AI- Welding Process

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Welding Process

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WELDING  PROCESSES

Earliest known form of welding, called forge welding, dates back to 2000  B.C.

Forge  welding  is  a  primitive  process  of joining metals by heating and hammering until the metals are fused (mixed) together

Now limited to the blacksmith trade.

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Welding Processes

Shielded metal arc welding-SMAW Gas metal arc welding-GTAW Flux cored arc welding-FCAW Gas tungston arc welding-GTAW Plasma arc welding-PAW Submerged arc welding-SAW Oxy acetylene welding-OAW

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Difference between Processes

Shielding usedWeld metal compositionJoint preparation & fit-upType of electrode/filler –metalWelding pattern

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Factors that affect weld Quality Shielding quality Base metal quality Filler-metal quality Base metal weld metal compatibility Heat input Electrode size Travel speed Current and Voltage Welding position Environment for welding

Wind, temperature & fit-up Knowledge & Experience of welders

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Filler metals filler  metal or  material

material added to fill-up the space in between two welding pieces during the welding process

Two types of filler metals commonly used welding  rods welding  electrodes.

welding rod  refers to a form of filler metal that does not conduct an electric current during welding process The  purpose  of  a  welding  rod  is to supply filler

metal to the joint. used for gas welding.

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Electrode

Electrode component that conducts the current from the electrode

holder to the metal being welded. Electrode types:   consumable  and  non-consumable.

Consumable electrodes provide a path for the current and also supply filler metal to

the joint.   Eg.electrode  used  in  shielded metal-arc  welding.

Non-consumable  electrodes used as a conductor for the electrical current-GTAW filler metal for GTAW, hand fed consumable welding rod.

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FLUXES Base metal has always impurities, called oxides

result from oxygen combining with metal & other contaminants in the base metal.

if these oxides are not removed a faulty weld may result

Fluxes Cleaning agents that dissolve oxides and release trapped

gases combines with impurities in the base metal, floating them

away in the form of a heavy slag which shields the weld from the atmosphere.

allow the filler metal and the base metal to be fused formulated for a specific base metal on the expected

welding temperature Available in the form of a paste, powder, or liquid

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ARC WELDING common to all arc-welding processes

a heat source, filler metal, and shielding source of heat

by arcing of an electrical current between two contacts. concentration of heat

less  heat  spread  reduces  buckling  and  warping increases depth of penetration  and   speeds  up  welding  operation

A distinct advantage of arc welding over gas welding more practical and economical than gas welding In gas welding flame spreads over a large area, causing

 heat  distortion

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Shielded Metal Arc Welding (SMAW)

performed by striking an arc between a coated-metal electrode and the base metal.

molten metal from the tip of the electrode flows together with the molten metal from the edges of the base metal to form a sound joint, process known as  fusion

The coating from the electrode forms a covering over  weld  deposit,  shielding  it  from contamination

common types of welding Oxy-fuel gas welding  (OFW)   arc  welding     resistance welding

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SMAW

high-quality welds are made rapidly at a low cost

Weld surfaces have valleys and ripples Makes interpretation difficult Discontinuities have random orientation

in the weld with other welding processes Contains entire spectrum of weld

discontinuities

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Oxy-fuel gas welding (OFW) Shielded metal arc welding (SMAW)

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GAS Metal Arc WELDING

source of heat oxy-fuel gas, such as acetylene, mixed with oxygen used in maintenance and repair works

Primary gases used helium, argon, carbon  dioxide or  a  mixture  of

 these gases Difference between SMAW & GMAW

type of shielding GTAW

both the arc and the molten puddle covered by a shield of inert gas.

The shield of inert gas prevents atmospheric contamination-producing  a  better  weld.

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GMAW Gas metal arc welding

No flux used Suitable for thin wall

sections < 10 mm Has Low base metal

penetration characteristics leading to

Incomplete penetration Cold lap Porosity -if loss of

shielding occurs Slag ???

Oxides in base metal may be drawn into the weld as slag

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GTAW

Gas Tungsten arc welding High quality welds with

good base metal penetration with operator skill

Discontinuities common to GTAW

Incomplete fusion Cold lap Porosity -if loss of

shielding occurs Tungsten inclusions

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FCAW

Flux cored arc welding

Has good penetrating capabilityDiscontinuities common to

FCAW

Slag Porosity

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SAW

Submerged arc welding

Has good penetrating capability Discontinuities common to SAW

Slag Lack of fusion

Follow welding direction and will be in Straight lines

Porosity

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Evaluation of weldments

Welding discontinuities

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Welding discontinuities

Discontinuities become defects when exceed welding standard

requirements Cracks Incomplete penetration (ICP) Incomplete fusion (LF & SWLF) Slag Inclusions (isolated & linear) Porosity

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Welding discontinuities Defective profile

Undercut overlap Under-fill Excess reinforcement Excess root reinforcement Root concavity burn through spatter

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Welding defects- Causes

Cracks Hydrogen Assisted cold cracking (HACC) Hydrogen induced cold cracking (HICC) solidification, liquation causes

Incomplete fusion Sidewall, inter run, root pass, weld toes ( cold

lap ) Electrode angle implicated or poor joint profile

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Welding defects- Causes

Porosity Gas entrapment / ejection poor shielding

Inclusions Slag, oxide, tungsten Usually operator induced

Defective weld profile / finish Under-weld, over-weld, lack of root bead, burn through,

undercut Usually operator induced

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weld defects

Incomplete sidewall fusion

Incomplete root fusion

Slag inclusion

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weld defects

Incomplete penetration

Cold lap

Undercut

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Solidification cracks

Crater crack

Longitudinal crack Centreline Crack

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Solidification cracking

Low melting point constituents Grain boundary segregation Strains arising during solidification

Expansion coefficient Differing between base material and weld

material Clad materials

Weld pool shape and size

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Methods of control

Preheat Slow down cooling rate between 800°C and 500°C

Remove hydrogen before weld cools below 150°C Stress relief immediately after welding Low temp temperature heat treatment (150°C to 250°C, known as out-gassing)

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Lamellar tearing

Separation or cracking along planes parallel to the principal plane of deformation.

Occurs in rolled sections mainly but can also occur in extrusions and forgings.

Does not occur in castings Not to be confused with plate lamination.

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Lamellar tearing