Fatigue in low pressure carbonitrided steels

Abstract

Low pressure carbonitriding and pressurized gas quenching heat treatments were conducted on four steel alloys: 20MnCr5, 20MnCr5 + B, SAE 8620 and SAE 8620 + Nb. Bending fatigue tests were performed with Brugger specimens to simulate fatigue on the tooth root of a gear. The heat treatment parameters were modified to ensure the surface hardness and effective case depths were comparable for the four alloys. The profiles for hardness, retained austenite, residual stress, carbon and nitrogen were comparable for all the alloys. However, the fatigue performance of each alloy was different. The highest endurance limit was attained by 20MnCr5+B, followed by 20MnCr5, SAE 8620+Nb, and SAE 8620. The differences in fatigue endurance limit occurred despite similar case depths and surface hardness, indicating that other features were relevant for the fatigue performance in this study. However, the difference between the 20MnCr5 endurance limits was minimal, suggesting the differences in B content in these alloys has little effect. Low magnitude tensile residual stresses were measured near the surface in all conditions. Additionally, non-martensitic transformation products (NMTPs) were observed to various extents near the surface on all the alloys, with a higher volume fraction in SAE 8620 + Nb and lower volume fraction in 20MnCr5 + B. However, there were no differences in retained austenite profiles between the conditions, and the retained austenite was mostly stable against deformation-induced transformation to martensite during fatigue testing near the endurance limit, which is in contrast to some studies on carburized steels where more significant transformation occurs. The presence of NMTPs and tensile residual stress could have occurred due to a low cooling rate during pressurized gas quenching. Residual stress profiles were very similar for all the alloys. Overall, it is interpreted that the difference in fatigue lives between the conditions is due to differences in chemical composition and prior austenite grain size. Alloys containing B and Nb had refined prior austenite grain sizes compared to their counterparts in each alloy class, and refinement in prior austenite grain size is established to improve fatigue performance in surface hardened steels. Fatigue cracks in all alloys nucleated on the surface through intergranular fracture of prior austenite grain boundaries. The variation in the chemical composition influences the grain boundary cohesion or embrittlement of the alloys. The segregation of elements such as boron, carbon, nitrogen, niobium, chromium, nickel and vanadium contribute to cohesion, and segregation of elements such as phosphorus, aluminum, manganese, silicon, copper and titanium contribute to embrittlement. The degree of segregation of each alloying element is unknown, though it is possible that the variation in the fatigue performance between 20MnCr5 alloys and SAE 8620 alloys is due to enhanced grain boundary cohesion from boron. Boron segregation was investigated through nano-SIMS in the low pressure carbonitrided 20MnCr5 + B specimens in the core and in the hardened case. Segregation of boron to prior austenite grain boundaries was apparent in the core region. However, clear boron segregation to the grain boundaries was not observed in the hardened case, indicating the carbonitriding heat treatment changes the boron distribution.