Investigation On Residual Stress In Dissimilar Steel Weld For Nuclear Steam Generator Safe-end

As a vital part in nuclear power equipment,nuclear steam generator has more complicated structures and principles.Weld,as a key technology to connect different parts has a role and influence cannot be ignored.It is the residual stress generated during the welding constitutes a threat to leak tightness and bearing strength of devices,which determine whether the entire nuclear power plant can operate safely and stably.For this reason,it has important significance and value to research for the steam generator welding residual stress.In this paper,the object being studied is the welded joint connecting the water chamber head coolant outlet pipe-nozzle of the AP1400 reactor pressure vessel to safeend pipe.The material of pipe-nozzle is ferritic low-alloy steel(SA508Gr.3Cl.2),and the safe-end pipe material is cast austenitic stainless steel(CF8A),the filler metal of buttering and butt weld uses primarily nickel-base alloy(Inconel 52M),and the technology of welding is hot-wire GTAW.Firstly,the residual stress in dissimilar steel welding joint was simulated based on two-dimensional and three-dimensional finite element analysis by SYSWELD.The overall distribution of residual stress can be obtained by the simulation,which can make more reasonable judgment for the effect of the actual residual stress.The result shows that the maximum axial residual stress,about 600 MPa,distributes the boundary area between the SA508 buttering and the base metal in outer wall.Hoop stress in weld zone is generally higher that axial stress.The process of 3D finite element simulation is accord with the actual welding procedure,and the magnitude of residual stress significantly decreased after machining a certain thickness along the inner and outer wall.Either 2D or 3D prediction,residual stress located in both sides of butt weld is close to the yield stress of base metal.So the locations have a big chance to stress corrosion cracking.Secondly,blind-hole method was taken to measure the residual stress of dissimilar steel joints.The measurement values were compared with the results of the finite simulation.And the results shows that measured values of circumferential and axial residual stress are all at a low level.Therefore,it has the smaller possibility to stress corrosion cracking at that positions.By the comparison of simulated and measured results,the axial residual stress values agree well with the measurement,but hoop stress has a larger difference due to the impact of cutting after welding.Finally,in order to better predict the stress distribution and evolution,the finite element model was extended to the actual structure,and 350?,17 MPa were imposed to simulate the actual service conditions.The results show that residual stress of extended model has a big difference with the test model and the stress field changes a lot before and after service.After applying service temperature and pressure,the impact of axial stress of two-dimensional model is more obvious,and stress dramatically decline,but little effect on hoop stress.Applied to the service temperature and pressure under the three-dimensional model has a greater impact on the position of the inner wall of the stress,in particular,can significantly reduce the magnitude of the axial stress.