Fatigue deformation mechanisms and dislocation model of 41Fe-25.5Ni-23.5Cr alloy welded joints under multistage heat treatment

Abstract

The fatigue behavior of 41Fe-25.5Ni-23.5Cr alloy welded joints subjected to different multistage post-weld heat treatment (PWHT) conditions was studied. The fatigue life of welded joints under PWHT1 (600 °C-1 h/1050 °C-1 h/950 °C-30 min) was significantly prolonged, which was attributed to the low cyclic hardening rate caused by the small cyclic yielding stress and effective stress. The microstructure analysis indicated that the twin boundaries in the fusion line maintained continuity with those in the base metal, causing the inhomogeneous fatigue deformation to be primarily distributed in the weld metal rather than the fusion line. The crystallographic orientation (001) of weld metal under PWHT1 remained unchanged during fatigue, the slip of bowed dislocation lines was the characteristics of fatigue deformation and the grain growth led to the decrease in weld metal deformation. The crystallographic orientation under PWHT2 (1050 °C-1 h) changed from (111) to (001), and the slip of tangled dislocation lines resulted in an accumulation of weld metal deformation. To study the impact of deformation behavior on fatigue properties, new dislocation slip models were developed based on the increase in dislocation density caused by the parabolic slip area and dislocation line length. The results of this study indicate that multistage PWHT is a remarkable approach to prolong the fatigue life of 41Fe-25.5Ni-23.5Cr alloy welded joints at high temperatures.