Abstract: In order to investigate the effect of quenching media on microstructure and impact property of the ion-nitrided 42CrMo steel, PAG aqueous solution and quenching oil was used respectively for quenching at 850 ℃ for 1 h and tempering at 580 ℃ for 1.5 h, and then ion-nitriding at 540 ℃ for 60 h was carried out. The microstructure, strength and toughness before and after ion-nitriding were studied by means of optical microscope (OM), scanning electron microscope (SEM), tensile test, impact test and microhardness tester. The results show that the microstructure of the 42CrMo steel is all martensite after quenching with PAG aqueous solution, while the oil quenched microstructure is a mixture of martensite and lower bainite, however, both of the microstructure of the steel after quenching and tempering with two quenching medias is fine and uniform tempered sorbite. After ion-nitriding, the nitriding layer of the 42CrMo steel with PAG aqueous solution quenching is thicker than that with oil quenching, and the microstructure at the core is not different from that after quenching and tempering, while the microstructure at the core of that with oil quenching shows obvious free ferrite. The impact absorbed energy at -20 ℃ of the 42CrMo steel after quenching and tempering with oil quenching is higher, while the impact absorbed energy at -20 ℃ of the nitrided 42CrMo steel with PAG aqueous solution is 13.8% higher than that with oil quenching.

Key words: ion-nitriding, quenching and tempering, impact property, quenching media