Bainite formation in low carbon Cr-Ni steels
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INTRODUCTION
BAINITE transformation in steels takes place in a broad temperature region, in which thermodynamic and kinetic behaviors vary greatly so that bainite is characterized by miscellaneous morphologies and complicated formation mechanisms. Up to now many topics on bainite transformation, such as the formation mechanism of ferritic component and the sources of carbide precipitates, remain controversial. [1] According to the shear mechanism, bainitic ferrite is an aggregate of discrete martensitic sub-units produced directly by shear, f2,3,4I Although Ko and CottrelI ~St suggested that carbide precipitation increased the driving force by reducing bulk strain, most investigatorsI2'41 now believe that bainitic ferrite forms with a carbon supersaturation, and partitioning of the carbon into residual austenite occurs immediately after ferrite formation. Aaronson I6] observed growth ledges on the broad faces of proeutectoid ferrite precipitated from austenite in Fe-C alloys by replica electron microscopy, and suggested that the diffusional growth of these ledges along the broad faces of a new phase resulted in thickening in the direction perpendicular to the broad faces. On the basis of a quantitative discussion on the ledge mechanism, 171Aaronson et al. predicted I~'81that Widmanst~itten and bainitic ferrite would 9 [91 grow by the ledge mechanism. Bee and Honeycombe, and Hsu et al. l~~ also provided some evidence for the ledge mechanism. The present authors Illl have earlier analyzed the mechanism of bainite formation thermodynamically. A detailed thermodynamic treatment of Fe-C alloys has shown that in the upper temperature region of bainite formation, the chemical free energy change of the reaction is insufficient for driving a shear transformation. The present paper attempts to investigate bainite formation in low alloy steels, and to clarify some problems on the formation mechanism of bainite. YIWEN MOU, formerly Graduate Student, Department of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, People's Republic of China, is Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. T. Y. HSU (Xu Zuyao) is Professor, Department of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, People's Republic of China. Manuscript submitted July 7, 1986.
METALLURGICALTRANSACTIONSA
EXPERIMENTAL
A low carbon Cr-Ni steel was used to investigate the morphology and crystallography of bainite. Table I lists its chemical composition. Specimens (5 x 10 x 15 mm) were austenitized at 1200 ~ for 20 minutes with oxidation-resistant covering, followed by isothermally reacting in nitrate salt baths at various temperatures between 625 ~ and 385 ~ for different times before finally being quenched into water. The various morphologies of bainite were distinguished by optical and electron microscopy. Orientation relationships between fertile and carbide, and the habit plane of bainitic ferrite were determined by transmission el
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