main types of flux for SAW are:

Bonded fluxes - produced by drying the ingredients, then bonding them with a low melting point compound such as a sodium silicate. Most bonded fluxes contain metallic deoxidisers which help to prevent weld porosity. These fluxes are effective over rust and mill scale.

Fused fluxes - produced by mixing the ingredients, then melting them in an electric furnace to form a chemical homogeneous product, cooled and ground to the required particle size. Smooth stable arcs, with welding currents up to 2000A and consistent weld metal properties, are the main attraction of these fluxes.

ApplicationsSAW is ideally suited for longitudinal and circumferential butt and fillet welds. However, because of high fluidity of the weld pool, molten slag and loose flux layer, welding is generally carried out on butt joints in the flat position and fillet joints in both the flat and horizontal-vertical positions. For circumferential joints, the workpiece is rotated under a fixed welding head with welding taking place in the flat position. Depending on material thickness, either single-pass, two-pass or multipass weld procedures can be carried out. There is virtually no restriction on the material thickness, provided a suitable joint preparation is adopted. Most commonly welded materials are carbon-manganese steels, low alloy steels and stainless steels, although the process is capable of welding some non-ferrous materials with judicious choice of electrode filler wire and flux combinations.

TIG Welding

Tungsten inert gas (TIG) welding became an overnight success in the 1940s for joining magnesium and aluminium. Using an inert gas shield instead of a slag to protect the weld pool, the process was a highly attractive replacement for gas and manual metal are welding. TIG has played a major role in the acceptance of aluminium for high quality welding and structural applications.

Process characteristicsIn the TIG process the arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or


helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding. Because the electrode is not consumed during welding, the welder does not have to balance the heat input from the arc as the metal is deposited from the melting electrode. When filler metal is required, it must be added separately to the weld pool.

Power sourceTIG must be operated with a drooping, constant current power source - either DC or AC. A constant current power source is essential to avoid excessively high currents being drawn when the electrode is short-circuited on to the workpiece surface. This could happen either deliberately during arc starting or inadvertently during welding. If, as in MIG welding, a flat characteristic power source is used, any contact with the workpiece surface would damage the electrode tip or fuse the electrode to the workpiece surface. In DC, because arc heat is distributed approximately one-third at the cathode (negative) and two-thirds at the anode (positive), the electrode is always negative polarity to prevent overheating and melting. However, the alternative power source connection of DC electrode positive polarity has the advantage in that when the cathode is on the workpiece, the surface is cleaned of oxide contamination. For this reason, AC is used when welding materials with a tenacious surface oxide film, such as aluminium.

Arc startingThe welding arc can be started by scratching the surface, forming a short-circuit. It is only when the short-circuit is broken that the main welding current will flow. However, there is a risk that the electrode may stick to the surface and cause a tungsten inclusion in the weld. This risk can be minimised using the 'lift arc' technique where the short-circuit is formed at a very low current level. The most common way of starting the TIG arc is to use HF (High Frequency). HF consists of high voltage sparks of several thousand volts which last for a few microseconds. The HF sparks will cause the electrode - workpiece gap to break down or ionise. Once an electron/ion cloud is formed, current can flow from the power source. Note: As HF generates abnormally high electromagnetic emission (EM), welders should be aware that its use can cause interference especially in electronic equipment. As EM emission can be airborne, like radio waves, or transmitted along power cables, care must be taken to avoid interference with control systems and instruments in the vicinity of welding. HF is also important in stabilising the AC arc; in AC, electrode polarity is reversed at a frequency of about 50 times per second, causing the arc to be extinguished at each polarity change. To ensure that the arc is reignited at each reversal of polarity, HF sparks are generated across the electrode/workpiece gap to coincide with the beginning of each half-cycle.

ElectrodesElectrodes for DC welding are normally pure tungsten with 1 to 4% thoria to improve arc ignition. Alternative additives are lanthanum oxide and cerium oxide which are claimed to give superior performance (arc starting and lower electrode consumption). It is important to select the correct electrode diameter and tip angle for the level of welding current. As a rule, the lower the current the smaller the electrode diameter and tip angle. In AC welding, as the electrode will be operating at a much higher temperature, tungsten with a zirconia addition is used to reduce electrode erosion. It should be noted that because of the large amount of heat generated at the electrode, it is difficult to maintain a pointed tip and the end of the electrode assumes a spherical or 'ball' profile.

Shielding gasShielding gas is selected according to the material being welded. The following guidelines may help:

Argon - the most commonly-used shielding gas which can be used for welding a wide range of materials including steels, stainless steel, aluminium and titanium.

Argon + 2 to 5% H2 - the addition of hydrogen to argon will make the gas slightly reducing, assisting the production of cleaner-looking welds without surface oxidation. As the arc is hotter and more constricted, it permits higher welding speeds. Disadvantages include risk of hydrogen cracking in carbon steels and weld metal porosity in aluminium alloys.

Helium and helium/argon mixtures - adding helium to argon will raise the temperature of the arc. This promotes higher welding speeds and deeper weld penetration. Disadvantages of using helium or a helium/argon mixture is the high cost of gas and difficulty in starting the arc.

ApplicationsTIG is applied in all industrial sectors but is especially suitable for high quality welding. In manual welding, the relatively small arc is ideal for thin sheet material or controlled penetration (in the root run


of pipe welds). Because deposition rate can be quite low (using a separate filler rod) MMA or MIG may be preferable for thicker material and for fill passes in thick-wall pipe welds. TIG is also widely applied in mechanised systems either autogenously or with filler wire. However, several 'off the shelf' systems are available for orbital welding of pipes, used in the manufacture of chemical plant or boilers. The systems require no manipulative skill, but the operator must be well trained. Because the welder has less control over arc and weld pool behavior, careful attention must be paid to edge preparation (machined rather than hand-prepared), joint fit-up and control of welding parameters.

Plasma Welding

Process characteristicsPlasma welding is very similar to TIG as the arc is formed between a pointed tungsten electrode and the workpiece. However, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. Plasma is then forced through a fine-bore copper nozzle which constricts the arc. Three operating modes can be produced by varying bore diameter and plasma gas flow rate:

Micro plasma: 0.1 to 15A.The micro plasma arc can be operated at very low welding currents. The columnar arc is stable even when arc length is varied up to 20mm.

Medium current: 15 to 200A.At higher currents, from 15 to 200A, the process characteristics of the plasma arc are similar to the

TIG arc, but because the plasma is constricted, the arc is stiffer. Although the plasma gas flow rate can be increased to improve weld pool penetration, there is a risk of air and shielding gas entrainment through excessive turbulence in the gas shield.

Keyhole plasma: over 100A.By increasing welding current and plasma gas flow, a very powerful plasma beam is created which can achieve full penetration in a material, as in laser or electron beam welding. During welding, the hole progressively cuts through the metal with the molten weld pool flowing behind to form the weld bead under surface tension forces. This process can be used to weld thicker material (up to 10mm of stainless steel) in a single pass.

Power sourceThe plasma arc is normally operated with a DC, drooping characteristic power source. Because its unique operating features are derived from the special torch arrangement and separate plasma and shielding gas flows, a plasma control console can be added on to a conventional TIG power source. Purpose-built plasma systems are also available. The plasma arc is not readily stabilised with sine wave AC. Arc reignition is difficult when there is a long electrode to work piece distance and the plasma is constricted, Moreover, excessive heating of the electrode during the positive half-cycle causes balling of the tip which can disturb arc stability. Special-purpose switched DC power sources are available. By imbalancing the waveform to reduce the duration of electrode positive polarity, the electrode is kept sufficiently cool to maintain a pointed tip and achieve arc stability.

Arc starting


Although the arc is initiated using HF, it is first formed between the electrode and plasma nozzle. This 'pilot' arc is held within the body of the torch until required for welding then it is transferred to the workpiece. The pilot arc system ensures reliable arc starting and, as the pilot arc is maintained between welds, it obviates the need for HF which may cause electrical interference.

ElectrodeThe electrode used for the plasma process is tungsten-2%thoria and the plasma nozzle is copper. The electrode tip diameter is not as critical as for TIG and should be maintained at around 30-60 degrees. The plasma nozzle bore diameter is critical and too small a bore diameter for the current level and plasma gas flow rate will lead to excessive nozzle erosion or even melting. It is prudent to use the largest bore diameter for the operating current level. Note: too large a bore diameter, may give problems with arc stability and maintaining a keyhole.

Plasma and shielding gasesThe normal combination of gases is argon for the plasma gas, with argon plus 2 to 5% hydrogen for the shielding gas. Helium can be used for plasma gas but because it is hotter this reduces the current rating of the nozzle. Helium's lower mass can also make the keyhole mode more difficult.

Applications

Micro plasma weldingMicro plasma was traditionally used for welding thin sheets (down to 0.1 mm thickness), and wire and mesh sections. The needle-like stiff arc minimises arc wander and distortion. Although the equivalent TIG arc is more diffuse, the newer transistorised (TIG) power sources can produce a very stable arc at low current levels.

Medium current weldingWhen used in the melt mode this is an alternative to conventional TIG. The advantages are deeper penetration (from higher plasma gas flow), and greater tolerance to surface contamination including coatings (the electrode is within the body of the torch). The major disadvantage lies in the bulkiness of the torch, making manual welding more difficult. In mechanised welding, greater attention must be paid to maintenance of the torch to ensure consistent performance.

Keyhole weldingThis has several advantages which can be exploited: deep penetration and high welding speeds. Compared with the TIG arc, it can penetrate plate thicknesses up to l0mm, but when welding using a single pass technique, it is more usual to limit the thickness to 6mm. The normal methods are to use the keyhole mode with filler to ensure smooth weld bead profile (with no undercut). For thicknesses up to 15mm, a vee joint preparation is used with a 6mm root face. A two-pass technique is employed and here, the first pass is autogenous with the second pass being made in melt mode with filler wire addition. As the welding parameters, plasma gas flow rate and filler wire addition (into the keyhole) must be carefully balanced to maintain the keyhole and weld pool stability, this technique is only suitable for mechanised welding. Although it can be used for positional welding, usually with current pulsing, it is normally applied in high speed welding of thicker sheet material (over 3 mm) in the flat position. When pipe welding, the slope-out of current and plasma gas flow must be carefully controlled to close the keyhole without leaving a hole.

The oxyacetylene process

Process featuresOxyacetylene welding, commonly referred to as gas welding, is a process which relies on combustion of oxygen and acetylene. When mixed together in correct proportions within a hand-held torch or blowpipe, a relatively hot flame is produced with a temperature of about 3,200 deg.C. The chemical action of the oxyacetylene flame can be adjusted by changing the ratio of the volume of oxygen to acetylene. Three distinct flame settings are used, neutral, oxidising and carburising.


Neutral flame

Oxidising flame

Carburising flameWelding is generally carried out using the neutral flame setting which has equal quantities of oxygen and acetylene. The oxidising flame is obtained by increasing just the oxygen flow rate while the carburising flame is achieved by increasing acetylene flow in relation to oxygen flow. Because steel melts at a temperature above 1,500 deg.C, the mixture of oxygen and acetylene is used as it is the only gas combination with enough heat to weld steel. However, other gases such as propane, hydrogen and coal gas can be used for joining lower melting point non-ferrous metals, and for brazing and silver soldering.

EquipmentOxyacetylene equipment is portable and easy to use. It comprises oxygen and acetylene gases stored under pressure in steel cylinders. The cylinders are fitted with regulators and flexible hoses which lead to the blowpipe. Specially designed safety devices such as flame traps are fitted between the hoses and the cylinder regulators. The flame trap prevents flames generated by a 'flashback' from reaching the cylinders; principal causes of flashbacks are the failure to purge the hoses and overheating of the blowpipe nozzle. When welding, the operator must wear protective clothing and tinted coloured goggles. As the flame is less intense than an arc and very little UV is emitted, general-purpose tinted goggles provide sufficient protection.

Operating characteristicsThe action of the oxyacetylene flame on the surface of the material to be welded can be adjusted to produce a soft, harsh or violent reaction by varying the gas flows. There are of course practical limits as to the type of flame which can be used for welding. A harsh forceful flame will cause the molten weld pool to be blown away, while too soft a flame will not be stable near the point of application. The blowpipe is therefore designed to accommodate different sizes of 'swan neck copper nozzle which allows the correct intensity of flame to be used. The relationship between material thickness, blowpipe nozzle size and welding speed, is shown in the chart. When carrying out fusion welding the addition of filler metal in the form of a rod can be made when required. The principal techniques employed in oxyacetylene welding are leftward, rightward and all-positional rightward. The former is used almost exclusively and is ideally suited for welding butt, fillet and lap joints in sheet thicknesses up to approximately 5mm. The rightward technique finds application on plate thicknesses above 5mm for welding in the flat and horizontal-vertical position. The all-positional rightward method is a modification of the rightward technique and is ideally suited for welding steel plate and in particular Pipework where positional welding, (vertical and overhead) has to be carried out. The rightward and all- positional rightward techniques enable the welder to obtain a uniform penetration bead with added control over the molten weld pool and weld metal. Moreover, the welder has a clear view of the weld pool and can work in complete freedom of movement. These techniques are very highly skilled and are less frequently used than the conventional leftward technique.

Equipment for Oxyacetylene WeldingEssential equipment componentsTorch


The basic oxyacetylene torch comprises: torch body (or handle) two separate gas tubes (through the handle connected to the hoses) separate control valves mixer chamber flame tube welding tip

NB The cutting torch requires two oxygen supplies to the nozzle, one mixed with fuel gas for preheating and a separate oxygen flow for cutting. HosesHoses are colour-coded red for acetylene and blue (UK) or green (US) for oxygen. Oxygen fittings on the hose have a right-hand thread while acetylene is left-handed. Gas regulatorsThe primary function of a gas regulator is to control gas pressure. It reduces the high pressure of the bottle-stored gas to the working pressure of the torch, and this will be maintained during welding. The regulator has two separate gauges: a high pressure gauge for gas in the cylinder and a low pressure gauge for pressure of gas fed to the torch. The amount of gas remaining in the cylinder can be judged from the high pressure gauge. The regulator, which has a pressure adjusting screw, is used to control gas flow rate to the torch by setting the outlet gas pressure. Note Acetylene is supplied in cylinders under a pressure of about 15 bars psi but welding is carried out with torch gas pressures typically up to 2 bars.Flame trapsFlame traps (also called flashback arresters) must be fitted into both oxygen and acetylene gas lines to prevent a flashback flame from reaching the regulators. Non-return spring-loaded valves can be fitted in the hoses to detect/stop reverse gas flow. Thus, the valves can be used to prevent conditions leading to flashback, but should always be used in conjunction with flashback arresters. A flashback is where the flame burns in the torch body, accompanied by a whistling sound. It will occur when flame speed exceeds gas flow rate and the flame can pass back through the mixing chamber into the hoses. Most likely causes are: incorrect gas pressures giving too low a gas velocity, hose leaks, loose connections, or welder techniques which disturb gas flow.Identification of gas cylindersAn oxygen cylinder is colour-coded black and the acetylene cylinder is maroon. Oxygen and acetylene are stored in cylinders at high pressure. Oxygen pressure can be as high as 230 bars. Acetylene, which is dissolved in acetone contained in a porous material, is stored at a much lower pressure, approximately 15 bars. The appropriate regulator must be fitted to the cylinders to accommodate cylinder pressures. To avoid confusion, oxygen cylinders and regulators have right-hand threads and acetylene cylinders and regulators have left-hand ones.Typical gas pressures and flow rates for C-Mn steel:

Steel thickness (mm)

Nozzle size

Acetylene Oxygen

Pressure (bar)

Consumption (l/min)

Pressure (bar)

Consumption (l/min)

0.90 1 0.14 0.50 0.14 0.50

1.20 2 0.14 0.90 0.14 0.90

2.00 3 0.14 1.40 0.14 1.40

2.60 5 0.14 2.40 0.14 2.40

3.20 7 0.14 3.30 0.14 3.30

4.00 10 0.21 4.70 0.21 4.70

5.00 13 0.28 6.00 0.28 6.00

6.50 18 0.28 8.50 0.28 8.50

8.20 25 0.42 12.00 0.42 12.00

10.00 35 0.63 17.00 0.63 17.00

13.00 45 0.35 22.00 0.35 22.00


25.00 90 0.63 42.00 0.63 42.00

Selection of correct nozzlesWelding torches are generally rated according to thickness of material to be welded. They range from light duty (for sheet steel up to 2mm in thickness) to heavy duty (for steel plate greater than 25mm in thickness). Each torch can be fitted with a range of nozzles with a bore diameter selected according to material thickness. Gas pressures are set to give correct flow rate for nozzle bore diameter. Proportions of oxygen and acetylene in the mixture can be adjusted to give a neutral, oxidising or carburising flame. (See the description of oxyacetylene processes) Welding is normally carried out using a neutral flame with equal quantities of oxygen and acetylene.

Equipment safety checksBefore commencing welding it is wise to inspect the condition and operation of all equipment. As well as normal equipment and workplace safety checks, there are specific procedures for oxyacetylene. Operators should verify that:

flashback arresters are present in each gas line hoses are the correct colour, with no sign of wear, as short as possible and not taped together regulators are the correct type for the gas a bottle key is in each bottle (unless the bottle has an adjusting screw)

It is recommended that oxyacetylene equipment is checked at least annually - regulators should be taken out of service after five years. Flashback arresters should be checked regularly according to manufacturer's instructions and, with specific designs, it may be necessary to replace if flashback has occurred. For more detailed information the following legislation and codes of practice should be consulted:

UK Health and Safety at Work Act 1974 Pressure Systems and Transportable Gas Containers Regulations British Compressed Gases Association, Codes of Practice BOC Handbook

Equipment for MMA Welding

Although the manual metal arc (MMA) process has relatively basic equipment requirements, it is important that the welder has a knowledge of operating features and performance to comply with welding procedures for the job and, of course, for safety reasons.

