Microstructure Evolution in Medium Mn, High Al Low-Density Steel During Different Continuous Cooling Regimes

Abstract

The objective of the present work is to investigate the microstructure evolution in Fe-12 Mn-13 Al-4.8 Ni-0.8 C wt.% steel after two-phase heat treatment, followed by different cooling regimes. The alloy was homogenized at 1200 ℃ for 3 h followed by forging and water quenching to room temperature. Thereafter it was heat-treated in a two-phase region and cooled to room temperature through different cooling modes viz. water quenching (WQ), air cooling (AC) and furnace cooling (FC). The microstructure of the heat-treated and cooled samples was investigated through optical microscopy, scanning electron microscope (SEM), energy dispersive spectrometry (EDS), X-ray diffractometry (XRD) and electron back-scattered diffraction (EBSD). The WQ sample was found to have a duplex microstructure consisting primarily of δ-ferrite and austenite, however, a minor α-ferrite (fine grain) phase was also detected inside austenite. In the AC sample, a small amount of α-ferrite and intragranular k-carbide was formed, along with δ-ferrite and austenite as the major phases. The α-ferrite formed within austenite in the WQ and AC samples was found to nucleate and grow at austenite twin boundaries and triple junctions. While in the FC sample, a significant amount of α-ferrite and k-carbide was found to form due to the decomposition of austenite. Interestingly two types of k-carbide viz. modulated intragranular type and lamellar type were found to form in the FC sample.