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Newly designed gears are subject to higher loads that demand a steel that is capable of greater pitting resistance. The application of shot peening to gears has been increasing to improve tooth root strength, but pitting resistance had not been necessarily high. This study examines the effect of alloying additions mainly on tempering resistance and the formation of a non-martensitic layer. The developed high Si-Mo type steel shows excellent pitting resistance, even in shot peened gears, as compared to that of conventional steels due to high tempering resistance and the thin, uniform non-martensitic layer. This new steel is of practical use in some multi-speed automatic transmission gears.

Gas carburizing is a surface hardening method that is used for automobile steel parts such as gears. The conventional gas carburizing(at 1223K) takes longer time. Shortening the carburizing time will contribute greatly to the energy saving and CO2 reduction of parts production. The new carburizing process at higher temperature,1273K, and vacuum is made without adding reheating and quenching process. That process can shorten treatment time to one-fourth of conventional process. The new alloy steel for high-temperature vacuum carburizing is developed in order to prevent the grain coarsening. This paper describes the outline of this alloy steel.