Deformation of metastable austenite and resulting properties during the ausform-finishing of 1 pct carburized AlSl 9310 steel gears

The process of ausform-finishing in gears involves the deformation of metastable austenite. A critical step in optimizing the deformation process is to determine the link between material deformation behavior and final material properties, such as hardness and microstructure. To this end, uniaxial compression testing was carried out on 1 pct carburized AISI 9310 steel specimens in the low-temperature ausforming regime (85 °C to 230 °C). The work-hardening response of metastable austenite and its relation to the hardness and microstructure was determined from these experiments. High work-hardening rates (work-hardening exponent n=0.4 to 0.7) were caused by deformation-induced transformation of metastable austenite to either martensite or bainite or both. It is postulated that, at the ausforming temperatures in the neighborhood of 230 °C, bainite formed at the highest achievable strains of 50 pct while oriented martensite (loading induced) was detectable at lower strains of 20 pct. The hardness of the resulting ausformed microstructure increased with degree of straining and with reduction in temperature of ausforming. An X-ray determination of the retained austenite content showed that austenite tends to stabilize even after minimal ausforming. A transmission electron microscopy study on ausformed specimens showed the presence of microtwinning and high-dislocation densities. The effect of processing parameters on fatigue response under rolling contact conditions is discussed given current understanding and available fatigue data.