Very early stage rolling contact fatigue crack growth in pearlitic rail steels

▶ Very early stage RCF crack growth in rail steels is very microstructurally influenced. ▶ Initially cracks grow along favourably aligned, strained prior-austenite (PA) grain boundaries. ▶ RCF crack initiation favoured at PA boundaries with pro-eutectoid (PE) ferrite. ▶ Very short RCF crack growth is restricted/halted at PA boundaries when un-favourably aligned. ▶ 3D early RCF crack growth and deformed PA grain shapes are linked to ratcheting flakes. This paper reports on metallurgical characterisation of small cracks in pearlitic steels, using observations from both twin disc tests and rails removed from service, plus methods to determine three-dimensional (3D) aspects of such cracks. Tests to varying fractions of rolling contact fatigue (RCF) life have been assessed, based on twin disc tests where “average 100% RCF life” has been defined by eddy current detection of cracks between 200 and 800 μm in depth. Contact surface features and microstructural alterations have been mapped against near-surface strain and compared with worn rail examinations. Initial crack growth along favourably aligned and strained, prior-austenite (PA) grain boundaries, particularly if the boundary contains pro-eutectoid (PE) ferrite, was found to be restricted or halted at the PA grain limit or where the boundary altered to be less-favourably aligned. For samples with up to 55% RCF life, initial crack propagation length was still less than the average PA grain size. 3D aspects of this early growth and the deformed PA grain shapes were found to be linked to ratcheting flakes and flake cracks on test disc and rail contact surfaces. The results have shown how initial RCF crack propagation is very microstructurally influenced.