Designing ductile martensitic steel microstructures via localised austenite formation controlled by tailored chemical gradients

This thesis explores two ways of designing compliant metastable austenite within a hard martensitic matrix to create a multi-phase blended microstructure that combines high strength and ductility, enhanced by transformation-induced plasticity. The first strategy comprises quenching and tempering treatments to promote austenite reversion and partitioning, applied to a range of medium carbon stainless steels, systematically screened by the rapid alloy prototyping method. Significant variations in austenite parameters are controlled by additions of Cr, Mn and Si, dictating austenite stability and mechanical properties. In a follow-up, the coincidence of austenite reversion and carbide precipitation during tempering is established both in-situ and ex-situ, as well as the exploitable kinetic window discussed. The second strategy is built on the novel concept of exploiting the substantial enrichment in alloying elements, inherent to carbide precipitation, for microstructure design. To this effect, carbides are employed as a temporary vessel phase, to be deliberately nucleated, grown, and dissolved within three steps of heat treatment, in order to build up and inherit pronounced local enrichments in alloying elements. Precise tuning of carbide dissolution by flash heating is shown to evoke local depressions in the martensite start temperature, in a morphological image of the previously tailored carbides, implementing a self-organising dispersion of retained austenite upon quenching. The derived blended microstructure consists of retained carbides, enveloped by austenite shells, forming a unique core-shell-type composite microstructure that offers spectacular mechanical properties, based on austenite stability parameters that are locally tunable through bulk heat treatment.