Effects of Tempering Treatment After Carbonitriding on the Integrity and the Scratch Resistance of 25CrMo4 Steel Surfaces

Tempering treatment allows to produce a tough steel with an optimal hardness, ductility, and toughness. In our experiments, a tempering treatment was applied at two temperatures (200 ℃ and 550 ℃) after a gas carbonitriding treatment of 25CrMo4 steel. The impact of tempering temperatures on the microstructure, mechanical and scratch behavior was investigated using X- ray diffractometry, microhardness measurements and scratch testing. The phase structure and cross-sections of the modified layers were also investigated by using an optical microscope. The obtained results showed that the microstructure consists mainly essentially of residual austenite and martensite whose proportions depend on the tempering treatment after carbonitriding. Moreover, the tempering treatment decreased the microhardness of the carbonitrided layer from 800 HV0.1 of the C1 state (i.e., austenization −870°/7 h, atmosphere between 0.8 and 1% carbon potential, quenching-oil) to 769 HV0.1 for the C11 state (i.e., austenization −870°/7 h, atmosphere between 0.8 and 1% Carbon potential, oil quench and. Temper. −200 ℃/1 h) and 612 HV0.1 for the C12 state (i.e., austenization −870°/7 h, with an atmosphere having a carbon potential ranging between 0.8 and 1%, quenching-oil and tempering −550 ℃/1 h), Scratch resistance was achieved using a progressive charge mode. The critical charges dropped from 12.7 ± 0.23, for the C1 state, to 7.3 ± 0.21 for the C12 state.