Influence of the compound layer on the rolling contact fatigue properties of nitrided medium-carbon steel at the elevated temperature

Abstract

Influence of the compound layer (CL) on the rolling contact fatigue (RCF) properties of plasma nitrided steel was experimentally investigated at the elevated temperature. The results show that with the removal of its CL, the rated life (L10), the median life (L50) and characteristic life (L63.2) of the nitrided steel increased by 157.9%, 57.9%, and 54.0%, respectively. Spalling and delamination are two main RCF failure modes of the nitrided steel independent on the CL. Spalling was caused by the mutual collection of two ring-type cracks or a ring-type crack and the main crack perpendicular to the rolling direction, while delamination was associated with the convergence of the ring-type crack and the subsurface main crack which was induced by not only the maximum shear stress but also the vein-like structure. Early spalling of CL made more severe stress concentration, which accelerated the initiation and propagation of the ring-type crack and then significantly reduced the RCF life of the nitrided steel. Consequently, a brittle CL is suggested to be removed to improve the service life of the industrial parts made of the nitrided steel and operating under the alternating stress.