Due to the evolution of cleanness of bearing steels, the failures observed in service conditions move from deep initiated spalling on non-metallic inclusions to surface damage due to machining marks, lack or contaminated lubrication (dents). To take into account, by the most economic way, these request for more demanding applications, ASCOMETAL, SNR et VALTI have developed a new steel grade [1,2] based on the classical analysis of 100Cr6 (SAE 52100), with higher characteristics in terms of in-service properties (sustaining high loading, high temperature and contaminated lubrication) without deteriorated steelmaking or forming process by an optimised composition of the steel.

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Powder metallurgy (PM) for bearing steel manufacturing was introduced several decades ago and mainly aimed at limiting segregation effects in high-alloy grades. Despite the significant potential of this relatively new process for producing high-performance bearing steels, its use in commercial applications is still very limited today. It is thought that the slow acceptance of this promising technology is partially due to a lack of understanding of how modern PM steels compare to conventional ingot metallurgical steels. Most of the comparative studies published on this topic have only focused on a few key mechanical properties, which are rarely related to the microstructure. For this study, several variants of M50 were produced using ingot metallurgical and PM processes. This grade was chosen as its performance is well known to be limited by segregation, and it could therefore benefit from a PM process route. The evolution of the microstructure during manufacturing, from solidification to tempering, was carefully investigated. After heat treatment, toughness, hardness, and rolling contact fatigue (RCF) life were measured. RCF tests were performed using a ball-on-rod configuration to compare the performance of the alloys as well as to evaluate the microstructural changes during testing. Differences in the populations of stress raisers (primary carbides and nonmetallic inclusions) were also assessed and used to explain the variations in RCF fatigue lives.

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