Comparison of fatigue crack growth rate: Pearlitic rail versus bainitic rail

In recent years, the possibility of using bainitic steels in railways has aroused considerable interest due to the promising mechanical properties and performance in service. Concerning fatigue performance, the published research does not unequivocally indicate the bainitic rails' advantage over conventional pearlitic rails. Therefore, the investigations' incentive were to compare the fatigue crack growth rate of the pearlitic and bainitic rails. The result of FCGR tests was surprising because, in terms of Paris's law, both materials exhibited comparable fatigue crack rate (regardless of cycle asymmetry) and transcrystalline fracture mechanism. It was found that the primarily fatigue crack of the bainitic rail was propagated between the bainitic ferrite sheaves with different orientations. There was also no indication of the strain-induced transformation of blocky retained austenite into martensite, suggesting the high stability of refined austenite blocks. In the case of the pearlitic rail, the primary fatigue crack propagated along the pearlite colony boundaries. Both rails are characterized by a different mode of fracture in the final fracture region. A quasi-cleavage fracture characterized the bainitic rail, while the pearlitic rail exhibited the typical cleavage fracture. The tests' results indicate that the comparative assessment of both materials should also take into account other functional properties, such as abrasive wear resistance and rolling contact fatigue.