Dependence of microstructure on the cooling rate for a modified austenite medium Mn steel

Abstract

An austenite medium Mn steel reinforced with well distributed γ -(Fe, Mn)3C granular eutectics (abbreviated EAMC) has been prepared by modifying with Si-containing combined agent [1]. The experiment observations and theoretical analysis indicate that the pseudo-eutectics (non-eutectic composition) are formed in the liquid pockets among the primary austenite dendrites at the later stage of solidification. The previous investigation indicates that the cooling rate has notable effect on the microstructure of EAMC, as well as the scales of eutectics such as the volume fraction ( f ) and diameter (d), and the appearance of others phases such as the pearlite and carbides. In the directional solidification experiments, one may study precisely the nucleation time and position of each phase and the dependence of microstructure on growth rate (cooling rate). Therefore, directional solidification of EAMC has been carried out using Vertical Bridgman method. The microstructure scales of the eutectics as a function of growth rate (V ) is described and discussed in the present work. The EAMC master samples modified with the controlled αk were prepared by melting weighed quantities of pig iron, steel scraps, Fe-Mn alloy (Fe-80%Mn1.0%Si-1.2%C), and Ca-Si agent (Fe-28%Ca-65%Si2.4%Al-0.8%C). After allowing time for melt homogenization, molten alloy was poured into the prepared bar molds (100 mm in length 7 mm ID and 8.5 mm OD). The chemical composition of the bars is presented (wt%) as follows: 1.25C, 1.06Si, 6.60Mn, 1.06Al, 0.10Ca, 0.02S, 0.02P, and balance Fe. Directional solidification was performed using a high temperature gradient Bridgman-type unit in the protective argon atmosphere. Detailed descriptions of the unit were given elsewhere [2]. Specimens were directionally solidified at different growth rate (2.18– 500 μms−1) with a constant temperature gradient about 800 Kcm−1. They were pulled rapidly into Ga-In reservoir after 30 mm steady growth. Samples were ground, polished, and then etched in a solution of 4 ml HNO3 and 96 ml C2H5OH. A video-image digital analysis system (VIDAS) and Vickers durometer (VD) were used in the work. VD analyses indicate that the microhardness of eutectic (Hv 560–800) is less than that of carbide (Hv 1200–1400). VIDAS observations show that the color of eutectic is white, but that of carbide is black in the optical. Thus, the eutectic and carbide can be differentiated by color and/or microhardness. The volume fraction of eutectics ( f ) can be gained directly by using VIDAS. The diameter (d) of eutectic is