Essential equipmentThe main components of the equipment required for welding are:

power source electrode holder and cables welder protection fume extraction

Tools required include: a wire brush to clean the joint area adjacent to the weld (and the weld itself after slag removal); a chipping hammer to remove slag from the weld deposit; and, when removing slag, a pair of clear lens goggles or a face shield to protect the eyes (lenses should be shatter-proof and noninflammable).

Power sourceThe primary function of a welding power source is to provide sufficient power to melt the joint. However with MMA the power source must also provide current for melting the end of the electrode to produce weld metal, and it must have a sufficiently high voltage to stabilise the arc. MMA electrodes are designed to be operated with alternating current (AC) and direct current (DC) power sources. Although AC electrodes can be used on DC, not all DC electrodes can be used with AC power sources.As MMA requires a high current (50-30OA) but a relatively low voltage (10-50V), high voltage mains supply (240 or 440V) must be reduced by a transformer. To produce DC, the output from the


transformer must be further rectified. To reduce the hazard of electrical shock, the power source must function with a maximum no-load voltage, that is, when the external (output) circuit is open (power leads connected and live) but no arc is present. The no-load voltage rating of the power source is as defined in BS 638 and must be in accordance with the type of welding environment or hazard of electrical shock. The power source may have an internal or external hazard reducing device to reduce the no-load voltage; the main welding current is delivered as soon as the electrode touches the workpiece. For welding in confined spaces, you should use a low voltage safety device to limit the voltage available at the holder to approximately 25V.There are four basic types of power source:

AC transformer DC rectifier AC/DC transformer-rectifier DC generator

AC electrodes are frequently operated with the simple, single phase transformer with current adjusted by means of tappings or sliding core control. DC rectifiers and AC/DC transformer-rectifiers are controlled electronically, for example by thyristors. A new generation of power sources called inverters is available. These use transistors to convert mains AC (50Hz) to a high frequency AC (over 500 Hz) before transforming down to a voltage suitable for welding and then rectifying to DC. Because high frequency transformers can be relatively small, principal advantages of inverter power sources are undoubtedly their size and weight when the source must be portable.

Electrode holder and cablesThe electrode holder clamps the end of the electrode with copper contact shoes built into its head. The shoes are actuated by either a twist grip or spring-loaded mechanism. The clamping mechanism allows for quick release of the stub end. For efficiency the electrode has to be firmly clamped into the holder, otherwise poor electrical contact may cause arc instability through voltage fluctuations. Welding cable connecting the holder to the power source is mechanically crimped or soldered. It is essential that good electrical connections are maintained between electrode, holder and cable. With poor connections, resistance heating and, in severe cases, minor arcing with the torch body will cause the holder to overheat. Two cables are connected to the output of the power source, the welding lead goes to the electrode holder and the current return lead is clamped to the workpiece. The latter is often wrongly referred to as the earthlead. A separate earth lead is normally required to provide protection from faults in the power source. The earth cable should therefore be capable of carrying the maximum output current of the power source.Cables are covered in a smooth and hard-wearing protective rubberised flexible sheath. This oil and water resistant coating provides electrical insulation at voltages to earth not exceeding 100V DC and AC (rms value). Cable diameter is generally selected on the basis of welding current level, As these electrode types are When welding, the welder air movement should be from duty cycle and distance of the work from the power source. The higher the current and duty cycle, the larger the diameter of the cable to ensure that it does not overheat (see BS 638 Pt 4). If welding is carried out some distance from the power source, it may be necessary to increase cable diameter to reduce voltage drop.

Care of electrodesThe quality of weld relies upon consistent performance of the electrode. The flux coating should not be chipped, cracked or, more importantly, allowed to become damp. StorageElectrodes should always be kept in a dry and well-ventilated store. It is good practice to stack packets of electrodes on wooden pallets or racks well clear of the floor. Also, all unused electrodes which are to be returned should be stored so they are not exposed to damp conditions to regain moisture. Good storage conditions are 10 degrees C above external air temperature. As the storage conditions are to prevent moisture from condensing on the electrodes, the electrode stores should be dry rather that warm. Under these conditions and in original packaging, electrode storage time is practically unlimited. It should be noted that electrodes are now available in hermetically sealed packs which obviate the need for drying. However, if necessary, any unused electrodes must be redried according to manufacturer's instructions. Drying of electrodesDrying is usually carried out following the manufacturer's recommendations and requirements will be determined by the type of electrode. Cellulosic coatings


As these electrode coatings are designed to operate with a definite amount of moisture in the coating, they are less sensitive to moisture pick-up and do not generally require a drying operation. However, in cases where ambient relative humidity has been very high, drying may be necessary. Rutile coatingsThese can tolerate a limited amount of moisture and coatings may deteriorate if they are overdried. Particular brands may need to be dried before use. Basic and basic/rutile coatingsBecause of the greater need for hydrogen control, moisture pick-up is rapid on exposure to air. These electrodes should be thoroughly dried in a controlled temperature drying oven. Typical drying time is one hour at a temperature of approximately 150 to 300 degrees C but instructions should be adhered to. After controlled drying, basic and basic/rutile electrodes must be held at a temperature between 100 and 150 degrees C to help protect them from re-absorbing moisture into the coating. These conditions can be obtained by transferring the electrodes from the main drying oven to a holding oven or a heated quiver at the workplace.Protective clothingWhen welding, the welder must be protected from heat and light radiation emitted from the arc, spatter ejected from the weld pool, and from welding fume. Hand and head shieldFor most operations a hand-held or head shield constructed of lightweight insulating and non-reflecting material is used. The shield is fitted with a protective filter glass, sufficiently dark in colour and capable of absorbmg the harmful infrared and ultraviolet rays. The filter glasses conform to the strict requirements of BS 679 and are graded according to a shade number which specifies the amount of visible light allowed to pass through - the lower the number, the lighter the filter. The correct shade number must be used according to the welding current level, for example:

Shade 9 - up to 40A Shade 10 - 40 to 80A Shade 11 - 80 to 175A Shade 12 - 175 to 300A Shade 13 - 300 to 500A

ClothingFor protection against sparks, hot spatter, slag and burns, a leather apron and leather gloves should be worn. Various types of leather gloves are available, such as short or elbow length, full fingered or part mitten. Fume extractionWhen welding within a welding shop, ventilation must dispose harmlessly of the welding fume. Particular attention should be paid to ventilation when welding in a confined space such as inside a boiler, tank or compartment of a ship. Fume removal should be by some form of mechanical ventilation which will produce a current of fresh air in the immediate area. Direction of the air movement should be from the welder's face towards the work. This is best achieved by localised exhaust ventilation using a suitably designed hood near to the welding area.

Further informationPlease refer to: BS 638 Arc welding power sources, equipment and accessories BS 679 Filters, cover lenses and backing lenses for use during welding and similar operations.

Equipment for MIG WeldingThe MIG process is a versatile welding technique which is suitable for both thin sheet and thick section components. It is capable of high productivity but the quality of welds can be called into question. To achieve satisfactory welds, welders must have a good knowledge of equipment requirements and should also recognise fully the importance of setting up and maintaining component parts correctly. Essential equipmentIn MIG the arc is formed between the end of a small diameter wire electrode fed from a spool, and the work piece. Main equipment components are:


power source wire feed system conduit gun

The arc and weld pool are protected from the atmosphere by a gas shield. This enables bare wire to be used without a flux coating (required by MMA). However, the absence of flux to 'mop up' surface oxide places greater demand on the welder to ensure that the joint area is cleaned immediately before welding. This can be done using either a wire brush for relatively clean parts, or a hand grinder to remove rust and scale. The other essential piece of equipment is a wire cutter to trim the end of the electrode wire. Power sourceMIG is operated exclusively with a DC power source. The source is termed a flat, or constant current, characteristic power source, which refers to the voltage/welding current relationship. In MIG, welding current is determined by wire feed speed, and arc length is determined by power source voltage level (open circuit voltage). Wire burn-off rate is automatically adjusted for any slight variation in the gun to workpiece distance, wire feed speed, or current pick-up in the contact tip. For example, if the arc momentarily shortens, arc voltage will decrease and welding current will be momentarily increased to burn back the wire and maintain pre-set arc length. The reverse will occur to counteract a momentary lengthening of the arc. There is a wide range of power sources available, mode of metal transfer can be:

dip spray pulsed

A low welding current is used for thin-section material or welding in the vertical position. The molten metal is transferred to the work piece by the wire dipping into the weld pool. As welding parameters will vary from around 100A \ 17V to 200A \ 22V (for a 1.2mm diameter wire), power sources normally have a current rating of up to 350A. Circuit inductance is used to control the surge in current when the wire dips into the weld pool (this is the main cause of spatter). Modern electronic power sources automatically set the inductance to give a smooth arc and metal transfer. In spray metal transfer, metal transfers as a spray of fine droplets without the wire touching the weld pool. The welding current level needed to maintain the non short-circuiting arc must be above a minimum threshold level; the arc voltage is higher to ensure that the wire tip does not touch the weld pool. Typical welding parameters for a 1.2mm diameter wire are within 250A \ 28V to 400A \ 35V. For high deposition rates the power source must have a much higher current capacity: up to 500A.The pulsed mode provides a means of achieving a spray type metal transfer at current levels below threshold level. High current pulses between 25 and 100Hz are used to detach droplets as an alternative to dip transfer. As control of the arc and metal transfer requires careful setting of pulse and background parameters, a more sophisticated power source is required. Synergic pulsed MIG power sources, which are advanced transistor-controlled power sources, are preprogrammed so that the correct pulse parameters are delivered automatically as the welder varies wire feed speed.Welding current and arc voltage ranges for selected wire diameters operating with dip and spray metal transfer:

Wire diameter (mm)

Dip transfer Spray transfer

Current (A)

Voltage (V)

Current (A)

Voltage (V)

0.6 30 - 80 15 - 18

0.8 45 - 180 16 - 21 150 - 250 25 - 33

1.0 70 - 180 17 - 22 230 - 300 26 - 35

1.2 100 - 200 17 - 22 250 - 400 27 - 35

1.6 120 - 200 18 - 22 250 - 500 30 - 40

Wire feed system


The performance of the wire feed system can be crucial to the stability and reproducibility of MIG welding. As the system must be capable of feeding the wire smoothly, attention should be paid to the feed rolls and liners. There are three types of feeding systems:

pinch rolls push-pull spool on gun

The conventional wire feeding system normally has a set of rolls where one is grooved and the other has a flat surface. Roll pressure must not be too high otherwise the wire will deform and cause poor current pick up in the contact tip. With copper coated wires, too high a roll pressure or use of knurled rolls increases the risk of flaking of the coating (resulting in copper build up in the contact tip). For feeding soft wires such as aluminium dual-drive systems should be used to avoid deforming the soft wire. Small diameter aluminium wires, 1mm and smaller, are more reliably fed using a push-pull system. Here, a second set of rolls is located in the welding gun - this greatly assists in drawing the wire through the conduit. The disadvantage of this system is increased size of gun. Small wires can also be fed using a small spool mounted directly on the gun. The disadvantages with this are increased size, awkwardness of the gun, and higher wire cost.ConduitThe conduit can measure up to 5m in length, and to facilitate feeding, should be kept as short and straight as possible. (For longer lengths of conduit, an intermediate push-pull system can be inserted). It has an internal liner made either of spirally-wound steel for hard wires (steel, stainless steel, titanium, nickel) or PTFE for soft wires (aluminium, copper). GunIn addition to directing the wire to the joint, the welding gun fulfils two important functions - it transfers the welding current to the wire and provides the gas for shielding the arc and weldpool. There are two types of welding guns: 'air' cooled and water cooled. The 'air' cooled guns rely on the shielding gas passing through the body to cool the nozzle and have a limited current-carrying capacity. These are suited to light duty work. Although 'air' cooled guns are available with current ratings up to 500A, water cooled guns are preferred for high current levels, especially at high duty cycles.Welding current is transferred to the wire through the contact tip whose bore is slightly greater than the wire diameter. The contact tip bore diameter for a 1.2mm diameter wire is between 1.4 andt 1.5mm. As too large a bore diameter affects current pick up, tips must be inspected regularly and changed as soon as excessive wear is noted. Copper alloy (chromium and zirconium additions) contact tips, harder than pure copper, have a longer life, especially when using spray and pulsed modes.Gas flow rate is set according to nozzle diameter and gun to workpiece distance, but is typically between 10 and 30 l/min. The nozzle must be cleaned regularly to prevent excessive spatter build-up which creates porosity. Anti-spatter spray can be particularly effective in automatic and robotic welding to limit the amount of spatter adhering to the nozzle.Protective equipmentA darker glass than that used for MMA welding at the same current level should be used in hand or head shields. Recommended shade number of filter for MIG/MAG welding:

Shade number

Welding current A

MIG Heavy metal

MIG Light metal

MAG

10 under 100 under 100 under 80

11 1001 - 175 100 - 175 80 - 125

12 175 - 300 175 - 250 125 - 175

13 300 - 500 250 - 350 175 - 300

14 over 500 350 - 500 300 - 500

15 over 500 over 450


Equipment for Submerged-arc WeldingThe submerged-arc welding(SAW) process is similar to MIG where the arc is formed between a continuously-fed wire electrode and the workpiece, and the weld is formed by the arc melting the workpiece and the wire. However, in SAW a shielding gas is not required as the layer of flux generates the gases and slag to protect the weld pool and hot weld metal from contamination. Flux plays an additional role in adding alloying elements to the weld pool. Essential equipmentEssential equipment components for SAW are:

power source wire gun flux handling protective equipment

As SAW is a high current welding process, the equipment is designed to produce high deposition rates. Power sourceSAW can be operated using either a DC or an AC power source. DC is supplied by a transformer-rectifier and AC is supplied by a transformer. Current for a single wire ranges from as low as 200A (1.6mm diameter wire) to as high as 1000A (6.0mm diameter wire). In practice, most welding is carried out on thick plate where a single wire (4.0mm diameter) is normally used over a more limited range of 600 to 900A, with a twin wire system operating between 800 and 1200A. In DC operation, the electrode is normally connected to the positive terminal. Electrode negative (DCEN) polarity can be used to increase deposition rate but depth of penetration is reduced by between 20 and 25%. For this reason, DCEN is used for surfacing applications where parent metal dilution is important. The DC power source has a 'constant voltage' output characteristic which produces a self-regulating arc. For a given diameter of wire, welding current is controlled by wire feed speed and arc length is determined by voltage setting.AC power sources usually have a constant-current output characteristic and are therefore not self-regulating. The arc with this type of power source is controlled by sensing the arc voltage and using the signal to control wire feed speed. In practice, for a given welding current level, arc length is determined by wire burnoff rate, i.e. the balance between the welding current setting and wire feed speed which is under feedback control.Square wave AC square wave power sources have a constant voltage output current characteristic. Advantages are easier arc ignition and constant wire feed speed control.Welding gunSAW can be carried out using both manual and mechanised techniques. Mechanised welding, which can exploit the potential for extremely high deposition rates, accounts for the majority of applications. Manual weldingFor manual welding, the welding gun is similar to a MIG gun, with the flux which is fed concentrically around the electrode, replacing the shielding gas. Flux is fed by air pressure through the handle of the gun or from a small hopper mounted on the gun. The equipment is relatively portable and, as the operator guides the gun along the joint, little manipulative skill is required. However, because the operator has limited control over the welding operation (apart from adjusting travel speed to maintain the bead profile) it is best used for short runs and simple filling operations. Mechanised welding - single wire

As SAW is often used for welding large components, the gun, wire feeder and flux delivery feed can be mounted on a rail, tractor or boom manipulator. Single wire welding is mostly practised using DCEP


even though AC will produce a higher deposition rate for the same welding current. AC is used to overcome problems with arc blow, caused by residual magnetism in the workpiece, jigging or welding machine.Wire stickout, or electrode extension - the distance the wire protrudes from the end of the contact tip - is an important control parameter in SAW. As the current flowing between the contact tip and the arc will preheat the wire, wire burnoff rate will increase with increase in wire stickout. For example, the deposition rate for a 4mm diameter wire at a welding current of 700A can be increased from approximately 9 kg/hr at the normal 32mm stickout, to 14 kg/hr at a stickout length of 178mm. In practice, because of the reduction in penetration and greater risk of arc wander, a long stickout is normally only used in cladding and surfacing applications where there is greater emphasis on deposition rate and control of penetration, rather than accurate positioning of the wire.For most applications, electrode stickout is set so that the contact tube is slightly proud of the flux layer. The depth of flux is normally just sufficient to cover the arc whose light can be seen through the flux.Recommended and maximum stickout lengths:

Wire diameter mm

Current range A

Wire stickout

Normal mm

Maximum mm

0.8 100 to 200 12 -

1.2 150 to 300 20 -

1.6 200 to 500 20 -

2.0 250 to 600 25 63

3.2 350 to 800 30 76

4.0 400 to 900 32 128

4.75 450 to 1000 35 165

Mechanised welding - twin wireTandem arc connectionsSAW can be operated with more than one wire. Although up to five wires are used for high deposition rates, e.g.

in pipe mills, the most common multi-wire systems have two wires in a tandem arrangement. The leading wire is run on DCEP to produce deep penetration. The trailing wire is operated on AC which spreads the weld pool, which is ideal for filling the joint. AC also minimises: interaction between the arcs, and the risk of lack of fusion defects and porosity through the deflection of the arcs (arc blow). The wires are normally spaced 20mm apart so that the second wire feeds into the rear of the weld pool. Gun angle

In manual welding, the gun is operated with a trailing angle, i.e. with the gun at an angle of 45 degrees (backwards) from the vertical. In single wire mechanised welding operations, the gun is perpendicular to the workpiece. However, in twin wire operations the leading gun is normal to the workpiece, with the trailing gun angled slightly forwards between an angle of 60 and 80 degrees. This reduces disturbance of the weld pool and produces a smooth weld bead profile.

Flux handlingFlux should be stored in unopened packages under dry conditions. Open packages should be stored in a humidity-controlled store. While flux from a newly-opened package is ready for immediate use, flux which has been opened and held in a store should first be dried according to manufacturer's instructions. In small welding systems, flux is usually held in a small hopper above the welding gun. It is fed automatically (by gravity or mechanised feed) ahead of the arc. In larger installations the flux is stored in large hoppers and is fed with compressed air. Unused flux is collected using a vacuum hose and returned to the hopper.


