Bainite in Silicon Steels: A New Composition-Property approach. Part 2

H. K. D. H. Bhadeshia and D. V. Edmonds

Abstract

Recent work on the mechanism of the bainite transformation has shown that the extent of transformation to bainitic ferrite, and the carbon content of the remaining austenite, can be estimated thermodynamically. This paper is concerned with the application of this work to the development of a quantitative relationship between the composition and some important mechanical properties of silicon containing steels, which can be transformed isothermally to an aggregate of bainitic ferrite and carbon enriched retained austenite only. It is demonstrated that the method has predictive capabilities, and forms the basis of further work (Part 2 of this study) on the theoretical design, optimization and testing of two promising steel compositions.

This research explores the thermodynamic principles used to enhance the mechanical properties of silicon-based steels through bainitic transformation. By applying a specific mathematical criterion to control the volume fraction of bainitic ferrite and the stability of retained austenite, the authors developed two specific alloy compositions with improved impact toughness and tensile strength. The study utilizes X-ray analysis and dark field imaging to demonstrate that reducing blocky austenite-which is prone to brittle martensitic transformation-leads to better performance under stress. Through isothermal transformation experiments, the data confirms that these microstructural adjustments produce high-strength materials that outperform traditional tempered martensite structures. These findings validate a predictive model for designing tougher steels by precisely manipulating their chemical composition and phase transitions.

Metal Science, Vol. 17, 1983, pp. 420-425.

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