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Bainite

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Bainite is formed at cooling rates slower than that for martensite formation and faster than that for ferrite and pearlite formation. There are two forms of bainite, known as upper and lower bainite.

Upper bainite generally forms at temperatures between 550 and 400°C. There are several proposed formation mechanisms, based on the carbon content and transformation temperature of the steel, resulting in slightly different morphologies. Low carbon steels exhibit fine bainitic laths, nucleated by a shear mechanism at the austenite grain boundaries. Carbon solubility in bainitic ferrite is much lower than in austenite, so carbon is rejected into the austenite surrounding the bainitic ferrite laths. When the carbon concentration in the austenite is high enough, cementite nucleates as discrete particles or discontinuous stringers at the ferrite/austenite interfaces. As the carbon content increases, the cementite filaments become more continuous, and at high carbon contents, the bainitic ferrite laths are finer with the cementite stringers more numerous and more continuous. The structure can appear more like pearlite, and is termed 'feathery' bainite.

Lower bainite generally forms at temperatures between 400 and 250°C, although the precise changeover temperature between upper and lower bainite depends on the carbon content of the steel. The transformation nucleates, like upper bainite, by partial shear. The lower temperature of this transformation does not allow the diffusion of carbon to occur so readily, so iron carbides are formed at approximately 50-60° to the longitudinal axis of the main lath, contiguously with the bainitic ferrite. With low levels of carbon, the carbide may precipitate as discrete particles, following the path of the ferrite/austenite interface. However, the overall mechanism of lower bainite formation is independent of carbon content in the main. The appearance of lower bainite strongly resembles that of martensite, but lower bainite is formed by a mixture of shear and diffusional processes rather than just shear.

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