Nitrogen solubility in liquid iron

Nitrogen dissolves in liquid iron. When the melt cools, it can no longer be bound and escapes as gas – that creates pores in the weld. This is especially an issue with chromium-alloyed steels because chromium promotes nitrogen absorption. So you have to guide the melt calmly and controlled so that the nitrogen can escape cleanly before solidification. Cooling too fast is poison; the nitrogen stays in the weld and you get holes.