Note: Care must be taken in recycling unused flux, particularly regarding the removal of slag and metal dust particles. The presence of slag will change the composition of the flux which, together with the wire, determines the composition of the weld metal. The presence of fine particles can cause blockages in the feeding system.Protective equipmentUnlike other arc welding processes, SAW is a clean process which produces minimum fume and spatter when welding steels. (Some noxious emissions can be produced when welding special materials.) For normal applications, general workshop extraction should be adequate. Protective equipment such as a head shield and a leather apron are not necessary. Normal protective equipment (goggles, heavy gloves and protective shoes) are required for ancillary operations such as slag removal by chipping or grinding. Special precautions should be taken when handling flux - a dust respirator and gloves are needed when loading the storage hoppers.

Equipment for TIG Welding

Job Knowledge for Welders No. 6 describes the TIG welding process. Using an inert gas shield instead of a slag to protect the weld pool, this technology is a highly attractive alternative to gas and manual metal arc welding and has played a major role in the acceptance of high quality welding in critical applications.

Essential equipmentIn TIG, the arc is formed between the end of a small diameter tungsten electrode and the work piece. The main equipment components are:

power source torch backing system protective equipment

Power sourceThe power source for TIG welding can be either DC or AC but in both the output is termed a drooping, or constant current, characteristic; the arc voltage / welding current relationship delivers a constant current for a given power source setting. If the arc voltage is slightly increased or decreased, there will be very little change in welding current. In manual welding, it can accommodate the welder's natural variations in arc length and, in the event of the electrode touching the work, an excessively high current will not be drawn which could fuse the electrode to the work piece. The arc is usually started by HF (High Frequency) sparks which ionise the gap between the electrode and the work piece. HF generates airborne and line transmitted interference, so care must be taken to avoid interference with control systems and instruments near welding equipment. When welding is carried out in sensitive areas, a non-HF technique, touch starting or 'lift arc', can be used. The electrode can be short circuited to the work piece, but the current will only flow when the electrode is lifted off the surface. There is, therefore, little risk of the electrode fusing to the work piece surface and forming tungsten inclusions in the weld metal. For high quality applications, using HF is preferred.

DC power sourceDC power produces a concentrated arc with most of the heat in the work piece, so this power source is generally used for welding. However, the arc with its cathode roots on the electrode (DC electrode negative polarity), results in little cleaning of the work piece surface. Care must be taken to clean the surface prior to welding and to ensure that there is an efficient gas shield. Transistor and inverter power sources are being used increasingly for TIG welding. The advantages are:

the smaller size makes them easily transported arc ignition is easier special operating features, e.g. current pulsing, are readily included the output can be pre-programmed for mechanised operations

The greater stability of these power sources allows very low currents to be used particularly for micro-TIG welding and largely replaced the plasma process for micro-welding operations.


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AC power sourceFor materials such as aluminium, which has a tenacious oxide film on the surface, AC power must be employed. By switching between positive and negative polarity, the periods of electrode positive will remove the oxide and clean the surface. The figure shows current and voltage waveforms for (sine wave) AC TIG welding. Disadvantages of conventional, sine wave AC compared with DC are:

the arc is more diffuse HF is required to reignite the arc at each current

reversal excessive heating of the electrode makes it

impossible to maintain a tapered point and the end becomes balled

Square wave AC, or switched DC, power sources are particularly attractive for welding aluminium. By switching between polarities, arc reignition is made easier so that the HF can be reduced or eliminated. The ability to imbalance the waveform to vary the proportion of positive to negative polarity is important by determining the relative amount of heat generated in the work piece and the electrode. To weld the root run, the power source is operated with the greater amount of positive polarity to put the maximum heat into the work piece.For filler runs a greater proportion of negative polarity should be used to minimise heating of the electrode. By using 90% negative polarity, it is possible to maintain a pointed electrode. A balanced position (50% electrode positive and negative polarities) is preferable for welding heavily oxidised aluminium.

TorchThere is a wide range of torch designs for welding, according to the application. Designs which have the on/off switch and current control in the handle are often preferred to foot controls. Specialised torches are available for mechanised applications, e.g. orbital and bore welding of pipes.

ElectrodeFor DC current, the electrode is tungsten with between 2 and 5% thoria to aid arc initiation. The electrode tip is ground to an angle of 600 to 900 for manual welding, irrespective of the electrode diameter. For mechanised applications as the tip angle determines the shape of the arc and influences the penetration profile of the weld pool, attention must be paid to consistency in grinding the tip and checking its condition between welds. For AC current, the electrode is either pure tungsten or tungsten with a small amount (up to 0.5%) of zirconia to aid arc reignition and to reduce electrode erosion. The tip normally assumes a spherical profile due to the heat generated in the electrode during the electrode positive half cycle.

Gas shieldingA gas lens should be fitted within the torch nozzle, to ensure laminar gas flow. This will improve gas protection for sensitive welding operations like welding vertical, corner and edge joints and on curved surfaces.

Backing systemWhen welding high integrity components, a shielding gas is used to protect the underside of the weld pool and weld bead from oxidation. To reduce the amount of gas consumed, a localised gas shroud for sheet, dams or plugs for tubular components is used. As little as 5% air can result in a poor weld bead profile and may reduce corrosion resistance in materials like stainless steel. With gas backing systems in pipe welding, pre-weld purge time depends on the diameter and length of the pipe. The flow rate/purge time is set to ensure at least five volume changes before welding. Stick on tapes and ceramic backing bars are also used to protect and support the weld bead. In manual stainless steel welding, a flux-cored wire instead of a solid wire can be used in the root run. This protects the underbead from oxidation without the need for gas backing.

Inserts


A pre-placed insert can be used to improve the uniformity of the root penetration. Its main use is to prevent suck-back in an autogenous weld, especially in the overhead position. The use of an insert does not make welding any easier and skill is still required to avoid problems of incomplete root fusion and uneven root penetration. Protective equipmentA slightly darker glass should be used in the head or hand shield than that used for MMA welding. Recommended shade number of filter for TIG welding:

Shade number

Welding current A

9 less than 20

10 20 to 40

11 40 to 100

12 100 to 175

13 175 to 250

14 250 to 400

The article was prepared by Bill Lucas.

Weld defects/imperfections in welds - lack of sidewall and inter-run fusion

This article describes the characteristic features and principal causes of lack of sidewall and inter-run fusion. General guidelines on best practice are given so that welders can minimise the risk of imperfections during fabrication.

IdentificationLack of fusion imperfections can occur when the weld metal fails

To fuse completely with the sidewall of the joint (Fig. 1) To penetrate adequately the previous weld bead (Fig. 2).

Fig. 1. Lack of side wall fusion

Fig. 2. Lack of inter-run fusion


Demagnetising a pipe

CausesThe principal causes are too narrow a joint preparation, incorrect welding parameter settings, poor welder technique and magnetic arc blow. Insufficient cleaning of oily or scaled surfaces can also contribute to lack of fusion. These types of imperfection are more likely to happen when welding in the vertical position.

Joint preparationToo narrow a joint preparation often causes the arc to be attracted to one of the side walls causing lack of side wall fusion on the other side of the joint or inadequate penetration into the previously deposited weld bead. Too great an arc length may also increase the risk of preferential melting along one side of the joint and cause shallow penetration. In addition, a narrow joint preparation may prevent adequate access into the joint. For example, this happens in MMA welding when using a large diameter electrode, or in MIG welding where an allowance should be made for the size of the nozzle.

Welding parametersIt is important to use a sufficiently high current for the arc to penetrate into the joint sidewall. Consequently, too high a welding speed for the welding current will increase the risk of these imperfections. However, too high a current or too low a welding speed will cause weld pool flooding ahead of the arc resulting in poor or non-uniform penetration.

Welder techniquePoor welder technique such as incorrect angle or manipulation of the electrode/welding gun, will prevent adequate fusion of the joint sidewall. Weaving, especially dwelling at the joint sidewall, will enable the weld pool to wash into the parent metal, greatly improving sidewall fusion. It should be noted that the amount of weaving may be restricted by the welding procedure specification limiting the arc energy input, particularly when welding alloy or high notch toughness steels.

Magnetic arc blowWhen welding ferromagnetic steels lack of fusion imperfections can be caused through uncontrolled deflection of the arc, usually termed arc blow. Arc deflection can be caused by distortion of the magnetic field produced by the arc current (Fig. 3), through:

residual magnetism in the material through using magnets for handling earth's magnetic field, for example in pipeline welding position of the current return

The effect of welding past the current return cable which is bolted to the centre of the place is shown in Fig. 4. The interaction of the magnetic field surrounding the arc and that generated by the current flow in the plate to the current return cable is sufficient to deflect the weld bead. Distortion of the arc current magnetic field can be minimised by positioning the current return so that welding is always towards or away from the clamp and, in MMA welding, by using AC instead of DC. Often the only effective means is to demagnetise the steel before welding.

Fig. 3. Interaction of magnetic forces causing arc deflection

Fig. 4. Weld bead deflection in DC MMA welding caused by welding past the current return connection


Best practice in preventionThe following fabrication techniques can be used to prevent formation of lack of sidewall fusion imperfections:

use a sufficiently wide joint preparation select welding parameters (high current level, short arc length, not too high a welding speed) to

promote penetration into the joint side wall without causing flooding ensure the electrode/gun angle and manipulation technique will give adequate side wall fusion use weaving and dwell to improve side wall fusion providing there are no heat input restrictions if arc blow occurs, reposition the current return, use AC (in MMA welding) or demagnetise the

steel

Acceptance standardsThe limits for incomplete fusion imperfections in arc welded joints in steel are specified in BS EN 25817 (ISO 5817) for the three quality levels (see Table). These types of imperfection are not permitted for Quality Level B (stringent) and C (intermediate). For Quality level D (moderate) they are only permitted providing they are intermittent and not surface breaking. For arc welded joints in aluminium, long imperfections are not permitted for all three quality levels. However, for quality levels C and D, short imperfections are permitted but the total length of the imperfections is limited depending on the butt weld or the fillet weld throat thickness.

Acceptance limits for specific codes and application standards

Application Code/Standard

Acceptance limit

Steel ISO 5817:1992

Level B and C not permitted.Level D intermittent and not surface breaking.

Aluminium ISO 10042:1992

Levels B, C, D.Long imperfections not permitted.Levels C and D.Short imperfections permitted.

Pressure vessels

BS5500:1997 Not permitted

Storage tanks BS2654:1989 Not permitted

Pipework BS2633:1987 'l' not greater than 15mm(depending on wall thickness)

Line pipe API 1104:1983

'l' not greater than 25mm(less when weld length <300mm)

Detection and remedial actionIf the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub-surface imperfections, detection is by radiography or ultrasonic inspection. Ultrasonic inspection is normally more effective than radiography in detecting lack of inter-run fusion imperfections. Remedial action will normally require their removal by localised gouging, or grinding, followed by re-welding as specified in the agreed procedure.

If lack of fusion is a persistent problem, and is not caused by magnetic arc blow, the welding procedures should be amended or the welders retrained.

This information was prepared by Bill Lucas with help from Gene Mathers.

Copies of other articles in the 'Job knowledge for welders' series can be found under Practical Joining Knowledge or by using the search engine.


Defects/imperfections in welds - porosity

The characteristic features and principal causes of porosity imperfections are described. Best practice guidelines are given so welders can minimise porosity risk during fabrication.

IdentificationPorosity is the presence of cavities in the weld metal caused by the freezing in of gas released from the weld pool as it solidifies. The porosity can take several forms:

distributed surface breaking pores wormhole crater pipes

Cause and prevention

Distributed porosity and surface poresDistributed porosity (Fig. 1) is normally found as fine pores throughout the weld bead. Surface breaking pores (Fig. 2) usually indicate a large amount of distributed porosity

Fig. 1. Uniformly distributed porosity

Fig. 2. Surface breaking pores (T fillet weld in primed plate)

Cause Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal.Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding. As little as 1% air entrainment in the shielding gas will cause distributed porosity and greater than 1.5% results in gross surface breaking pores. Leaks in the gas line, too high a gas flow rate, draughts and excessive turbulence in the weld pool are frequent causes of porosity.Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes or the workpiece surface. Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen.Surface coatings like primer paints and surface treatments such as zinc coatings, may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides (see Fig 2). Special mention should be made of the so-called weldable (low zinc) primers. It should not be necessary to remove the primers but if the primer thickness exceeds the manufacturer's recommendation, porosity is likely to result especially when using welding processes other than MMA.

Prevention


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The gas source should be identified and removed as follows:

Air entrainment - seal any air leak - avoid weld pool turbulence - use filler with adequate level of deoxidants - reduce excessively high gas flow - avoid draughts

Hydrogen - dry the electrode and flux - clean and degrease the workpiece surface

Surface coatings - clean the joint edges immediately before welding - check that the weldable primer is below the recommended maximum thickness

WormholesCharacteristically, wormholes are elongated pores (Fig. 3) which produce a herring bone appearance on the radiograph.

Cause Wormholes are indicative of a large amount of gas being formed which is then trapped in the solidifying weld metal. Excessive gas will be formed from gross surface contamination or very thick paint or primer coatings. Entrapment is more likely in crevices such as the gap beneath the vertical member of a horizontal-vertical, T joint which is fillet welded on both sides.When welding T joints in primed plates it is essential that the coating thickness on the edge of the vertical member is not above the manufacturer's recommended maximum, typically 20µ, through over-spraying.

PreventionEliminating the gas and cavities prevents wormholes.

Gas generation - clean the work piece surfaces - remove any coatings from the joint area - check the primer thickness is below the manufacturer's maximum

Joint geometry - avoid a joint geometry which creates a cavity

Crater pipeA crater pipe forms during the final solidified weld pool and is often associated with some gas porosity.

Cause This imperfection results from shrinkage on weld pool solidification. Consequently, conditions which exaggerate the liquid to solid volume change will promote its formation. Switching off the welding current will result in the rapid solidification of a large weld pool.In TIG welding, autogenous techniques, or stopping the wire before switching off the welding current, will cause crater formation and the pipe imperfection.

PreventionCrater pipe imperfection can be prevented by removing the stop or by welder technique.

Removal of stop - use run-off tag in butt joints - grind out the stop before continuing with the next electrode or depositing the subsequent weld run


Elongated pores or wormholes

Welder technique - progressively reduce the welding current to reduce the weld pool size - add filler (TIG) to compensate for the weld pool shrinkage

Porosity susceptibility of materialsGases likely to cause porosity in the commonly used range of materials are listed in the Table.

Principal gases causing porosity and recommended cleaning methods

Material Gas Cleaning

C Mn steel Hydrogen, Nitrogen and Oxygen

Grind to remove scale coatings

Stainless steel Hydrogen Degrease + wire brush + degrease

Aluminium and alloys

Hydrogen Chemical clean + wire brush + degrease + scrape

Copper and alloys Hydrogen, Nitrogen Degrease + wire brush + degrease

Nickel and alloys Nitrogen Degrease + wire brush + degrease

Detection and remedial action

If the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub surface imperfections, detection is by radiography or ultrasonic inspection. Radiography is normally more effective in detecting and characterising porosity imperfections. However, detection of small pores is difficult especially in thick sections. Remedial action normally needs removal by localised gouging or grinding but if the porosity is widespread, the entire weld should be removed. The joint should be re-prepared and re-welded as specified in the agreed procedure.

Defects/imperfections in welds - slag inclusions

Prevention of slag inclusions by grinding between runs

The characteristic features and principal causes of slag imperfections are described.

Identification

Slag is normally seen as elongated lines either continuous or discontinuous along


Fig. 1. Radiograph of a butt weld showing two slag lines in the weld root

the length of the weld. This is readily identified in a radiograph, Fig 1. Slag inclusions are usually associated with the flux processes, ie MMA, FCA and submerged arc, but they can also occur in MIG welding.

CausesAs slag is the residue of the flux coating, it is principally a deoxidation product from the reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. When the next layer is deposited, the entrapped slag is not melted out. Slag may also become entrapped in cavities in multi-pass welds through excessive undercut in the weld toe or the uneven surface profile of the preceding weld runs, Fig 2. As they both have an effect on the ease of slag removal, the risk of slag imperfections is influenced by

Type of flux Welder technique

The type and configuration of the joint, welding position and access restrictions all have an influence on the risk of slag imperfections.

Fig. 2. The influence of welder technique on the risk of slag inclusions when welding with a basic MMA (7018) electrode

a) Poor (convex) weld bead profile resulted in pockets of slag being trapped between the weld runs

b) Smooth weld bead profile allows the slag to be readily removed between runs

Type of fluxOne of the main functions of the flux coating in welding is to produce a slag which will flow freely over the surface of the weld pool to protect it from oxidation. As the slag affects the handling characteristics of the MMA electrode, its surface tension and freezing rate can be equally important properties. For welding in the flat and horizontal/vertical positions, a relatively viscous slag is preferred as it will produce a smooth weld bead profile, is less likely to be trapped and, on solidifying, is normally more easily removed. For vertical welding, the slag must be more fluid to flow out to the weld pool surface but have a higher surface tension to provide support to the weld pool and be fast freezing. The composition of the flux coating also plays an important role in the risk of slag inclusions through its effect on the weld bead shape and the ease with which the slag can be removed. A weld pool with low oxygen content will have a high surface tension producing a convex weld bead with poor parent metal wetting. Thus, an oxidising flux, containing for example iron oxide, produces a low surface tension weld


pool with a more concave weld bead profile, and promotes wetting into the parent metal. High silicate flux produces a glass-like slag, often self detaching. Fluxes with a lime content produce an adherent slag which is difficult to remove.

The ease of slag removal for the principal flux types are: Rutile or acid fluxes - large amounts of titanium oxide (rutile) with some silicates. The oxygen

level of the weld pool is high enough to give flat or slightly convex weld bead. The fluidity of the slag is determined by the calcium fluoride content. Fluoride-free coatings designed for welding in the flat position produce smooth bead profiles and an easily removed slag. The more fluid fluoride slag designed for positional welding is less easily removed.

