Translated Abstract
As two important processes, welding and heat treatment plays a vital role in manufacture of the nuclear power plant equipment. Local PWHT is usually performed to eliminate the residual stress when it is impractical to heat treat the whole vessel in a furnace. However different countries have different criterion of local PWHT. Numerical simulation that could simulate the welding and Local PWHT temperature field and stress-strain field well, and has a great practical significance and economic value in the optimization of heat treatment process, control heat treatment distortion, and the improvement of the overall quality of structures.
In this study, the numerical method such as thermo-elastic-plastic creep FEM is used, and it is compared with the experiments. The thick-walled pipes of steam generator with girth weld and repair weld are analyzed under different conditions of local PWHT. The indirect method is applied to analyze the welding temperature field and stress-strain field. The element birth and death technology is used to simulate the materials filling process in the multi-pass welding process. In the structure analysis coupled with trasient temperature fields, the temperatures of the nodes are loaded as the loads in structure. Combining with creep behaviour at high temperatures of material, the residual stresses of the component is simulated for the welding and PWHT. An analysis program about thick-walled pipes welding and PWHT is developed based on the ANSYS platform. And the effect of the heated width and insulation band width on residual stress distribution in thick-walled pipes under 595~620 ℃ and 6 h heat treatment was discussed. On the basis of the analysis, the local heat treatment specification for thick-walled pipe is summarized. And the changes of the stress distribution about thick-walled pipes with girth in different local PWHT standards are compared. As well as different sizes of thick-wall pipes with girth are analyzed under the optimal local PWHT heating scheme. The local PWHT heating scheme about repair weld is also disscused.
The results indicate that residual stress relief mechanisms in post-weld heat treatment include plastic deformation induced stress relief and creep strain induced stress relaxation. Most of the residual stress reduction already occurred during the first few hours after reaching PWHT temperature. Thermal stress is caused during local PWHT of welds in thick-walled pipe. The effect of soak band on welding residual stress can be determined by heated band. The residual stress after local PWHT is greatly affected by heated band width during local PWHT of girth weld in thick-walled pipes. The residual stress of weld and the surrounding will be reduced with the increase of heated band width. But when the heated band is wide enough (half of HB= ), the weld residual stress changes little with the increasing heated band width and the value is basically equal to the overall heat treatment(about 47MPa). The insulating band can relieve temperature gradient on the edge of the heating area, so that the stress on the edge of the heating area is reduced. In the existing local PWHT standards, AWS D10.10 is more suitable for thick-wall pipes. The processing parameters are （ ）and during local PWHT of girth weld in thick-walled pipe. And the processing parameters are and with local 360-degree band during local PWHT of repair weld in thick-walled pipe. By compared with the simulation results the measured value of residual stress for the thick-walled pipes through the blind hole method shows that the finite element model can be used to analyze the behavior of weld and PWHT and determine the local PWHT processing parameters.
Translated Keyword
[Creep, Girth weld, Local PWHT, Numerical simulation, Repair weld]
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