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Article

Near-Ms Austempering of Carbide-Free Bainitic Steel: Effects on Phase Transformation, Microstructure and Mechanical Properties

by
Haoqing Zheng
1,
Zhixiang Liu
2,*,
Hua Fan
3,
Xiao Hu
1,4,
Guanqiao Su
1,
Yang Jin
1,
Hongwei Wang
3,
Tao Xie
1 and
Xuefei Huang
2,*
1
Panzhihua Iron and Steel Research Institute Co., Ltd., Pangang Group, Panzhihua 617000, China
2
College of Materials Science and Engineering, Sichuan University, Chengdu 610065, China
3
Pangang Group Xichang Steel & Vanadium Co., Ltd., Xichang 615000, China
4
School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China
*
Authors to whom correspondence should be addressed.
Crystals 2025, 15(12), 1044; https://doi.org/10.3390/cryst15121044
Submission received: 31 October 2025 / Revised: 27 November 2025 / Accepted: 5 December 2025 / Published: 8 December 2025
(This article belongs to the Section Crystalline Metals and Alloys)
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Abstract

The parameters of the austempering process play a crucial role in governing the microstructure and mechanical properties of carbide-free bainitic (CFB) steel. In this study, a CFB steel was austempered at temperatures close to its martensite start (Ms = 372 °C) temperature to investigate the bainitic transformation kinetics, microstructure, and mechanical properties. To identify the optimal strength–ductility combination, austempering was carried out at 360 °C, 380 °C, and 400 °C for comparison. The results show that austempering slightly below Ms (360 °C) produces the highest yield-to-tensile strength ratio and a good strength–ductility balance. Dilatometry curves indicate that the onset of bainite transformation occurs fastest when austempering slightly below Ms. The stronger transformation driving force and the presence of athermal martensite are the primary reasons. The enhanced thermodynamic driving force and increased nucleation density promote the formation of a larger amount of bainitic laths. Electron backscatter diffraction (EBSD) analysis reveals that the retained austenite blocks are finest after austempering at 360 °C, which helps alleviate the ductility loss associated with the reduction in retained austenite content.
Keywords: austempering; bainite transformation kinetics; microstructure; tensile properties austempering; bainite transformation kinetics; microstructure; tensile properties

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MDPI and ACS Style

Zheng, H.; Liu, Z.; Fan, H.; Hu, X.; Su, G.; Jin, Y.; Wang, H.; Xie, T.; Huang, X. Near-Ms Austempering of Carbide-Free Bainitic Steel: Effects on Phase Transformation, Microstructure and Mechanical Properties. Crystals 2025, 15, 1044. https://doi.org/10.3390/cryst15121044

AMA Style

Zheng H, Liu Z, Fan H, Hu X, Su G, Jin Y, Wang H, Xie T, Huang X. Near-Ms Austempering of Carbide-Free Bainitic Steel: Effects on Phase Transformation, Microstructure and Mechanical Properties. Crystals. 2025; 15(12):1044. https://doi.org/10.3390/cryst15121044

Chicago/Turabian Style

Zheng, Haoqing, Zhixiang Liu, Hua Fan, Xiao Hu, Guanqiao Su, Yang Jin, Hongwei Wang, Tao Xie, and Xuefei Huang. 2025. "Near-Ms Austempering of Carbide-Free Bainitic Steel: Effects on Phase Transformation, Microstructure and Mechanical Properties" Crystals 15, no. 12: 1044. https://doi.org/10.3390/cryst15121044

APA Style

Zheng, H., Liu, Z., Fan, H., Hu, X., Su, G., Jin, Y., Wang, H., Xie, T., & Huang, X. (2025). Near-Ms Austempering of Carbide-Free Bainitic Steel: Effects on Phase Transformation, Microstructure and Mechanical Properties. Crystals, 15(12), 1044. https://doi.org/10.3390/cryst15121044

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