Formation of austenite during scuffing failure of SAE 4340 steel

Scuffing failure was induced in samples of SAE 4340 steel which had been quenched and annealed at several temperatures. Examination of worn areas using grazing-incidence X-ray diffraction revealed an austenitic phase at volume fractions of up to 60%. No dependence of austenite fraction on depth was observed for depths of up to 5.0 μm. No clear trend in austenite fraction after scuffing as a function of tempering temperature was found. One wear test was interrupted precisely during scuffing, and the worn areas of that sample showed 15% austenite. A fully scuffed sample of the same tempering temperature contained 60% austenite, while analysis of original material and contact areas prior to scuffing initiation revealed no austenite. Observations from the present work suggest that austenite formation was caused by scuffing and not the other way around. Austenite may have formed due to a rapid austenitization process, mechanical dissolution of carbides, or a diffusionless transformation. Once formed, the austenite may have been stabilized by stress or by small crystallite size, but not likely by compositional change. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the US Department of Energy under Contract W-31-109-ENG-38.