Abstract
In this study, the low-pressure carbonitriding (LPCN) response of four different steel alloys is investigated. The influence of unprotected boron is evaluated by comparing the microstructure and fatigue performance of 20MnCr5 with and without boron additions after LPCN. Similarly, the influence of Nb microalloying is assessed by comparing 8620 steel with and without Nb additions. Low-pressure carbonitriding heat treatments were developed to achieve case depths of 0.65-0.75 mm in each alloy, thus allowing the influence of alloying and microstructure to be explored independent of case depth. The hardness and case microstructure are correlated with bending fatigue response measured with Brugger fatigue specimens, which are designed to simulate the root of a gear tooth. The 20MnCr5 alloys had the highest fatigue strength, and the 8620 alloys had the lowest fatigue strength, despite having the same hardness profile and case depth. Reductions in prior austenite grain size and increased surface hardness appeared to have a positive effect on the endurance limit of the steel alloys. Retained austenite volume fraction on the surface of the steel alloys as well as non-martensitic transformation products subsurface in the case did not have an apparent direct contribution to fatigue performance.
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