Duplex Hardening of Steels for Aeroengine Bearings

Duplex hardening is a process in which conventional heat-treatments such as secondary or case hardening are followed up by surface nitriding to achieve exceptional properties, particularly in the context of aeroengine bearings. Hardness values in excess of 1 000 HV can in principle be achieved, and although the hardness decreases as a function of depth below the surface, some level of enhancement can occur to a depth of up to 0.2 mm. The process can be applied to both through hardened and case hardened bearing steels such as M50 and M50NiL respectively, with concomitant increases in rolling contact fatigue resistance and tribological properties. The process has not yet been universally adopted and is ripe for a critical assessment.