In this paper, the role of cold treatment (before tempering and after tempering) on the fatigue properties of 18CrNiMo7-6 carburized steel are systematically investigated, and the fatigue crack initiation mechanism of 18CrNiMo7-6 carburized steel is clarified through the observation of fatigue fracture surfaces. In addition, the changes of hardness, microstructure and residual stress during fatigue test were observed in situ. The results revealed that after cold treatment (after tempering), the fatigue limit increased from 830 MPa to 870 MPa. The fracture of cold treatment group after tempering was caused by inclusions. On the contrary, the cold treatment before tempering is not beneficial to the fatigue property. Furthermore, the cracking induced by high density twinned martensite was observed in the cold treatment group before tempering. Finally, the retained austenite content of the surface layer after the cold treatment (before tempering) decreased significantly. However, under the action of cyclic load, the high hardness and residual compressive stress of the surface layer continue to decrease due to the cyclic softening and stress relaxation effects, respectively. The poor persistence of hardness and residual compressive stress is not conducive to the fatigue properties.

中文翻译：

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冷处理对18CrNiMo7-6渗碳钢疲劳性能的影响

本文系统研究了冷处理（回火前和回火后）对18CrNiMo7-6渗碳钢疲劳性能的作用，并通过疲劳断口观察阐明了18CrNiMo7-6渗碳钢疲劳裂纹的萌生机理。表面。此外，还现场观察了疲劳试验过程中硬度、显微组织和残余应力的变化。结果显示，冷处理后（回火后），疲劳极限从830 MPa提高到870 MPa。冷处理组回火后断裂是由于夹杂物造成的。相反，回火前的冷处理则不利于疲劳性能。此外，在回火前的冷处理组中观察到由高密度孪晶马氏体引起的开裂。最后，冷处理后（回火前）表层残余奥氏体含量显着下降。然而，在循环载荷的作用下，表层的高硬度和残余压应力分别由于循环软化和应力松弛效应而不断降低。硬度和残余压应力的持久性差，不利于疲劳性能。