Effects of Subzero Treatments on the Bending Fatigue Performance of Carburized SAE-4320 and SAE-9310 Steels 960313

The effects of subzero treatments on the bending fatigue performance of carburized gear steels were investigated by cantilever bending fatigue testing. Specimens were machined from SAE-4320 and SAE-9310 bar stock steel, gas-carburized, quenched, tempered at 175°C, subzero cooled to -73°C and -196°C, and tempered at 175°C. Bending fatigue specimens were characterized by light metallography to determine microstructure and prior austenite grain size, x-ray diffraction for residual stress and retained austenite contents, microhardness testing, and scanning electron microscopy to evaluate fatigue crack initiation, propagation and overload. Refrigeration treatments caused additional transformation of retained austenite and increased surface hardness and compressive residual stresses. Bending fatigue endurance limits for the SAE-4320 specimens were determined to be 1310 MPa for the as-carburized condition, 1170 MPa for the -73°C condition, and 1280 MPa for the -196°C condition. Endurance limits for the SAE-9310 specimens were determined to be 1170 and 1070 MPa for the as-carburized and -73°C conditions, respectively. The lower endurance limits of the subzero treated specimens are attributed to retained austenite to martensite transformation and the formation of microcracks in the carburized case layer due to subzero treatments. The importance of tensile residual stresses in the austenite remaining after subzero cooling is also discussed.