Basic fluxes - the high proportion of calcium carbonate (limestone) and calcium fluoride (fluospar) in the flux reduces the oxygen content of the weld pool and therefore its surface tension. The slag is more fluid than that produced with the rutile coating. Fast freezing also assists welding in the vertical and overhead positions but the slag coating is more difficult to remove.

Consequently, the risk of slag inclusions is significantly greater with basic fluxes due to the inherent convex weld bead profile and the difficulty in removing the slag from the weld toes especially in multi-pass welds.

Welder techniqueWelding technique has an important role to play in preventing slag inclusions. Electrode manipulation should ensure adequate shape and degree of overlap of the weld beads to avoid forming pockets which can trap the slag. Thus, the correct size of electrode for the joint preparation, the correct angle to the workpiece for good penetration and a smooth weld bead profile are all essential to prevent slag entrainment. In multi-pass vertical welding, especially with basic electrodes, care must be taken to fuse out any remaining minor slag pockets and minimise undercut. When using a weave, a slight dwell at the extreme edges of the weave will assist sidewall fusion and produce a flatter weld bead profile.Too high a current together with a high welding speed will also cause sidewall undercutting which makes slag removal difficult.It is crucial to remove all slag before depositing the next run. This can be done between runs by grinding, light chipping or wire brushing. Cleaning tools must be identified for different materials eg steels or stainless steels, and segregated.When welding with difficult electrodes, in narrow vee butt joints or when the slag is trapped through undercutting, it may be necessary to grind the surface of the weld between layers to ensure complete slag removal.

Best practiceThe following techniques can be used to prevent slag inclusions:

Use welding techniques to produce smooth weld beads and adequate inter-run fusion to avoid forming pockets to trap the slag

Use the correct current and travel speed to avoid undercutting the sidewall which will make the slag difficult to remove

Remove slag between runs paying particular attention to removing any slag trapped in crevices Use grinding when welding difficult butt joints otherwise wire brushing or light chipping may be

sufficient to remove the slag.

Acceptance standardsSlag and flux inclusions are linear defects but because they do not have sharp edges compared with cracks, they may be permitted by specific standards and codes. The limits in steel are specified in BE EN 25817 (ISO 5817) for the three quality levels. Long slag imperfections are not permitted in both butt and fillet welds for Quality Level B (stringent) and C (moderate). For Quality Level D, butt welds can have imperfections providing their size is less than half the nominal weld thickness. Short slag related imperfections are permitted in all three quality levels with limits placed on their size relative to the butt weld thickness or nominal fillet weld throat thickness. Article prepared by Bill Lucas with help from Gene Mathers and Colin Eileens.


Defects - solidification cracking

Weld repair on a cast iron exhaust manifold

A crack may be defined as a local discontinuity produced by a fracture which can arise from the stresses generated on cooling or acting on the structure. It is the most serious type of imperfection found in a weld and should be removed. Cracks not only reduce the strength of the weld through the reduction in the cross section thickness but also can readily propagate through stress concentration at the tip, especially under impact loading or during service at low temperature.

Identification

Visual appearanceSolidification cracks are normally readily distinguished from other types of cracks due to the following characteristic factors:

they occur only in the weld metal they normally appear as straight lines along the centreline of the weld bead, as shown in Fig. 1,

but may occasionally appear as transverse cracking depending on the solidification structure solidification cracks in the final crater may have a branching appearance as the cracks are 'open', they are easily visible with the naked eye

Fig. 1 Solidification crack along the centre line of the weld

On breaking open the weld, the crack surface in steel and nickel alloys may have a blue oxidised appearance, showing that they were formed while the weld metal was still hot.

MetallographyThe cracks form at the solidification boundaries and are characteristically inter dendritic. The morphology reflects the weld solidification structure and there may be evidence of segregation associated with the solidification boundary.


CausesThe overriding cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include:

insufficient weld bead size or shape welding under high restraint material properties such as a high impurity content or a relatively large amount of shrinkage on

solidification. Joint design can have a significant influence on the level of residual stresses. Large gaps between component parts will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Therefore, weld beads with a small depth-to-width ratio, such as formed in bridging a large gap with a wide, thin bead, will be more susceptible to solidification cracking, as shown in Fig. 2. In this case, the centre of the weld which is the last part to solidify, is a narrow zone with negligible cracking resistance.

Fig. 2 Weld bead penetration too small

Segregation of impurities to the centre of the weld also encourages cracking. Concentration of impurities ahead of the solidifying front weld forms a liquid film of low freezing point which, on solidification, produces a weak zone. As solidification proceeds, the zone is likely to crack as the stresses through normal thermal contraction build up. An elliptically shaped weld pool is preferable to a tear drop shape. Welding with contaminants such as cutting oils on the surface of the parent metal will also increase the build up of impurities in the weld pool and the risk of cracking.As the compositions of the plate and the filler determine the weld metal composition they will, therefore, have a substantial influence on the susceptibility of the material to cracking.

SteelsCracking is associated with impurities, particularly sulphur and phosphorus, and is promoted by carbon whereas manganese and silicon can help to reduce the risk. To minimise the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As a general rule, for carbon-manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%. Weld metal composition is dominated by the consumable and as the filler is normally cleaner than the metal being welded, cracking is less likely with low dilution processes such as MMA and MIG. Plate composition assumes greater importance in high dilution situations such as when welding the root in butt welds, using an autogenous welding technique like TIG, or a high dilution process such as submerged arc welding.In submerged arc welds, as described in BS 5135 (Appendix F), the cracking risk may be assessed by calculating the Units of Crack Susceptibility (UCS) from the weld metal chemical composition (weight %):UCS = 230C* + 190S + 75P + 45Nb - 12.3Si - 5.4Mn - 1C* = carbon content or 0.08 whichever is higher Although arbitrary units, a value of <10 indicates high cracking resistance whereas >30 indicates a low resistance. Within this range, the risk will be higher in a weld run with a high depth to width ratio, made at high welding speeds or where the fit-up is poor. For fillet welds, runs having a depth to width ratio of about one, UCS values of 20 and above will indicate a risk of cracking. For a butt weld, values of about


25 UCS are critical. If the depth to width ratio is decreased from 1 to 0.8, the allowable UCS is increased by about nine. However, very low depth to width ratios, such as obtained when penetration into the root is not achieved, also promote cracking.

AluminiumThe high thermal expansion (approximately twice that of steel) and substantial contraction on solidification (typically 5% more than in an equivalent steel weld) means that aluminium alloys are more prone to cracking. The risk can be reduced by using a crack resistant filler (usually from the 4xxx and 5xxx series alloys) but the disadvantage is that the resulting weld metal is likely to have non-matching properties such as a lower strength than the parent metal.

Austenitic Stainless SteelA fully austenitic stainless steel weld is more prone to cracking than one containing between 5-10% of ferrite. The beneficial effect of ferrite has been attributed to its capacity to dissolve harmful impurities which would otherwise form low melting point segregates and consequently interdendritic cracks. Therefore the choice of filler material is important to suppress cracking so a type 308 filler is used to weld type 304 stainless steel.

Best practice in avoiding solidification crackingApart from the choice of material and filler, the principal techniques for minimising the risk of welding solidification cracking are:

Control joint fit-up to reduce gaps. Before welding, clean off all contaminants from the material Ensure that the welding sequence will not lead to a build-up of thermally induced stresses. Select welding parameters and technique to produce a weld bead with an adequate depth to

width ratio, or with sufficient throat thickness (fillet weld), to ensure the weld bead has sufficient resistance to the solidification stresses (recommend a depth to width ratio of at least 0.5:1).

Avoid producing too large a depth to width ratio which will encourage segregation and excessive transverse strains in restrained joints. As a general rule, weld beads whose depth to weld ratio exceeds 2:1 will be prone to solidification cracking.

Avoid high welding speeds (at high current levels) which increase the amount of segregation and the stress level across the weld bead.

At the run stop, ensure adequate filling of the crater to avoid an unfavourable concave shape.

Acceptance standardsAs solidification cracks are linear imperfections with sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817). Crater cracks are permitted for quality level D.

Detection and remedial actionSurface breaking solidification cracks can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic or radiographic examination techniques. Most codes will specify that all cracks should be removed. A cracked component should be repaired by removing the cracks with a safety margin of approximately 5mm beyond the visible ends of the crack. The excavation is then re-welded using a filler which will not produce a crack sensitive deposit.

Defects - hydrogen cracks in steels - identification


Preheating to avoid hydrogen cracking

Hydrogen cracking may also be called cold cracking or delayed cracking. The principal distinguishing feature of this type of crack is that it occurs in ferritic steels, most often immediately on welding or after a short time after welding.In this issue, the characteristic features and principal causes of hydrogen cracks are described.Identification

Visual appearanceHydrogen cracks can be usually be distinguished due to the following characteristics:

In C-Mn steels, the crack will normally originate in the heat affected zone (HAZ) but may extend into the weld metal (Fig 1).

Cracks can also occur in the weld bead, normally transverse to the welding direction at an angle of 45° to the weld surface. They are essentially straight, follow a jagged path but may be non-branching.

In low alloy steels, the cracks can be transverse to the weld, perpendicular to the weld surface, but are non-branching and essentially planar.

Fig. 1 Hydrogen cracks originating in the HAZ (note, the type of cracks shown would not be expected to form in the same weldment)

On breaking open the weld (prior to any heat treatment), the surface of the cracks will normally not be oxidised, even if they are surface breaking, indicating they were formed when the weld was at or near ambient temperature. A slight blue tinge may be seen from the effects of preheating or welding heat.

MetallographyCracks which originate in the HAZ are usually associated with the coarse grain region, (Fig 2). The cracks can be inter granular, trans granular or a mixture. Inter granular cracks are more likely to occur in the harder HAZ structures formed in low alloy and high carbon steels. Trans granular cracking is more often found in C-Mn steel structures.


In fillet welds, cracks in the HAZ are usually associated with the weld root and parallel to the weld. In butt welds, the HAZ cracks are normally oriented parallel to the weld bead.

Fig. 2 Crack along the coarse grain structure in the HAZ

CausesThere are three factors which combine to cause cracking:

hydrogen generated by the welding process a hard brittle structure which is susceptible to cracking residual tensile stresses acting on the welded joint

Cracking is caused by the diffusion of hydrogen to the highly stressed, hardened part of the weldment.In C-Mn steels, because there is a greater risk of forming a brittle microstructure in the HAZ, most of the hydrogen cracks are to be found in the parent metal. With the correct choice of electrodes, the weld metal will have a lower carbon content than the parent metal and, hence, a lower carbon equivalent (CE). However, transverse weld metal cracks can occur especially when welding thick section components.In low alloy steels, as the weld metal structure is more susceptible than the HAZ, cracking may be found in the weld bead.The effects of specific factors on the risk of cracking are::

weld metal hydrogen parent material composition parent material thickness stresses acting on the weld heat input

Weld metal hydrogen contentThe principal source of hydrogen is the moisture contained in the flux ie the coating of MMA electrodes, the flux in cored wires and the flux used in submerged arc welding. The amount of hydrogen generated is determined mainly by the electrode type. Basic electrodes normally generate less hydrogen than rutile and cellulosic electrodes. It is important to note that there can be other significant sources of hydrogen eg moisture from the atmosphere or from the material where processing or service history has left the steel with a significant level of hydrogen. Hydrogen may also be derived from the surface of the material or the consumable.Sources of hydrogen will include:

oil, grease and dirt rust paint and coatings cleaning fluids

Parent metal compositionThis will have a major influence on harden ability and, with high cooling rates, the risk of forming a hard brittle structure in the HAZ. The harden ability of a material is usually expressed in terms of its carbon content or, when other elements are taken into account, its carbon equivalent (CE) value.


The higher the CE value, the greater the risk of hydrogen cracking. Generally, steels with a CE value of <0.4 are not susceptible to HAZ hydrogen cracking as long as low hydrogen welding consumables or processes are used. Parent material thicknessMaterial thickness will influence the cooling rate and therefore the hardness level, microstructure produced in the HAZ and the level of hydrogen retained in the weld. The 'combined thickness' of the joint, ie the sum of the thicknesses of material meeting at the joint line, will determine, together with the joint geometry, the cooling rate of the HAZ and its hardness. Consequently, as shown in Fig. 3, a fillet weld will have a greater risk than a butt weld in the same material thickness.

Fig.3 Combined thickness measurements for butt and fillet joints

Stresses acting on the weldThe stresses generated across the welded joint as it contracts will be greatly influenced by external restraint, material thickness, joint geometry and fit-up. Areas of stress concentration are more likely to initiate a crack at the toe and root of the weld. Poor fit-up in fillet welds markedly increases the risk of cracking. The degree of restraint acting on a joint will generally increase as welding progresses due to the increase in stiffness of the fabrication.

Heat inputThe heat input to the material from the welding process, together with the material thickness and preheat temperature, will determine the thermal cycle and the resulting microstructure and hardness of both the HAZ and weld metal. A high heat input will reduce the hardness level.Heat input per unit length is calculated by multiplying the arc energy by an arc efficiency factor according to the following formula:

V = arc voltage (V)A = welding current (A)S = welding speed (mm/min)k = thermal efficiency factorIn calculating heat input, the arc efficiency must be taken into consideration. The arc efficiency factors given in BS EN 1011-1: 1998 for the principal arc welding processes, are:

Submerged arc(single wire)

1.0

MMA 0.8MIG/MAG and flux cored 0.8


wireTIG and plasma 0.6In MMA welding, heat input is normally controlled by means of the run-out length from each electrode which is proportional to the heat input. As the run-out length is the length of weld deposited from one electrode, it will depend upon the welding technique eg weave width /dwell.Bill Lucas prepared this article with help from Gene Mathers and Dave Abson.

Defects - hydrogen cracks in steels - prevention and best practice

Preheating of a jacket structure to prevent hydrogen cracking

In this issue, techniques and practical guidance on the avoidance of hydrogen cracks are described.

Preheating, interpass and post heating to prevent hydrogen crackingThere are three factors which combine to cause cracking in arc welding:

hydrogen generated by the welding process a hard brittle structure which is susceptible to cracking residual tensile stresses acting on the welded joint

In practice, for a given situation (material composition, material thickness, joint type, electrode composition and heat input), the risk of hydrogen cracking is reduced by heating the joint.

PreheatPreheat, which slows the cooling rate, allows some hydrogen to diffuse away and prevents a hard, crack-sensitive structure being formed. The recommended levels of preheat for carbon and carbon manganese steel are detailed in BS 5135. (Nb a draft European standard Pr EN 1011-2 is expected to be introduced in 2000). The preheat level may be as high as 200°C for example, when welding thick section steels with a high carbon equivalent (CE) value.

Interpass and post heatingAs cracking rarely occurs at temperatures above ambient, maintaining the temperature of the weldment during fabrication is equally important. For susceptible steels, it is usually appropriate to maintain the preheat temperature for a given period, typically between 2 to 3 hours, to enable the hydrogen to diffuse away from the weld area. In crack sensitive situations such as welding higher CE steels or under high restraint conditions, the temperature and heating period should be increased, typically 250-300°C for three to four hours. Post weld heat treatment (PWHT) may be used immediately on completion of welding ie without allowing the preheat temperature to fall. However, in practice, as inspection can only be carried out at ambient temperature, there is the risk that 'rejectable,' defects will only be found after PWHT. Also, for highly hardenable steels, a second heat treatment may be required to temper the hard microstructure present after the first PWHT.Under certain conditions, more stringent procedures are needed to avoid cracking than those derived from the nomograms for estimating preheat in BS 5135. Appendix E of this standard mentions the following conditions:

a. high restraintb. thick sections ( approximately 50mm)c. low carbon equivalent steels (CMn steels with C 0.1% and CE approximately 0.42)


d. 'clean' or low sulphur steels (S approximately 0.008%), as a low sulphur and low oxygen content will increase the hardenability of a steel.

e. alloyed weld metal where preheat levels to avoid HAZ cracking may be insufficient to protect the weld metal. Low hydrogen processes and consumables should be used. Schemes for predicting the preheat requirements to avoid weld metal cracking generally require the weld metal diffusible hydrogen level and the weld metal tensile strength as input.

Use of austenitic and nickel alloy weld metal to prevent crackingIn situations where preheating is impractical, or does not prevent cracking, it will be necessary to use an austenitic consumable. Austenitic stainless steel and nickel electrodes will produce a weld metal which at ambient temperature, has a higher solubility for hydrogen than ferritic steel. Thus, any hydrogen formed during welding becomes locked in the weld metal with very little diffusing to the HAZ on cooling to ambient. A commonly used austenitic MMA electrode is 23Cr:12Ni (eg from BS 2926:1984). However, as nickel alloys have a lower coefficient of thermal expansion than stainless steel, nickel austenitic electrodes are preferred when welding highly restrained joints to reduce the shrinkage strain. Figure 1 is a general guide on the levels of preheat when using austenitic electrodes. When welding steels with up to 0.2%C, a preheat would not normally be required. However, above 0.4%C a minimum temperature of 150°C will be needed to prevent HAZ cracking. The influence of hydrogen level and the degree of restraint are also illustrated in the figure.

Fig.1 Guide to preheat temperature when using austenitic MMA electrodes at 1-2kJ/mma) low restraint (e.g. material thickness <30mm)b) high restraint (e.g. material thickness >30mm)

Best practice in avoiding hydrogen cracking

Reduction in weld metal hydrogenThe most effective means of avoiding hydrogen cracking is to reduce the amount of hydrogen generated by the consumable, ie by using a low hydrogen process or low hydrogen electrodes. Welding processes can be classified as very low, low, medium or high depending on the amount of weld metal hydrogen produced:

Very low

<5ml/100g

Low 5 - 10ml/100g

Medium 10 - 15ml/100g

High >15ml/100g

Figure 2 illustrates the relative amounts of weld metal hydrogen produced by the major welding processes. MMA, in particular, has the potential to generate a wide range of hydrogen levels. Thus, to achieve the lower values, it is essential that basic electrodes are used and they are baked in accordance with the


manufacturer's recommendations. For the MIG process, cleaner wires will be required to achieve very low hydrogen levels.

