Microstructure and mechanical properties of a medium-carbon bainitic steel treated by quenching and two-step bainitic isothermal transformation below Ms

ABSTRACT

A quenching and two-step bainitic isothermal transformation below Ms (Q-2BT) process was proposed for a medium-carbon steel. The evolution of microstructure and its correlation with mechanical properties were investigated and compared with the samples subjected to a quenching and one-step bainitic isothermal transformation below Ms (Q-1BT) process. Results showed that the Q-2BT sample has a finer martensite/bainite/retained austenite multiphase microstructure than the Q-1BT ones because of the formation of smaller bainite and the decomposition of large untransformed austenite blocks. Meanwhile, an optimal strength (1582 MPa), elongation (20.2%) and toughness (110 J) combination can be obtained after the Q-2BT process. The significantly improved impact toughness can be attributed to refined microstructure, increased filmy retained austenite as well as a high proportion of high-angle boundaries (70.7%). In addition, the Q-2BT sample exhibits a longer hardening plateau and a higher hardening exponent (0.084) because of the larger amount of retained austenite (11.3%) and more filmy austenite with high stability.

KEYWORDS:

• Multiphase microstructure
• retained austenite
• bainitic steel
• impact toughness
• work hardening behaviour

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Additional information

Funding

This research was financially supported by the National Natural Science Foundation of China [grant number 52003181), the Sichuan Science and Technology Program [grant number 2021YJ0555], the Natural Science Basic Research Plan in Shaanxi Province of China [grant number 2021JQ-852], the Open Research Subject of Key Laboratory of Fluid and Power Machinery (Xihua University), Ministry of Education [grant number LTDL2021-006], the Opening Project of Material Corrosion and Protection Key Laboratory of Sichuan Province [grant number 2021CL16] and the Scientific Research Foundation of Xi’an Aeronautical University [grant number 2020KY0215].