5 Unit 2- Welding Quality and Defects

8/13/2019 5 Unit 2- Welding Quality and Defects

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MANUFACTURINGTECHNOLOGY ISUBCODE: MEC230

Unit 2Welding (Metal Joining) Processes

Metal Forming Processes


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Unit 2 : Welding Quality, Defects and Inspection


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Welding Quality and Defects:Weld Quality: Concerned with obtaining an acceptable weld joint that is strong

and absent from defects Also concerned with the methods of inspecting and testing the

joint to assure its qualityTopics:

Residual stresses and distortion Welding defects

Inspection and testing methods


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Residual Stresses and Distortion: Rapid heating and cooling in localized regions during FW result

in thermal expansion and contractionthat cause residual stresses These stresses, in turn, cause distortion and warpage Distortion is the alteration of the original shape or other

characteristic of an object.

Warpage is a distortion where the surfaces do not follow theintended shape of the design.

Situation in welding is complicated because: Heating is very localized

Melting of base metals in these regions


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Residual Stresses and Distortion:(a) Butt welding two plates

(b) Shrinkage(c) Residual stress patterns

(d) Likely warping of weldment


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Techniques to Minimize Warpage: Welding fixturesto physically restrain parts Heat sinks to rapidly remove heat Selection of welding conditions (speed, amount of filler metal

used, etc.) to reduce warpage

Preheating base parts

Stress relief heat treatmentof welded assembly Proper design of weldment


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Welding Defects:

The defects in the weld can be defined as irregularities in theweld metal produced due to Incorrect welding parameters

Wrong welding procedures

Wrong combination of filler metal and parent metal.

Defects may be on the surface or inside the weld metal

Certain defects such as cracks are never tolerated but otherdefects may be acceptable within permissible limits.

Welding defects may result into the failure of components under

service condition, leading to serious accidents and causing theloss of property and sometimes also life.


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Welding Defects:Various welding defects can be classified into groups such as, Cracks

Cavities

Solid inclusions

Imperfect shape or unacceptable contour

Incomplete fusion

Inadequate penetration

Miscellaneous defects


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Welding Cracks: Cracks may be of micro or macro size and may appear in the

weld metal or base metal or base metal and weld metalboundary

Different categories of cracks are

Longitudinal Cracks

Transverse Cracks

Cracks in the Weld Crater

Cracks occur when localized stressesexceed the ultimatetensilestrength of material

These stresses are developed due to shrinkage duringsolidification of weld metal


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Welding Cracks: Various forms of welding cracks


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Porosities: Welding Cavities: Two defect types, similar to defects found in

castings: Porosity and Shrinkage voids Porosity - small voids in weld metal formed by gases entrapped

during solidification

Shrinkage voids - cavities formed by shrinkage duringsolidification Different Forms of Porosities


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Porosity

The gases are generated from flux or coating constituents ofelectrode or shielding gases during welding or from absorbedmoisture in the coating

Caused by inclusion of atmospheric gases, sulfur in weld metal,

or surface contaminants

Rust, dust, oil and grease present on the surface of work pieces

or on electrodes are also source of gases during welding

Porosity may be easily prevented if work pieces are properly

cleaned from rust, dust, oil and grease Porosity can also be controlled by avoiding high welding

currents, faster welding speeds and long arc lengths

Flux and coated electrodes are properly baked


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Solid Inclusions : Solid inclusions may be in the form of slag or any other non-

metallic material entrapped in the weld metal as these may notable to float on the surface of the solidifying weld metal

During arc welding flux either in the form of granules or coating

after melting, reacts with the molten weld metal removing

oxides and other impurities in the form of slag and it floats onthe surface of weld metal due to its low density

If the molten weld metal has high viscosity or too lowtemperature or cools rapidly then the slag may not be releasedfrom the weld pool and may cause inclusion

Most common form is slag inclusions generated during AW

processes that use flux


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Solid Inclusions : Other forms:metallic oxides that form during welding of certain

metals such as aluminum, which normally has a surface coatingof Al2O3

Slag inclusion can be prevented if proper groove is selected, all

the slag from the previously deposited bead is removed, too high

or too low welding currents and long arcs are avoided.

Slag Inclusion in Weldments


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Lack of Fusion : Lack of fusion is the failure to fuse together either the base metal

and weld metal or subsequent beads in multipass weldingbecause of failure to raise the temperature of base metal or

previously deposited weld layer to melting point during welding.

Lack of fusion can be avoided by properly cleaning of surfaces to

be welded, selecting proper current, proper welding techniqueand correct size of electrode.

