STRUCTURE AND GROWTH OF WIDMANSTAETTEN FERRITE AND BAINITE.
CASE INST OF TECH CLEVELAND OHIO DEPT OF METALLURGY
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Transmission microscopy has been used to demonstrate that Widmanstaetten ferrite, upper bainite and lower bainite can be distinguished on the basis of their substructures. Widmanstaetten ferrite forms as single crystals whereas upper and lower bainite develop as aggregates of smaller substructural units. The structural transition between acicular ferrite and upper bainite identifies a structural B sub s temperature which agrees with that determined by kinetic techniques. Correlation of optical and transmission micrographs provides indirect evidence that both upper and lower bainite grow by the repeated nucleation of substructural units which propagate rapidly to a limited size. This leads to the concept that both forms of bainite form initially with substantial supersaturation and the kinetics of carbide precipitation from supersaturated ferrite contributes significantly to the differences between the two forms of bainite. It is proposed that growth rates of bainite measured by hot stage microscopy refer to the rate at which substructural units nucleate rather than to the rate at which a unique interface advances under either diffusion or interface control. Author