钢轨踏面马氏体白层组织分析

Microstructure analysis of martensite white etching layers on rails

用扫描电镜(SEM)对长期运行后的U71Mn钢轨踏面马氏体白层组织进行了分析。结果得出,曲线段钢轨踏面白层较直线段白层厚度深,硬度更高。随距表面距离增加,曲线段钢轨踏面白层内原奥氏体晶粒分别呈等轴状、长条状和非常细小的等轴状,原奥氏体晶粒内的马氏体形态随深度的增加,由以片状马氏体为主逐渐转变为以板条状为主的混合马氏体;而直线段钢轨踏面白层内原奥氏体晶粒均为长条状,且长轴方向平行于踏面,原奥氏体晶粒内为以板条状马氏体为主的混合马氏体。结合分析结果和可能的运行情况,作者认为,出现上述结果的主要原因是在钢轨使用过程中,由于蠕滑等原因造成表面温度急剧升高,超过了奥氏体的形成温度,使得组织中的碳化物发生不同程度的溶解,形成的奥氏体晶粒形状和大小,以及随后的快速冷却过程中得到的马氏体形态也与这种快速冷热循环有关。

White etching layers （WELs） with martensite microstructure on U71Mn steel rails were investigated by scanning electron microscopy （ SEM）. The results show that the WELs at the curved parts of the rails exhibit higher thickness and hardness compared with those of the straight line parts. With the increase of depth from the surface, the prior austenite grains in the WELs of the curved part present the equiaxed, long strip and very fine equiaxcd characters, respectively. Meanwhile, the martensite type in these grains is transformed from plate martensite to lath martensite. However, the prior austenitc grains in the WELs of the straight part all present a long strip shape, and their long axis direction is parallel to the rail surface, and in which the martensite is mainly composed of lath martensite. The observation of the microstructure suggests that the temperature above austenitizing temperature in the surface layer of the rails enhanced due to creeping slip results in the carbides dissolving inhomogeneonsly formation of the austenite with different sizes and shapes, and the austcnite is transformed to the martensite with different morphology by the rapid cooling subsequently.