Mechanism of stress relief cracking in a granular bainitic steel

Abstract

Coarse-grained heat-affected zone (CGHAZ) of a low alloyed, granular bainitic steel T24 was simulated in a Gleeble apparatus. The stress relief of the CGHAZ was analyzed by annealing the samples. The morphology and behavior of the microstructure near the grain boundaries during stress relief were investigated by means of focused ion beam, in situ tensile testing, transmission electron microscopy, scanning electron microscopy and electron back-scatter diffraction. It was observed that there were large martensite/austenite islands distributed along the grain boundaries of CGHAZ. During stress relief at elevated temperature, the retained austenite at the grain boundaries decomposed into M3C carbides and a ferrite forming softening zone. Together with the stress relief, piled up of dislocations occurred within the ferrite in the area adjacent to the ferrite/M3C interface, which resulted in high level of stress accumulation and caused crack initiation along the grain boundaries. These results indicate that the stress relief induced the grain boundary crack is controlled by other mechanisms rather than the creep-like grain boundary sliding.