Brittle-to-ductile transition in martensite - bainite steels

Abstract

To understand the influence of composite structures on the brittle-to-ductile transition (BDT) in low-carbon martensite-bainite steel, this study examines the temperature dependence of the impact absorbed energy in five types of steel with identical chemical compositions: fully martensitic steel, fully bainitic steel, and martensite-bainite steels containing 4%, 15%, and 55% bainite fractions, respectively. The BDT temperature was found to be highest for fully martensitic steel, followed by martensite–4% bainite, martensite–15% bainite, martensite–55% bainite, and fully bainitic steel. Despite the significant differences in bainite volume fractions, the BDT temperatures of martensite–15% bainite, martensite–55% bainite, and fully bainitic steel were similar, indicating that the BDT temperature trend cannot be solely explained by the composite rule. To elucidate the trend in BDT temperature based on the shielding theory, the temperature dependence of the 0.2% proof stress was measured for each type of steel. Optical micrographs and the temperature dependence of effective stress showed that dislocations in the bainite phase were preferentially activated, playing a dominant role in determining the yielding behaviour and BDT in the martensite-bainite steels. This behaviour was also associated with the network structure of bainite surrounding the martensite, where the bainite phase underwent plastic deformation immediately after yielding in the steels under investigation.