Influence of a Nonuniform Microstructure on the Rolling Contact Fatigue Properties of ER8 Steel Wheel

To mitigate the damage caused by rolling contact fatigue (RCF) on wheels, RCF tests are conducted to obtain the RCF limit of the surface area of an ER8 wheel tread that comprised a nonuniform microstructure. The microstructure topography of the RCF area is observed with optical microscopy, a scanning electron microscopy, and a transmission electron microscopy. The hardness, elasticity, and plasticity of different areas of the nonuniform microstructure are analyzed with a nanoindentation instrument. The results show that, besides pearlite and proeutectoid ferrite (matrix), a large amount of upper bainite is present. This component, therefore, interrupts the continuity of the wheel steel matrix, leading to a nonuniform microstructure and an RCF limit of 1074 MPa, which is lower than that of a wheel worn to the limit area and that comprised a normal microstructure. Furthermore, cracks mainly initiate and propagate at the boundary of the upper bainite and matrix. Although the hardness and the elasticity of the upper bainite are higher than those of the matrix, the plasticity is lower. Furthermore, under the effect of the contact stress, the difference between their deformations leads to stress concentration at the boundary, facilitating the initiation and propagation of cracks in steel wheels and accelerating the damage due to RCF.