Papers by Author: Timothy D. Bigg

Abstract: The novel heat treatment concept of Quenching and Partitioning (Q&P) offers exciting prospects for the production of higher strength steel products with enhanced formability from a microstructure containing retained austenite and martensite. The Q&P process hinges on an interrupted quench and partitioning step at intermediate temperatures whereby the untransformed austenite can be thermodynamically stabilised by enrichment of carbon from the supersaturated martensite. Although the concept is similar to that producing carbide-free bainite in TRIP-assisted steel, Q&P offers the advantage of separating the ferrite formation and austenite enrichment stages of the process. While the concept is readily understood, the details of microstructural evolution during interrupted quenching and partitioning steps are difficult to study and are generally inferred from dilatometry or metallographic examination after a final quench back to room temperature. Consequently, in this study, alloying has been used to develop a model alloy in which the sequential steps of heat treatment can be separated for closer, more direct inspection by neutron diffraction techniques.

Abstract: The novel non-equilibrium heat treatment procedure known as Quenching and Partitioning (Q&P) may offer the prospect of higher strength steel products with enhanced formability based upon martensitic microstructures containing controlled quantities of carbon-enriched retained austenite. The Q&P process requires an interrupted quench and isothermal annealing (partitioning) step at intermediate temperatures, whereby untransformed austenite can be thermodynamically stabilised by carbon migration from supersaturated martensite regions. The concept is comparable to that producing carbide-free bainite, for example, in TRIP-assisted steel, although Q&P allows separation of the ferrite formation and austenite enrichment stages of the process. However, although the Q&P concept is readily understood, evolution of the microstructure during interrupted quenching and partitioning has been inferred indirectly from dilatometer studies and metallographic examination after final quenching to room temperature. Consequently, a model alloy was developed in which the sequential steps of heat treatment could be separated for direct inspection by conventional metallography, X-ray diffraction and neutron diffraction techniques.