Phase transformation-induced grain refinement in rapidly solidified ultra-high-carbon steels

Abstract

Superplastic forming offers a promising approach for reducing the cost of high-performance metal components with complex shapes. Severe thermomechanical deformation is one method for producing the fine grain structure needed to permit superplastic forming economically. Our approach to generating fine-grained microstructures is by cyclic heat treatment of rapidly solidified material. First, a metastable structure is produced by rapid quenching of the liquid metal. Then, solid-state phase transformations at modest temperatures are employed to refine this structure. In the ultra-high-carbon steels (UHCS) studied, the brittle as-cast structure of martensite and austenite was transformed, after cyclic heat treatment, to a ductile mixture of 1-µm ferrite and 0.25-µm carbide. Varying the heat-treat temperatures by 100 °C within the transformation range had little effect on the scale of the microstructure. Higher C resulted in coarser carbide spheroids, addition of Al refined the microstructure, and the finest mean carbide size was obtained with an intermediate level (5 pct) of Cr. Refinement of the martensite plates retained austenite via cyclic tempering and austenitization was found to be the key step in the overall mechanism for phase transformation-induced grain refinement in rapidly solidified UHCS.