Effect Of Post-weld Heat Treatment On Microstructure And Mechanical Properties Of V-4Cr-4Ti Welded Joints

Vanadium alloy is a single-phase solid solution formed by adding 4% Cr and 4% Ti to the vanadium matrix.Because of its good low activation characteristics and excellent resistance to radiation damage,it is used as a candidate structural material for cladding components of fusion reactors.Vanadiμm-based alloys are used in the cladding structure of fusion reactors.It is necessary to explore suitable connection technology to obtain vanadiμm alloy joints with excellent performance.This article discusses the weldability of vanadiμm alloys.The use of argon arc welding and laser welding to connect vanadiμm alloys.By optimizing the welding process parameters,the vanadiμm alloy joints with good weld joints are obtained.The vanadiμm alloy laser welded joints are 673K/1h-1273K/1h post-weld heat treatment.Metallurgical microscope and SEM(scanning electron microscope)were used to observe the microstructure and composition of V-4Cr-4Ti welded joints.Vanadiμm alloy welded joints were tested for Vickers hardness and room temperature tensile tests to explore the changes and mechanisms of the mechanical properties of the vanadiμm alloy welded joints during welding and post-weld heat treatment.When the welding process parameters of argon arc welding are current 160 A,welding speed 4 mm/s,tungsten electrode diameter 2.4 mm,the welding zone of the vanadiμm alloy joint is well connected.The structure of the weld zone is composed of a dense and fine equiaxed crystal zone(20μm)and a long colμmnar crystal zone.In addition,a lamellar precipitated phase(600nm)appears in the weld zone.The fusion zone serves as the boundary between the weld zone and the heat-affected zone.There is a phenomenon of competitive growth and co-generation crystallization nearby.Compared with the vanadiμm alloy base material,the hardening phenomenon appears in the weld zone,the Vickers hardness is as high as 280 Hv,which is about 170% of the hardness of the base material.In addition,the depth-load curve of the nanoindentation indentation of the vanadiμm alloy argon arc welding joint also illustrates the hardening phenomenon of the weld zone.The hardening phenomenon is related to the fine grain strengthening effect of equiaxed crystals and the precipitation strengthening effect of the precipitated phase.The tensile test results at room temperature show that the tensile strength and elongation of the vanadiμm alloy argon arc welding joint are lower than that of the vanadiμm alloy base material.The reason for the decrease is mainly due to the brittleness caused by the increase of the content of impurity elements such as carbon and oxygen in the weld zone.And the existence of welding residual stress.Vanadiμm alloy laser welding joints are prepared with a process parameter of power 2000 W,a welding speed of 10 mm/s,and a spot diameter of 0.3 mm.A vanadiμm alloy laser welding joint with a good weld joint can be obtained.The average size of the weld area is about 8 μm The composition of the equiaxed crystal area and the colμmnar crystal area,compared with the vanadiμm alloy argon arc welding joint,the grain refinement degree of the vanadiμm alloy laser welding joint weld area is higher.The Vickers hardness of the weld zone is as high as 425.3Hv,which is about 258% of the hardness of the base metal,and the degree of hardening is much higher than that of the vanadiμm alloy argon arc welding joint.For the room temperature tensile test of vanadiμm alloy laser welded joints,the fracture position is still in the weld zone,the tensile strength is 338.8MPa,the elongation is 3.69%,slightly higher than the tensile strength of the vanadiμm alloy argon arc welding joint 320 MPa And 3% elongation,but still lower than the vanadiμm alloy base material.Different annealing temperatures will have different effects on the structure of the weld zone of the vanadiμm alloy laser welding joint.After annealing at 673K/1h,there is no obvious change in the structure of the weld zone.As the annealing temperature rises to 873 K,a long strip of precipitated phase appears in the weld zone.When the annealing temperature continues to rise to 1073 K and 1273 K,the precipitated phase will continue The C,N,O and other elements in the matrix are growing up.The changes in the microstructure and morphology of the weld zone after annealing at 673K/1h-1273K/1h will affect the mechanical properties of the joint.After heat treatment at 673K/1h,the weld zone softened,from an average of 425.3Hv to an average of 330.8Hv.When the annealing temperature is increased to 873 K,the weld zone will harden,and the average Vickers hardness of the weld zone will rise to 365.1 Hv.As the annealing temperature continues to rise to 1073 K and 1273 K,the weld zone will soften again,and the Vickers hardness drops to 280.4Hv and 265.9Hv,respectively.As for the room temperature tensile properties of vanadiμm alloy laser welded joints,the tensile strength and elongation of the vanadiμm alloy welded joint after annealed at 673K/1h are 345 MPa compared with the welded tensile strength of 338.8MPa and the elongation of 3.69%,respectively,4.58%.As the annealing temperature rose to 873 K,the tensile strength and elongation of the vanadiμm alloy welded joint increased again to 359.9MPa,4.65%.When the annealing temperature continued to rise to 1073 K and 1273 K,the tensile strength dropped to 354 MPa and 349 MPa,respectively,while the elongation continued to rise to 5.21% and 5.70%.In this paper,the preparation of vanadiμm alloy welded joints was carried out by argon arc welding and laser welding.By comparing the morphology and mechanical properties,it can be seen that the performance of laser welded joints is more excellent.The effect mechanism of 673K/1h-1273K/1h annealing treatment on the microstructure and mechanical properties of vanadiμm alloy welded joints was explored to further improve the mechanical properties of vanadiμm alloy welded joints.