Research On Welding Process And Post-weld Heat Treatment Of SA738 Steel For Nuclear Containment Vessel

Low alloy high strength steel SA738 Gr.B was used for the third-generation nuclear power AP1000.It was mainly used for steel safety shell welding,which together with concrete safety shell forms the final barrier for nuclear power safety.It is an important task to study the welding performance of SA738 Gr.B steel to ensure the good performance of joints in service.In this study,CO2 gas shielded welding method was adopted to study the influence of different welding processes on the microstructure performance of SA738 Gr.B weld joints.According to the characteristics of the microstructure performance of weld joints,different linear energy and post-weld heat treatment processes were adopted to improve the comprehensive performance of weld joints.The research content is expected to further enrich the basic theory and technology of SA738 Gr.B steel welding.Firstly,the effect of linear energy on weld formation was studied.1G welding position: the surface of the welding bead had higher deposition rate;2G welding position: the higher deposition rate was obtained at low linear energy,but the weld formation was more poor at 1.75 k J·mm^-1 and 2.0k J·mm^-1;3G welding position: the surface of weld bead was in good condition at 2.5k J·mm^-1 and 3.0k J·mm^-1,however,weld bead surface presented larger splash at 4.0k J·mm^-1,which indicated that the weld formation was relatively poor.The hardness in the HAZ was higher than weld zone,and the hardness of weld zone was higher than base material.Secondly,the influence of SA738 Gr.B steel welding process on the joint performance was studied.The results showed that 1G flat welding: when the linear energy was 1.5k J·mm^-1,the tensile strength and impact power were 705 MPa and 167 J at WZ and 197 J at HAZ,respectively.2G horizontal welding: when the line energy was 1.5k J·mm^-1,the tensile strength and impact power were 707 MPa and 177 J at WZ and 237 J at HAZ,respectively.3G vertical welding: when the line energy was 3k J·mm^-1,the tensile strength and impact power were the 671 MPa and 141 J at WZ and 242 J at HAZ,respectively.The weld microstructure was mainly lower bainite and acicular ferrite.Bainite was the main structure in the HAZ.Pro-eutectoid ferrite and oxide inclusion were the main causes of cleavage fracture.Finally,the influence of post-weld heat treatment process on joint performance was investigated.The results indicated that after heat treatment,the tensile strength of weld joints at different positions were gradually decreased,which showed that stress relieving annealing after welding would lead to a certain degree of softening.The impact toughness of weld joint was improved greatly,which was higher than the minimum 54 J for required by technology.The impact performance of HAZ was stable at different welding positions.Blocky ferrites were incre ased in the WZ and HAZ after heat treatment.Blocky ferrites were the main reason for the decrease of tensile properties of weld joints after heat treatment.It is suggested that linear energy of 1.5k J·mm^-1 should be used for flat welding and horizontal we lding,and linear energy of 3.0k J·mm^-1 should be used for vertical welding.