WELD DEFECTS AND DISCONTINUITIES
Posted: 12 Oct 2010, 05:11
TYPES OF WELD DISCONTINUITIES
Causes:
1. Oxyacetylene welding: flame not properly adjusted, carbon excess in base metal, unkilled steel, excessive distance flame-to-piece, excess of acetylene.
2. Manual electric welding: too long arc, dirty joint preparation, moisture in welding room and in electrode, windy welding area.
3. Argon-arc welding: too long arc, lack of inert gas or impure inert gas.
4. Submerged arc welding: lack of protective flow.
Inclusions
A. Slag Inclusions - oxide or non-metallic inclusions in welding seam. Inclusion of any shape and in any direction.
Causes:
1. Manual elctric welding: Lack of cleaning of electrode slag between each passes, irregular shape of previous passes, unskilled welder, irregular edges setting.
2. Argon-arc welding: not likely
3. Submerge arc welding: previous passes with excessive convex shape.
B. Tungsten inclusions - rounded or irregular tungsten (metallic) inclusions
Causes:
1. Argon-arc welding: tungten electrode contact with base metal in welding operation.
Insufficient throat (underfill) - a depression on the face of a weld or root surface below the surface plane of the adjacent base metal.
Causes:
The welder or welding operator failed to fill the weld joint completely. On pipe welds, underfill at the root of the weld is called internal concavity.
Inadequate Joint Penetration - unfilled root face of the joint. Incomplete weld penetration.
Causes:
Inadequate joint penetration may result from improper joint design and wrong electrode or filler wire used.
1. Oxyacetylene welding: irregular edges setting, too large welding rod, fast welding speed rate, cold bead.
2. Manual electric welding: Insufficient current value, irregular edges setting, too large welding rod, fast welding rate, unfavorable working condition.
3. Argon-arc welding: irregular edges setting, too large filler wire, current too low, fast welding rate.
4. Submerged arc welding: insufficient current or voltage welding values, fast deposition rate.
Incomplete fusion - failure of the liquid weld metal to fuse into the total face area of the joint.
Causes:
Incomplete fusion is usually caused by insufficient application of heat to all faces of the joint. However, incomplete fusion can also be caused by the presence of oxides which inhibit fusion by remaining tightly secured to the base metal.
1. Oxyacetylene welding: too large welding rod, too powerful blowpipe, fast deposition rate, unfavorable welding condition.
2. Manual electric welding: too large welding rod, improper joint preparation, welding current too low.
3. Argon-arc welding: too large filler wire, fast deposition rate, welding current too low.
4. Submerged arc welding: too large filler wire, welding current too low.
Overlap - is the protrusion of weld metal beyond the toe, face, or root of the weld joint without fusion. the resulting discontinuity is a severe mechanical notch on the surface. similar to incomplete fusion but different location where the fusion failed to take place.
Causes:
overlap is caused by the inability of the weld metal to fuse with the surface, especially when tightly adhering oxides cover the base metal. Overlap results from lack of control of the welding process in the form of insufficient heat, improper selection of welding materials, or improper prepation of the joint (failure to remove mill scale or other surface coatings).
Undercut - is a surface discontinuityfrom melting of the base metalat the junction of the weld and base metal. It takes the form of a mechanical notch at the fusion boundary.
Causes:
Undercut is caused by the application of excessive heat (excessive welding current) which melts away base metal.
1. Oxyacetylene welding: fast deposition rate, too powerful blowpipe, wrong blowpipe position
2. Manual electric welding: current too high, too high welding arc, wrong electrode.
3. Argon-arc welding: welding speed too fast, wrong torch position.
4. Submerged arc welding: excessive current, welding speed too fast.
Cracks - linear ruptures of metal under stress. Although somitimes wide, they are often very narrow separations in the weld or adjacent base metal.
Causes:
1. Oxyacetylene welding: internal shrinkage, incorrect pre or post heating in alloyed welded steels, faulty heat treatment.
2. Manual electric welding: same as with Oxyacetylene
3. Argon-arc welding: same as with Oxyacetylene
4. Submerged arc welding: same as with Oxyacetylene
Discontinuities are imperfections in welds. Defects are unacceptable discontinuities. Ideally, a sound weld should have no discontinuities at all, but welds are not perfect; imperfection do exsist in varrying degrees.
