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Metallurgy of welding materials
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ME 328.3 E5 - Welding Metallurgy
Purpose:
To become more familiar with the weldingprocess and its effects on the material
To look at the changes in microstructure and thehardness in the Heat Affected Zone (HAZ)
Welding defects, their cause and preventativemeasures
Industrial radiography techniques
Definitions:
Welding is the joining of multiple pieces of metalby the use of heat and or pressure. A union ofthe parts is created by fusion or recrystallizationacross the metal interface. Welding can involvethe use of filler material, or it can involve nofiller.
What commercial and technological importance does welding have?
Provides a permanent joint Weld joint can be stronger than parent material
If the filler material has superior strength characteristics and propertechniques are used
Usually the most economical way to join components Can be done in the field away from a factory
Limitations? Expensive in terms of labour cost Most welding processes involve the use high energy, are
inherently dangerous Welds are permanent bonds, not allowing for convenient
disassembly The welded joint can suffer from certain quality defects
that are difficult to detect, these defects can reduce thequality of the joint
Types:
Arc Welding A fusion welding process in which the coalescence of the metals is
achieved by the heat from an electric arc between an electrodeand the work.
Shielded Metal Arc Welding (SMAW) An arc welding process that uses a consumable electrode
consisting of a filler metal rod coated with chemicals thatprovide flux and shielding.
Gas Metal Arc Welding (GMAW) Arc welding process in which the electrode is a consumable bare
metal wire and shielding is accomplished by flooding the areawith gas.
Submerged Arc Welding Arc welding process that uses a continuous, consumable bare
wire electrode, arc shielding is provided by a cover of granularflux.
Resistance Welding A fusion welding process that utilizes a combination of heat and
pressure to accomplish coalescence, the heat being generatedby electrical resistance to current flow at the junction to bewelded
Oxyacetylene Welding A fusion welding process performed by a high-temperature flame
from a combustion of acetylene and oxygen
HEAT25.12HEAT2
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Fusion Weld Joint
Fusion Zone A mixture of filler metal and base metal that has completely
melted High degree of homogeneity among the component metals that
have been melted during welding The mixing of these components is motivated largely by
convection in the molten weld pool
Weld Interface The narrow boundary that separates the fusion zone and the
heat affected zone This interface consists of a thin band of base metal that was
melted or partially melted (localized melting within the grains)during the welding process, but immediately solidified before anymixing could take place
Heat Affected Zone (HAZ) The metal in this region has experienced temperature below its
melting point, but high enough to change the microstructure This metal consists of the base metal which has undergone a
heat treatment due to the welding temperatures, so that itsproperties have been altered.
The amount of metallurgical damage in the HAZ depends on theamount of heat input, peak temp reached, distance from fusionzone, time at elevated temp, cooling rate, and the metalsthermal properties
Heat Affected Zone (HAZ) contd The effect on the mechanical properties is usually negative, and
it is most often the region of the weld joint where failure occurs Unaffected Base Metal Zone
Where no metallurgical change has occurred The base metal surrounding the HAZ is likely to be in a state of
high residual stress, due to the shrinkage in the fusion zone
Weld Defects:1. Cracks
DetectionSurface: Visual examination, magnetic particle, dye or
fluorescent penetrant inspection
Internal: Ultrasonic flaw detection, radiography
Solidification Cracking
Causes: Large depth/width ratio of weld
bead High arc energy and/or preheat Sulphur, phosphorus or niobium
pick-up from parent metal
Hydrogen Induced HAZ Cracking
Causes: Hardened HAZ coupled with the
presence of hydrogen diffused fromweld metal
Susceptibility increases with theincreasing thickness of sectionespecially in steels with high carbonequivalent composition
Can also occur in weld metal Increase welding heat beneficial Preheating sometimes necessary Control of moisture in consumables
and cleanliness of weld prepdesirable
Lamellar Tearing
Causes: Poor ductility in through-thickness
direction in rolled plate due to non-metallic inclusions
Occurs mainly in joints having weldmetal deposited on plate surfaces
Prior buttering of surface beneficialfor susceptible plate
Reheat Cracking Occurs in creep resisting and some
thick section structural low alloy steels during post weld heat treatment
Causes: Poor creep ductility in HAZ
coupled with thermal stress Accentuated by severe notches
such as preexisting cracks, or tears at weld toes, or unfused root of partial penetration weld
Heat treatment may need to include low temperature soaking
Grinding or peening weld toes after welding can be beneficial
X 35
X 200
1. CavitiesDetection
Surface: Visual inspection
Internal: Ultrasonic flaw detection, radiography
Worm Holes Resulting from the entrapment of gas
between the solidifying dendrites ofweld metal, often showing herringbonearray ( B )
