The purpose of this study is to understand in detail the micro-mechanical behavior of martensite and ferrite microphases in a medium-silicon 35CHGSA dual-phase (DP) steel by varying the volume fractions of martensite and ferrite phase constituents. Accordingly, martensite-ferrite DP microstructures containing different phase fractions were prepared using a step-quenching heat treatment process that comprised of austenitizing at 900 ºC for 15 min, followed by holding at 720 ºC in the intercritical austenite-ferrite phase region for various durations from 1 to 30 min and finally, water-quenching to room temperature. Light microscopy combined with microhardness measurements were supplemented by electron microscopy and XRD-analysis in order to evaluate the micro-mechanical behavior of martensite and ferrite microphases in relation to the evolving microstructure. The experimental observations illustrate that both the microhardness data of martensite and ferrite are considerably affected by the progress of ferrite formation in the microstructures. The hardness of ferrite diminished sharply from 352 to 217 HV5g with the change in volume fraction of ferrite from 3 to 13 %, beyond which it increased abnormally to 245 HV5g with a further marginal increase in volume fraction to 15 %. The hardness of martensite too varied significantly depending on the progress of ferrite formation in the intercritical range. At the beginning, the martensite microhardness increased with the increase in volume fraction of ferrite until reaching a peak hardness value that was significantly higher than that of the direct water-quenched (DWQ) fully martensitic microstructure. The hardness dropped with further increase in volume fraction of ferrite as a result of longer holding times. These observations are rationalized in part to the mutual interaction of martensite with ferrite, and the variation of martensite and ferrite alloying concentrations, developed as a result of ferrite formation during intercritical holding at 720 ºC

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