5 unit 2- welding quality and defects
TRANSCRIPT
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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