Thermochemical treatment

This is case hardening. You put the component into a medium that releases atoms at elevated temperature, which diffuse into the surface and form a hard layer. This makes the surface more wear-resistant or corrosion-resistant, but the core remains tough. If you don't pay attention during welding, you destroy the layer again. The welding heat softens the hardened zone or creates unwanted microstructures. So you have to keep the heat input low and, if in doubt, rebuild the layer after welding.