Fig.2 General relationships between potential hydrogen and weld metal hydrogen levels for arc welding processes

General guidelines

The following general guidelines are recommended for the various types of steel but requirements for specific steels should be checked according to BS 5135 or BS EN 1011: Mild steel (CE <0.4)

- readily weldable, preheat generally not required if low hydrogen processes or electrodes are used - preheat may be required when welding thick section material, high restraint and with higher levels of hydrogen being generated

C-Mn, medium carbon, low alloy steels (CE 0.4 to 0.5) - thin sections can be welded without preheat but thicker sections will require low preheat levels and low hydrogen processes or electrodes should be used

Higher carbon and alloyed steels (CE >0.5) - preheat, low hydrogen processes or electrodes, post weld heating and slow cooling required.

More detailed guidance on the avoidance of hydrogen cracking is described in BS 5135. Practical TechniquesThe following practical techniques are recommended to avoid hydrogen cracking:

clean the joint faces and remove contaminants such as paint, cutting oils, grease use a low hydrogen process if possible dry the electrodes (MMA) or the flux (submerged arc) in accordance with the manufacturer's

recommendations reduce stresses on the weld by avoiding large root gaps and high restraint if preheating is specified in the welding procedure, it should also be applied when tacking or

using temporary attachments preheat the joint to a distance of at least 75mm from the joint line ensuring uniform heating

through the thickness of the material measure the preheat temperature on the face opposite that being heated. Where this is

impractical, allow time for the equalisation of temperature after removing the preheating before the temperature is measured

adhere to the heat input requirements


maintain heat for approximately two to four hours after welding depending on crack sensitivity In situations where adequate preheating is impracticable, or cracking cannot be avoided,

austenitic electrodes may be used Acceptance standardsAs hydrogen cracks are linear imperfections which have sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817).

Detection and remedial actionAs hydrogen cracks are often very fine and may be sub-surface, they can be difficult to detect. Surface-breaking hydrogen cracks can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic or radiographic examination techniques. Ultrasonic examination is preferred as radiography is restricted to detecting relatively wide cracks parallel to the beam. Most codes will specify that all cracks should be removed. A cracked component should be repaired by removing the cracks with a safety margin of approximately 5mm beyond the visible ends of the crack. The excavation is then re-welded.To make sure that cracking does not re-occur, welding should be carried out with the correct procedure, ie preheat and an adequate heat input level for the material type and thickness. However, as the level of restraint will be greater and the interpass time shorter when welding within an excavation compared to welding the original joint, it is recommended that a higher level of preheat is used (typically by 50°C).

ReferencesBS 5135:1984 Arc Welding of Carbon and Carbon Manganese Steels Pr EN 1011-1:1998 Welding - Recommendations for Welding of Metallic MaterialsPart 1- General Guidance for Arc WeldingPart 2- Arc Welding of Ferritic SteelsBS EN ISO 13916: 1997 Welding - Guidance on the Measurement of Preheating Temperature, Interpass Temperature and Preheat Maintenance TemperatureN Bailey et al, Welding steels without hydrogen cracking, Woodhead Publishing, 1993


Defects - lamellar tearing

BP Forties platform lamellar tears were produced when attempting the repair of lack of root penetration in a brace weld

Lamellar tearing can occur beneath the weld especially in rolled steel plate which has poor through-thickness ductility. The characteristic features, principal causes and best practice in minimising the risk of lamellar tearing are described.

Identification

Visual appearanceThe principal distinguishing feature of lamellar tearing is that it occurs in T-butt and fillet welds normally observed in the parent metal parallel to the weld fusion boundary and the plate surface , (Fig 1). The cracks can appear at the toe or root of the weld but are always associated with points of high stress concentration.

Fracture faceThe surface of the fracture is fibrous and 'woody' with long parallel sections which are indicative of low parent metal ductility in the through-thickness direction, (Fig 2).

Fig. 1. Lamellar tearing in T butt weld


Fig. 2. Appearance of fracture face of lamellar tear

MetallographyAs lamellar tearing is associated with a high concentration of elongated inclusions oriented parallel to the surface of the plate, tearing will be transgranular with a stepped appearance.

CausesIt is generally recognised that there are three conditions which must be satisfied for lamellar tearing to occur:

1. Transverse strain - the shrinkage strains on welding must act in the short direction of the plate ie through the plate thickness

2. Weld orientation - the fusion boundary will be roughly parallel to the plane of the inclusions3. Material susceptibility - the plate must have poor ductility in the through-thickness direction

Thus, the risk of lamellar tearing will be greater if the stresses generated on welding act in the through-thickness direction. The risk will also increase the higher the level of weld metal hydrogen

Factors to be considered to reduce the risk of tearingThe choice of material, joint design, welding process, consumables, preheating and buttering can all help reduce the risk of tearing.

MaterialTearing is only encountered in rolled steel plate and not forgings and castings. There is no one grade of steel that is more prone to lamellar tearing but steels with a low Short Transverse Reduction in Area (STRA) will be susceptible. As a general rule, steels with STRA over 20% are essentially resistant to tearing whereas steels with below 10 to 15% STRA should only be used in lightly restrained joints (Fig. 3). Steels with a higher strength have a greater risk especially when the thickness is greater than 25mm. Aluminium treated steels with low sulphur contents (<0.005%) will have a low risk.Steel suppliers can provide plate which has been through-thickness tested with a guaranteed STRA value of over 20%.


Fig. 3. Relationship between the STRA and sulphur content for 12.5 to 50mm thick plate

Joint DesignLamellar tearing occurs in joints producing high through-thickness strain, eg T joints or corner joints. In T or cruciform joints, full penetration butt welds will be particularly susceptible. The cruciform structures in which the susceptible plate cannot bend during welding will also greatly increase the risk of tearing. In butt joints, as the stresses on welding do not act through the thickness of the plate, there is little risk of lamellar tearing.As angular distortion can increase the strain in the weld root and or toe, tearing may also occur in thick section joints where the bending restraint is high.Several examples of good practice in the design of welded joints are illustrated in Fig. 4.

As tearing is more likely to occur in full penetration T butt joints, if possible, use two fillet welds, Fig. 4a.

Double-sided welds are less susceptible than large single-sided welds and balanced welding to reduce the stresses will further reduce the risk of tearing especially in the root, Fig. 4b

Large single-side fillet welds should be replaced with smaller double-sided fillet welds, Fig. 4c Redesigning the joint configuration so that the fusion boundary is more normal to the

susceptible plate surface will be particularly effective in reducing the risk, Fig. 4d

Fig. 4 Recommended joint configurations to reduce the risk of lamellar tearing

Fig. 4a

Fig. 4b

Fig. 4c

Fig. 4d


Weld sizeLamellar tearing is more likely to occur in large welds typically when the leg length in fillet and T butt joints is greater than 20mm. As restraint will contribute to the problem, thinner section plate which is less susceptible to tearing, may still be at risk in high restraint situations.

Welding processAs the material and joint design are the primary causes of tearing, the choice of welding process has only a relatively small influence on the risk. However, higher heat input processes which generate lower stresses through the larger HAZ and deeper weld penetration can be beneficial. As weld metal hydrogen will increase the risk of tearing, a low hydrogen process should be used when welding susceptible steels.

ConsumableWhere possible, the choice of a lower strength consumable can often reduce the risk by accommodating more of the strain in the weld metal. A smaller diameter electrode which can be used to produce a smaller leg length, has been used to prevent tearing. A low hydrogen consumable will reduce the risk by reducing the level of weld metal diffusible hydrogen. The consumables must be dried in accordance with the manufacturer's recommendations.

PreheatingPreheating will have a beneficial effect in reducing the level of weld metal diffusible hydrogen. However, it should be noted that in a restrained joint, excessive preheating could have a detrimental effect by increasing the level the level of restraint produced by the contraction across the weld on cooling. Preheating should, therefore, be used to reduce the hydrogen level but it should be applied so that it will not increase the amount of contraction across the weld.

ButteringButtering the surface of the susceptible plate with a low strength weld metal has been widely employed. As shown for the example of a T butt weld (Fig. 5) the surface of the plate may be grooved so that the buttered layer will extend 15 to 25mm beyond each weld toe and be about 5 to 10mm thick.

Fig. 5. Buttering with low strength weld metal

a) general deposit on the surface of the susceptible plate

b) in-situ buttering


In-situ buttering ie where the low strength weld metal is deposited first on the susceptible plate before filling the joint, has also been successfully applied. However, before adopting this technique, design calculations should be carried out to ensure that the overall weld strength will be acceptable.Acceptance standardsAs lamellar tears are linear imperfections which have sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817).

Detection and remedial actionIf surface-breaking, lamellar tears can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic examination techniques but there may be problems in distinguishing lamellar tears from inclusion bands. The orientation of the tears normally makes them almost impossible to detect by radiography.

Defects/imperfections in welds - reheat cracking

Brittle fracture in CrMoV steel pressure vessel probably caused through poor toughness, high residual stresses and hydrogen cracking

The characteristic features and principal causes of reheat cracking are described. General guidelines on 'best practice' are given so that welders can minimise the risk of reheat cracking in welded fabrications.

Identification

Visual appearanceReheat cracking may occur in low alloy steels containing alloying additions of chromium, vanadium and molybdenum when the welded component is being subjected to post weld heat treatment, such as stress relief heat treatment, or has been subjected to high temperature service (typically 350 to 550°C). Cracking is almost exclusively found in the coarse grained regions of the heat affected zone (HAZ) beneath the weld, or cladding, and in the coarse grained regions within the weld metal. The cracks can often be seen visually, usually associated with areas of stress concentration such as the weld toe.Cracking may be in the form of coarse macro-cracks or colonies of micro-cracks.A macro-crack will appear as a 'rough' crack, often with branching, following the coarse grain region, (Fig. 1a). Cracking is always inter granular along the prior austenite grain boundaries (Fig. 1b). Macro-cracks in the weld metal can be oriented either longitudinal or transverse to the direction of welding. Cracks in the HAZ, however, are always parallel to the direction of welding.


Fig.1a. Cracking associated with the coarse grained heat affected zone

Fig.1b. Intergranular morphology of reheat cracks

Micro-cracking can also be found both in the HAZ and within the weld metal. Micro-cracks in multipass welds will be found associated with the grain coarsened regions which have not been refined by subsequent passes.

CausesThe principal cause is that when heat treating susceptible steels, the grain interior becomes strengthened by carbide precipitation forcing the relaxation of residual stresses by creep deformation at the grain boundaries. The presence of impurities which segregate to the grain boundaries and promote temper embrittlement eg sulphur, arsenic, tin and phosphorus, will increase the susceptibility to reheat cracking.The joint design can increase the risk of cracking. For example, joints likely to contain stress concentration, such as partial penetration welds, are more liable to initiate cracks.The welding procedure also has an influence. Large weld beads are undesirable as they produce a coarse grained HAZ which is less likely to be refined by the subsequent pass and therefore will be more susceptible to reheat cracking.

Best practice in preventionThe risk of reheat cracking can be reduced through the choice of steel, specifying the maximum impurity level and by adopting a more tolerant welding procedure / technique.

Steel choiceIf possible, avoid welding steels known to be susceptible to reheat cracking. For example, A 508 Class 2 is known to be particularly susceptible to reheat cracking whereas cracking associated with welding and cladding in A508 Class 3 is largely unknown. The two steels have similar mechanical properties but A508 Class 3 has a lower Cr content and a higher manganese content. Similarly, in the higher strength, creep resistant steels, an approximate ranking of their crack susceptibility is as follows:

5 Cr 1Mo lower risk

2.25Cr 1 Mo


0.5Mo B

0.5Cr 0.5Mo 0.25V

higher risk

Thus, in selecting a creep resistant, chromium molybdenum steel, 0.5Cr 0.5Mo 0.25V steel is known to be susceptible to reheat cracking but the 2.25Cr 1Mo which has a similar creep resistance, is significantly less susceptible.Unfortunately, although some knowledge has been gained on the susceptibility of certain steels, the risk of cracking cannot be reliably predicted from the chemical composition. Various indices, including

G1, PSR and Rs, have been used to indicate the susceptibility of steel to reheat cracking. Steels which

have a value of G of less than 2, PSR less than zero or Rs less than 0.03, are less susceptible to reheat cracking

G1 = 10C + Cr + 3.3Mo + 8.1V - 2

PSR = Cr +Cu + 2Mo + 10V +7Nb + 5Ti - 2

Rs = 0.12Cu +0.19S +0.10As + P +1.18Sn + 1.49Sb

Impurity levelIrrespective of the steel type, it is important to purchase steels specified to have low levels of trace elements (antimony, arsenic, tin and phosphorus). It is generally accepted that the total level of impurities in the steel should not exceed 0.01% to minimise the risk of temper embrittlement. Welding procedure and techniqueThe welding procedure can be used to minimise the risk of reheat cracking by

Producing the maximum refinement of the coarse grain HAZ Limiting the degree of austenite grain growth Eliminating stress concentrations

The procedure should aim to refine the coarse grained HAZ by subsequent passes. In butt welds, maximum refinement can be achieved by using a steep sided joint preparation with a low angle of attack to minimise penetration into the sidewall, (Fig 2a). In comparison, a larger angle V preparation produces a wider HAZ limiting the amount of refinement achieved by subsequent passes, (Fig 2b). Narrow joint preparations, however, are more difficult to weld due to the increased risk of lack of sidewall fusion.

Fig.2a. Welding in the flat position - high degree of HAZ refinement


Fig.2b. Welding in the horizontal/vertical position - low degree of HAZ refinement

Refinement of the HAZ can be promoted by first buttering the surface of the susceptible plate with a thin weld metal layer using a small diameter (3.2mm) electrode. The joint is then completed using a larger diameter (4 - 4.8mm) electrode which is intended to generate sufficient heat to refine any remaining coarse grained HAZ under the buttered layer.The degree of austenite grain growth can be restricted by using a low heat input. However, precautionary measures may be necessary to avoid the risk of hydrogen assisted cracking and lack-of-fusion defects. For example, reducing the heat input will almost certainly require a higher preheat temperature to avoid hydrogen assisted cracking.The joint design and welding technique adopted should ensure that the weld is free from localised stress concentrations which can arise from the presence of notches. Stress concentrations may be produced in the following situations:

welding with a backing bar a partial penetration weld leaving a root imperfection internal weld imperfections such as lack of sidewall fusion the weld has a poor surface profile, especially sharp weld toes

The weld toes of the capping pass are particularly vulnerable as the coarse grained HAZ may not have been refined by subsequent passes. In susceptible steel, the last pass should never be deposited on the parent material but always on the weld metal so that it will refine the HAZ. Grinding the weld toes with the preheat maintained has been successfully used to reduce the risk of cracking in 0.5Cr 0.5Mo 0.25V steels.

Residual Stress


Magnitude Of Stresses- A Simple Analogy



Strain Age Embrittlement

This phenomenon applies to carbon and low alloy steel. It involves ferrite forming a compound with nitrogen; iron-nitride (Fe4N). Temperatures around 250°C, will cause a fine precipitation of this


compound to occur. It will tend to pin any dislocations in the structure that have been created by cold work or plastic deformation.

Strain ageing increases tensile strength but significantly reduces ductility and toughness.

Modern steels tend to have low nitrogen content, but this is not necessarily true for welds. Sufficient Nitrogen, approximately 1 to 2 ppm, can be easily picked up from the atmosphere during welding.

Weld root runs are particularly at risk because of high contraction stresses causing plastic deformation. This is why impact test specimens taken from the root or first pass of a weld can give poor results.

Additions of Aluminium can tie up the Nitrogen as Aluminium Nitride, but weld-cooling rates are too fast for this compound to form successfully. Stress relief at around 650 degrees C will resolve the problem.

HOW TO AVOID PWHT

The above picture is of a new pressure vessel that failed during its hydraulic test. The vessel had been stress relieved, but some parts of it did not reach the required temperature and consequently did not experience adequate tempering. This coupled with a small hydrogen crack, was sufficient to cause catastrophic failure under test conditions. It is therefore important when considering PWHT or its avoidance, to ensure that all possible failure modes and their consequences are carefully considered before any action is taken.

The post weld heat treatment of welded steel fabrications is normally carried out to reduce the risk of brittle fracture by: -

Reducing residual Stresses. These stresses are created when a weld cools and its contraction is restricted by the bulk of the material surrounding it. Weld distortion occurs when these stresses exceed the yield point. Finite element modelling of residual stresses is now possible, so that the complete welding sequence of a joint or repair can be modelled to predict and minimise these stresses.

Tempering the weld and HAZ microstructure. The microstructure, particularly in the HAZ, can be hardened by rapid cooling of the weld. This is a major problem for low and medium alloy steels containing chrome and any other constituent that slow the austenite/ferrite transformation down, as this will result in hardening of the micro structure, even at slow cooling rates.

The risk of brittle fracture can be assessed by fracture mechanics. Assuming worst-case scenarios for all the relevant variables. It is then possible to predict if PWHT is required to make the fabrication


safe. However, the analysis requires accurate measurement of HAZ toughness, which is not easy because of the HAZ’s small size and varying properties. Some approximation is possible from impact tests, providing the notch is taken from the point of lowest toughness.

If PWHT is to be avoided, stress concentration effects such as: - backing bars, partial penetration welds, and internal defects in the weld and poor surface profile, should be avoided. Good surface and volumetric NDT is essential. Preheat may still be required to avoid hydrogen cracking and a post weld hydrogen release may also be beneficial in this respect (holding the fabrication at a temperature of around 250C for at least 2 hours, immediately after welding).

Nickel based consumables can often reduce or remove the need for preheat, but their effect on the parent metal HAZ will be no different from that created by any other consumable, except that the HAZ may be slightly narrower. However, nickel based welds, like most austenitic steels, can make ultrasonic inspection very difficult.

Further reduction in the risk of brittle fracture can be achieved by refining the HAZ microstructure using special temper bead welding techniques.

