Microstructural Evolution Of CrNiMoV Steel For Nuclear Power Rotors During Heating And Cooling Process

In this dissertation,microstructural evolution of CrNiMoV steel for nuclear power rotors during heat treatment process were studied,which provides valuable reference for the formulation of heat treatment processing.Firstly,the continuous cooling transformation of CrNiMoV steel was studied.Based on the CCT curve,microstructural characterization and hardness test,the evolution of microstructure upon continuous cooling at different rates were obtained.The result shows that as the cooling rate decreases form 100?/s to 0.02?/s,the products of supercooled austenite transformation are martensite,martensite+bainite and bainite respectively,pearlite transformation doesn't occur in the range of measured cooling rates.As the austenitizing temperature raises from850?to 1050?,M_s temperature decreases from 389?to 352?,while M_f temperature does not change significantly.With the increase of carbon content,both M_s and B_s temperatures decrease significantly.Secondly,the effects of high temperature pre-tempering,heating rate,holding time and isothermal temperature on grain refining of CrNiMoV steel were investigated.The results show that structure heredity is effectively inhibited in the untempered sample heated at>300?/h,while high temperature pre-tempering treatment can delay the nucleation and growth of needle-shaped austenite,which lowers the critical heating rate.When holding at temperature above 840?,the austenite will spontaneously recrystallize to form non-oriented fine grains,which can be an effective method to inhibit structure heredity.As the isothermal temperature raises,the incubation period of spontaneous recrystallization will be shorten.Finally,the austenite grain growth behavior during continuous heating and isothermal process was studied.The relationship between average grain size and heating rate,holding time and isothermal temperature was obtained.When the isothermal temperature is constant,the change of austenite grain size is parabolic with the holding time;when the holding time is constant,the average austenite grain size is exponential with the isothermal temperature.Several mathematical models of grain growth behavior during 1000?-1250?were established,and the Sallans&Whiteman model agrees best with the experimental data.