Duplex Hardening for Aerospace Bearing Steels

Abstract

A new process, duplex hardening, was developed, for two steels which are already widely used for aerospace applications, M50 and M50 NiL. The rolling contact fatigue and sliding wear performance of duplex hardened bearing components were evaluated on M50 and M50NiL steels. The rolling contact fatigue and sliding wear tests performed under the development program demonstrate superior performance of duplex hardened M50 and M50 NiL over conventional treated M50 and M50NiL bearing components. The bearing rig tests performed in this study also demonstrate superior bearing performance of duplex hardened M50 and M50NiL steels over conventional heat treated M50 resp. M50NiL steels. Under boundary lubricating conditions, the duplex hardened M50 and M50NiL bearings did not even fail after 1500 hrs. In tests with predamaged races, duplex hardened M50 bearing races demonstrated more than ten times the calculated L10-life. The tests on duplex hardened M50NiL bearings were interrupted after 1.500 hrs. runtime with no bearing failure.

Duplex Hardening of Steels for Aeroengine Bearings

“…Figure 7 illustrates the hardness profile of M50NiL after plasma nitriding. 6,24,25,33) The hardness achieved with the higher nitriding temperature and nitrogen potential is, as might be expected, greater, given the higher likelihood of intragranular precipitation (Fig. 4).…”

“…33) In order to avoid this intergranular precipitate, a low temperature and low nitrogen potential has been preferred by other workers. 6,24,25) It has been claimed 23,24,34) that both M50 and M50NiL can be plasma nitrided to a depth of 150 μm without compound layer formation, while achieving a surface hardness of about 1 100-1 250 HV ( Table 2). It is also suggested in that claim that the optimum depth for the improvement of rolling contact fatigue using nitriding is between 100-150 μm.…”

“…22) Figure 3 also shows that the time required to achieve a given diffusion layer thickness can be reduced by using a different gas mixture and a slightly greater process temperature. An alternative gas mixture of 25% ammonia in hydrogen has also been used to nitride M50, 6) but requires a long treatment time (60 h at 500°C), and the precipitation of white compound on the grain boundaries has been reported.…”

“…The reported surface hardness achieved was 900-950 HV for both M50 and M50NiL. 24) For example, in other work 6) involving nitriding at a temperature of 500°C and a longer time 40-70 h, a final grinding process is needed to eliminate the compound layer.…”

“…[3][4][5][6][7] Secondary hardening 2 of the component is necessary, since it is essential to attain the proper hardness before the start of the nitrding process. The goal is to achieve a superior tribological resistance through an increased surface hardness, and possibly by the raising the level of compressive residual stress in the region near the surface.…”