Austenitic stainless steelsAustenitic stainless steels have high ductility, low yield stress and relatively high ultimate tensile strength, when compare to a typical carbon steel. A carbon steel on cooling transforms from Austenite to a mixture of ferrite and cementite. With austenitic stainless steel, the high chrome and nickel content suppress this transformation keeping the material fully austenite on cooling (The Nickel maintains the austenite phase on cooling and the Chrome slows the transformation down so that a fully austenitic structure can be achieved with only 8% Nickel). Heat treatment and the thermal cycle caused by welding, have little influence on mechanical properties. However strength and hardness can be increased by cold working, which will also reduce ductility. A full solution anneal (heating to around 1045°C followed by quenching or rapid cooling) will restore the material to its original condition, removing alloy segregation, sensitisation, sigma phase and restoring ductility after cold working. Unfortunately the rapid cooling will re-introduce residual stresses, which could be as high as the yield point. Distortion can also occur if the object is not properly supported during the annealing process. Austenitic steels are not susceptible to hydrogen cracking, therefore pre-heating is seldom required, except to reduce the risk of shrinkage stresses in thick sections. Post weld heat treatment is seldom required as this material as a high resistance to brittle fracture; occasionally stress relief is carried out to reduce the risk of stress corrosion cracking, however this is likely to cause sensitisation unless a stabilised grade is used (limited stress relief can be achieved with a low temperature of around 450°C ). Austenitic steels have a F.C.C atomic structure which provides more planes for the flow of dislocations, combined with the low level of interstitial elements (elements that lock the dislocation chain), gives this material its good ductility. This also explains why this material has no clearly defined yield point, which is why its yield stress is always expressed as a proof stress. Austenitic steels have excellent toughness down to true absolute (-273°C), with no steep ductile to brittle transition. This material has good corrosion resistance, but quite severe corrosion can occur in certain environments. The right choice of welding consumable and welding technique can be crucial as the weld metal can corrode more than the parent material. Probably the biggest cause of failure in pressure plant made of stainless steel is stress corrosion cracking (S.C.C). This type of corrosion forms deep cracks in the material and is caused by the presence of chlorides in the process fluid or heating water/steam (Good water treatment is essential ), at a temperature above 50°C, when the material is subjected to a tensile stress (this stress includes residual stress, which could be up to yield point in magnitude). Significant increases in Nickel and also Molybdenum will reduce the risk. Stainless steel has a very thin and stable oxide film rich in chrome. This film reforms rapidly by reaction with the atmosphere if damaged. If stainless steel is not adequately protected from the atmosphere during welding or is subject to very heavy grinding operations, a very thick oxide layer will form. This thick oxide layer, distinguished by its blue tint, will have a chrome depleted layer under it, which will impair corrosion resistance. Both the oxide film and depleted layer must be removed, either mechanically (grinding with a fine grit is recommended, wire brushing and shot blasting will have less


effect), or chemically (acid pickle with a mixture of nitric and hydrofluoric acid). Once cleaned, the surface can be chemically passivated to enhance corrosion resistance, (passivation reduces the anodic reaction involved in the corrosion process). Carbon steel tools, also supports or even sparks from grinding carbon steel, can embed fragments into the surface of the stainless steel. These fragments can then rust if moistened. Therefore it is recommended that stainless steel fabrication be carried out in a separate designated area and special stainless steel tools used where possible. If any part of stainless-steel is heated in the range 500 degrees to 800 degrees for any reasonable time there is a risk that the chrome will form chrome carbides (a compound formed with carbon) with any carbon present in the steel. This reduces the chrome available to provide the passive film and leads to preferential corrosion, which can be severe. This is often referred to as sensitisation. Therefore it is advisable when welding stainless steel to use low heat input and restrict the maximum interpass temperature to around 175°, although sensitisation of modern low carbon grades is unlikely unless heated for prolonged periods. Small quantities of either titanium (321) or niobium (347) added to stabilise the material will inhibit the formation of chrome carbides.

To resist oxidation and creep high carbon grades such as 304H or 316H are often used. Their improved creep resistance relates to the presence of carbides and the slightly coarser grain size associated with higher annealing temperatures. Because the higher carbon content inevitably leads to sensitisation, there may be a risk of corrosion during plant shut downs, for this reason stabilised grades may be preferred such as 347H. The solidification strength of austenitic stainless steel can be seriously impaired by small additions of impurities such as sulphur and phosphorous, this coupled with the materials high coefficient of expansion can cause serious solidification cracking problems. Most 304 type alloys are designed to solidify initially as delta ferrite, which has a high solubility for sulphur, transforming to austenite upon further cooling. This creates an austenitic material containing tiny patches of residual delta ferrite, therefore not a true austenitic in the strict sense of the word. Filler metal often contains further additions of delta ferrite to ensure crack free welds. The delta ferrite can transform to a very brittle phase called sigma, if heated above 550°C for very prolonged periods (Could take several thousand hours, depending on chrome level. A duplex stainless steel can form sigma phase after only a few minutes at this temperature) The very high coefficient of expansion associated with this material means that welding distortion can be quite savage. I have seen thick ring flanges on pressure vessel twist after welding to such an extent that a fluid seal is impossible. Thermal stress is another major problem associated with stainless steel; premature failure can occur on pressure plant heated by a jacket or coils attached to a cold veesel. This material has poor thermal conductivity, therefore lower welding current is required (typically 25% less than carbon steel) and narrower joint preparations can be tolerated. All common welding processes can be used successfully, however high deposition rates associated with SAW could cause solidification cracking and possibly sensitisation, unless adequate precautions are taken.


To ensure good corrosion resistance of the weld root it must be protected from the atmosphere by an inert gas shield during welding and subsequent cooling. The gas shield should be contained around the root of the weld by a suitable dam, which must permit a continuous gas flow through the area. Welding should not commence until sufficient time has elapsed to allow the volume of purging gas flowing through the dam to equal at least the 6 times the volume contained in the dam (EN1011 Part 3 Recommends 10). Once purging is complete the purge flow rate should be reduced so that it only exerts a small positive pressure, sufficient to exclude air. If good corrosion resistance of the root is required the oxygen level in the dam should not exceed 0.1%(1000 ppm); for extreme corrosion resistance this should be reduced to 0.015% (150 ppm). Backing gasses are typically argon or helium; Nitrogen Is often used as an economic alternative where corrosion resistance is not critical, Nitrogrn + 10% Helium is better. A wide variety of proprietary pastes and backing materials are available than can be use to protect the root instead of a gas shield. In some applications where corrosion and oxide coking of the weld root is not important, such as large stainless steel ducting, no gas backing is used.

Carbon content:

304 L grade Low Carbon, typically 0.03% Max 304 grade Medium Carbon, typically 0.08% Max 304H grade High Carbon, typically Up to 0.1% The higher the carbon content the greater the yield strength. (Hence the stength advantage in using stabilised grades) Typical Alloy Content 304 316 316 Ti 320 321 347 308 309

(18-20Cr, 8-12Ni) (16-18Cr, 10-14Ni + 2-3Mo) (316 with Titanium Added) (Same as 316Ti) (17-19Cr, 9-12Ni + Titanium) (17-19Cr, 9-13Ni + Niobium) (19-22Cr, 9-11Ni) (22-24Cr, 12-15Ni)

304 + Molybdenum 304 + Moly + Titanium - 304 + Titanium 304 + Niobium 304 + Extra 2%Cr 304 + Extra 4%Cr + 4% Ni

All the above stainless steel grades are basic variations of a 304. All are readily weldable and all have matching consumables, except for a 304 which is welded with a 308 or 316, 321 is welded with a 347 (Titanium is not easily transferred across the arc) and a 316Ti is normally welded with a 318. Molybdenum has the same effect on the microstructure as chrome, except that it gives better resistance to pitting corrosion. Therefore a 316 needs less chrome than a 304. 310 (24-26Cr,19-22Ni) True Austenitic. This material does not transform to ferrite on

cooling and therefore does not contain delta ferrite. It will not suffer sigma phase embrittlement but can be tricky to weld.

904L (20Cr,25Ni,4.5Mo) Super Austenitic Or Nickel alloy. Superior corrosion resistance providing they are welded carefully with low heat input (less than 1 kJ/mm recommended) and fast travel speeds with no weaving. Each run of weld should not be started until the metal temperature falls below 100°C. It is unlikely that a uniform distribution of alloy will be achieved throughout the weld (segregation), therefore this material should either be welded with an over-alloyed consumable such as a 625 or solution annealed after welding, if maximum corrosion resistance is required.

Carbon Steel To Austenitic SteelWhen a weld is made using a filler wire or consumable, there is a mixture in the weld consisting of approximately 20% parent metal and 80% filler metal alloy ( percentage depends on welding process, type of joint and welding parameters). Any reduction in alloy content of 304 / 316 type austenitics is likely to cause the formation of matensite on cooling. This could lead to cracking problems and poor ductility. To avoid this problem an overalloyed filler metal is used, such as a 309, which should still form austenite on cooling providing dilution is not excessive. The Shaeffler diagram can be used to determine the type of microstructure that can be expected when a filler metal and parent metal of differing compositions are mixed together in a weld.


The Shaeffler Diagram

The Nickel and other elements that form Austenite, are plotted against Chrome and other elements that form ferrite, using the following formula:-

Nickel Equivalent = %Ni + 30%C + 0.5%Mn

Chrome Equivalent = %Cr + Mo + 1.5%Si + 0.5%Nb

Example, a typical 304L = 18.2%Cr, 10.1%Ni, 1.2%Mn, 0.4%Si, 0.02%C

Ni Equiv = 10.1 + 30 x 0.02 + 0.5 x 1.2 = 11.3 Cr Equiv = 18.2 + 0 + 1.5 x 0.4 + 0 = 18.8

A typical 309L welding consumable Ni Equiv = 14.35, Cr Equiv = 24.9

The main disadvantage with this diagram is that it does not represent Nitrogen, which is a very strong Austenite former.


Ferrite NumberThe ferrite number uses magnetic attraction as a means of measuring the proportion of delta ferrite present. The ferrite number is plotted on a modified Shaeffler diagram, the Delong Diagram. The Chrome and Nickel equivalent is the same as that used for the Shaeffler diagram, except that the Nickel equivalent includes the addition of 30 times the Nitrogen content.

Examples


The Shaeffler diagram above illustrates a carbon steel C.S , welded with 304L filler. Point A represents the anticipated composition of the weld metal, if it consists of a mixture of filler metal and 25% parent metal. This diluted weld, according to the diagram, will contain martensite. This problem can be overcome if a higher alloyed filler is used, such as a 309L, which has a higher nickel and chrome equivalent that will tend to pull point A into the austenite region. If the welds molten pool spans two different metals the process becomes more complicated. First plot both parent metals on the shaeffler diagram and connect them with a line. If both parent metals are diluted by the same amount, plot a false point B on the diagram midway between them. (Point B represents the microstructure of the weld if no filler metal was applied.)

Next, plot the consumable on the diagram, which for this example is a 309L. Draw a line from this point to false point B and mark a point A along its length equivalent to the total weld dilution. This point will give the approximate microstructure of the weld metal. The diagram below illustrates 25% total weld dilution at point A, which predicts a good microstructure of Austenite with a little ferrite.


The presence of martensite can be detected by subjecting a macro section to a hardness survey, high hardness levels indicate martensite. Alternatively the weld can be subjected to a bend test ( a side bend is required by the ASME code for corrosion resistant overlays), any martensite present will tend to cause the test piece to break rather than bend.

However the presence of martensite is unlikely to cause hydrogen cracking, as any hydrogen evolved during the welding process will be absorbed by the austenitic filler metal.

Evaluating Dilution


Causes Of High Dilution High Travel Speed. Too much heat applied to parent metal instead of on filler metal. High welding Current. High current welding processes, such as Submerged Arc Welding can

cause high dilution. Thin Material. Thin sheet TIG welded can give rise to high dilution levels. Joint Preparation. Square preps generate very high dilution. This can be reduced by carefully

buttering the joint face with high alloy filler metal.

Weldability of Austenitic Steels

The steels of type ASTM 304, 316, 304L, and 316L have very good weldability. The old problem of inter granular corrosion after welding is very seldom encountered today. The steels suitable for wet corrosion either have carbon contents below 0.05% or are niobium or titanium stabilised.


They are also very unsusceptible to hot cracking, mainly because they solidify with a high ferrite content. The higher-alloy steels such as 310S and N08904 solidify with a fully austenitic structure when welded. They should therefore be welded using a controlled heat input. Steel and weld metal with high chromium and molybdenum contents may undergo precipitation of brittle sigma phase in their microstructure if they are exposed to high temperatures for a certain length of time. The transformation from ferrite to sigma or directly from austenite to sigma proceeds most rapidly within the temperature range 750-850 °C. Welding with a high heat input leads to slow cooling, especially in light-gauge welds. The weld's holding time between 750-850 °C then increases, and along with it the risk of sigma phase formation.

Weldability of Ferritic Steels

These steels are generally more difficult to weld than austenitic steels. This is the main reason they are not used to the same extent as austenitic steels.

The older types, such as AISI 430, had greatly reduced ductility in the weld. This was mainly due to strong grain growth in the heat-affected zone (HAZ), but also due to precipitation of martensite in the HAZ. They were also susceptible to inter granular corrosion after welding. These steels are therefore often welded with preheating and post-weld annealing. Today's ferritic steels of type S44400 and S44635 have considerably better weldability due to low carbon and nitrogen contents and stabilisation with titanium/niobium. However, there is always a risk of unfavourable grain enlargement if they are not welded under controlled conditions using a low heat input. They do not normally have to be annealed after welding.

Weldability of Ferritic-Austenitic Steels

Today's ferritic-austenitic steels have considerably better weldability than earlier grades. They can be welded more or less as common austenitic steels.

were also susceptible to ferrite grain growth in the heat affected zone (HAZ) and poor ferrite to austenite transformation, resulting in reduced ductility. Today's steels, which have higher nickel content and are alloyed with nitrogen, exhibit austenite transformation in the HAZ that is sufficient in most cases. However, extremely rapid cooling after welding, for example in a tack or in a strike mark, can lead to unfavourably high ferrite content. Extremely high heat input can also lead to besides being susceptible to inter granular corrosion, the old steels heavy ferrite grain growth in the HAZ.When welding UNS S31803 (AvestaPolarit 2205) in a conventional way (0.6-2.0 kJ/mm) and using filler metals at the same time, a satisfactory ferrite-austenite balance can be obtained. For the super duplex stainless steel (AvestaPolarit SAF 2507) a somewhat different heat input is recommended (0.2-1.5 kJ/mm). The reason for lowering the minimum value is that this steel has much higher nitrogen content than 2205. The nitrogen favours a fast reformation of austenite, which is important when welding with a low heat input. The maximum level is lowered in order to minimize the risk of secondary phases.The steels are welded with ferritic-austenitic or austenitic filler metals. Welding without filler metal is not recommended without subsequent quench annealing. Nitrogen affects not only the microstructure, but also the weld pool penetration. Increased nitrogen content reduces the penetration into the parent metal. To avoid porosity in TIG welding it is recommended to produce thin beads. To achieve the highest possible pitting corrosion resistance at the root side in ordinary 2205 weld metals, the root gas should be Ar+ N2 or Ar+ N2 + H2. The use of H2 in the shielding gas is not recommended when welding super duplex steels. When welding 2205 with plasma, a shielding gas containing Ar+5% H2 is sometimes used in combination with filler metal and followed by quench annealing.


A typical macroscopic image of a weld bead. These steels are welded with matching or austenitic super-alloyed filler metal

(such as AvestaPolarit P 5).

Weldability of Martensitic Steels

The quantity of martensite and its hardness are the main causes of the weldability problems encountered with these steels. The fully martensitic steels are air hardening. The steels are therefore very susceptible to hydrogen embrittlement.

By welding at an elevated temperature (= the steel's Ms temperature), the HAZ can be kept austenitic and tough throughout the welding process. After cooling, the formed martensite must always be tempered at about 650-850 °C, preferably as a concluding heat treatment. However, the weld must first have been allowed to cool to below about 150 °C.

Martensitic-austenitic steels, such as 13Cr/6Ni and 16Cr/5Ni/2Mo, can often be welded without preheating and without post-weld annealing. Steels of the 13Cr/4Ni type with low austenite content must, however, be preheated to a working temperature of about 100 °C.

If optimal strength properties are desired, they can be heat treated at 600 °C after welding. The steels are welded with matching or austenitic filler metals.

Standards - Approval of welding procedures, welders and welding operators

For a given application, the main way of ensuring adequate weld quality is to specify the procedure and the skill level of the welding operator. Here, the alternative routes for welding procedure approval are described together with the requirements for welder or welding operator approval.

Routes to welding procedure approvalThe key document is the Welding Procedure Specification (WPS) which details the welding variables to be used to ensure a welded joint will

achieve the specified levels of weld quality and mechanical

The WPS is supported by a number of documents (eg a record of how the weld was made, NDE, mechanical test results) which together comprise a welding approval record termed the WPAR (EN288) or PQR (ASME).In both the European and ASME standards, there are a number of 'essential variables' specified which, if changed, may affect either weld quality or mechanical properties. Therefore, a change in any of the essentials will invalidate the welding procedure and will require a new approval test to be carried out. The essential variables are detailed in the relevant specification but include material type, welding process, thickness range and sometimes welding position.

The route followed to produce a WPS in EN 288 and the responsibilities of the manufacturer and the Examiner/Examining Body are shown in Fig. 1.The most common method of gaining approval is to carry out an approval test as described in EN 288 Pt3 (steels) and Pt4 (aluminium and its alloys). The manufacturer initially drafts a preliminary welding procedure (pWPS) which is used by one of the manufacturer's competent welders to prove that it is capable


AC TIG welding of aluminium cryogenic pressure vesselCourtesy of Air Products PLC

Fig. 1. Stages in welding and welder approval

of producing a welded joint to the specified levels of weld quality and mechanical properties. The welding procedure approval record (WPAR) is a record of this weld. If the WPAR is approved by the Examiner, it is used to finalise one or more WPSs which is the basis for the Work Instructions given to the welder.It is noteworthy that the welder carrying out a satisfactory welding procedure approval test is approved for the appropriate range of approval given in the relevant standard (EN 287, ASME IX or AWS D1.1).