Causes: The gas may arise from
contamination of surfaces to bewelded, or be prevented fromescaping from beneath the weld byjoint crevices
Uniformly Distributed Porosity Resulting from the entrapment of gas
in solidified weld metal Causes:
Gas may originate from dampness or grease on consumables or workpiece, or by nitrogen contamination from the atmosphere
If the weld wire used contains insufficient deoxidant it is also possible for carbon monoxide to cause porosity
Restart Porosity
Causes: Unstable arc conditions at weld
start, where weld pool protection may be incomplete and temperature gradients have not had time to equilibrate, coupled with inadequate manipulative technique to allow for this instability
Surface Porosity
Causes: Excessive contamination from
grease, dampness, or atmosphere entrainment
Occasionally caused by excessive sulphur in consumables or parent metal
Crater Pipes Resulting from shrinkage at the end
crater of a weld run Causes:
Incorrect manipulative technique or current decay to allow for crater shrinkage
Solid Inclusions Detection - normally revealed by radiography
Linear Slag Inclusions Cause:
Incomplete removal of slag in multi-pass welds often associated with the presence of undercut or irregular surfaces in underlying passes
Isolated Slag Inclusions Causes:
Normally by the presence of mill scale and/or rust on prepared surfaces, or electrodes with cracked or damaged coverings
Can also arise from isolated undercut in underlying passes of multi-pass welds
1. Lack of Fusion and Penetration
Detection This type of defect tends to be sub surface and is therefore
detectable only by ultrasonics or X-ray methods Lack of side wall fusion which penetrates the surface may be
detected using magnetic particle, dye or fluorescent penetrant inspection
Cause Incorrect weld conditions (eg. low current) and/or incorrect
weld preparation (eg. root face too large) Both cause the weld pool to freeze too rapidly
Lack of side-wall fusion Lack of root fusion Lack of inter-run fusion
Lack of penetration
Imperfect Shape Detection - all shape defects can be determined by visual inspections
Linear Misalignment Cause:
Incorrect assembly or distortion during fabrication
Excessive Reinforcement
Causes: Deposition of too much weld metal,
often associated with in adequate weld preparation
Incorrect welding parameters Too large of an electrode for the
joint in question
Overlap
Causes: Poor manipulative technique Too cold a welding conditions
(current and voltage too low)
Undercut Results from the washing away of edge
preparation when molten Causes:
Poor welding technique Imbalance in welding conditions
Undercut Results from the washing away of edge
preparation when molten Causes:
Poor welding technique Imbalance in welding conditions
Excessive Penetration Causes:
Incorrect edge preparation providing insufficient support at the weld root
Incorrect welding conditions (too high of current)
The provision of a backing bar can alleviate this problem in difficult circumstances
Root Concavity Causes:
Shrinkage of molten pool at weld root, due to incorrect root preparation or too cold of conditions
May also be caused by incorrect welding technique
1. Miscellaneous Faults
Arc Strikes Cause:
Accidental contact of an electrode or welding torch with a plate surface remote from the weld
Usually result in small hard spots just beneath the surface which may contain cracks, and are thus to be avoided
Spatter
Causes: Incorrect welding conditions
and/or contaminated consumables or preparations, giving rise to explosions within the arc and weld pool
Globules of molten metal are thrown out, and adhere to the parent metal remote from the weld
Copper Pick-Up
Causes: Melting of copper contact tube in
MIG welding due to incorrect welding conditions
X 275
PROCEDURE1. Students are provided with weldments of approximately 0.4% C
steel. The first weldment was prepared without preheat treatment. The electrode used produces a large amount of hydrogen which diffuses into the weld metal. The second was preheated to 150C. An electrode with relatively low hydrogen content was used. For each of these samples:
a) Examine the microstructure of the weldments in a traverse from weld metal to parent metal, sketching about five different areas. Using the Fe-C diagram and your knowledge of the phase transformations in steel, comment on the microstructures describing the time-temperature history and how this history resulted in the observed structure.
b) Conduct a microhardness traverse across the HAZ and correlate the hardness with the microstructure observed in (a).
2. Some radiographs of weld defects are provided. Examine these radiographs and describe the defects responsible, citing ways of avoiding the problem.
RadiographsID # Position Comments Results PageQ13 1gf Shallow undercut by cap pass AcceptableQ18 4gf Incompletefusion at the root FailQ10 1gf Incompletefusion at the root FailH2 4gf Incompletefusion at the root & slag throughout FailH1 1gf Porosity throughout FailJ3 4gf Slag inclusions Acceptable
F10 1gf Slag inclusions FailF2 2g Incompletefusion at the root FailF7 3gf Minor slag Acceptable983 2g Slag inclusions Acceptable983 3gf Slag inclusions at the root & inner passes Fail982 3gf Slag inclusions Fail852 2g No defects Acceptable852 3gf Slag inclusion at the root & porosity Fail850 4gf Minor slag & film scratch Acceptable 5
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