Microstructure stability in wheel steel: A case of thermal-accumulated damage capacity in pearlite and low-carbon bainite

Abstract

Novel train rails/wheels with low-carbon bainitic microstructure have drawn much attention recently owing to their good match of strength and toughness in mechanical properties from retained austenite and microstructure multi-orientation, as a supplement to the existing pearlitic wheels. However, the heat-affected zone is found on the pearlitic and bainitic wheel tread after 100,000 km of test running, while several severe failure cracks were found in both pearlite and bainite wheel treads, particularly in the pearlitic one. Their service advantage in accumulated damage resistance caused by alternating thermal/force loads in service is controversial and their excellent mechanical properties sometimes correspond not to expected fatigue resistance. Thus, the thermal-accumulated microstructure stability of low-carbon bainitic treated by 560 °C tempering, with similar initial strength and hardness levels compared with pearlitic wheel steel was investigated. The comparison of the two results shows that the bainitic structure composed of bainite ferrite and cementite shows relatively stronger crack resistance and better hardness stability (∼355 HV). Grain boundary strengthening and solid solution strengthening in bainite ferrite still play a dominant role in its stability. While pearlitic structure demonstrates a stronger tendency to fatigue cracking and the hardness decreases from ∼ 355.2 HV to ∼ 336.0 HV due to spheroidizing evolution and lamellar fracture, accompanied by element segregation and two-phase lattice mismatching. These comparisons mean that the thermal-accumulated service performance of wheel steel is not solely determined by original higher mechanical properties, but is close to the contribution of microstructure stability for service sustainability.