Objective High silicon titanium-containing austenitic stainless steel is used as shell material in nuclear reactor fuel components due to its excellent radiation resistance and high-temperature mechanical properties. By adjusting element content based on 316 stainless steel, 1515 stainless steel improved service performance and became the third-generation shell material of the fast neutron reactor fuel component. Welding is an essential method for building shell components during manufacturing. For the welding of 1515 stainless steel, the traditional arc welding method is prone to defects, such as thermal cracks, intergranular corrosion, and joint embrittlement, and porosity defects of laser-welding lead to failure risks during service conditions, especially for incomplete penetration jointing. Thus, decreasing welding defects of shell components and obtaining welding joints, which meet technical requirements, are the main task for the welding of 1515 stainless steel. In this study, shell components are welded using the laser-arc hybrid welding method. By process optimized, welded joints without pores, cracks, and other defects, and mechanical properties met technical requirements of a nuclear reactor.
Methods In this study, 1515 stainless steel with backing plate is welded using the laser-arc hybrid welding method. First, the welding orthogonal test is designed based on the preliminary process exploration, and optimized welding parameters are obtained. The quantity and distribution of pores in welded joint are detected by radiographic testing(RT) ray detection and the influence of laser power; welding speed and defocus on the porosity of the weld joint are investigated. Second, the microstructure of the welded joint is observed using the optical microscope and mechanical properties of tensile. Besides, bending and hardness are tested. Finally, the tensile fracture section is observed through a scanning electron microscope.
Results and Discussions Welding speed and defocus have a strong influence on the porosity of the welded joint. Increasing welding speed and defocus can significantly reduce porosity when the laser power and arc current remained even welded joints with almost no porosity can be obtained (Table 2) under laser power 3.0 kW, arc current 130 A, welding speed 1.8 m/min, and defocus +20 mm. The overall shape of the welded joint cross section is wide at the top and narrow at the bottom. The weld grains gradually decreased from top to bottom. This is due to the higher heat input on the upper part of the welded joint than its lower part. However, the welded joint consists of the weld zone, fusion line, and heat-affected zone. The microstructure of the weld zone is single austenite. There are many thick columnar crystals in the center of the weld zone, and the crystal growth direction is along the direction perpendicular to the fusion line. The grain size of the weld zone near the fusion line is reduced, and there are small equiaxed and cellular crystals. The microstructure of the heat-affected zone is the same as that of the base material; however, the grain size is slightly larger than the base material (Fig. 4) due to the welding heat input. In the tensile test of the welded joint, the tensile strength is (607±12)MPa, which is about 73% of the tensile strength of the base material. It meets the requirement of the nuclear reactor for the welded joint tensile strength of the shell material (≥520 MPa). The average elongation of the joint after fracture is 6.5%. The tensile fracture position is at the center of the weld (Fig. 5). Besides, the surface of the tensile fracture is covered with a large number of tear edges and small dimples (Fig. 6), indicating that the fracture of the welded joint is a typical ductile fracture. In the bending test, no cracks are found on the surface and root of the weld (Fig. 7) after 180° face and back bending of the welded joints. Besides, the hardness of the weld zone is 160 HV0.1, which is lower than other areas of the welding joint, and is about 60% of the hardness of the base material. The hardness of the heat-affected zone is between the weld zone and base metal (Fig. 8).
Conclusions With the optimal parameters, the laser-arc hybrid welding of high silicon titanium-containing austenitic stainless steel is completed, and well-formed welded joints are obtained. By increasing the welding speed and defocus, the porosity of the weld can be significantly reduced. Besides, welded joints with almost no porosity can be obtained under laser power of 3.0 kW, arc current 130 A, welding speed 1.8 m/min, and defocus +20 mm. The welded joint consists of the weld, fusion, and heat-affected zones. The microstructure of the weld zone is single austenite, and the grain size gradually decreases from top to bottom on the cross section. The microstructure of the heat-affected zone is the same as that of the base material; however, the grain size is slightly larger than that of the base material. The average tensile strength of the welded joint is 607 MPa, which is about 73% of the tensile strength of the base material, and the average elongation after fracture is 6.5%. There are many tear edges and dimples in the tensile fracture, which is a typical ductile fracture. The hardness of the weld is 160 HV0.1, which is about 60% of the hardness of the base material. The results of 180° face and back bends are qualified. It means that joint mechanical properties meet the technical requirements for shell material in nuclear reactors.