EN 288 also permits the following options for procedure approval:

Welding procedure test Approved welding consumable Previous welding experience Standard welding procedure Pre-production welding test The conventional procedure test (as specified in Parts 3 or 4) does not always need to be carried out to gain approval. But alternative methods have certain limits of application regarding, for example, welding processes, materials and consumables as specified in the appropriate application standard or contract agreement.The welding procedure test method of approval is often a mandatory requirement of the Application Standard. If not, the contracting parties can agree to use one of the alternative methods. For example, a welding procedure specification can be approved in accordance with the requirements of Part 6 (previous experience) on condition that the manufacturer can prove, with appropriate documentation, that the type of joint has previously been welded satisfactorily.The American standard, ASME IX requires a welding procedure test (PQR) but AWS D1.1 will allow the use of pre-qualified procedures within the limits detailed in the specification.Welder approvalThe welder approval test is carried out to demonstrate that the welder has the necessary skill to produce a satisfactory weld under the conditions used in production as detailed in the approved WPS or Work Instruction. As a general rule, the test piece approves the welder not only for the conditions used in the test but also for all joints which are considered easier to weld. As the welder's approval test is carried out on a test piece which is representative of the joint to be welded, it is independent of the type of construction. The precise conditions, called 'essential variables', must be specified in the approval test eg material type, welding process, joint type, dimensions and welding position. The extent of approval is not necessarily restricted to the conditions used for the test but covers a group of similar materials or a range of situations which are considered easier to weld.It is important to note that a number of Amendments and Corrigenda have now been issued which affect the range of approval (see list of Relevant Standards).In EN 287, the certificate of approval testing is issued under the sole responsibility of the Examiner / Examining Body. The welder approval certificate remains valid subject to the requirements of the application standard. In EN 287, it can be extended at six monthly intervals by the employer for up to two years provided the welder has been successfully welding similar joints. After two years, prolongation of the welder's qualification will need approval of the Examiner who will require proof that his or her performance has been of the required standard during the period of validity. As the Examiner will normally examine the company's records on the welder's work and tests as proof that he has maintained his skill, it is essential that work records are maintained by the company.It should also be noted that EN 287 requires records of tests ie half yearly documentation about X-ray or ultrasonic inspections or test reports on fracture tests must be maintained with the welder's approval certificate (tests on production welds will satisfy this requirement). Failure to comply will necessitate a retest.American standards have similar requirements although the extent of approval of the welding variables are different to those of EN 287.

Welding operator approval

When required by the contract or application standard, the welding operators responsible for setting up and/or adjustment of fully mechanised and automatic equipment must be approved but the personnel operating the equipment do not need approval. In clarifying the term 'welding operator', personnel who are using the equipment (loading and unloading robotic equipment or operating a resistance welding machine) do not require approval.

As specified in EN 1418, approval of operators of equipment for fusion welding and resistance weld equipment setters can be based on: welding a procedure test pre-production welding test or production test production sample testing or a function test.


It should be noted that the methods must be supplemented by a functional test appropriate to the welding unit. However, a test of knowledge relating to welding technology which is the equivalent of 'Job knowledge for welders' in EN 287 is recommended but not mandatory.Prolongation of the welding operator approval is generally in accordance with the requirements of EN 287. The welding operator's approval remains valid for two years providing the employer/welding co-ordinator confirms that there has been a reasonable continuity of welding work (period of interruption no longer than six months) and there is no reason to question the welding operator's knowledge.The validity of approval may be prolonged for further periods of two years by the examiner / examining body providing there is proof of production welds of the required quality, and appropriate test records maintained with the operator's certificate.

When working to ASME IX, operators for both mechanised and automatic welding equipment require approval. The essential variables are different to those in welder approval.

Relevant StandardsEN 287: Part 1. Steels (Amendment 9665, August 1997) (Amendment 9804, January 1998) (Corrigenda No 1, April 1998) Part 2.Aluminium and alloys(Amendment No 9733, November 1997)(Corrigenda No 1 June, 1998)EN 288: Part 3. Steels(Amendment No 9736, November 1997)(Corrigenda No 1, June 1998)EN 1418 : 1998 Welding personnel - Approval testing of welding operators for fusion welding and resistance weld setters for fully mechanised and automatic welding of metallic materials

Introduction to Corrosion Failures

The identification of the factors associated with the forms of corrosion can guide failure investigators. A listing of the most important factors would ensure that engineers with little or no corrosion training are made aware of the complexity and multitude of variables involved. Inexperienced investigators would be reminded of critical variables that may otherwise be overlooked.


Corrosion FormsRecommendations for designing sound corrosion control into components and systems require that a special care is given to the forms or types of corrosion that can occur. However, one has to realize that one type of corrosion can lead into a series of worsening reactions where other types of corrosion attack can become the dominating factors. A simple galvanic situation aggravated by the presence of an electrolyte can cause devastating effects as was observed in the supporting saddles of the Statue of Liberty during its restoration. Consider this simple example taken from the light chamber in the tallest lighthouse ever built in Canada. The different forms of corrosion represent corrosion phenomena categorized according to their appearance:

Group 1 - readily identifiable by ordinary visual examination

Uniform corrosion Pitting Crevice corrosion Galvanic corrosion

Group 2 - may require supplementary means of examination

Erosion corrosion Cavitation Fretting corrosion Intergranular corrosion Exfoliation Dealloying (selective leaching)

Group 3 - verification is usually required by microscopy (optical, electron microscopy etc.)

Environmental cracking Stress Corrosion Cracking (SCC) Corrosion fatigue Hydrogen embrittlement


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Uniform Corrosion

Uniform corrosion is characterized by corrosive attack proceeding evenly over the entire surface area, or a large fraction of the total area. General thinning takes place until failure. On the basis of However, uniform corrosion is relatively easily measured and predicted, making disastrous failures relatively rare. In many cases, it is objectionable only from an appearance standpoint. As corrosion occurs uniformly over the entire surface of the metal component, it can be practically controlled by cathodic protection, use of coatings or paints, or simply by specifying a corrosion allowance. In other cases uniform corrosion adds color and appeal to to a surface. Two classics in this respect are the patina created by naturally tarnishing copper roofs and the rust hues produced on weathering steels.tonnage wasted, this is the most important form of corrosion. The breakdown of protective coating systems on structures often leads to this form of corrosion. Dulling of a bright or polished surface, etching by acid cleaners, or oxidation (discoloration) of steel are examples of surface corrosion. Corrosion resistant alloys and stainless steels can become tarnished or oxidized in corrosive environments. Surface corrosion can indicate a breakdown in the protective coating system, however, and should be examined closely for more advanced attack. If surface corrosion is permitted to continue, the surface may become rough and surface corrosion can lead to more serious types of corrosion. An example of uniform corrosion damage on a rocket assisted artillery projectile is shown here.

Artistic rust formed on a weathering steel sculpture

Weathering steel has the unique characteristic that as it corrodes under proper conditions, it forms a dense and tightly adherent oxide barrier that seals out the atmosphere and retards further corrosion. This is in contrast to other steels that, as they corrode, they form a coarse, porous and flaky oxide that allows the atmosphere to continue penetrating the steel. Although cleaning and handling of the material can affect the short term appearance of the product, the overall corrosion resistance and ultimate appearance of weathering steel is not affected by cleanliness.

The early rust forms in discrete crystallites that are fine, red and diffusely reflecting, like hematite. The massive recrystallized layer is a shiny blue, approaching the blue-black of specular hematite. Thus portions of weathering steel that have seen different amounts of wetting and drying will have different degrees of recrystallized oxide and will have different appearances. Most weathering steel sculptures in most environments provide surfaces that see varied amounts of wetting and drying. Runoff of water from upper portions of a sculpture tend to produce long-lasting streaks or other patterns of redder oxide on lower portions. Similarly, in wetter climates the overall color of weathering steel sculptures will have generally have an overall redder cast relative to those exposed in drier climates.

The appearance of weathering steel can also be affected by other factors. During recrystallization the rust will trap particulate matter on the surface. If this material is colored it will contribute to the appearance of the rust. For example, in dirty industrial atmospheres the rust on weathering steel can be almost black due to the incorporation of airborne dirt. Chemical cleaning treatments such as acids can convert the hydrated iron oxide to other iron compounds of different color or appearance. In atmospheres with significant content of sulfur oxides deposits of white to yellow ferrous sulfate may appear in the rust on weathering steel.


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In some climates organic growth such as moss may be present and affect the appearance of the rust. Discolored areas on a weathering steel sculpture could be due to any of the variety of factors described above, or excessive corrosion. The rust layer on weathering steel in many climates does not consume a significant amount of steel in its formation, so removal in most cases should not affect the strength of the work. However, in some cases of inappropriate design crevices or pockets will trap water and the continual presence of water leads to excessive corrosion evidenced by rust flaking or observable metal loss.

These should be sealed or coated to provide protection, and may need reinforcement if there has been significant steel loss. In the case of discoloration of rust due to other causes, if the rust were to be removed without a change in some factor in the environment the rust would eventually return to the original discolored appearance. (reference)

Causes of CorrosionA point of view proposed by Professor Staehle is that all engineering materials are reactive chemically and that the strength of materials depends totally upon the extent to which environments influence the reactivity and subsequent degradation of these materials. In order to define the strength of an engineering material for a corrosion based design it is essential to define the nature of the environments affecting the material over time.

Material factor

Bulk chemical composition Micro Grain boundary composition structure Surface condition

Environment Factor

Nominal environment definition Type Chemistry concentration phase conductivity Local environment definition Velocity thin layer wetting wetting and drying cycles heat transfer boiling wear and fretting deposits

Stress Factor

Stress definition mean stress maximum stress minimum stress constant load/constant strain strain rate plane stress/plane strain modes I, II, II biaxial cyclic frequency wave shape Sources of stress Intentional Residual corrosion wedging


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thermal cycling

Geometry Factor

Discontinuities which intensify stress Generation of galvanic potentials Chemical crevices Settling of solids Restricted geometries leading to concentration cells

Temperature FactorTime Factor

Changes in GB chemistry Changes in microstructure Changes in surface deposits, chemistry or/and heat transfer resistance Development of surface defects, pitting or/and erosion Development of occluded cells

Corrosion Information

Humans are used to working with imprecise information. They naturally accept vague use of language, making continuous interpretations of the information they receive based upon context. This section introduces a generic corrosion framework linking mechanistic principles leading to corrosion damage with the observable signs of a corrosion attack.

Sixteen recognized corrosion experts accepted to complete an opinion poll listing the main sub-factors and the common forms of corrosion. The responses were then analyzed.

The most important factors leading to different forms of corrosion failures according to an expert survey

An obvious application of these results would be to help optimize the number of fields required for the development of an efficient knowledge based system.


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Welder Performance Qualification (WPQ)

Materials The purpose of this test is to determine the welders ability to deposit a sound weld therefore the base material is not considered as critical as it is in the PQR. Hence a performance test on any material in P groups 1 to 11 approves all those groups and sub groups, also P34 and P4X (P40-P49). Providing a compatible consumable exists with the same F number used in the qualification test. (QW423.1) Note a single sided weld is classed as a weld without backing and a double sided weld or weld with sealing run is classed as a weld with backing

Consumables The F number cannot be changed without re qualification of the welder except that for performance qualification only using SMAW (MMA) F numbers up to and including 4 approve all lesser F numbers for double sided or welds with backing only. One Consumable from F41 To F45 approves any of these consumables, except SAW. Ref. QW404.11. Note 'A' numbers do not apply to welder approval tests.

Variables For each welding process there is a list of essential variables in QW352 to QW357 and QW360 for welding operators, these are not necessarily the same as the ones for the PQR. Essential variables cannot be changed. Explanations of all these variables is given in section IV of the code.

Diameter and Thickness Ranges Diameter limits for all circular welds including groove welds, branch welds and fillet welds is given in QW452.3. there are no upper limits on diameters approved and pipe covers plate Note for branch welds the diameter considered for the above limits is the one containing the weld preparation. Thickness limits, groove welds. The thickness limit only applies to the deposited weld metal thickness not the plate thickness and any groove weld approves all fillet weld sizes. For t greater than 12.5mm there is no restriction on the size that can be welded (Providing the test weld deposit contains at least 3 layers of weld). Thickness limits, fillet welds. A test on plate greater than 3/16" approves all base metal thicknesses and fillet weld sizes ref. QW452.5. (Note the above diameter limits apply unless the fillet weld is qualified by a groove weld)

Joint Configuration Joint geometry, a double V (or U) is considered the same as a joint with backing and does not qualify a single V (or U) without backing, but a single full penetration joint without backing qualifies all joint configurations.

Approval Range Extent of approval is very well explained in QW461.9. Take particular note of welding positions which are also explained in QW461, for example to qualify a fillet weld in the normal horizontal-vertical position with a groove weld, the groove weld must be qualified in at least the 2G position. The welding positions defined in QW461.1.& QW461.2 should be referred to in the WPS. The position designations: 1G ,2G ,3G ,4G ,5G ,6G (Groove Welds) and 1F ,2F ,3F ,4F (Fillet Welds) are test positions

Period of Validity/Renewal of Qualifications (QW 322.2) Providing the welder uses the process for which he is qualified and there is no reason to question his ability then his qualification lasts indefinitely. If the welder does not use the welding process for which he is qualified for a period of 6 months or more then he must perform a new test in pipe or plate, any parent material, thickness and position, if successful all the welder approvals for that welding process are renewed in one test.

Testing Requirements Test requirements for groove welds QW452 consists of either:-

One face bend and one root bend except for welding positions 5G & 6G which require 4 bends (Ref QW452.1 Note 4). If the plate exceeds 3/8" side bends may be used. See QW 466 for precise details and exceptions.

Note:- Bend Tests can in most cases be replaced by Radiography {See Below}.


Radiography is optional and must be supplemented by bend tests when using GMAW (MIG/MAG) with dip transfer (Short Circuiting Arc) or when welding some special materials. Ref. QW304. Note:- Ultrasonic Examination in lieu of Radiography is not permitted

Test requirements for fillet welds in plate ref. QW452.5:-

One macro section (QW 184) and One fracture test (QW182). The location where each specimen has to be taken is defined in QW463

Radiography Ref QW 191 A length of at least 6" must be examined for plate or the entire circumference for pipe. If the pipe circumference is less than 6" then more samples must be welded up to a maximum of 4. Ref QW 302.2.

Visual Examination Ref QW 302.2 & QW 190 Performance test coupons must show complete joint penetration with full fusion of the weld metal and base metal. The welder performance test must follow a properly qualified W.P.S. Once qualified the welder must always work within the extent of approval of any properly qualified W.P.S. and his W.P.Q. The welder who qualifies the P.Q.R. is automatically approved within the limits specified in QW304, QW305 and QW303. Ref QW301.2.

Specialist Processes Such as corrosion resistant overlay or hard facing are covered in QW 453. Procedure variables are defined with all procedure variables in QW252 and in QW380 for welder approval. Min base thickness approved = size welded or 1", QW 453 Min Deposit Size Approved:- Point Where Chemical analysis taken No upper limit QW402.16 (462.5a) Welding Positions QW405.4 Performance Qualification approves all deposit thickness’ No min.QW381

Ch Welding services

Welding is the process of permanently joining two or more metal parts, by melting both materials. The molten materials quickly cool, and the two metals are permanently bonded. Spot welding and seam welding are two very popular methods used for sheet metal parts.Advantage Fabricated Metals' welding staff has decades of complex welding experience. Our skilled welding staff provides a full range of welding services. We can provide welding services ranging from structural and plate welding to light gauge tubular steels, stainless, and aluminum.We jig and fixture components to be welded to ensure parts are correctly aligned before being joined providing quality and repeatability.

We provide:

TIG (Tungsten Inert Gas) MIG (Metal Inert Gas) Stick, and Oxy-Acetylene

services plus weld & cut services.

Every welding job's requirements are different, but Advantage Fabricated Metals’ welders often:

grind the welds polish the welds fill in voids descale the welds prepare the welded surfaces for painting, and prime the prepared surface

after the welding process is completed.We will complete your custom metal fabricating component welding project in our 80,000 square foot manufacturing facility. Advantage Fabricated Metals has the equipment to do the job right!We carry the latest equipment and have a welding staff with high levels of welding training that is backed by decades of industrial welding experience.Let Advantage Fabricated Metals handle all your custom metal fabricating welding/cutting jobs. Whether your custom components will be used in conveyer lines, racking equipment, stainless and aluminum tables, platforms, mixing tanks, storage containers, tubular frames, truck frames, or whatever custom application, call us at 1-773-650-1390 or fill out our contact form to get help with your industrial welding needs!


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characteristics of the MIG welding process

MIG welding

Advantage Fabricated Metals performs a number of welding processes. The two most common welding processes we use include TIG, an acronym for Tungsten Inert Gas welding and MIG, an acronym for Metal Inert Gas welding. TIG is also referred to as GTAW (Gas Tungsten Arc Welding) and Heliarc®. MIG also is referred to as GMAW (Gas Metal Arc Welding). We also provide oxy-acetylene welding.The "Metal" in Gas Metal Arc Welding refers to the wire that is what is used to start the arc. It is shielded by inert gas and the feeding wire also acts as the filler rod. A semi-automatic process, it is fairly easy to learn and use.

MIG:

Uses a consumable wire electrode during the welding process that is fed from a spool, Provides a uniform weld bead, Produces a slag-free weld bead, Uses a shielding gas, usually – argon, argon - 1 to 5% oxygen, argon - 3 to 25% CO2 and a combination

argon/helium gas, Is considered a semi-automatic welding process, Allows welding in all positions, Requires less operator skill than TIG welding, Allows long welds to be made without starts or stops, Needs little cleanup.

The illustration that follows provides a look at a typical MIG welding process showing an arc that is formed between the wire electrode and the workpiece. During the MIG welding process, the electrode melts within the arc and becomes deposited as filler material. The shielding gas that is used prevents atmospheric contamination from and protects the weld during solidification. The shielding gas, forms the arc plasma, stabilizes the arc on the metal being welded, shields the arc and molten weld pool, and allows smooth transfer of metal from the weld wire to the molten weld pool.

