Chromium carbide precipitation kinetics

Chromium carbides precipitate at the grain boundaries at certain temperatures. This strips chromium from the surrounding steel, which kills the corrosion resistance. That's called chromium depletion, and the result is intergranular corrosion, i.e., cracks along the grains. Especially with stainless steels, you have to keep the heat input low and cool quickly so you don't stay long in the critical temperature range. If the part gets too hot or cools too slowly, you're in trouble.