The lower bainite transformation and the significance of carbide precipitation
H. K. D. H. Bhadeshia
Abstract
The nature and significance of carbide precipitation accompanying the lower bainite transformation in steels has been examined. It is found that lower bainitic cementite nucleates and grows from within supersaturated ferrite, and that such precipitation is strongly inconsistent with the concept of 'interphase precipitation' at the ferrite-austenite transformation interface. Crystallographic determinations suggest that the lattice invariant shear of lower bainite may not be directly responsible for the occurrence of the characteristic single carbide variant. It is suggested that it is the transformation strain associated with a lower bainite platelet that stimulates the development of a particular carbide variant which is compatible with the relief of this strain. The transition temperature from upper to lower bainite has been tentatively rationalised on thermodynamic grounds and in the context of the BS and MS temperatures. It seems that the appearance of epsilon carbide in some lower bainites but cementite in others can be understood in terms of a theory of martensite tempering due to Kalish and Cohen.
