Fatigue behavior of two carburized low alloy steels

The bending fatigue behavior of carburized low alloy steels is critical to the performance of gearing. Several factors influence the fatigue behavior including: surface finish, alloy composition, carburizing conditions and quench rate. The purpose of this study is to evaluate the influence of alloy content on bending fatigue performance of two steel alloys. The steels under evaluation were modified 8620 and 4615 alloys, machined into bend bars with a notch and carburized to yield a case depth of approximately 0.35 mm. The samples were fatigued, under a fully reversed condition (R = {minus}1), in 4-point bending. Optical and electron microscopic evaluation revealed the presence of internal oxides and NMTP (non-martensitic transformation products) at the surface of the modified 8620 alloy, the result of oxidation of Mn, Cr and Si during the carburization process. The modified 4615 alloy showed a similar depth of oxidation without NMTP. The bending fatigue performance of the modified 4615 alloy was 210% better (on a stress basis) than the modified 8620 alloy. The near surface oxidation and NMTP was removed by electrochemical polishing from both materials. The fatigue resistance of the 8620 was improved substantially, while the 4615 was nearly unchanged. NMTP is one of themore » major detriments to the fatigue behavior of carburized low alloy steels.« less