1993 Volume 34 Issue 1 Pages 69-75
It was found in the previous studies that Ni–Cr–Fe overlay weld alloys containing NbC particles, formed by a plasma transferred arc welding process, had excellent corrosion and wear resistances. Clarification of solidification process of alloys is essential to make clear the mechanism of such improvement of properties. In this study, the solidification structures of alloys containing 5∼60 mass%Fe and 40 vol%NbC have been examined by means of SEM, TEM, EPMA and XRD.
A part of the NbC particles dissolved in a molten alloy caused the crystallization of NbC and M23C6. The morphologies and amounts of these carbides changed largely at 20 mass%Fe. As the Fe content of alloy increased to 20 mass%Fe, the amount of NbC increased and that of M23C6 decreased. However, the amount of carbide deposited was almost constant above 20 mass%Fe. The observations of microstructure revealed that solidification proceeded in the sequence, primary NbC→eutectic(γ+NbC)→eutectic(γ+NbC+M23C6) in alloys with more than 20 mass%Fe. In alloys with iron below 20 mass%, the solidification occurred in the order, primary NbC→primary M23C6→eutectic (γ+M23C6)→eutectic (γ+M23C6+NbC)→eutectic(γ+NbC)→eutectic(γ+α+NbC). The difference in the crystallization manner suggested that the free energy for the formation of carbide should change depending upon the activities of Cr and Nb contents in the molten alloy.
