MICROSTRUCTURE OF HIGH-STRENGTH STEEL REFINED WITH INTRAGRANULARLY NUCLEATED WIDMANSTATTEN FERRITE

A method has been developed to refine the microstructure of clean, high strength martensitic steels by partitioning the austenite grain structure with intragranularly nucleated plates of Widmanstatten ferrite. The plates were induced to nucleate heterogeneously on oxide particles present in the steels, by first forming uniform, thin layers of inactive allotriomorphic ferrite at the austenite grain surfaces. This effectively removed the austenite grain boundaries as potential sites for the nucleation of Widmanstatten ferrite, which consequently nucleated intragranularly on impurity oxide particles. The intragranularly nucleated ferrite plates were found to radiate in many directions from each oxide particle, in a manner which subdivided the remaining austenite into fine blocks. Quenching after this partial transformation to allotriomorphic and Widmanstatten ferrite resulted in the martensitic decomposition of the blocks of residual austenite, leading to a significantly refined overall microstructure.