Surface hardening

Case hardening means only the outer layer of a part is hardened, while the core stays tough. You do this, for example, with flame or induction hardening when the surface will see wear but the part must not break. When welding, you have to be careful not to soften the hardened layer again. If you weld near a hardened zone, the heat can destroy the hardness. Clean the area well beforehand, and check the hardness after welding, otherwise you'll quickly get cracks or a too-soft spot.