Microstructure and micro strain in the CGHAZ of 2.25Cr-1.6W steel after intercritical heat treatment

Abstract

Multiple microscopic analysis techniques, such as transmission electron microscope (TEM), electron backscatter diffraction (EBSD) and transmission-EBSD (T-EBSD), were used to study the microstructure and micro strain in the coarse-grained heat-affected zone (CGHAZ) of 2.25Cr-1.6W steel after strain-to-fracture (STF) tests under intercritical heat treatment temperature. This study was a supplement of previous research, which was helpful for understanding the mechanism of reduced susceptibility of reheat cracking by intercritical heat treatment. The result showed that transformation products were retained austenite and twin martensite which had many variants with different crystallographic orientation. Twin boundaries were found both in recrystallized grains and matrix near the recrystallized grains. They were not conventinal parallelling interfaces and the misorientation was not exactly equal to 60°. Transformation resulted in large strain in recrystallized grains and the strain concentration kept away from the prior austenite grain boundaries (PAGBs). Besides transformation strain, the position of recrystallization was another factor which led to different mechanisms of reduced susceptibility to ductility-dip cracking (DDC) in Ni-based alloy and reheat cracking in 2.25Cr-1.6W steel, respectively.