| Electric power industry is the basis of economy and society, and the thermal power generation will still play an important role in China even in the world in next several decades. The super/ultra-supercritical advanced power plants with high thermodynamic efficiency will be a solution to the problems of the increasing demand for electricity and the ensuing environmental pollution. P92 steel has been widely used in the super/ultra-supercritical advanced power plants for its excellent high temperature capabilities. As an efficient method, welding is widely used to join components or pipes under pressure-bearing and high temperature conditions in thermal power plants. Almost all the structures made of P92 steel are performed by welding process. Due to the composition of P92, the microstructure in both fusion zone and heat affected zone(HAZ) are martensite after welding, which will potentially result in cold cracking. The Type IV cracking occurs around the HAZ of the P92 welded joint is the main premature failure. Although the mechanism of Type IV cracking is still not clear, a number of researchers suggest that the welding residual stresses(WRS) in the welded joint can contribute to Type IV cracking. On the other hand, after weld repair for P92 structures, WRS cannot be mitigated by the process of post-weld heat treatment(PWHT). Therefore, we should pay more attention to the WRS in such repaired joints. Based on the two points above, the WRS in P92 welded joint became one of the hottest topics currently.In this study, based on the specialized welding software SYSWELD, a ―thermal-metallurgical-mechanical‖ computational procedure taking the solid-state phase transformation into consideration was developed to simulate temperature field, microstructural state field and residual stress induced by welding process. Through observing the microstructure and measuring the micro-hardness of the joint, the effectiveness of the simulated microstructure field were verified. Moreover, WRS were measured by blind-hole method and were compared with the existing results measured by neutron diffraction technique. By comparing the simulation result and the measured data, the effectiveness and the calculation accuracy of the computational approach developed in this study was confirmed. In the simulation of single-pass joint performed by TIG welding, the effects of volumetric change, yield strength change and phase transformation induced plasticity(TRIP) on WRS were analyzed through different computation cases. In the double-pass joint, the relationship of temperature-microstructure-stress and the evolution of residual stress were clarified, and the mechanism how the solid-state phase transformation influence WRS were illustrated as well. In the four-pass joint, the WRS distribution was predicted, meanwhile, the influence of inter-pass temperature on the WRS were also analyzed through using different simulation cases. The WRS in the P92 thick-plate multi-pass butt-welded joints was predicted.The results showed that the volumetric change due to austenite-martensite transformation has a significant influence on the WRS. The volumetric change not only changes the magnitude of longitudinal WRS, but also alters the sign of the longitudinal WRS in the weld zone and HAZ. The yield strength variation also has an important effect in WRS. The influence of TRIP seems to be inferior to the former two fact ors, but it can relax both longitudinal and transverse residual stresses to some extent. In the multi-pass butt-welded joints of P92 steel, the cover passes have more important effects on the WRS than the former passes. Moreover, the last pass seems to have the most significant effect on the WRS.Through using the ―thermal-metallurgical-mechanical‖ computational procedure with considering the solid-state phase transformation in this study, the WRS in the P92 thick-plate multi-pass butt-welded joints were simulated. The result of residual stress obtained by numerical simulation will be meaningful and helpful to structural integrity analysis and life assessment. In addition, the computational approach developed by this study can be used to predict the WRS in other low transformations temperature steels. |
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