Rolling Contact Fatigue Performance of Carbonitrided Pyrowear 675 at 218°C with MIL-PRF-23699G

Abstract

This research presents the rolling contact fatigue (RCF) life performance of carbonitrided (CN) martensitic stainless steel Pyrowear 675 (P675). P675 was heat-treated using a patented single-step carburizing-nitriding process. The CN specimens were tempered at 316°C (low temperature tempered: LTT) and 496°C (high temperature tempered: HTT). The CN Pyrowear 675 specimens were evaluated for RCF life using a ball-on-rod tester at a maximum Hertzian stress of 5.5 GPa in a hybrid configuration (with silicon nitride rolling elements) and at a temperature of 218°C using three gas turbine engine lubricants conforming to MIL-PRF-23699G. The RCF performance of the CN P675 was compared to case-carburized P675 (both LTT and HTT) and M50, evaluated under identical test conditions. Both versions of CN P675 showed significant improvement in fatigue life and wear resistance compared to M50 and case-carburized P675. RCF rod specimens were also analyzed for microstructural changes. The microstructural changes in CN P675 showed only the formation of dark etching regions (DERs). Microstructural changes in the form of white etching regions (WERs) or white etching cracks (WECs) were observed with M50 and case-carburized P675. In addition to operating conditions (load, speed, and temperature), the results indicate that bearing material, heat treatment, and lubricant play an important role in microstructural changes.