Stress-strain analysis of creep deterioration in heat affected weld zone in high Cr ferritic heat resistant steel

Abstract

The present study investigated creep cracking occurring in fine grained heat affected zones (FGHAZ) in welded joints of high Cr ferritic heat resistant steel at high temperature and low applied stress, based on experimental and analytical methods. Initially, a creep test on a single pass welded specimen was performed at 923 K and 90 MPa and results showed that cracking occurred in the FGHAZ. Further, a finite element model (FEM) was proposed to investigate the stress-strain distribution in the welded joint during the creep process. Good agreement on specimen deformation was observed between calculated and measured results. Analytical results showed that the FGHAZ position has high equivalent strain, high first principal stress, and high hydrostatic pressure. Base metal (BM) has a strong structural constraint on the FGHAZ region; weld metal (WM) has a strong structural constraint on the coarse grained heat affected zone (CGHAZ). Further, it was found that the creep void distribution, which was measured by a creep experiment, has a similar trend to the equivalent strain distribution, which was calculated using the model. This phenomenon suggested an important role for equivalent strain in the occurrence of creep voids. On the other hand, the high hydrostatic pressure is considered to accelerate void coalescence and the occurrence of type IV cracking. In addition, simulation results showed that a narrow FGHAZ can decrease the equivalent strain in the FGHAZ and consequently reduce the occurrence of creep voids.