Heat treatment processes are used to change the properties of steel in gears. There are various types of heat treatments including annealing, normalizing, quenching, tempering, and surface hardening processes. For gears, common hardening processes include through hardening, case hardening using carburizing, nitriding or carbo-nitriding, and surface hardening using induction hardening or flame hardening. Case hardening produces a hard wear-resistant surface layer while keeping the core ductile, to increase pitting resistance without risk of tooth breakage. Surface hardening selectively hardens only parts of the gear like the tooth flanks, roots and tips.