Investigation of carbon and silicon partitioning on ferrite hardening in a medium silicon low alloy ferrite-martensite dual-phase steel

In this paper, the micromechanical behavior of ferrite microphase was evaluated in conjunction with carbon and silicon partitioning occurred during prior austenite to ferrite phase transformation using microhardness measurements supplemented by light observation and field-emission scanning electron microscopy equipped with X-ray energy dispersive spectroscopy (EDS). For this purpose, at first, the samples were austenitized at 900°C for 15 min and then air-cooled (normalized) to room temperature in order to develop more starting homogeneous microstructural features in the proposed heat-treated samples. The wide variety of ferrite-martensite dual-phase (DP) samples containing different volume fractions of ferrite and martensite microphases developed using step-quenching heat treatment processes at 750, 720, 700, and 680°C for 5 min isothermal holding time with the subsequent water quenching after being austenitized at 900°C for 15 min in the same conditions as to the direct water-quenched (WQ) samples. The experimental results showed that, for a particular ferrite grain in a particular ferrite-martensite DP samples, the ferrite location nearer to the ferrite-martensite interfaces was accompanied with a significantly lower carbon and silicon centrations, while the associated ferrite hardening response was abnormally higher in comparison to that of the central regions of ferrite grains. This abnormal higher trend in ferrite hardness with lower carbon and silicon concentrations was attributed to the higher ferrite/martensite interaction of ferrite area adjacent to the martensite generated during martensitic phase transformation.