Chromium Carbide Formation
The chemical bonding of chromium and carbon at elevated temperatures, which depletes chromium from the matrix. This can lead to local susceptibility to corrosion at grain boundaries if alloying is insufficient.
Chromium carbide formation happens when the steel is in the critical temperature range and carbon combines with chromium at the grain boundaries. This strips the surrounding area of chromium, and exactly there the steel loses its corrosion protection – that's called grain decay or intergranular corrosion. When welding, the risk is especially high because the heat-affected zone hits exactly that critical range. If you're not careful, you'll get cracks later or the material will rust away at the grain boundaries even though it's otherwise stainless. So you have to make sure the steel doesn't hang around too long in that critical range. You do that through heat management and cooling. For thin sheets, a copper backing or rapid cooling often helps; for thicker parts, you need to watch the interpass temperature. And if you have the choice, go with a low-carbon or stabilized grade – then you won't have the problem in the first place.