Effects of Ni and Cr on Cryogenic Impact Toughness of Bainite/Martensite Multiphase Steels

In the present research, the effects of Nickel (Ni) and Chromium (Cr) on cryogenic impact toughness (CIT) of low-carbon bainite/martensite multiphase steels [processed by two different cooling processes: isothermal transformation process (ITP) and continuous cooling process (CCP)] were investigated. It was found that due to the formation of carbides during isothermal treatment, the addition of Ni and Cr yielded no significant improvements in CIT. However, during CCP treatment, the addition of Ni manifested a considerable enhancement in CIT, whereas the addition of both Ni and Cr caused a decrease in CIT. Further, after ITP treatment, the microstructure of all steels consisted of bainite and martenite, while Ni + Cr steel contained the largest amount of bainite. The microstructures of the CCP-treated steels mainly also consisted of bainite and martensite, but no retained austenite and carbides were observed, thus resulting in a superior CIT.