Brittle-to-ductile transition in martensite – bainite steel

To understand the effect of composite structure on brittle-to-ductile transition (BDT) in low-carbon martensite–bainite steel, this study investigates the temperature dependence of the impact absorbed energy in five types of steel having the same chemical composition: fully martensitic steel, fully bainitic steel, and martensite–bainite steel with bainite fractions of 4%, 15%, and 55%, respectively. The BDT temperature was the highest for fully martensitic steel, followed by those of the martensite–4% bainite, martensite–15% bainite, martensite–55% bainite, and full bainitic steel. The BDT temperatures of martensite–15% bainite, martensite–55% bainite, and fully bainitic steel were close despite the large differences in their bainite volume fractions, suggesting that the trend of BDT temperatures cannot be explained simply based on the rule of mixture. The temperature dependence of 0.2% proof stress was measured in each steel to understand the trend of BDT temperature based on the shielding theory. Optical micrographs and the temperature dependence of effective stress indicated that the dislocations in bainite were preferentially activated and governed the yielding and BDT in the martensite-barite steels. This phenomenon was also linked to the network structures of bainite surrounding the martensite, where bainite was subjected to plastic deformation immediately after yielding in the employed steel.