Residual hydrogen in weld metal
The hydrogen fraction remaining in the weld metal after solidification, which must be minimized by post-weld heat treatment. High residual contents endanger the structural integrity of the joint.
Residual hydrogen is the hydrogen that goes into the metal during welding and remains after cooling. It mainly comes from moisture in the shielding gas, on the wire, or on the workpiece. Too much of it makes the weld brittle and can lead to cracks, often only later. You don't want that. The remedy is degassing: keep the part warm after welding or let it cool slowly so that the hydrogen can diffuse out. Depending on the material and plate thickness, you have to do this for a longer or shorter time. For critical steels, this is mandatory, otherwise you'll have trouble.