The Effect of Compositional Heterogeneity on the Martensite Start Temperature of a High Strength Steel During Rapid Austenitisation and Cooling

Many low alloy steels are compositionally heterogeneous due to the preferential segregation of alloying elements to the dendritic or interdendritic regions when the steel is first solidified (microsegregation). This segregation is often ignored when using CCT diagrams to predict the phase transformation behaviour of steels, but may be of importance in some industrially-relevant cases. This work focuses on the martensite start temperature (M_s) of Super CMV, a high strength aerospace steel, after rapid austenitisation treatment to various peak temperatures from 900 °C to 1300 °C. It was found that the average M_s temperature increased with increasing peak temperature (and prior austenite grain size) at peak temperatures of 1100°C and above, which is to be expected conventionally. However, at peak temperatures of below 1100°C, the M_s temperature increased with decreasing prior-austenite grain size. It is proposed that this was due to the presence of non-dissolved carbides in these conditions, particularly in enriched bands of material, which deplete the matrix and hence raise M_s.