Versatility is the major benefit of the MIG welding process. It is capable of joining most types of metals and it can be performed in most positions, even though flat horizontal is most optimum.Even though it is considered welding’s most versatile process, there are a number of problems associated with MIG welding. These include:

Burnback Metal hardening Reduction in metal fatigue strength Poor appearance Creation of cracks and porosity Reduction of corrosion resistance - in the welding zone Heavily oxidized weld deposit Irregular wire feed Porosity Unstable arc


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The most common welds are illustrated below. They include the:

lap joint butt joint T-joint, and the edge joint

MIG is used to weld many materials, and different gases are uses to form the arc depending on the materials to be welded together. Argon is normally used to weld mild steel, aluminum, titanium, and alloy metals. Helium is used to weld mild steel and titanium in high speed process and also copper and stainless steel. Carbon dioxide is most often used to weld carbon and low allHome > Metal Fabricating Tips & Facts > TIG Welding

TIG weldingAdvantage Fabricated Metals performs a number of welding processes. The two most common welding processes we use include TIG, an acronym for Tungsten Inert Gas welding and MIG, an acronym for Metal Inert Gas welding. TIG is also referred to as GTAW (Gas Tungsten Arc Welding) and Heliarc®. MIG also is referred to as GMAW (Gas Metal Arc Welding). We also provide oxy-acetylene welding.TIG welding is also called GTAW (Gas Tungsten Arc Welding) and Heliarc® welding. Heliarc® was the trade name given to the process by Linde's when it was introduced decades ago. The arc is started with a tungsten electrode shielded by inert gas and filler rod is fed into the weld puddle separately. The gas shielding that is required to protect the molten metal from contamination and amperage are supplied during the TIG welding operation.TIG welding is a slower process than MIG, but it produces a more precise weld and can be used at lower amperages for thinner metal and can even be used on exotic metals. TIG welding is a commonly used high quality welding process. TIG welding has become a popular choice of welding processes when high quality, precision welding is required. The TIG welding process requires more time to learn than MIG. It is similar in technique to gas welding.

Characteristics of the TIG welding process

TIG:

Uses a non-consumable tungsten electrode during the welding process, Uses a number of shielding gases including helium (He), argon (Ar), and carbon dioxide (CO2), May harden the welded materials, May reduce fatigue strength, Is easily applied to thin materials, May leave a poor appearance, May reduce corrosion resistance at the weld, May create cracks and porosity in the materials welded, Produces very high-quality, superior welds, Welds can be made with or without filler metal, Provides precise control of welding variables (i.e. heat), Welding yields low distortion, Leaves no slag or splatter.

In TIG welding, an arc is formed between a non-consumable tungsten electrode and the metal being welded. Gas is fed through the torch to shield the electrode and molten weld pool. If filler wire is used, it is added to the weld pool separately. The illustration that follow provide a schematic showing how the TIG welding processworks.



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The most common TIG welds are illustrated below. They include the:

butt joint, lap joint, and the T-joint

The following illustration shows these TIG-welded joints:

The TIG welding process utilizes a number of shielding gases including:

argon argon + hydrogen argon/helium helium

Argon is superior for welding metals. It operates at a higher arc voltage, makes the arc start more easily, and is commonly used to weld mild steel, aluminum and titanium.Helium is generally added to increase heat input (increase welding speed or weld penetration) and is used for high speed welding of mild steel and titanium. Helium offers a smaller heat affected zone and therefore, penetrates metals deeply. It also can increase the welding speed up to 40%. Helium is also commonly used to weld stainless steel and copper.Hydrogen will result in cleaner looking welds and also increase heat input, however, hydrogen may promote porosity or hydrogen cracking. The argon/helium combination gas is used for a hotter arc in welding aluminum and aluminum alloys. It is also used in automatic welding applications.

Even though TIG is a commonly used welding process, there are a number of limitations. These include:

TIG requires greater welder dexterity than MIG or stick welding, TIG yields lower deposition rates, TIG is more costly for welding thick metal sections.

oy steels. Magnesium and cast iron are other metals commonly welded used the MIG process.

Oxy-acetylene welding


Advantage Fabricated Metals performs a number of welding processes. The two most common welding processes we use include TIG, an acronym for Tungsten Inert Gas welding and MIG, an acronym for Metal Inert Gas welding. TIG is also referred to as GTAW (Gas Tungsten Arc Welding) and Heliarc®. MIG also is referred to as GMAW (Gas Metal Arc Welding). We also provide oxy-acetylene welding.Oxy-acetylene welding is a very common welding process. The use of oxygen and acetylene as welding gases dates back to the 1890’s. The combination of oxygen and acetylene produces a flame temperature over 6000 degrees Fahrenheit making it ideal for welding and cutting.

Characteristics of the oxy-acetylene welding process

Characteristics of the oxy-acetylene welding process include:

The use dual oxygen and acetylene gases stored under pressure in steel cylinders, Its ability to switch quickly to a cutting process, by changing the welding tip to a cutting tip, The high temperature the gas mixture attains, The use of regulators to control gas flow and reduce pressure on both the oxygen and acetylene tanks, The use of double line rubber hoses to conduct the gas from the tanks to the torch, Melting the materials to be welded together, The ability to regulate temperature by adjusting gas flow.

The illustration that follows provides a look at a typical oxy-acetylene welding process. The welding tip is mounted on the end of the torch handle and fuel and gas mixture pass through it to feed the flame. Welding tips have only one hole while cutting tips have a centrally located hole with a number of smaller holes located around it in a circular pattern. During cutting, the oxygen comes from the center hole and the preheat flames come from the holes around the center hole.

In oxy-acetylene welding, the flame produced by the combination of the gases melts the metal faces of the workpieces to be joined, causing them to flow together. A filler metal alloy is normally added and sometimes be used to prevent oxidation and to facilitate the metal union.The molten metal has a tendency to pop and splatter as heat is applied and oxygen reacts with the superheated metal. It is critical that operators using the oxy-acetylene welding or cutting process wear proper gloves and use approved safety goggles or face shield. The goggles and/or face shield protect the eyes from sparks and flying hot metal particles. The goggles or face shield use special lenses to protect the eyes form light damage. A variety of lenses are used depending on the type of welding or cutting that needs to be done, the type of material, and the thickness of the material. If protective eye shielding is not used, painful burns can occur on the surface of the eye, and could result in permanent damage.View an overview of our welding services any of the metal forming processes offered by Advantage Fabricated Metals.For more information about Advantage Fabricated Metals and the metal fabricating services we provide, please fill out our contact form or call us at 1-773-650-1390.

Stick or MSAW welding

Advantage Fabricated Metals performs a number of welding processes. The two most common welding processes we use include TIG, an acronym for Tungsten Inert Gas welding and MIG, an acronym for Metal Inert Gas welding. TIG is also referred to as GTAW (Gas Tungsten Arc Welding) and Heliarc®. MIG also is referred to as GMAW (Gas Metal Arc Welding). We also provide oxy-acetylene welding and stick or MSAW welding.Shielded Metal Arc Welding (SMAW) is frequently referred to as "stick" or "covered electrode" welding. Stick welding is among the most widely used welding processes. The flux covering the electrode melts during welding. This forms the gas and slag to shield the arc and molten weld pool. The slag must be chipped off the weld bead after welding. The flux also provides a method of adding scavengers, deoxidizers, and alloying elements to the weld metal.

When an arc is struck between the metal rod (electrode) and the workpiece, both the rod and workpiece surface melt to form a weld pool. Simultaneous melting of the flux coating on the rod will form gas and slag which protects the weld pool from the surrounding atmosphere. The slag will solidify and cool and must be chipped off the weld bead once the weld run is complete (or before the next weld pass is deposited). The process allows only short lengths of weld to be produced before a new electrode needs to be inserted in the holder. Weld penetration is low and the quality of the weld deposit is highly dependent on the skill of the welder.

Process characteristics of Shielded Metal Arc Welding (SMAW/Stick)SMAW welding:

Uses a electrode rod that is quickly consumed, Uses equipment that is simple, inexpensive, and highly portable, Uses an electrode that provides and regulates its own flux, Provides all position flexibility, Is less sensitive to wind or drafts,






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Yields a weld with a variable quality and appearance based on operator skill,

During the SMAW welding process the arc is established, the flux coating on the rod disintegrates and then forms a gas that shields the weld from the atmosphere. The slag that is produced by the flux coating prevents the weld metal from oxidizing.Equipment required to perform the SMAW welding process includes a constant current power source that supplies the power to the consumable rod electrode.The SMAW welding process typically is capable of producing three types of welded joints. They are:

Butt joint Lap joint, and T-joint.

The illustration below shows these three common welded joints.


Flux-coated electrodes are available in many core wire diameters and lengths. Matching the electrode properties to the base materials as a general rule for choosing the type of electrode. Available electrodes types include aluminum bronze, bronze, mild steel, nickel, and stainless steel. Materials commonly welded using the SMAW process include mild steel, cast iron, and stainless steel.

There are some problems with SMAW welding. These include:

Arc Blow Arc Stability Excessive spatter Incorrect weld profile Porosity

Rough surface

Crack originating at Gusset Plate

Thermal Image of Crack at Gusset Plate(Heated from Right)

Gradient Image of Crack at Gusset Plate


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Crack in T-Weld

Gradient Image of Crack in T-Weld

In the most challenging example, a crack had grown in the flange above a stiffener.

Crack in top flange

Thermal Image of Crack in Flange(Heated from Right)

The thermal image of the crack (left) shows the effects of the crack on heat conduction in the girder. The structure was heated to the right, which induced a large thermal gradient at the face of the crack.

Gradient Image of Crack in Flange

The gradient image (left) shows the result of the full process and clearly indicates the crack, which after being coated with bridge paint was invisible to the naked eye.


WELD CRACKS, DEFINITIONS, CAUSES, PREVENTION & REPAIR faq330-85

ARC CRACKS

Definition: A depression left at the termination of the weld where the weld pool is left unfilled.

Cause: Improper weld termination techniques

Repair: If no cracks exist, simply fill in the crater. Generally welding from beyond the crater back into the crater.

Longitudinal Crack

Definition: A crack running in the direction of the weld axis. May be found in the weld or base metal.

Cause: Preheat or fast cooling problem. Also caused by shrinkage stresses in high constraint areas.

Prevention: Weld toward areas of less constraint. Also preheat to even out the cooling rates.

Repair: Remove and reweld

Transverse Crack

Definition: A crack running into or inside a weld, transverse to the weld axis direction.

Cause: Weld metal hardness problem

Crater Crack

Definition: A crack, generally in the shape of an “X” which is found in a crater. Crater cracks are hot cracks.

Cause: The center of the weld pool becomes solid before the outside of the weld pool, pulling the center apart during cooling

Prevention: Use crater fill, fill the crater at weld termination and/or preheat to even out the cooling of the puddle

Throat Crack

Definition: A longitudinal crack located in the weld throat area.

Cause: Transverse Stresses, probably from shrinkage. Indicates inadequate filler metal selection or welding procedure. May be due to crater crack propagation.

Prevention: Correct initial cause. Increasing preheat may prevent it. be sure not to leave a crater. Use a more ductile filler material.

Repair: Remove and reweld using appropriate procedure. Be sure to correct initial problem first.

Toe Crack

Definition: A crack in the base metal beginning at the toe of the weld

Cause: Transverse shrinkage stresses. Indicates a HAZ brittleness problem.

Prevention: Increase preheat if possible, or use a more ductile filler material.


Root Crack

Definition: A crack in the weld at the weld root.Cause: Transverse shrinkage stresses. Same as a throat crack.

Prevention: Same as a throat crack

Underbead Crack

Definition: A crack in the unmelted parent metal of the HAZ.

Cause: Hydrogen embrittlement

Prevention: Use LOW HYDROGEN electrodes and/or preheat

Repair: (only found using NDT). Remove and reweld.

Hot Crack

Definition: A crack in the weld that occurs during solidification.

Cause: Micro stresses from weld metal shrinkage pulling apart weld metal as it cools from liquid to solid temp.

Prevention: Preheat or use a low tensile filler material.

Repair: Remove and reweld, correct problem first, preheat may be necessary, increase weld size.

Cold Crack

Definition: A crack that occurs after the metal has completely solidified

Cause: Shrinkage, Highly restrained welds, Discontinuities

Prevention: Preheat, weld toward areas of less constraint, use a more ductile weld metal

Repair: Remove and reweld, correct problem first, preheat may be necessary.

Steel Alloys

Below is a list of some SAE-AISI designations for Steel (the xx in the last two digits indicate the carbon content in hundredths of a percent)

Carbon Steels 10xx Plain Carbon11xx Resulfurized12xx Resulfurized and

rephosphorizedManganese steels 13xx Mn 1.75Nickel steels 23xx Ni 3.525xx Ni 5.0Nickel Chromium Steels 31xx Ni 1.25 Cr 0.65-0.8032xx Ni 1.75 Cr 1.07


33xx Ni 3.50 Cr 1.50-1.5734xx Ni 3.00 Cr 0.77Chromium Molybdenum steels

41xx Cr 0.50-0.95 Mo 0.12-0.30Nickel Chromium Molybdenum steels

43xx Ni 1.82 Cr 0.50-0.80 Mo 0.2547xx Ni 1.05 Cr 0.45 Mo 0.20 – 0.3586xx Ni 0.55 Cr 0.50 Mo 0.20Nickel Molybdenum steels 46xx Ni 0.85-1.82 Mo 0.2048xx Ni 3.50 Mo 0.25Chromium steels 50xx Cr 0.27- 0.6551xx Cr 0.80 – 1.05

Effects of Elements on Steel

Steels are among the most commonly used alloys. The complexity of steel alloys is fairly significant. Not all effects of the varying elements are included. The following text gives an overview of some of the effects of various alloying elements. Additional research should be performed prior to making any design or engineering conclusions.

Carbon has a major effect on steel properties. Carbon is the primary hardening element in steel. Hardness and tensile strength increases as carbon content increases up to about 0.85% C as shown in the figure above. Ductility and weldability decrease with increasing carbon.

Manganese is generally beneficial to surface quality especially in resulfurized steels. Manganese contributes to strength and hardness, but less than carbon. The increase in strength is dependent upon the carbon content. Increasing the manganese content decreases ductility and weldability, but less than carbon. Manganese has a significant effect on the hardenability of steel.

Phosphorus increases strength and hardness and decreases ductility and notch impact toughness of steel. The adverse effects on ductility and toughness are greater in quenched and tempered higher-carbon steels. Phosphorous levels are normally controlled to low levels. Higher phosphorus is specified in low-carbon free-machining steels to improve machinability.

Sulfur decreases ductility and notch impact toughness especially in the transverse direction. Weldability decreases with increasing sulfur content. Sulfur is found primarily in the form of sulfide inclusions. Sulfur levels are normally controlled to low levels. The only exception is free-machining steels, where sulfur is added to improve machinability.


Silicon is one of the principal deoxidizers used in steelmaking. Silicon is less effective than manganese in increasing as-rolled strength and hardness. In low-carbon steels, silicon is generally detrimental to surface quality.

Copper in significant amounts is detrimental to hot-working steels. Copper negatively affects forge welding, but does not seriously affect arc or oxyacetylene welding. Copper can be detrimental to surface quality. Copper is beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20%. Weathering steels are sold having greater than 0.20% Copper.

Lead is virtually insoluble in liquid or solid steel. However, lead is sometimes added to carbon and alloy steels by means of mechanical dispersion during pouring to improve the machinability.

Boron is added to fully killed steel to improve hardenability. Boron-treated steels are produced to a range of 0.0005 to 0.003%. Whenever boron is substituted in part for other alloys, it should be done only with hardenability in mind because the lowered alloy content may be harmful for some applications.

Boron is a potent alloying element in steel. A very small amount of boron (about 0.001%) has a strong effect on hardenability. Boron steels are generally produced within a range of 0.0005 to 0.003%. Boron is most effective in lower carbon steels.

Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance, to increase hardenability, or to improve high-temperature strength. As a hardening element, Chromium is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes increased strength. Chromium is a strong carbide former. Complex chromium-iron carbides go into solution in austenite slowly; therefore, sufficient heating time must be allowed for prior to quenching.

Nickel is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution in ferrite, strengthening and toughening the ferrite phase. Nickel increases the hardenability and impact strength of steels.

Molybdenum increases the hardenability of steel. Molybdenum may produce secondary hardening during the tempering of quenched steels. It enhances the creep strength of low-alloy steels at elevated temperatures.

Aluminum is widely used as a deoxidizer. Aluminum can control austenite grain growth in reheated steels and is therefore added to control grain size. Aluminum is the most effective alloy in controlling grain growth prior to quenching. Titanium, zirconium, and vanadium are also valuable grain growth inhibitors, but there carbides are difficult to dissolve into solution in austenite.

Zirconium can be added to killed high-strength low-alloy steels to achieve improvements in inclusion characteristics. Zirconium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.


Niobium (Columbium) increases the yield strength and, to a lesser degree, the tensile strength of carbon steel. The addition of small amounts of Niobium can significantly increase the yield strength of steels. Niobium can also have a moderate precipitation strengthening effect. Its main contributions are to form precipitates above the transformation temperature, and to retard the recrystallization of austenite, thus promoting a fine-grain microstructure having improved strength and toughness.

Titanium is used to retard grain growth and thus improve toughness. Titanium is also used to achieve improvements in inclusion characteristics. Titanium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.

Vanadium increases the yield strength and the tensile strength of carbon steel. The addition of small amounts of Niobium can significantly increase the strength of steels. Vanadium is one of the primary contributors to precipitation strengthening in microalloyed steels. When thermomechanical processing is properly controlled the ferrite grain size is refined and there is a corresponding increase in toughness. The impact transition temperature also increases when vanadium is added.

All microalloy steels contain small concentrations of one or more strong carbide and nitride forming elements. Vanadium, niobium, and titanium combine preferentially with carbon and/or nitrogen to form a fine dispersion of precipitated particles in the steel matrix.


TWI Welding

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