Study On Thermal Deformation Behavior And Microstructures And Properties Of 405 Ferritic Stainless Steel For Nuclear Power

405 ferritic stainless steel is used as the heat exchange tube support plate in nuclear power.It is necessary to study 405 ferritic stainless steel viewing the point of the rapid development of nuclear power in China.In this paper,405 ferritic stainless steel was used as the experiment material.The hot deformation behavior of 405 ferritic stainless steel was studied by single pass compression experiment.The effects of quenching and tempering treatment,cycle quenching and thermo-mechanical treatments on the microstructure and properties of 405 ferritic stainless steel were studied by using OM,SEM,TEM,EBSD and EPMA,which can providing a reference for improvement of mechanical properties and industrial processing conditions.The main contents and innovations of this paper are as follows:(1)The hot deformation behavior and microstructural evolution of 405 ferritic stainless steel were studied by hot compression tests under different temperatures and strain rates.The flow stress curves under different deformation conditions were obtained.Based on the flow stress curves,the constitutive model was established by the Arrhenius-type equation coupled with the effect of strain and it can better predict the flow behavior of hot deformation.The relationship between critical strain for dynamic recrystallization and peak strain was determined,and finally the dynamic recrystallization kinetics for experimental steel was established.The processing maps of experiemental steel were developed to predict the occurenece of flow instability.(2)The effects of austenitizing temperature and time and tempering temperature and time on the microstructure and properties of experimental steel were studied.The results showed that the mechanical properties of expertmental steel after different austenitizing processes were mainly controlled by the content and grain size of austenite durng austenitizing.When the austenitizing temperature was lower or the holding time was shorter,the content of austenite was lower but the grain size was smaller.When the austenitizing temperature was relatively high or the holding time was relatively long,the austenite grain size increased,which led to the coarsening of martensite structure.Hence,the strength of experimental steel increased firstly and then decreased,and the toughness gradually decreased with the increase of austenitizing temperature and time.The mechanical properties after different tempering processes were controlled by the martensite and precipitate during tempering.Increasing tempering temperature and time can promote the decreasing of dislocation density and softening of martensite and the formation of precipitate and the decreasing of carbon content in martensite.When tempering temperature was relatively high or time was relatively long,the martensite lath obviously coarsened,and the precipitate fraction and size significantly increased.Finally,the strength decreased and the ductility increased while the toughness increased first and then decreased with the increasement of tempering temperature and time.After austenitizing at 980? for 1h,and tempering at 730? for 2h,the best combination of strength and toughness was obtained.The tensile strength was 576MPa,and the impact energy was 133J at-23?.(3)The effect of cycle quenching temperature and time on the micro structure and properties of experimental steel were studied.With the increasement of austenitizing temperature and time,the microstructure coarsened after the first stage of cycle quenching.When the cycle quenching temperature was too low or the time was too short,the cycle quenching process was similar to the tempering process,which improved the toughness and reduced the strength.When the cycle quenching temperature was too high or the time was too long,the microstructure coarsed,which improved the strength and decreased the toughness.After austenitizing at 970? for 1h,austenitizing at 970? for 40 min and then tempering at 730? for 15 min,the experimental steel possessed higher strength,with the strength of 676MPa and impact energy of 60J at-23?.After austenitizin at 970? for 1h,austenitizin at 920? for 15 min,and then tempering at 730? for 15 min,the experimental steel possessed higher toughness,with the strength of 622MPa and impact energy of 85J at-23?.(4)The effect of rolling and tempering temperature during thermo-mechanical treatment on the microstructure and properties of experiemtanl steel were studied.With the increasement of rolling temperature,recovery was easy to occur,which is beneficial to reducing the dislocation density,coarsening sub-grain,decreasing the nucleation site of precipitates and increasing the size of precipates.Hence,the strength decreased and toughness increased.Increasing the tempering temperature can accelerates the softening and increases the precipitates size.Therefore,the strength decreased and toughness increased.After rolling at 640?,and tempering at 680? for 1h,the experimental steel possessed high strength,with strength of 670MPa,and impact energy of 50J at-23?.After rolling at 740?,and tempering at 730? for 1h,the experimental steel possessed high toughness,with strength of 574MPa,and impact energy of 63J at-23?.(5)During industrial production,the strength of experimental steel was 50 MPa lower than that of pilot production.It can be attributed to the fact that the tempering time in the industrial production formulated based on experience operation was relatively long than that in the pilot production.Longer tempering time can promote the occurrence of softening,the decrease of dislocation denstiy,the decomposition of martensite,and the coarsening of precipitate and thus decrease the strength.