| With the development of mechanical manufacturing, the requirement of bearing fatigue properties are higher than before. The surface chemical heat treatment process of a single element had been unable to meet the fatigue properties very well. Therefore, composite chemical heat treatment process developed gradually as a kind of advanced surface hardening technology. The process has an effect on improving the fatigue properties of the bearing and prolonging the service life of bearing.In this paper, the influence of quenching temperature and tempering temperature on microstructure and mechanical properties of the medium-carbon chrome molybdenum steel was researched, as well as the rolling contact fatigue properties after carbonitriding treatment. moreover, the rotating bending fatigue properties and the rolling contact fatigue properties of the low-carbon CrNiMoV steel was studied with different duplex-thermochemical processing. optical microscope, phase shift microXAM-3D, SEM, EDS, XRD and TEM analysis means wes used to observe the microstructure, the second phase, and the fracture morphology of the tested steel. The fatigue properties of the tested steel was tested by the rotating bending fatigue experiment and rolling contact fatigue experiment with different duplex-thermochemical processing.The core of the medium-carbon chrome molybdenum steel had good comprehensive mechanical properties when quenching temperature was 990℃, cryogenic treatment temperature was-73℃ and tempering temperature was 220℃.The tensile strength, yield strength,rockwell hardness, shock absorption function reduction of area and break elongation of the tested steel were 2053 MPa,1806 MPa,54.2 HRC,25 J,30%,10%. And the grain size of the tested steel was 37.58 microns.After carbonitriding, the sueface highest microhardness and the effective depth of carbonitriding layer of the medium-carbon chrome molybdenum steel were 781 HV and 1.1 mm. The microstructure of carbonitriding layer was acicular martensite and the carbonitriding layer of hardness curve was changed quite gentle. The carbides of carbonitriding layer were small and dispersed. moreover,the carbonitriding layer structure was ideal. The medium-carbon chrome molybdenum steel had a good rolling contact fatigue performance with carbonitriding. when the contact stress was 4.5 GPa, the probability which cyclic times supass 107 was 83.3%. The major cause of fatigue failure was Al2O3 inclusions, and the number, size, shape of Al2O3 inclusions had a distinct influence on the fatigue life of the tested steel.After the first kind of duplex-thermochemical processing, the highest permeability layer microhardness and infiltration depth of the low-carbon CrNiMoV steel were 885 HVand 1.7 mm, and the depth of the composite hardening layer was 0.30 mm, the residual compressive stress was 287.52 MPa. Surface microhardness of duplex hardening was more 93 HV higher than the carburizing and the residual compressive stress on the surface of tested steel with duplex hardening increased 102.59 MPa. Compared with carburizing, the rotating bending fatigue limit of the tested steel with duplex hardening raised nearly 200 MPa, and the rotating bending fatigue performance significantly improved.After the other kind of duplex-thermochemical processing, the highest permeability layer hardness and infiltration depth of the low-carbon CrNiMoV steel were 1153 HV and 1.83 mm, and the depth of the composite hardening layer was 0.28 mm. The microhardness changed quickly in the duplex hardening layer, there was many vein structure that distributed along the grain boundary in the duplex hardening layer.and the distribution range of vein structure was 0-0.19 mm.when the contact stress was 4.5 GPa, the probability which cyclic times of tested steel supass 106 was 100%.The results showed that the micro cracks and the vein structure in the duplex hardening layer ware the main cause of rolling contact fatigue failure.
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