Carbonitriding
Diffusion of carbon and small amounts of nitrogen into the surface layer at austenitizing temperatures prior to subsequent quenching.
Carbonitriding is a case hardening process where you introduce carbon and nitrogen simultaneously into the surface. You do this by bringing the workpiece to austenitizing temperature in a suitable bath or gas mixture and then quenching. This gives you a hard, wear-resistant case while the core remains tough. When welding, you have to be careful: if you weld on an already carbonitrided surface, you burn off the layer and get cracks or porosity in the heat-affected zone. The hardening is gone after welding, so you either have to remove it beforehand or reapply it afterwards. Too much carbon or nitrogen also makes the layer brittle and prone to cracking, so stick to the specified process times and temperatures.