Effect Of Tempering Process On Residual Austenite Transformation In M50Steel

In order to make the residual austenite transforms completely, the traditionalheat treatment of M50steel is several high temperature tempering after quenching.But in the other hand, as the tough phase, austenite does some benefits to thedelaying of crack propagation and improves the fatigue life of bearings to someextent. This paper takes another heat treatment process to reserve some residualaustenite and improves the stability of the austenite by carbon partitioning toincrease its carbon content. It does some researches about the effect of temperingprocess on the retain austenite transformation like its content and carbon content. Atthe same time, it does some observation of the morphology and distribution ofresidual austenite in different heat treatment states. It also studies the stability of theresidual austenite by a combination quenching-tempering-carbon partitioning heattreatment process.The content of the residual austenite of the quenching state M50steel is about15%as the austenitizing temperature is1090oC. There are two sorts of austenitemorphology in the quenching state steel. One has the long block form with a widthabout300nm and the other is in film state with the width about50nm. Studies haveshown that at the relatively low tempering temperature300oC and450oC, even theholding time increases to4h, the content of residual austenite stay the same, butthere are significant changes of the carbon content of the residual austenite. At the300oC tempering temperature, the carbon content of residual austenite increasesfrom0.64%to the peak value0.92%as the holding time increases from30s to10min. When the holding time increases further to2h and4h, the carbon contentdecreases to0.84%. As the tempering temperature increases to450oC, even theholding time is30s, the carbon partitioning has done and the carbon content of theresidual austenite is0.89%. As the holding time increases, the carbon content ofaustenite appears the tendency to decrease. For instance, when the holding time is4h, the carbon content of austenite is0.68%.When the tempering temperature is relatively high, the residual austenite startsto decompose. As the holding time is0.5h and1h at500oC, the content of residualaustenite is about9%and the content decreases to4%further as the holding timeincreases to2h. At the same time, the carbon content of residual austenite decreaseswith the increasing of the holding time0.5h is0.68%and1h is0.52%. As thetempering temperature ascends to530oC, the content of retain austenite is only6%when the holding time is just0.5h.1h holding time makes the austenite transform almost completely. At550oC tempering temperature, even the holding time is0.5h,the content of austenite is beyond determination. Meanwhile, the TEM observationof the sample which is tempered at500oC for2h shows that the original long blockaustenite has transformed and it becomes more fine residual austenite which isparallel distributed and gets sandwiched between lath martensite. At the same time,the precipitation of fine alloy-carbides exists.When a300oC10min carbon partitioning heat treatment is inserted betweenthe first500oC2h tempering and the second530oC tempering process, the contentof residual austenite is almost the same as the530oC second tempering holding timeis0.5h and1h. The austenite can still be observed by XRD analysis although thecontent is little. But if there isn’t the300oC partitioning heat treatment process, theaustenite peak can’t be observed in the XRD results when the530oC temperingholding time is1h. So the300oC carbon partitioning heat treatment improves thestability of the residual austenite.