Types of Lack of Fusion


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Inadequate or Incomplete Penetration: Incomplete penetration means that the weld depth is not upto

the desired levelor root faces have not reached to melting pointin a groove joint.

Causes :

Low currents or larger arc lengths Large root face or small root gap

Too narrow groove angles are used then it results into poor

penetration

Types of Inadequate Penetration


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Imperfect Shape : Imperfect shape means the variation from the desired shape andsizeof the weld bead During welding a notch (undercut) is formed either on one side

of the weld bead or both sides in which stresses tend to

concentrate and it can result in the early failure of the joint

Main reasons for undercutting are the excessive welding

currents, long arc lengths and fast travel speeds

Underfilling may be due to low currents, fast travel speeds andsmall size of electrodes

Overlap may occur due to low currents, longer arc lengths andslower welding speeds


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Imperfect Shape :Various Imperfect Shapes of Welds


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Imperfect Shape : Excessive reinforcement is formed if high currents, slow travel

speeds and large size electrodes are used

Excessive root penetration and sag can occur if excessive highcurrents and slow travel speeds are used for relatively thinner

members

Distortionis caused because of shrinkage occurring due to largeheat input during welding


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Inspection and Testing Methods: Visual inspection

Nondestructive evaluation

Destructive testing

Miscellaneous Defects :Various miscellaneous defects may be

Multiple arc strikes i.e. several arc strikes are one behind the

other

Splash, grinding and chipping marks

Oxidized surface in the region of weld Unremoved slag and misalignment of weld beads


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Visual Inspection: Most widely used welding inspection method

Human inspector visually examines the weld

Conformance to dimensions, warpage

Cracks, cavities, incomplete fusion, and other surface defects

Limitations: Only surface defects are detectable

Welding inspector must also decide if additional tests are

warranted


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Nondestructive Evaluation (NDE) Tests: Ultrasonic testing - high frequency sound waves through

specimen to detect cracks and inclusions

Radiographic testing - x-rays or gamma radiation providephotograph of internal flaws

Dye-penetrant and fluorescent-penetrant tests - to detect smallcracks and cavities at part surface

Magnetic particle testing- iron filings sprinkled on surface revealsubsurface defects by distorting magnetic field in part


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Destructive Evaluation (DE) or Testing: Tests in which weld is destroyed either during testing or to

prepare test specimen

Mechanical tests - purpose is similar to conventional testingmethods such as hardness, tensile tests, shear tests, etc

Metallurgical tests- preparation of metallurgical specimens (e.g.,photomicrographs) of weldment to examine metallic structure,

defects, extent and condition of heat affected zone, and similar

phenomena


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Mechanical Tests in Welding:(a) Tension-shear test,

(b) fillet break test,

(c) tension-shear of spot weld, and

(d) peel test for spot weld


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Weldability: Capacity of a metal or combination of metals to be welded into a

suitable structure, and resulting weld joint to possess therequired metallurgical properties to perform satisfactorily in

intended service

Good weldability characterized by: Ease with which welding is accomplished

Absence of weld defects

Strength, ductility, and toughness in welded joint


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Weldability Factors Welding Process: Some metals or metal combinations can be readily welded by

one process but are difficult to weld by others

Example: stainless steel readily welded by most AW and RW

processes, but difficult to weld by OFW

Weldability Factors Base Metal Some metals melt too easily; e.g., aluminum

Metals with high thermal conductivity transfer heat away from

weld, which causes problems; e.g., copper High thermal expansion and contraction in metal causes

distortion problems

Dissimilar metals pose problems in welding when their physical

and/or mechanical properties are substantially different


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Other Factors Affecting Weldability: Filler metal must be compatible with base metal(s)

In general, elements mixed in liquid state that form a solid

solution upon solidification do not cause a problem

Surface conditions

Moisture can result in porosity in fusion zone Oxides and other films on metal surfaces can prevent adequate

contact and fusion


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Design Considerations in Welding: Design for welding - product should be designed from the start

as a welded assembly, not as a casting or forging or other formedshape

Minimum parts - welded assemblies should consist of fewestnumber of parts possible

Example: for bending usually more cost efficient to performsimple bending operations on a part than to weld an assembly

from flat plates and sheets


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Arc Welding Design Guidelines: Good fit-up of parts - to maintain dimensional control and

minimize distortion

Machining is sometimes required to achieve satisfactory fit-up

Assembly must allow access for welding gun to reach welding

area

Design of assembly should allow flat welding to be performed as

much as possible, since this is the fastest and most convenient

welding position


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Arc Welding Positions: Welding positions defined here for groove welds: (a) flat, (b)

horizontal, (c) vertical, and (d) overhead

5 Unit 2- Welding Quality and Defects

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