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.
Starting temperature of martensitic transformation
Scanning electron microscope
Transmission electron microscope
Martensite and austenite
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The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (NSFC) (Nos. 51874216 and 51704217), The Major Projects of Technology Innovation of Hubei Province (No. 2017AAA116), The project of Science and Technology Plan of Wuhan (No. 2018010402011187) and Hebei Joint Research Fund for Iron and Steel (No.E2018318013).