What is welding defects? Before going to welding defects, we should understand what is welding?
Welding is a process of fabrication. This joins different materials, mainly metals or non-metals like thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. However such as brazing and soldering, are used with lower temperature techniques, which do not melt the base metal.
In a summary any shortcomings to welding are called welding defects. Some of the welding defects are listed below.
- Lack of penetration
- Lack of fusion
- Undercut
- Slag inclusion
- Porosity
- Crack
- Faulty weld size & profile
- Distortion
Lack of penetration
This defect occurs at the root of the joint when the weld metal fails to reach it or weld metal fails
to fuse completely the root faces of the joint. As a result, a void remains at the root zone, which
may contain slag inclusions.
Cause: –
A. Use of incorrect size of electrode in relation to the form of joint.
B. Low welding current.
C. Faulty fit-up and inaccurate joint preparation.
Lack of fusion.
Lack of fusion is defined as a condition where boundaries of unfused metal exist between the
Weld metal & base metal or between the adjacent layers of weld –metals.
Cause: –
A. Presence of scale, dirt, oxide, slag and other non-metallic substance which prevents
the weld metal to reach melting temperature.
B. Improper deslagging between the weld pass.
Precaution: –
A. Keep the weld joint free from scale, dirt, oxide, slag and other non- metallic substance.
B. Use adequate welding current.
C. Deslag each weld pass thoroughly.
D. Place weld passes correctly next to each other.
Undercut
This defect appears as a continuous or discontinuous groove at the toes of a weld pass and is
located on the base metal or in the fusion face of a multipass weld. It occurs prominently on the
edge of a fillet weld deposited in the horizontal position.
Cause: –
A. Excessive welding current.
B. Too high speed of arc travel.
C. Wrong electrode angle.
Rectification: –
The defect is rectified by filling the undercut groove with a weld pass. If undercut is deep &
contains slag, it should be chipped away before rewelding.
Slag Inclusion
Non–metallic particles of comparatively large size entrapped in the weld metal are termed
as slag inclusion.
Cause: –
A. Improper cleaning of slag between the deposition of successive passes.
B. Presence of heavy mill scale, loose rust, dirt, grit & other substances present on the
surface of base metal.
Precaution: –
A. Clean the slag thoroughly between the weld pass.
B. Keep the joint surface (especially gas cut surface) and bare filler wire perfectly clean.
C. Avoid undercut & gaps between weld pass.
D. Use proper welding consumables.
Porosity
The presence of gas pores in a weld caused by entrapment of gas during solidification is
termed as porosity. The pores are in the form of small spherical cavities either clustered locally
or scattered throughout the weld deposit. Sometimes entrapped gas give rise to a single large
cavity called Blowholes.
Cause: –
A. Chemically imperfect welding consumables, for example, deficient in deoxidiser.
B. Faulty composition of base material or electrode, for example, high sulphur content.
C. Presence of oil, grease, moisture and mill scale on the weld surface.
D. Excessive moisture in the electrode coating or submerged-arc flux.
E. Inadequate gas shielding or impure gas in a gas –shielded process.
F. Low welding current or too long an arc.
G. Quick-freezing of weld deposit.
Crack
Fracture of the metal is called crack. Two types of cracks: – Cold crack & Hot crack.
Cold crack usually occur in HAZ of the base metal when this zone becomes hard and brittle due
to rapid cooling after the weld metal has been deposited & sufficient hydrogen has been
absorbed by the weld metal from the arc atmosphere.
Precaution: –
A. Use of low carbon equivalent materials.
B. Higher heat input during welding.
C. Preheating.
D. Use of low hydrogen electrode.
Faulty weld size and profile
A weld is considered faulty if it has lack of reinforcement, excessive reinforcement or irregular
Profile.
Distortion
Because a weldment is locally heated (by most welding processes), the temperature distribution in the weldment is not uniform and changes take place as welding processes. Typically, the weld metal and the base metal heat-affected zone immediately adjacent to it are at a temperature substantially above that of the unaffected base metal.
As the molten pool solidifies and shrinks, it begins to exert shrinkage stresses on the surrounding weld metal and heat-affected zone area. When it first solidifies, this weld metal is hot, relatively weak, and can
exert little stress. As it cools to ambient temperature, however, the shrinkage of the weld metal exerts increasing stress on the weld area and eventually reaches the yield point of the base metal and the heat-affected zone. Residual stresses in weldments have two major effects. First, they produce distortion, and second, they may be the cause of premature failure in weldments.
Distortion is caused when the heated weld region contracts nonuniformly, causing shrinkage in one part of the weld to exert eccentric forces on the weld cross section. The distortion may appear in butt joints as both longitudinal and transverse shrinkage or contraction, and as angular change (rotation) when the face of the weld shrinks more than the root.
Distortion in fillet welds is similar to that in butt welds: transverse and longitudinal shrinkage as well as angular distortion results from the unbalanced nature of the stresses in these welds
