Research On The Effect Of Ultrasonic Impact Treatment On The Welding Residual Stress Of Nuclear Power Steel

With the improvement of welding safety performance requirements in different industries,especially in the nuclear power industry,welding of large thick plate complex structure parts with high strength level is often characterized by complex distribution and peak value of welding residual stress due to the excessive heat input of multi-layer and multi-pass welding.Aiming at the problem that it is difficult to eliminate welding residual stress by heat treatment for large nuclear power structures,this paper adopts ultrasonic shock treatment and grinding treatment to regulate and control the residual stress,which can not only conveniently and quickly eliminate welding residual stress,but also can clear impact pits to meet the requirements of surface coating.Therefore,it is of far-reaching significance to detect and regulate welding residual stress.In this paper,the welding joints of S32101,Q345B and SA738Gr.B with a thickness of 25~50mm were taken as the research objects,and the parameter optimization,layer by layer grinding process tests of the UIT in different areas and the back of the UIT were carried out.The small hole method and X-ray diffraction method were used to test the surface residual stress of the welding joints.The appropriate ultrasonic impact parameters of nuclear power materials were determined and the impact points per unit area and the impact frequency on the impact effect were studied.The influence of different grinding area,different grinding amount and different size on the stress distribution is studied.The influence of ultrasonic shock treatment on the distribution of residual stress field was studied.The results of parameter optimization test and impact factors of ultrasonic impact treatment showed that the impact strength and coverage rate were the main controlling factors to ensure the impact effect of ultrasonic impact treatment.SA738Gr.B and Q345B materials had the best impact effect when the impact strength was 15s/cm~2 and the coverage rate reached 100%.When the impact strength of S32101 material is 5s/cm~2 and the coverage reaches 100%,the impact effect is the best.With the increase of the number of impact pins,the impact frequency acting on the component increases gradually,but not linearly.Under the same excitation frequency,the impact frequency on the component is related to the number of impact pins,and the compressive stress effect is related to the impact strength;The test results of the whole grinding of the test plate with different polishing areas,different grinding amounts and different sizes after the UIT show that the surface stress of the test plate fluctuates greatly no matter 1/3 or 1/2 of the impact area is polished,while the overall grinding stress changes little and the trend is stable.After polishing only 1/2 of the impact zone and the whole impact zone,the longitudinal stress in the impact zone changes to a tensile stress of about 100MPa,and the effect of residual stress treatment is similar to that of heat treatment of welded parts.It can not only control the residual stress after welding,but also solve the problem that it is difficult to heat treatment for large engineering components and clear pits to meet the requirement of surface coating.It is recommended that after ultrasonic impact treatment,weld residual height should be directly polished to the level of base metal.With the decrease of plate thickness,the effect of overall polishing treatment is more significant.The test results of the post-hitting stress in different areas show that the final stress state of the test plate is the"pull-press-pull"stress alternation,no matter it is the single impact welded toe or the full cover impact.But it can effectively reduce the stress concentration in the welding toe part.Under the condition of limited conditions,it can improve the strength and fatigue resistance of the welded joint to a certain extent.