Numerical Simulation And Experimental Research On Residual Stress Of Expansion Welding Of Steam Generator Tube And Tube Sheet

Steam generator is an important equipment in nuclear power plant,which not only plays the role of heat exchange,but also blocks the possible leakage of radioactive core coolant.The heat transfer tube is connected with the tubesheet by expanding and welding process.The process makes residual stress exist in the expanding boundary and welding joint of the inner wall of the tube.In addition,the strong corrosive medium flows in the tube,and the tubesheet is in the working environment of high temperature and high pressure,which is prone to stress cracking failure.Therefore,it is of great significance to study the distribution and variation of residual stress in tubesheet to ensure the safe operation of nuclear power equipment.In this paper,tubesheet is taken as the research object.(1)Firstly,based on the elastic-plastic mechanics,the elastic-plastic deformation of tubesheet is calculated,and the expansion pressure range of tubesheet material is 176 mpa ?347mpa.Then,the expansion process of steam generator tubesheet is simulated by ANSYS.The results show that: the expansion zone of tube sheet is in the state of compressive stress,and the axial residual stress of inner and outer wall of transition zone changes from compressive stress to tensile stress;with the increase of expansion pressure,the sealing range gradually widens,and the expansion pressure increases linearly with the peak value of pressure ring,expansion rate and pull-out strength;the expansion length has little effect on the expansion rate at low expansion pressure,more than 280 MPa,and increases with the increase of expansion pressure With the increase of expansion length,the expansion rate increases significantly.(2)The thermal mechanical coupling numerical simulation was used to study the distribution of residual stress in the inner and outer wall of tube sheet and joint area under the process of expanding before welding.The results show that the radial,circumferential and axial stresses in the inner wall of the tube present wave crest and valley changes;when the weld temperature increases or decreases,the radial displacement of the outer wall of the tube end is caused,resulting in the disappearance of the stress ring,15 mm away from the joint,resulting in residual contact The residual contact pressure decreases by 15% at 300 MPa.(3)Based on the test standard of blind hole method,the strain release coefficients a and B were compared by fitting,and the validity of forward and reverse drilling test under the fitting coefficient was verified through the forward and reverse drilling test of beam in the process of loading and unloading of beam with equal strength,with the error less than 10%;the blind hole method stress test was carried out on the inner wall of tube sheet and joint area through the verified a and B coefficients,and X-ray analysis was carried out The results show that the deviation of the three methods is less than 20 MPa.Comparing the numerical simulation results with the test results,the stress distribution of the two methods is consistent,which verifies the effectiveness of the simulation.(4)The thermal mechanical coupling numerical simulation was used to study the residual stress distribution of the inner and outer wall of the tube sheet and the joint area under the process of welding before expanding,and the stress distribution under the process of expanding and welding after expanding was compared.The results show that under the condition of increasing the expanding pressure evenly,the process contact stress change of welding before expanding is the largest,and the effect of expanding on the mode of expanding before welding is the smallest Therefore,in actual operation,when the actual operation of expansion pressure is difficult,it is recommended to optimize the process of welding before expansion.On the contrary,when the pull-out strength and sealing performance of the pipe are high,the process of welding before expansion is better than that of uniform expansion and welding after expansion.(5)The performance of Tubesheet in service was studied.After the simulation of expansion welding,the design temperature of primary side was 343? and the pressure was17.1?.The results show that the working load can improve the sealing performance and pullout strength of the joint to a certain extent,and the temperature load can reduce the effectiveness of the joint,The results show that the contact stress value of the secondary side is less affected by the working conditions,the stress ring area of the primary side is narrowed and the joint performance is reduced by using the post expansion welding process;the contact stress value of the primary side joint area appears by using the post expansion welding process,and the joint performance is improved.Finally,some measures are put forward to prevent stress corrosion cracking of tube and tubesheet,such as heat treatment after welding and increasing the content of Cr and Ni in the material.