ME 328.3 E5 - Welding Metallurgy

Purpose:

• To become more familiar with the welding process and its effects on the material

• To look at the changes in microstructure and the hardness in the Heat Affected Zone (HAZ)

• Welding defects, their cause and preventative measures

• Industrial radiography techniques

Definitions:

• Welding is the joining of multiple pieces of metal by the use of heat and or pressure. A union of the parts is created by fusion or recrystallization across the metal interface. Welding can involve the use of filler material, or it can involve no filler.

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 proper techniques 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 the quality 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 electrode and 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 that provide 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 area with gas

• Submerged Arc Welding– Arc welding process that uses a continuous, consumable bare

wire electrode, arc shielding is provided by a cover of granular flux

• Resistance Welding– A fusion welding process that utilizes a combination of heat and

pressure to accomplish coalescence, the heat being generated by electrical resistance to current flow at the junction to be welded

• Oxyacetylene Welding– A fusion welding process performed by a high-temperature flame

from a combustion of acetylene and oxygen

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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 any mixing 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 its properties have been altered.

– The amount of metallurgical damage in the HAZ depends on the amount of heat input, peak temp reached, distance from fusion zone, time at elevated temp, cooling rate, and the metal’s thermal properties

• Heat Affected Zone (HAZ) cont’d– 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. CracksDetection

Surface: 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 from weld metal

– Susceptibility increases with the increasing thickness of section especially in steels with high carbon equivalent composition

– Can also occur in weld metal– Increase welding heat beneficial– Preheating sometimes necessary– Control of moisture in consumables

and cleanliness of weld prep desirable

Lamellar Tearing

• Causes:– Poor ductility in through-thickness

direction in rolled plate due to non-metallic inclusions

– Occurs mainly in joints having weld metal deposited on plate surfaces

– Prior buttering of surface beneficial for 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

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2. CavitiesDetection

Surface: Visual inspection

Internal: Ultrasonic flaw detection, radiography

Worm Holes• Resulting from the entrapment of gas

between the solidifying dendrites of weld metal, often showing ‘herringbone’ array ( B )

• Causes:– The gas may arise from

contamination of surfaces to be welded, or be prevented from escaping from beneath the weld by joint 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

3. 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

4. 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

5. 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

5. 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

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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 150˚C. 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.

Radiographs

ID # 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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