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Mechanisms of fatigue crack propagation in a Q&P-processed steel

机译：Q＆P处理钢中疲劳裂纹扩展的机理

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摘要

The fatigue crack propagation (FCP) behavior of a quenched and partitioned (Q&P), low-alloyed CSiCrMn steel containing 12% of retained austenite is investigated in Delta K-controlled tests at different R-ratios and compared with its quenched and tempered (QT) counterpart, containing 4% of retained austenite. Characteristic features of the crack wake and FCP surfaces are examined by scanning electron microscopy and related to the presence or absence of martensitic transformation. Comparing the FCP behavior of the Q&P condition with its QT counterpart, the fatigue threshold Delta K-th is higher, and fatigue crack growth rates in the Paris-regime are lower, for Q& P. With an increase of Delta K, Paris-lines intersect and, at high Delta K, the fatigue crack growth resistance of the Q&P condition becomes inferior. Such FCG behavior is attributed to the amount and stability of retained austenite: at low Delta K, contributions of the martensitic transformation are negligible, and thus a higher fraction of retained austenite accounts for an increased FCG resistance. With increasing Delta K, an increasing percentage of retained austenite in the cyclic plastic zone at the crack tip is transformed, providing brittle pathways for fast crack propagation and thus lowering FCG resistance. Analysing the crack surface, cleavage-like intergranular crack propagation can be directly related to martensitic transformation: in the threshold region, FCP occurs predominantly in a transgranular mode and the percentage of cleavage planes increases as Delta K increases. This study is the first one that examines the FCP behavior in a Q&P-processed steel and one of very few which deal with the near-threshold FCP behavior in TRIP steels.
机译：通过Delta K控制试验研究了在不同R比下淬火和分配（Q＆P）的低合金CSiCrMn钢（其残余奥氏体含量为12％）的疲劳裂纹扩展（FCP）行为，并将其与淬火和回火（QT）进行了比较），其中含有4％的残余奥氏体。裂纹尾流和FCP表面的特征通过扫描电子显微镜检查，并与是否存在马氏体相变有关。将Q＆P条件的FCP行为与其QT对应的FCP行为进行比较，对于Q＆P，疲劳阈值Delta K-th较高，并且巴黎区域的疲劳裂纹增长率较低。随着Delta K的增加，巴黎线相交，并且在高Delta K时，Q＆P条件的抗疲劳裂纹扩展性变差。这种FCG行为归因于残余奥氏体的数量和稳定性：在低Delta K时，马氏体相变的贡献可忽略不计，因此，残余奥氏体的比例越高，说明FCG阻力越大。随着ΔK的增加，裂纹尖端的循环塑性区中残留奥氏体的百分比增加，从而为脆性裂纹的快速传播提供了脆性途径，从而降低了FCG的抵抗力。分析裂纹表面，类似裂隙的晶间裂纹扩展可能与马氏体转变直接相关：在阈值区域，FCP主要以跨晶模式发生，并且裂隙面的百分比随Delta K的增加而增加。这项研究是第一个检验Q＆P处理钢中FCP行为的研究，也是处理TRIP钢中接近阈值FCP行为的极少数研究之一。

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