Welding Technology And Residual Stresses Analysis Of Tube-plate Structure With Multi-welds

Recently, some stress corrosion cracks (SCC) are found in the welding structures withthe nozzle in pressurized water reactors (PWR) and boiling water reactor (BWR). So moreattentions are paid to ensure safety of nuclear power operation. It is well known that threemajor factors, namely materials properties, caustic media and stress status, have largecontributions to SCC. It is recognized that the tensile residual stress on the surface of thewelding structure largely increases the risk of initiating SCC. It is very important for thewelding structure with the nozzle in the nuclear reactor to evaluate and predict the weldingresidual stresses in the dissimilar metal tube-plate welding structure (TPWS) by the numericalsimulation method.To optimize the welding technology of the large TPWS with multi-welds, this paper appliedthe welding thermal conduction, the welding thermal elastic-plastic analysis theory and theANSYS finite element analysis software, proposed the finite element model (FEM) and thealgorithm about heat source loading and the passes filled by the “birth and death” elementmethod, based on the numerical simulation of the TPWS in the temperature field and thewelding residual stresses field. They were suitable for the numerical simulation of thetube-plate welding structure with T-shape sections (TPWST) in the residual stresses field. Theeffect of the groove angle, the welding sequence and the overlaying on the intersection ofthree weld beads on the residual stresses was analyzed. The FEM and the algorithm wereapplied to simulate the temperature field and the residual stresses field of the low alloy highstrength steels S690and SS316L stainless steels dissimilar metal TPWST. The residualstresses on the surface of the welding specimen were measured by blind-hole method. Theapplicability of the FEM and the algorithm was verified by the experiment in the numericalsimulation and prediction of the welding residual stresses in the TPWST.To the low alloy high strength steels TPWS, the simulation results showed that, the grooveangle from40°to50°, the Von-Mises equivalent stress, hoop and axial residual stressdistribution was improved to some extent, while deterioration of the radial residual stressdistribution was not obvious. The welding sequence had great influence on the peak values ofthe equivalent stress, the radial, hoop and axial residual stress. The alternative welding sequence was better than the other. The equivalent stress, the radial, hoop and axial residualstress of the amended alternative welding sequence were almost same as these of thealternative welding sequence. In the actual welding operation, the large groove angle canensure penetration of the weld bead, and the amended alternative welding sequence madeturnover of the welding part reduce. To simplify the welding operation and improve the weldbead quality, it was good to apply the welding technology of the50°groove angle and theamended alternative welding sequence.The simulation results showed that, to the TPWST, the overlaying on the intersection ofthree weld beads decreased obviously the peak values of the equivalent stress and the axialresidual stress at the toes of the girth weld, and had the little effect on the peak values of theradial and hoop residual stress. The overlaying decreased obviously the peak values of theequivalent stress and the axial residual stress on the intersection of the weld beads, anddecreased the peak value of the hoop residual stress to some extent, but the peak value of theradial residual stress increased. So the austenitic overlaying on the intersection of the girthweld and the weld bead of T-shape sections was the effective method to reduce the weldingresidual stresses after welding to the TPWST. The overlaying was finished after all weldbeads.To verify the feasibility of the3D FEM and the algorithm of the S690and SS316Ldissimilar metal TPWST, this paper compared simulation results of the welding residualstresses with testing data, the both accorded each other. It showed that the FEM and thealgorithm can be used to predict the residual stress distribution of the TPWST and theapplicability of them was verified by the experiment. From the simulation results and test data,the residual stress distribution of the S690and SS316L dissimilar metal TPWST was knownwith the welding technology of50°groove angle and the amended alternative weldingsequence. The compression stress occurred on the heat affected zone and the toe of the girthweld on the plate. The low tensile stress occurred on the tube. The peak value of the residualtensile stress was on the girth weld close to the plate. The maximum value of the equivalentstress in measurement was269MPa, which was on the girth weld and2mm from the fusionline of the girth weld on the plate.