服役U71Mn钢轨疲劳裂纹萌生及扩展分析

Analysis of Fatigue Crack Initiation and Propagation on U71Mn Steel Rail during Service

采用HV-1000型显微维氏硬度计、光学显微镜、扫描电子显微镜和能谱仪等手段,研究了服役后U71Mn钢轨表层力学性能、轨距角处的显微组织以及裂纹萌生和扩展机理.结果表明:钢轨表层轨距角处的硬化程度最严重,并且该区域沿列车运行方向呈现出较为严重的剪切塑性变形,裂纹容易在该处萌生.钢轨轨距角处的裂纹萌生受到表层切应力分布状态、表面摩擦因数以及表层夹杂物等的综合影响,钢轨表层受到循环应力的反复作用,在加工硬化的作用下材料局部发生硬化,塑韧性降低,另外钢轨表层有大量长条状的MnS夹杂物,这些因素都极大地促进了疲劳裂纹的萌生.轨距角处的裂纹沿着钢轨表层剪切塑性变形的方向扩展,主要方式是穿晶扩展,裂纹间隙中较多的氧化物对裂纹扩展有较大的促进作用.

The mechanical properties, microstructure, and crack initiation and propagation of U71Mn steel rail after service were studied by the techniques of microsclerometer, optical microscope, scanning e- lectron microscope （SEM）, and energy disperse spectroscopy. The results indicate that the hardening de- gree on the gauge corner of the rail is the most serious, and the plastic deformation is very serious in this section. Thus, the crack may initiate on the gauge corner, and the crack initiation is influenced by the shear stress, surface friction coefficient and inclusions. Meanwhile, the microstructure on the surface will deform again and again under the cyclic stress, and the breakdown of the grains will harden the materials and decrease the toughness and ductility. Moreover, there are many lath-shape MnS inclusions in the rail, which all promote the initiation of the crack. The crack in the rail propagates along the surface plastic de- formation, and the path of crack growth is transgranular. There is much iron oxide in the gap of the crack, which can also promote the propagation of the crack.