Microstructural Evolution during the Novel Quenching and Partitioning (Q&P) Heat Treatment of Steel

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.