Study On The Mechanism Of Radiation Damage Resistance Of 316L Stainless Steel Welding Joint

Nuclear power equipment not only needs to operate in the corrosive environment of high temperature,high pressure and cooling medium,but also some key components will suffer radiation damage caused by neutron irradiation.The improvement of the high temperature mechanical properties,corrosion resistance and radiation damage resistance of nuclear power structural materials is one of the important means to ensure the safe operation of nuclear power equipment.Welding technology is inevitably used in the manufacturing process of large components contained in nuclear power equipment,and the weld seam is the weakest part of the welded joint.Therefore,the study of radiation damage of welded joint has important guiding significance for improving the manufacturing process of nuclear power equipment and improving the safety of nuclear power equipment.Austenitic stainless steel is widely used as structural material in nuclear power equipment except a few special components.Over the years,domestic and foreign scholars have studied a lot of radiation damage of austenitic stainless steel,but there are few studies on the radiation damage of welded joints.In this paper,316L austenitic stainless steel was welded by fully automatic TIG welding.The microstructures and surface morphologies of the welded joints after irradiation were characterized by XRD,TEM and SEM.The effects of heat input and He⁺irradiation dosage on the microstructural evolution and surface morphology of 316L stainless steel welded joints were investigated.The surface hardness of welded joints before and after irradiation was characterized.The influence of heat input and irradiation dose on the increment of irradiation hardening of welded joints and its mechanism were discussed based on the theory of irradiation hardening.It was found that after irradiation with He⁺of 2×10¹⁷ n/cm²,a small amount of convex and tetrahedral holes were formed on the surface of the specimens,and their sizes ranged from 0.2 to0.6 um.Surface reliefwas formed by aggregation of He-V clusters,while tetrahedral voids wereformed by aggregation of stacking fault tetrahedron?SFT?and vacancy defect clusters.Heat input had only a slight effect on the size and density of holes and bumps on the surface of welded joints after irradiation.The increase of residual tensile stress caused by the decrease of heat input promoted the aggregation of interstitial atoms generated by irradiation,and the size and density of dislocation rings increased with the decrease of heat input?with the decrease of heat input,the average size of dislocation rings of the three groups of samples are 7.64108 nm,11.82726 nm and 13.20321 nm,respectively?made vacancy defects and He-V clusters aggregate to form bulges and tetrahedral holes with larger size and density.The test results of welded joints after irradiation with different doses showed that with the increase of irradiation doses,the concentration of He⁺in irradiated layer and the size and density of defects increased,which caused the surface of welded seam from the smooth surfaceto small-sized bumps and holes.After irradiation by He⁺of 1×10¹⁸ n/cm²,dense large-sized helium bubbles and a few holes were formed on the surface of the weld.Some of the bubbles also broke up and the surface peeled off.There wereno bubbles in the heat affected zone,but cracks initiated at grain boundaries to spall the whole grain surface.This was mainly due to the fact that the interface density of the weld was much higher than that of the heat-affected zone,which promoted the aggregation of helium bubbles at the interface.The TEM characterization results also showed that the size and density of dislocation rings and helium bubbles increased with the increase of dose,and large size SFT and dislocation entanglement also appeared at high dose.According to the theory of irradiation hardening and dislocation channel effect,it wasfound that the increase of dislocation ring size and density and grain refinement were caused by residual tensile stress.The hardness of the joint surface also increased with the increase of irradiation dose,whichwas caused by the increase of size and density of the dislocation ring and helium bubble with the increasement of irradiation dose.