2008 Volume 49 Issue 9 Pages 2016-2027
A model is developed to describe the formation and crystal structure of martensite in quenched Fe-C steels based on the extensive published literature on the subject. Unique changes in the properties and structure of martensite are shown to occur at 0.6 mass% C, designated as the H-point. The concept of primary and secondary martensite is introduced in order to indicate that two different, sequential, martensites will form during quenching of Fe-C steels above 0.6 mass% C. Below 0.6 mass% C, only primary martensite is created through the two sequential steps FCC → HCP followed by HCP → BCC. Primary martensite has a lath structure and is described as BCC iron containing a C-rich phase that precipitates during quenching. The HCP transition phase is critical in interpreting the two martensite structures based on the premise that the maximum solubility of C in the HCP phase is 0.6 mass%. Primary martensite continues to form at compositions greater than 0.6 mass% C with the creation of a carbon-rich BCT phase. This is followed by the start of secondary martensite which forms at the MS (martensite start temperature) and creates the traditional BCT plates adjoining retained austenite. Both martensites are predicted to co-exist at the highest C contents. A quantitative model, based on the specific volume of the various phases obtained after quenching, has been used to calculate the composition of the precipitated C-rich phase for a 0.88 mass% C steel. It is predicted that the carbon-rich phase is either diamond or η (Fe2C) carbide.