What are the common welding defects in welds? How are they formed?

Common defects in welds include porosity, slag inclusion, incomplete penetration, lack of fusion and cracks.

1) Air holes are cavities formed in the weld metal by absorbing excessive gas or gas generated by metallurgical reactions when the weld pool is high in the welding process, and it is too late to escape before cooling and solidification. The main reason for the formation is that the welding rod or flux is not dried before welding, and the dirt on the surface of the weldment is not cleaned up.

2) Incomplete penetration refers to the phenomenon that the base material at the root of the welded joint is not completely penetrated. The main reason is that the welding current is too small, the transport speed is too fast, or the welding specification is improper.

3) Unfused means that the filler metal and base material or filler metal and filler metal are not fused together. The main reasons for the lack of fusion are unclean grooves, too fast transport speed, too small welding current, improper welding rod angle, etc.

4) Slag inclusion: refers to the slag or non-metallic inclusions remaining in the weld metal after welding. The main reason for slag inclusion is that the welding current is too small, the welding speed is too fast, and the cleaning is not clean, resulting in the formation of slag or non-metallic inclusions too late to float. 

5) Crack: refers to the local gap in the heat-affected zone of the weld or base metal during or after welding. Cracks can be divided into hot cracks, cold cracks and reheat cracks according to their causes. Hot cracks are caused by improper welding process during welding; cold cracks are caused by excessive welding stress, high hydrogen content in the electrode flux, or excessive differences in the rigidity of the weldment, often after the weldment is cooled to a certain temperature So it is also called delayed crack; reheat crack is generally the crack produced by reheating the weldment after welding (stress relief heat treatment or other heating process).

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