Distribution And Modification Of Residual Stresses In Welded Supercritical Boiler Water Wall

The high residual stresses appears between the pipes and accessories during the process of welding in the 700 oC supercritical boiler water wall, and the high tensile residual stresses will deteriorate the properties of the weld samples. In order to keep the supercritical boiler water wall works well in the practical application, the investigation on distribution of residual stresses is very important and necessary. So in this thesis, the distribution of residual stresses on the supercritical boiler water wall will be investigated and improved, in order to optimize the residual stresses distribution on the weld area.Three dimensional finite element methods was used to model the process of welding. First, utilize the moving heat source to calculate the temperature field of weld, then model the mechanical properties according to the temperature field, at last, the residual stresses were obtained. At the same time, the residual stresses along the horizontal and longitudinal directions were investigated and discussed.The distribution of residual stresses of the supercritical boiler water wall before and after annealing were investigated via X-ray diffraction methods. The results revealed that the residual stresses along the horizontal and longitudinal directions in the weld area were low, and the differences between them was not obvious. Besides, after annealing, the residual stresses along the horizontal and longitudinal directions were reduced to 20% of the initial state, that was to say, most residual stresses were eliminated.The residual stresses and the relaxation during thermal exposure were the important reference for the practical application during the manufacture of workpieces. So the residual stresses relaxations of the supercritical boiler water wall during thermal exposure were investigated in this work. The results showed that the residual stresses were relaxed under high temperatures. Besides, the relaxation activation energy was obtained according to the calculation of dynamics of relaxation. The relaxation activation energy Q = 242 KJ.mol-1, which was similar with the activation energy of steel. The stresses relaxation were influenced by the diffusion of atoms and the movement of dislocation. While the hinder of atom diffusion and dislocation movement increases, the higher relaxation activation energy appears.Shot peening(SP) is an important surface treatment to improve the surface properties of mechanical components. In this work, the weld samples were treated using three SP intensities,0.15 Amm, 0.25 Amm and 0.35 Amm with glass balls, and the peened surface layer were characterized. The results reveal that after SP, compressive residual stresses, refined domain sizes, surface roughness and hardness were changed and improved, which resulted from the increased SP intensity. With the increase of SP intensities, the depth layer of compressive residual stresses increased, and the compact between steel balls and the materials were more severe, which resulted in the deformation of surface layer more serious.