The Influence of Heat Treat Process and Alloy on the Surface Microstructure and Fatigue Strength of Carburized Alloy Steel 1999-01-0600

Gas carburized and quenched low alloy steels typically produce surface microstructures which contain martensite, retained austenite and often NMTP's (non-martensitic transformation products). The NMTP's are caused by a reduction of surface hardenability in the carburizing process from loss of alloying elements to oxidation. Gas carburized low alloy steels such as SAE 8620 with NMTP's on the surface have been shown to have inferior bending fatigue properties when compared to more highly alloyed steels which do not form NMTP's, such as SAE 4615M. One method of minimizing the formation of oxides and eliminating NMTP formation during carburizing and quenching is to use plasma carburizing instead of conventional gas carburizing. In this study the microstructures and bending fatigue performance of plasma carburized SAE 8620 and SAE 4615M is compared to the same alloys conventionally gas carburized and quenched. Scanning electron and transmission electron microscopy was used to examine the surface phases and chemistry in detail. Plasma carburizing eliminated the NMTP's and most oxidation products in both SAE 8620 and SAE 4615M, whereas the gas carburized SAE 8620 and SAE 4615M contained substantial amounts oxides and the gas carburized SAE 8620 sample also contained surface NMTP's. The gas carburized SAE 4615M was much better in fatigue compared to the gas carburized SAE 8620. The fatigue performance of the plasma carburized SAE 8620 and SAE 4615M samples were nearly identical to each other except SAE 4615M was better in the low cycle fatigue regime. The elimination of the NMTP from the surface by plasma carburizing and quenching made the high cycle fatigue performance alloy independent.