Solidification of High Chromium Cast Iron Substituted by 25 to 70 mass%Ni for Fe

抄録

In (Fe, Ni)-Cr-C alloys, with Fe replaced by 25 mass%Ni–70 mass%Ni in high-chromium cast irons, the effects of Ni content on the solidification structure, solidification sequence, and liquidus surface diagram were investigated.
The microstructures of alloys consist of matrix and M₇C₃ carbides precipitated as primary and/or eutectic crystals. They resemble general high-chromium cast iron except for mostly austenitic matrix. According to EDS analyses of alloy contents in each phase of specimens, the Cr content in M₇C₃ carbide increases concomitantly with increasing Ni content, but that in γ exhibits almost no change.
Solidification begins by crystallization of primary austenite (γ) in hypoeutectic alloy and that of primary M₇C₃ carbide in hypereutectic alloy. Either case is followed by precipitation of (γ+M₇C₃) eutectic. However, graphite precipitates from melt in the alloys with high-C or high-Ni content and its precipitation region shifts to the low-C–high-Cr side as the Ni content increases. The (γ-M₇C₃) eutectic line on the liquidus surface diagram moves strongly to the low-C side with increased Ni content, but it returns to the high-C side when Ni contents become greater than 50 mass%. The diagrams produced using thermo-Calc software resemble those constructed based on experimental results.