The Effect Of Stress On Bainitic Transformation Kinetics And Transformation Plasticity Of HQ785 Steel

This paper has determined CCT curve and yield strangth at high temperature of HQ785 steel by Gleeble thermo-mechanical simulator.Transformation kinetics models during heat treatment have been studied and analyzed from theory to experiment, incorporating the process of bainite phase transformation,the effect of stress on bainite phase transformation and transformation plasticity during isothermal procedure have been studied.The main contents and results are as follows:(1)The experiments constructed CCT curves and hardness of experimental steel in austenite without deformation by dilatation method were made in Gleeble-2000 thermo-mechanical simulator.According to OM,there is wider bainite transformation range of experimental steel.The cooling rate has much influence on the transformation point and microstructure;cooling rate is lower than 5℃/s;the microstructure appears proeutectoid ferriteα_p,when cooling rare is in the range of 20 to 50℃/s,there will be granular bainite and lath bainite.Hardness value is HV310-350,with the increase cooling rate,hardness increases.(2)The transformation kinetics model and transformation plasticity of experimental steel on compression stress during isothermal transformation were studied.At the same isothermal temperature,with the stresses increase,bainit start temperature raised.The major reason is that stress enhanced transformation driving force.The constant k of transformation plasticity for experimental steel is not a constant.K reduces along with stress increasing.At the same time,isothermal temperature had also an influence on k, the value is comparatively large at 500℃.(3)The effect of tension/compression stress on the transformation kinetics at continuous cooling transformation have been studied.With continuous cooling transformation process B_s were increased;at the same cooling rate,the increased level on tension is remarkable;tension\compression stress enlarged temperature ranges of bainite transformation.Tension stress accelerated transformation during the entire bainite transformation.Under compression stress,the transformation was inhibited at the starting stage.(4)Variation of transformation rate with bainite volume fraction at continuous cooling have been discussed.Under tension stress,the transformation rate curve arise slower stage aboutξequaled to 0.2～0.5;Under compression stress,the increment of transformation rate with bainite volume fraction is slower,then growing quickly whenξequaled to 0.2.A maximum value of transformation rate appeared whenξequaled to 0.5.(5)The analysis has made effects of external stress on bainitic morphologies.At continuous cooling transformation,lath bainite increases and granular bainite decreases with stress increasing.On the contrary,lath bainite decreases and granular bainite increases with stress increasing at isothermal bainitic transformation.(6)Transmission electron microscope was used for observing microstructure of experimental steel.The results show that:during continuous cooling,the lath ferrites broaden with stress increasing and the dividing lines between laths clearer;under compression stress,lath ferrites thinner and longer,the orientation was random as stress adding.During two cooling methods,whether tension stress or compression stress,the retained austenite in steel reduced with stress increasing,had become from massive to streak or filminess.