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.

具有亚稳态奥氏体和精细微观结构的无碳化物贝氏体 (CFB) 钢已被证明是一种有前途的替代传统珠光体钢轨和车轮钢的材料，因为它具有出色的静态机械性能和使用性能。然而，对其在铁路车轴中的潜在应用的探索很少，微观结构对 CFB 钢疲劳裂纹扩展 (FCG) 行为的影响尚不清楚。在这项工作中，通过等温淬火和低温回火设计了一种含有残余奥氏体 (RA) 和细化贝氏体的 CFB 车轴钢。评价了常规高速铁路车桥钢（DZ2级）和CFB钢的静态力学性能、高周疲劳（HCF）性能和FCG性能。抗拉强度，CFB 钢的均匀伸长率和 HCF 强度分别比 DZ2 钢高 87%、24% 和 60%。两种车轴钢的 FCG 行为在巴黎法区几乎相同。在DZ2钢中，裂纹偏转和二次裂纹抑制了主裂纹的扩展，晶间裂纹扩展引起的晶粒桥接延迟了裂纹尖端的裂纹张开。裂纹在 CFB 钢中穿晶扩展。当裂纹扩展方向平行于贝氏体板条长轴时，裂纹优先沿该方向扩展。然而，当方向接近垂直时，在细贝氏体板条附近形成的锯齿形裂纹路径可以有效地阻止裂纹扩展。此外，