The plasma nutriding of tool and bearing steels

Abstract

There is some evidence to suggest that nitriding of alloy steels, in particular high speed tool steels, under carefully controlled conditions might sharply increase rolling contact fatigue resistance. However, the subsurface shear stresses developed in aerospace bearing applications tend to occur at depths greater than the usual case depths currently produced by nitriding. Additionally, case development must be limited with certain materials due to case spalling and may not always be sufficient to achieve the current theoretical depths necessary to ensure that peak stresses occur within the case. It was the aim of' this work to establish suitable to overcome this problem by plasma nitriding. To assist this development a study has been made of prior hardening treatment, case development, residual stress and case cracking tendency. M2 in the underhardened, undertempered and fully hardened and tempered conditions all responded similarly to plasma nitriding - maximum surface hardening being achieved by plasma nitriding at 450°C. Case development varied linearly with increasing treatment temperature and also with the square root of the treatment time. Maximum surface hardness of M5O and Tl steels was achieved by plasma nitriding in 15% nitrogen/85% hydrogen and varied logarithmically with atmosphere nitrogen content. The case-cracking contact stress varied linearly with nitriding temperature for M2. Tl and M5O supported higher stresses after nitriding in low nitrogen plasma atmospheres. Unidirectional bending fatigue of M2 has been improved up to three times the strength of the fully hardened and tempered condition by plasma nitriding for 16hrs at 400°C. Fatigue strengths of Tl and M5O have been improved by up to 30% by plasma nitriding for 16hrs at 450°C in a 75% hydrogen/25% nitrogen atmosphere.