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

摘要

The process of ausform-finishing in gears involves thedeformation of metastable austenite. A critical step inoptimizing the deformation process is to determine the linkbetween material deformation behavior and final materialproperties, such as hardness and microstructure. To this end,uniaxial compression testing was carried out on 1 pct carburizedAISI 9310 steel specimens in the low-temperature ausformingregime(85℃ to 230℃). The work-hardening response ofmetastable austenite and its relation to the hardness andmicrostructure was determined from these experiments. Highwork-hardening rates (work-hardening exponent n=0.4 to 0.7)were caused by deformation-induced transformation of metastableaustenite to either martensite or bainite or both. It ispostulated that, at the ausforming temperatures in theneighborhood of 230℃， bainite formed at the highest achievablestrains of 50 pct while oriented martensite (loading induced)was detectable at lower strains of 20 pct. The hardness of theresulting ausformed microstructure increased with degree ofstraining and with reduction in temperature of ausforming. AnX-ray determination of the retained austenite content showedthat austenite tends to stabilize even after minimal ausforming.A transmission electron microscopy study on ausformed specimensshowed the presence of microtwinnig and high-dislocationdensities. The effect of processing parameters on fatigueresponse under rolling contact conditions is discussed givencurrent understanding and available fatigue data.