High cycle fatigue properties and fatigue crack propagation behavior of a bainite railway axle steel

Abstract

Carbide-free bainite (CFB) steel with metastable austenite and refined microstructures has been demonstrated to be a promising alternative to conventional pearlitic rail and wheel steels, due to its outstanding static mechanical properties and performance in service. However, the exploration on its potential application in railway axles is scarce, and the effect of microstructures on the fatigue crack growth (FCG) behavior of CFB steels is not well understood. In this work, a CFB axle steel containing retained austenite (RA) and refined bainite is designed by austempering and low-temperature tempering. The static mechanical properties, high cycle fatigue (HCF) properties and FCG properties of the conventional high-speed railway axle steel (grade DZ2) and the CFB steel are evaluated. The tensile strength, uniform elongation and HCF strength of CFB steel are about 87%, 24% and 60% higher than that of the DZ2 steel, respectively. The FCG behavior of the two axle steels is almost equivalent in the Paris law region. In the DZ2 steel, crack deflection and secondary cracks inhibit the main crack growth, the grain bridging caused by the intergranular crack growth delays the crack opening at the crack tip. The crack propagates transgranularly in the CFB steel. When the crack propagation direction is parallel to the long axis of the bainite lath, the crack propagates preferentially along this direction. However, when the directions are nearly perpendicular, a zig-zag crack path formed near the fine bainitic lath can effectively prevent crack growth. Furthermore, the deformation induced martensitic transformation ahead of the crack tip enhances the crack-tip shielding.