Gears are typically heat treated to produce a wear resistant case on the surface of the gear tooth, with an impact resistant core in the middle of the tooth and body.  This combination of properties is most often accomplished by carburize hardening of carbon and alloy steel gears, and by induction hardening of large dimension gears.  For gear applications in which the surface and core requirements are the same, the alloy steel gears may be through hardened in batch oil quench furnaces.  Stainless steel parts may be bright hardened in hydrogen atmosphere belt furnaces or in vacuum furnaces.