40Cr steel is one of the most widely used steels in machinery manufacturing industry. After quenching and tempering, it has good comprehensive mechanical properties, good low temperature impact toughness and low notch sensitivity. The hardenability of the steel is good, and it can be quenched to the maximum when water quenched Ф 28 ~ 60mm, oil quenched to Ф 15～40mm。 When the hardness is 174 ~ 229hb, the relative machinability is 60%. The steel is suitable for making medium plastic mould.