Abstract: This paper describes a new heat treatment process for 40Cr steel. The steel is quenched at 1280℃ to get a coarse quenching lath martensitic substructure first, then a secondary quenching is carried out at 810℃ to get a fine lath martensite. The test results show that ths yield strength and impact toughness of the steel is increased obviously, and the temper brittleness under lower temperature(350 ℃ )is restrained.
The micromcchanism for improving mechanical properties and restraining temper brittleness is also discussed. It is concluded that the cementite in granular form is the primary source for improving mechanical properties and restraining temper brittleness.