Thermal stability of retained austenite in Quenching & Partitioning steels

The drive for ever more safety and fuel efficiency in the automotive industry led the industry to search for steels with enhanced strength and ductility. Promising candidates to satisfy these demands are steels with a microstructure consisting of martensite and significant fractions of retained austenite. One class of steels with such a microstructure are Quenching & Partitioning (Q&P) steels. The enhanced strength and ductility of Quenching & Partitioning steels is largely due to the presence of retained austenite. At elevated temperatures encountered during processes such as welding, hot dip galvanizing and paint baking, the possibility of retained austenite decomposition into thermodynamically more stable ferrite and carbides exists. Decomposition of retained austenite would have detrimental effects on the mechanical properties of Quenching & Partitioning steels. A requirement for practically applicable Quenching & Partitioning steels is therefore knowledge of the thermal stability of retained austenite against decomposition. In this work, multiple microstructures have been created in one particular steel alloy using Quenching & Partitioning processing. These microstructures have been characterized using X-Ray Diffraction, Scanning Electron Microscopy, Optical Microscopy and Electron Backscatter Diffraction. Special attention was paid to the morphology and carbon content of retained austenite. The response of retained austenite to isothermal and isochronal annealing has been investigated using dilatometry and thermomagnetic methods. The main finding of this work is that retained austenite in essence behaves as austenite which is higher in carbon content compared to the base alloy. Furthermore, the decomposition mechanisms of retained austenite have been successfully related to existing theory about austenite decomposition and mapped to a TTT-like diagram. Low-carbon retained austenite decomposes significantly quicker than high-carbon retained austenite.Mechanical Engineering: Materials Engineering and ApplicationsMaterials Science & EngineeringMechanical, Maritime and Materials Engineerin