Martensitic transformation of reversed austenite in a low-carbon 7Ni steel

Reverse transformation plays an important role in phase controlling of 7Ni steel. However, little attention has been paid on the martensitic transformation behaviour of this material. In this research, martensitic transformation kinetics and the martensite-start temperature (Ms) of reversed austenite is studied in a 7Ni steel by dilatometry and microstructure analysis. It is shown that the transformation kinetics can be described by an exponential Koistinen-Marburger (KM) equation rather than by the traditional KM equation. An abnormal phenomenon was found, namely that although reversed austenite is enriched with solute elements and has a smaller grain size, no distinct decrease of Ms occurred. This may be caused by a decreased critical driving force for martensitic nucleation, including those associated with strain and interfacial energies, as it was found that the orientation of fresh martensite is close to that of the surrounding tempered martensite.