研究了20CrNiMo钢微氮渗碳层中残余奥氏体的形态,及其在疲劳断裂过程中的转变和作用。结果表明:渗碳层残余奥氏体有薄膜状和块状两种形态,由于界面取向的影响,疲劳裂纹更易于穿过块状奥氏体,促使其发生应变诱发马氏体相变。在疲劳断裂过程中,裂纹面附近的残余奥氏体大部分转变成马氏体,这种转变是在断裂瞬间发生的,与疲劳力学条件无关。转变所引起的体积膨胀增强了裂纹闭合效应。相变诱发闭合是渗碳表层高碳区疲劳裂纹扩张行为的主要因素之一。
The morphology of the retained austenite in the carburized case overthe 20CrNiMo steel and its transformation during fatigue crack propagation throughthe case were investigated by using X-ray and TEM analysis. In the carburizedcase, both film and block shared retained austenite were found. The fatigue crackis inclined to pass through the block shaped retained austenite and thereby stimu-lates its strain-induced martensitic transformation, due to the effect of the orienta-tion relationship between the surfaces. During the process of the fatigue fracture,most of the retained austenite on the crack path are transformed into the marten-site, and the untransformed amounts on the fracture are remained less than 6%. Thetransformation of retained austenite, which is restricted mainly within the plasticzone, occurs only in the time of the fracture, and is independent of external stressesand stress ratios and cyclic numbers. The volume expansion accompanided by itstransformation creates an additional residual displacement of about 0.44μm oncracked surfaces, which is relevent in the magnitude to the plasticity-induced residualdisplacement. The phase transformation induced fatigue crack closure is believed tobe an important factor affecting the fatigue crack behaviors in the high carbonlayer of the carburized case.
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