Effects of Post Weld Heat Treatment Residual Stress on the Creep of Elbow Welded Joint

Abstract

It is well known that type IV crack emerges in the high temperature welded components. Although the welding residual stress will drop after post weld heat treatment (PWHT), the PWHT residual stress still remains in the welded joint. When the joint is in service at high temperature and under high pressure, the PWHT stress and pressure will induce the creep strain in the welded joint of the elbow of T91 boiler tubes. Usually, it is considered that the PWHT stress will relax at high temperature environment and is ignored by many researchers. In this paper, by the finite element analysis codes ABAQUS and the function of RESTART, a sequential coupling calculating program for creep based on welding residual stress has been developed by establishing the model of temperature field, residual stress and creep analysis. The welding residual stress and creep of elbow welded joint for T91 boiler tubes was simulated by the program. According to the finite element calculation results, the PWHT stress has a great influence on the T91tube creep. This method established the basis of calculating the effects of welding residual stress on the creep of welded joint. Firstly, the welding residual stress and the PWHT stress was simulated by ABAQUS. Based on the work, the stress distribution of the T91 tube elbow can be obtained under internal pressure and the PWHT stress. Secondly, for the elbow welded joint the creep under the internal pressure only and the creep under internal pressure and PHWT stress can be simulated by the ABAQUS codes. Then comparison of creep strain in the elbow welded joint with PWHT stress and with internal pressure is made in the paper. Although the stress induced by the PWHT and internal pressure would relax in a short time, the creep strain of the elbow welded joint is still influenced by initial stress state. The results shown that the PWHT stress can not be ignored for the creep analysis of the elbow welded joint. So it can be considered that the PWHT residual stress is one of possible failure factors for the elbow welded joint. The design of high temperature elbow welded joint need to consider the influence of the PWHT residual stress. This paper will provide a more comprehensive means for life assessment of the welded joint in service.