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, the average ferrite microhardness from the central location of ferrite grains toward the ferrite area near the prior α/γ interfaces has been increased from 122 to 145HV1g for DP samples treated at 720°C for 5 min holding time. The carbon and silicon concentrations from central regions of ferrite grains toward the α/γ interface are decreased from 5.97 to 4.14 EDSNs and 0.89 to 0.45 EDSNs, respectively, 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 the ferrite area adjacent to the martensite generated during martensitic phase transformation.