Investigation On Type IV Creep Cracking And Its Prediction Method For P92 Steel Welded Joints

The main steam tubes of ultra supercritical fossil-fired power plants are usually produced by P92 steel, and the high-temperature creep properties of the welded joints is one of the weak links. In this paper, the most dangerous cracking type-Type IV cracking of P92 steel welded joints was studied, involving the cracking mechanism, the evolution of creep damage and the numerical calculation prediction techniques.The various time creep test for welded joints of P92 steel had been conducted at 650℃under 70 MPa. When the total creep time reached to more than 7000 h, the Type IV cracking occurred. Then the Type IV cracking mechanism of P92 steel welded joint was investigated through the microstructure analysis and FEM analysis, respectively.The microstructure analysis indicates that the Type IV cracking process essentially is the creep damage evolution process of the fine-grain HAZ in the welded joint. The carbides which were located at the coarse austenite grain boundary of the original base metal, were not completely dissolved during the welding process, and were precipitated on the grain boundaries of FGHAZ after the welding. In the creep, the carbides will grow up and new coarse carbides will form in the grain boundaries, and may become the nucleation site of the creep voids, so the coarse carbides lead to the decline of the FGHAZ creep strength. As the creep voids growing, they will coalesce together and connect each other, and the grain boundaries will be separated from the substrates. As the separated grain boundaries increase, the micro crack will form on the original austenite boundaries, and lead to the final fracture of the welded joint.The creep specimens of the simulated FGHAZ and CGHAZ, which were prepared by heat treatment method, together with the weld metal specimens and base metal specimens, were tested in the uniaxial creep condition. The creep curves and creep parameters of the different zones of the welded joints were achieved and the modified K-R creep damage equations of the different micro zones were constructed. Then, the user material subroutine in ABAQUS-UMAT coupled with modified K-R damage equations was complied to calculate the creep damage in the P92 steel welded joint with a high accuracy.The FEM analysis results showed that Type IV cracking was related to the stress redistribution and the stress triaxiality in the welded joints, beside the tested stress level and the low creep strength of the FGHAZ. The stress redistribution could suppress the Type IV cracking, because it made the tensile stress decrease in the FGHAZ and made the tensile stress increase in the CGHAZ and base metal.In addition, the FGHAZ with low creep strength was located between the CGHAZ and base metal with high creep strength, so that the stress triaxiality of FGHAZ was higher than that of CGHAZ and that of base metal during the stress redistribution. The higher stress triaxiality could promote the initiation and growth of creep voids in FGHAZ, and accelerates of creep damage in FGHAZ. The FEM results also showed that the inner elements of welded joints specimen occurred firstly, and developed to the elements of outer surface. This can be explained that the stress triaxiality of the inner of the welded joint is higher than that of the outer surface.According to the analyses of Type IV creep cracking mechanism, adding a suitable number of normalizing treatments on the P92 steel before welding ,which can dissolve the carbides in the austenitic grain boundaries as many as possible, is thought to be a better choice to suppress the descend of Type IV cracking in the welded joint.