Ferrite hardening response in a low alloy ferrite-martensite dual phase steel

This paper is concerned to investigate in details the variation of ferrite hardening response in conjunction with carbon partitioning developed within ferrite during austenite to ferrite phase transformation in a low alloy ferrite-martensite dual phase (DP) steel. For this purpose, a wide variety of ferrite-martensite DP microstructures consisting different volume fractions of ferrite and martensite have been prepared using step quenching heat treatment processes at isothermal temperature of 600 °C for various holding times. Nanoindentation measurements have been supplemented by energy dispersive X-ray and microprobe wavelength-dispersive spectroscopic analyses to follow the variation of ferrite hardening response and its relation to the carbon concentration of ferrite in the ferrite-martensite DP microstructures. The experimental results showed that the ferrite hardening response is quite variable depending on the progress of ferrite formation in the ferrite-martensite DP microstructures. For a specific ferrite grain in a specific ferrite-martensite DP microstructure, the location nearer to the ferrite-martensite interfaces has been accompanied with a significant higher carbon concentration and simultaneously higher ferrite hardening response in comparison to that of central regions of ferrite grains. These results are rationalized with a higher concentration of carbon within ferrite developed as a consequence of higher carbon entrapment within defected ferrite area generated at early stage of austenite to ferrite phase transformation at more pre-existing defected area of prior austenite grain boundaries, and that the carbon diluted ferrite region is related to the rejection of carbon from interior ferrite area to the ferrite/austenite interfaces developed on the subsequent isothermal holding times.