Study On Welding Microstructure And Properties Control Of Q460 Steel For Nuclear

With the development of nuclear power plants,the pressure vessels of nuclear power plants are developing in a large-scale direction.This puts new demands on the steel for pressure vessel supports.The Q460 steel is used as a new generation of nuclear voltage vessel support steel,which has good mechanics and Anti-irradiation performance,but researches on its welding are few now.In this paper,the steel for Q460 support is selected as the research object.The SH-CCT of Q460 steel coarse-grained heat affected zone is drawn by L78 quenching dilatometer.According to the results of the CCT curve,the single-double pass test protocol was developed and tested,and a high temperature tensile test was performed by using a Gleeble thermal simulator.The equipment used in the test process includes laser confocal microscope,scanning electron microscope,Vickers hardness tester and so on.On the basis of the coarse-grained heat-affected zone,SH-CCT curve,single-double pass test,the coarse-grained heat-affected zone,and single-double-pass microstructure and properties were obtained.According to the analysis of the change of the high-temperature tensile-strain curve through high-temperature tensile test,the deformation mechanism of the material under high temperature tensile conditions is obtained,and the high temperature strength and shape are obtained.Through the indirect evaluation of the welding of nuclear Q460 steel and the analysis of SH-CCT curve,the nuclear Q460 steel has certain cold crack sensitivity and low thermal crack sensitivity.There are three phase transition zones in the coarse-grained heat-affected zone,when the welding t_8/5 is between 3s and 10s,the structure is lath martensite;When the welding t_8/5 is between 10 and 100s,the structure is martensite and shell.When the welding t_8/5 is between 100s and 1000s,the structure is bainite.When the welding t_8/5between 1000s and 16000s,the structure is bainite and pearlite.Through the analysis of high temperature stress-strain curve:the high temperature stress-strain curve is divided into three stages.As the tensile temperature increases,the tensile strength of the material decreases,and the difference between the highest tensile strength and the lowest tensile strength is 124 MPa.In the wake of the stretching temperature,the reduction of the area shows the trend of rising first and then decreasing.When the stretching temperature is1000℃,the reduction of the area is 85.7%,and when the stretching temperature is 1250℃,the reduction of the area is the lowest,being 34.5%.Through the analysis of the welding structure and performance in the single-pass heat affected zone,it is found that with the increase of welding t_8/5,the hardness value of the material decreases,and the phase transition of the material moves towards the direction of high temperature.In the coarse crystal region,when t_8/5 is 25s,the tissues obtained are martensite+bainite,when t_8/5 is 50s,the tissues is lower bainite,when t_8/5 is 150s,the tissues is Granular bainite.In the fine-grained region,when t_8/5 is 25s,obtaining the lower bainite,when t_8/5 is 50s¹50s,the granular bainite is obtained.In the critical phase transition region,when t_8/5 is 25s⁵0s,the obtained tissues are granular bainite.When t_8/5was 150s,the obtained tissues are pearlite+granular bainite.In the subcritical phase transition region,when t_8/5 is 25s¹50s,the obtained microstructure is tempered martensite.Through the analysis of the welding microstructure and properties in the double-pass heat affected zone,it is found that in the coarse grain reheat affected zone,because the microstructure in the welding heat affected zone is mostly non-equilibrium,the phenomenon of tissue inheritance occurs.In the CGCG thermal affected area,when t_8/5 is25s,the obtained tissues are lamellar martensite+bainite;when t_8/5 is at 50¹50s,the obtained tissues are granular bainite.In the FGCG heat-affected area,when t_8/5 was 25s¹50s,the obtained tissues are granular bainite.In the ICCG thermally affected region,when t_8/5 is 25s⁵0s,the granular bainite is obtained,and when t_8/5 is 150s,the granular bainite+pearlite is obtained.In the SCCG heat affected region,when t_8/5 was 25s¹50s,Granular bainite with greater influence on toughness was produced in the ICCG heat affected zone,which reduced the toughness of the reheat heat affected zone and required post-welding heat treatment.