Hydrogen Desorption Kinetics

Hydrogen in the weld metal is a known issue; it makes the material brittle and can lead to cracking. That's why you don't just let the weld cool down after welding, but keep it warm. By keeping it warm, the hydrogen gets time to diffuse out of the workpiece before the microstructure transforms. If you cool it down too fast, the hydrogen stays trapped and you risk cold cracking. So: after welding, cover the weld and the heat-affected zone and let it cool slowly, then you won't have any surprises.