High frequency quenched gears are usually made of carbon steel to low alloy steel (40,45,40 Costa Rica and 45 Mn2) with carbon content of 0.40% and 0.50%. In large-scale production, it is generally required to select carbon containing components to ensure the quality. The temperature limit of 45 Costa Rica steel is 0.42% ~ 0.47%, and that of 40 steel is 0.37% and 0.42%. After high frequency quenching and low temperature tempering, the quenching layers are mixed martensite, and the center is microstructure after heat treatment (normalization or tempering).