Retained Austenite in Bainitic Steels

Bainite is formed through the transformation of austenite, and a portion of the austenite remains untransformed within the bainitic microstructure as retained austenite. The performance of bainitic steels is often unstable because of the complex relationship among the transformation process, microstructure, and material properties, which is largely governed by retained austenite. This complexity has long challenged researchers and engineers. At the same time, it has sustained considerable scientific interest, leading to extensive research and many significant advances in the field of bainitic steels. The authors have devoted many years to the study of bainitic steels and have completed numerous research projects in this area. These include the National Excellent Youth Science Foundation project, Fundamental Research on Disruptive Technologies for High-Speed Railway Manufacturing; the National 863 Program project, Key Technologies for the Production of Special High-Performance Steels—Bearing Steels for Wind Turbines; the National Natural Science Foundation project, Microstructural Evolution, Crystallographic Characteristics, and Deformation Partitioning Behavior of Carbide-Free Bainitic Steels under Strain; the project Hydrogen Diffusion and Hydrogen Embrittlement Mechanisms in Clean High-Strength Arc-Shaped Bainitic Steels for Heavy Plates and Forgings; and the project Formation, Characteristics, and Effects of White-Etching Layers in Turnout Steels under Rolling/Sliding Contact Stress. The authors have also participated in several provincial research initiatives, including the Hebei Provincial High-Level Talent Program, the Hebei Provincial Joint Innovation and Entrepreneurship Team Leader Program, and the Hebei Provincial Development and Reform Commission's major project, Key Common Technology Research and Development for High-End Equipment and Advanced Manufacturing—Production Line for High-Strength Steel Used in Heavy-Duty Railway Tracks. Other notable projects include the Hebei Provincial Outstanding Youth Fund project, Research on the Macro- and Micromechanics of Bainitic Steel Frogs for High-Speed Railway Turnouts. More recently, the authors have been engaged in National Natural Science Foundation key projects entitled Design, Preparation, and Key Issues in the Application of Rapid Phase Transformation Nano-Bainitic Steels and Approaches and Mechanisms for Enhancing the Performance of Nano-Bainitic Bearing Steels through Multiphase Composite Synergy. Years of research have demonstrated that bainitic steels, particularly carbide-free bainitic steels, exhibit outstanding comprehensive mechanical properties. However, the retained austenite present in bainitic steels exerts a complex influence on both their mechanical behavior and service performance. This book systematically summarizes the relationships between the mechanical properties of bainitic steels and the composition, volume fraction, size, morphology, and substructure of retained austenite. It further discusses strategies and approaches for controlling retained austenite to achieve stable and reliable mechanical performance in bainitic steels. This book presents the major research achievements of the authors and their research team accumulated over many years. It also incorporates selected findings from published studies by other researchers, providing a comprehensive discussion of the morphology, stability, phase transformation, and mechanical effects of retained austenite in bainitic steels, as well as its influence on engineering applications. The authors hope that this volume will serve as a valuable reference for researchers and engineers working in this field. The authors gratefully acknowledge the financial support provided by national and provincial funding agencies for the research projects described in this book. They also extend their sincere appreciation to the many scholars, both in China and abroad, whose published work has contributed to the advancement of this field. Special thanks are due to all faculty members and students of the research team for their dedication and contributions to bainitic steel research. The authors also express their gratitude to Dr. Zhao Jiali, Dr. Kang Ji, and Dr. Wang Yanhui for their assistance in organizing and preparing parts of this book. Appreciation is further extended to Associate Researcher Zheng Chunli, Dr. Li Yanguo, Dr. Long Xiao'an, Dr. Wang Mengmeng, Dr. Chen Chen, and other members of the editorial and illustration team for their valuable support. Despite the authors' best efforts, this book may still contain shortcomings. Readers' constructive comments, criticisms, and suggestions are sincerely welcomed.