Study On P92Steel Welding Residual Stress And Its Effect On The High Temperature Life

Due to the excellent creep resistance at high temperature, P92steel has beenwidely used to manufacture main steam pipes in ultra-supercritical (USC) powerplants. These pipes are always made of welded joints and the residual stress wouldoccur in the welded joints after welding process and would inevitably affect thecomponents safety under longtime service. Thus, post weld heat treatment is neededto improve microstructure and modify the distribution of residual stress. Hence,understanding of residual stress status of the welded joint and its effect on the hightemperature creep life is important for the reliability assessment of high-temperaturestructural components.In the present study, X-ray diffraction method and indentation method wereemployed to measure the residual stress of the welded joints after welding andsubsequent post heat treatment. Then, the distribution of residual stress on the outersurface of welded joint of P92steel pipe is obtained. Furthermore, finite elementmethod was conducted to simulate the welding process and determine the temperaturedistribution after welding process and then obtain the residual stress. The calculatedresidual stress distribution was in agreement with the measured results. It means that agood numerical simulation technology for P92steel welding process could beachieved.The local post weld heat treatment technique applied on P92steel welded jointusing flexible porcelain electrical resistance method was investigated. In addition,FEM was employed to simulate the local post weld heat treatment process and toobtain the temperature distribution. Then, coupled with experimental results, the localheat treatment numerical simulation technique was developed to determine the actualheat treatment parameters in practice. The technique was also used to determine thespecific parameters of local heat treatment for the P92steel pipe with a largethickness and length, and various inner diameter sizes in practice. As a result, a newspecification on heated band and gradient control band was obtained, which modifiedthe current national standards or international standards.Furthermore, the residual stress could affect the creep strain development of P92steel pipes during service at high temperature and long time. In generally, the residualstress was higher than the service stress in practice. As a result, the amplitude of residual stress determined the creep deformation and in particular its effect on axialcreep strain was remarkable. Therefore, it is inevitable to exclude the effect ofresidual stress in the elevated temperature strength design.Creep crack growth tests were also carried out on the compact tension (CT)specimens with/without residual stress. The results revealed that due to the residualstress the creep crack growth rate was accelerated and the maximum creepdeformation was also accelerated compared with the specimen without residual stress.In addition, the creep strain under the residual stress coupled with tensile stress wasonly accelerated the creep deformation rather than the summation of the creep strainunder residual stress condition and tensile stress condition, respectively.Then, the prediction of creep crack growth under the residual stress coupled withtensile stress was obtained using parameter C*evaluating from reference stress, but itwas conservative compared with experimental data. Furthermore, a modifiedpredicted method which introduced a modified initial stress intensity factor toevaluate the parameter C*was employed and the calculated results exhibited wellagreement with the experimental data.