P91 steel is a typical steel used in the manufacture of boilers in ultra-supercritical power plants and heat exchangers in nuclear power plants. For the long-term serviced P91 steel pressurized structures, the main failure mode is the welded joint failure, especially the heat affected zone (HAZ) failure. Repair welding technique is an effective method for repairing such local defects. However, the thermal shock composed of high temperature and thermal stress in the repair welding process will pose a critical loading condition for the existing defects near the heat source which cannot be detected by conventional means. So, the evaluation of structural integrity for the welded joint in the thermal-mechanical coupling field is necessary. In this work, the crack propagation law in the HAZ for the P91 steel welded joint was investigated under repair welding thermal loads. The weld repair model of the P91 steel welded joint was established by ABAQUS. The transient temperature field and stress field in repair welding process were calculated by relevant user subroutines and sequential coupling simulation method. The residual stress was determined by the impact indentation strain method to verify the feasibility of the finite element (FE) model and simulation method. In order to obtain the crack propagation path, the elastoplastic fracture analysis of the welded joint with initial crack was performed based on the extended finite element method (XFEM). The influence of different welding linear energy on the crack propagation was analyzed. The results show that the cracks in the HAZ propagate perpendicular to the surface and tend to deflect to the welding seam under repair welding thermal loads. The crack propagation occurs in the early stage of cooling. Higher welding linear energy leads to larger HAZ and higher overall temperature. With the increase of welding linear energy, the length and critical distance of the crack propagation increase. Therefore, low welding linear energy can effectively inhibit the crack propagation in the HAZ. The above calculation and analysis provide a reference for the thermal shock damage analysis of repair welding process, which is of great significance to improving the safety and reliability of weld repaired components.

For long-serviced pressure equipment that is under severe working conditions such as a high temperature, high pressure, and corrosion, the material properties and structure will be unavoidably damaged or degraded, especially cracks and other damages at key positions such as welded joints, which seriously threaten the safe operation of the equipment. In order to promote the sustainable development of industries such as the chemical and petrochemical industries, remanufacturing technology has emerged worldwide, and various surface repair processes have also rapidly developed. As an important branch of surface repair technology, the high energy spark deposition (HESD) process is a new pulse cold welding repair technology developed from electro-spark deposition, which combines the advantages of multiple surface repair processes. The HESD process has the characteristics of a smaller heat affected zone and lower welding residual stress. It is a new type of repair method that is worthy of popularization and application. The process has been initially applied in the fields of surface modification and die steel repair. In this paper, the application of the HESD process to the repair of welded joints was introduced, the mechanical properties of the joints and the residual stress distribution after welding were analyzed, and the feasibility of HESD as a repair welding method for pressure structures was discussed. First, a numerical simulation of the temperature and stress field of HESD was proposed by using ABAQUS and the related subprograms, and the validity of the simulation results was verified by the residual stress test with the indentation strain method. Due to the precise control of the heat and pulse discharge working mode, the heat-affected zone and deformation caused by the HESD were extremely small, and the residual stress that was generated was low and only concentrated on the repair welding seam. Second, according to the numerical simulation and the test results of the mechanical properties of the welded joint, the optimal repair welding process parameters were obtained through the orthogonal experiment: peak current 45 A, pulse width 90 ms, and output voltage 10 V.