No abstract

Low pressure carbonitriding and pressurized gas quenching heat treatments were conducted on four steel alloys. Bending fatigue tests were performed, and 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 between alloys. Low magnitude tensile residual stresses were measured near the surface in all conditions. Additionally, nonmartensitic transformation products (NMTPs) were observed to various extents near the surface. However, there were no differences in retained austenite profiles, and retained austenite was mostly stable against deformation-induced transformation to martensite during fatigue testing, contrasting some studies on carburized steels. The results suggest that the observed difference in fatigue lives 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.

Low pressure carbonitriding (LPCN) has the potential to improve impact and fatigue strength, with gears being an example application, through the enrichment of nitrogen in addition to carburizing at higher heat treatment temperatures. In this study, the LPCN response of four different steel alloys is investigated. The influence of unprotected boron is evaluated by comparing the LPCN response of 20MnCr5 with and without boron additions. The influence of Nb microalloying is assessed by comparing the LPCN response of 8620 with and without Nb additions. Low pressure carbonitriding heat treatments were developed to achieve case depths of 0.65 to 0.75 mm in each alloy. The hardness and case microstructure are correlated to bending fatigue response measured with Brugger fatigue specimens, which are designed to simulate the root of a gear tooth.