- Porosity
Inclusions, both metallic and nonmetallic
Insufficient throat (underfill)
Inadequate joint penetration
Incomplete fusion
Overlap
Undercut
Cracks
Causes:
1. Oxyacetylene welding: flame not properly adjusted, carbon excess in base metal, unkilled steel, excessive distance flame-to-piece, excess of acetylene.
2. Manual electric welding: too long arc, dirty joint preparation, moisture in welding room and in electrode, windy welding area.
3. Argon-arc welding: too long arc, lack of inert gas or impure inert gas.
4. Submerged arc welding: lack of protective flow.
Inclusions
A. Slag Inclusions - oxide or non-metallic inclusions in welding seam. Inclusion of any shape and in any direction.
Causes:
1. Manual elctric welding: Lack of cleaning of electrode slag between each passes, irregular shape of previous passes, unskilled welder, irregular edges setting.
2. Argon-arc welding: not likely
3. Submerge arc welding: previous passes with excessive convex shape.
B. Tungsten inclusions - rounded or irregular tungsten (metallic) inclusions
Causes:
1. Argon-arc welding: tungten electrode contact with base metal in welding operation.
Insufficient throat (underfill) - a depression on the face of a weld or root surface below the surface plane of the adjacent base metal.
Causes:
The welder or welding operator failed to fill the weld joint completely. On pipe welds, underfill at the root of the weld is called internal concavity.
Inadequate Joint Penetration - unfilled root face of the joint. Incomplete weld penetration.
Causes:
Inadequate joint penetration may result from improper joint design and wrong electrode or filler wire used.
1. Oxyacetylene welding: irregular edges setting, too large welding rod, fast welding speed rate, cold bead.
2. Manual electric welding: Insufficient current value, irregular edges setting, too large welding rod, fast welding rate, unfavorable working condition.
3. Argon-arc welding: irregular edges setting, too large filler wire, current too low, fast welding rate.
4. Submerged arc welding: insufficient current or voltage welding values, fast deposition rate.
Incomplete fusion - failure of the liquid weld metal to fuse into the total face area of the joint.
Causes:
Incomplete fusion is usually caused by insufficient application of heat to all faces of the joint. However, incomplete fusion can also be caused by the presence of oxides which inhibit fusion by remaining tightly secured to the base metal.
1. Oxyacetylene welding: too large welding rod, too powerful blowpipe, fast deposition rate, unfavorable welding condition.
2. Manual electric welding: too large welding rod, improper joint preparation, welding current too low.
3. Argon-arc welding: too large filler wire, fast deposition rate, welding current too low.
4. Submerged arc welding: too large filler wire, welding current too low.
Overlap - is the protrusion of weld metal beyond the toe, face, or root of the weld joint without fusion. the resulting discontinuity is a severe mechanical notch on the surface. similar to incomplete fusion but different location where the fusion failed to take place.
Causes:
overlap is caused by the inability of the weld metal to fuse with the surface, especially when tightly adhering oxides cover the base metal. Overlap results from lack of control of the welding process in the form of insufficient heat, improper selection of welding materials, or improper prepation of the joint (failure to remove mill scale or other surface coatings).
Undercut - is a surface discontinuityfrom melting of the base metalat the junction of the weld and base metal. It takes the form of a mechanical notch at the fusion boundary.
Causes:
Undercut is caused by the application of excessive heat (excessive welding current) which melts away base metal.
1. Oxyacetylene welding: fast deposition rate, too powerful blowpipe, wrong blowpipe position
2. Manual electric welding: current too high, too high welding arc, wrong electrode.
3. Argon-arc welding: welding speed too fast, wrong torch position.
4. Submerged arc welding: excessive current, welding speed too fast.
Cracks - linear ruptures of metal under stress. Although somitimes wide, they are often very narrow separations in the weld or adjacent base metal.
Causes:
1. Oxyacetylene welding: internal shrinkage, incorrect pre or post heating in alloyed welded steels, faulty heat treatment.
2. Manual electric welding: same as with Oxyacetylene
3. Argon-arc welding: same as with Oxyacetylene
4. Submerged arc welding: same as with Oxyacetylene
Discontinuities are imperfections in welds. Defects are unacceptable discontinuities. Ideally, a sound weld should have no discontinuities at all, but welds are not perfect; imperfection do exsist in varrying degrees.