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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
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