Retained austenite
The metastable face-centered cubic phase that remains untransformed at room temperature after martensite formation, the amount of which can be reduced by cryogenic treatment.
After welding, some steels retain retained austenite in the microstructure that doesn't transform. That's normal, but if you don't treat it, it can cause problems later because the austenite is metastable and can transform under stress or at lower temperatures. To get it under control, you need to deep-freeze the workpiece in a targeted way. It's important to complete the cooling properly and not stop too early, otherwise you'll still have too much austenite and the part will be hard and crack-prone. The exact temperatures and times depend on the steel grade, so you need to follow the data sheet.