Spalling failure mechanism of a railway turnout made from carbide-free bainitic steel

Railway turnouts play a transition role between straight and branching rails and therefore suffer from spalling failure as a result of repeated rolling contacts with wheels, as well as shear and impact loads. In this paper, we present a comprehensive failure investigation on the spalling failure mechanism of an off-track turnout made from bainitic steel. The experimental research was focused on the rolling-induced microstructural evolution from various depths to the rail top which caused the cracking and spalling wear. The worn turnout was analysed both on the rail top and on metallographically polished cross-sections using a variety of methods including optical microscopy, microhardness testing, field-emission scanning electron microscopy and energy dispersive X-ray spectroscopy, transmission electron microscopy, and quantitative X-ray diffraction for crystallography and surface residual stress analysis. The results revealed that, the turnout was a low carbon and Si-Mn-Cr-Ni-Mo alloyed steel showing a dual-phase carbide-free microstructure of bainitic ferrite laths and retained inter-lath austenite. The rolling contact fatigue on the rail top resulted in the formation of spalling pits, cracking, extreme hardening, embrittlement, and the creation of a compressive residual stress of 400 MPa. The spalling failure was associated with a gradient microhardness profile to a depth of 1.5 mm, from HV0.1 7.3 GPa of the rail top to HV0.1 3.9 GPa of the bulk steel. Surface cracks were found to have propagated from the rail top to various depths up to 0.9 mm. The spalling failure was caused by deformation induced microstructure evolution within a depth of 0.2 mm from strain-free bainite to sub-micro and nano-scale laminates and eventually to quasi-amorphous rail top. Phenomena of transformation induced plasticity (TRIP) were found, not only including the decrease of retained austenite in the laminate structure, but also the formation of martensitic laths beneath the laminates.