Page 65 - CITS - Welder - Trade Theory

WELDER - CITS

Craters : Depression at the termination of a weld head or in the molten weld pool.
Melt-through : A condition resulting when the arc melts through the bottom of a joint welded from one side

Spatter : Metal particles expelled during welding which do not form a part of the weld.

Arc strikes (arc burns): Discontinuities consisting of any localized re melted metal, or change in the surface
profile of any part of a weld or base metal resulting from an arc.
Cracking
This can occur due just to thermal shrinkage or due to a combination of strain accompanying phase change and
thermal shrinkage.
In the case of welded stiff frames, a combination of poor design and inappropriate procedure may result in high
residual stresses and cracking. Where alloy steels or steels with a carbon content greater than about 0.2%
are being welded, self cooling may be rapid enough to cause some (brittle) marten site to form. This will easily
develop cracks.

To prevent these problems a process of pre-heating in stages may be needed and after welding a slow controlled
post cooling in stages will be required. This can greatly increase the cost of welded joins, but for high strength
steels, such as those used in petrochemical plant and piping, there may well be no alternative.
Types of Cracking
Solidification Cracking
This is also called centre line or hot cracking. They are called hot cracks because they occur immediately after
welds are completed and sometimes while the welds are being made. These defects, which are often caused by
sulphur and phosphorus, are more likely to occur in higher carbon steels.

A schematic diagram of a centre line crack is shown below

Distortion after welding
Distortion of parts after welding : (a) butt joints; (b) fillet welds. Distortion is caused by differential thermal expansion
and contraction of different parts of the welded assembly

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CITS : C G & M - Welder - Lesson 12&13