Fe-C-Cr-Nb合金の凝固組織

抄録

The freezing processes and solidification structures of Fe-5%Cr and Fe-15%Cr alloys with 0.5 to 4.7%C and 0 to 7.5%Nb were investigated metallographically. The distribution of alloying elements were also studied by use of E.P.M.A. to clarify the behavior of elements during solidification. The constitutional diagrams for liquidus surfaces of Fe-5%Cr-base alloys and Fe-15%Cr-base alloys were established. In the 5%Cr system primary δ, γ, NbC and M₃C phases and eutectic reactions of Liquid→(γ+NbC), Liquid→(γ+M₃C) and Liquid→(γ+NbC+M₃C) appeared. Primary δ, γ, NbC and M₇C₃ phases and eutectic reactions of Liquid→(γ+NbC), Liquid→(γ+M₇C₃) and Liquid→(γ+NbC+M₇C₃) occurred in the 15%Cr system. Primary niobium carbides grew dendritically and eutectic niobium carbides crystallized in the shape of petal. The amount of NbC increased and that of M₃C and M₇C₃ decreased with increasing niobium content of alloy. During solidification niobium was preferentially partitioned to primary and eutectic NbC, and chromium to M₇C₃ crystals. The partition ratios of niobium and chromium to primary γ were 0.02 to 0.01 and 0.93 to 0.72, respectively. They decreased with the increase in the carbon content of alloy. The addition of 0.3%Ti to the melt nodularized both primary and eutectic niobium carbides, because many tiny titanium carbide particles precipitated at higher temperatures nucleated the niobium carbides.