Abstract: Influence of nitriding time in QPQ technology on the microstructure and friction properties of an alloy cast iron was studied by means of OM, SEM, hardness test, friction and wear test methods. The results show that after QPQ treatment, the nitriding layer formed on the surface of the alloy cast iron is mainly composed of Fe_2-3N, Fe₂O₃ and FeO. There is a linear relationship between the square of the nitriding layer thickness and the nitriding time. Under the condition of nitriding salt bath at 580 ℃, the activation energy of nitrogen diffusion in the alloy cast iron is 70.07 kJ/mol. Within the nitriding time range of 90-150 min, with the increase of the nitriding time, the surface hardness value of the nitriding layer decreases from 522 HV0.05 to 441 HV0.05, while when the nitriding time is extended to 180 min, the surface hardness value rises back to 455 HV0.05. Compared with the untreated specimen, the QPQ-treated specimen has a smaller friction coefficient, and after the QPQ treatment, the furrows on the surface of the specimen become significantly shallower, the metal peeling at the surface is also significantly improved. As the nitriding time increases, the thickness of the nitriding layer increases and the material is stabilized. The QPQ specimen after 180 min nitriding has the best comprehensive properties, with surface hardness value of 455 HV0.05 and friction coefficient of 0.32.

Key words: QPQ technology, nitriding time, alloy cast iron, microstructure, friction and wear