Discussion
The presence of which gas in the weld metal has harmful effect leading to crack?
Answer: B
The gas that causes cracking is hydrogen.
Hydrogen dissolves in the molten weld metal, coming mainly from moisture in electrode coatings, damp flux or grease on the work. As the weld cools, the hydrogen diffuses into the hard heat-affected zone and causes hydrogen-induced cracking, also called cold or delayed cracking, which can appear hours after welding.
Oxygen and nitrogen are harmful too, causing oxides, porosity and brittleness, but cracking is the characteristic effect of hydrogen. That is why low hydrogen electrodes are baked before use.
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