Chromium carbide network
Continuous precipitation of chromium carbides along austenite grain boundaries under inappropriate cooling or annealing conditions. This depletes the adjacent solid solution of the chromium necessary for corrosion protection.
If the steel cools too slowly after welding, the chromium can react with the carbon and form carbides. These then form at the grain boundaries, and that's exactly where the steel starts to rust, because the chromium is bound there and is missing for corrosion protection. The structure also becomes coarser and more brittle. So you have to make sure that the weld and the heat-affected zone cool fast enough. For high-alloy chromium steels, that means: work cleanly, keep heat input low, and don't preheat too much. If you can't let the weld cool down after welding, then intermediate pass temperature management often helps to get through the critical zone quickly.