Reliability/unreliability of mixture rule in a low alloy ferrite-martensite dual phase steel

The aim of this paper is to investigate in details the relationship between the volume fractions of ferrite and martensite with the variation of hardness in a low alloy ferrite-martensite dual phase (DP) steel. For this purpose, a wide variety of ferrite-martensite DP samples consisting different volume fractions of ferrite and martensite have been developed using step quenching heat treatment cycle involving reheating at 860 °C for 60 min, soaking at 600 °C salt bath for various holding times followed by 70 °C hot oil quenching. Optical microscopy has been supplemented by electron microscopy and hardness measurements to follow microstructural changes and their relation to the variation in hardness. The results showed that there is a non-linear relationship between the hardness of DP samples with the volume fraction of phase constituents indicating that the mixture rule is not reliable in the ferrite-martensite DP microstructures. The unreliability of mixture rule is related to the variation of ferrite and martensite hardening responses developed in the DP samples. The DP microstructure consisting 6-7% volume fraction of continuous grain boundary ferrite in the vicinity of martensite has been associated with a remarkable higher hardness for both ferrite and martensite in comparison with the other DP microstructures. The higher martensite hardness is due to the higher carbon content of the remaining metastable austenite developed in the ferrite-austenite two phase field area, leading to the harder martensite formation on the subsequent 70 °C hot oil quenching. The harder ferrite grains have been developed as a consequence of more constraints induced in the ferrite grains during martensitic phase transformation. The higher martensite volume fraction in the vicinity of thinner continuous grain boundary ferrite networks has been associated with the harder ferrite formation.