Case hardening
Heat treatment process where the surface layer of low-carbon steel is enriched with carbon and then hardened, creating a wear-resistant surface with a tough core.
Case hardening is different: you introduce carbon or nitrogen into the surface layer by placing the part in a carbon- or nitrogen-releasing medium and heating it up. The carbon then diffuses into the surface and makes it hard and wear-resistant after hardening, while the core remains tough. When welding, you have to be careful not to melt away the surface layer or overheat it, otherwise you'll lose the hardness or get cracks. If you have to grind after welding, go carefully so you don't go too deep into the hardened zone. The exact temperatures and times depend on the material and the desired case depth, so you have to check the data sheet or the heat treatment specs.