An unusual structure of an As-cast 30% Cr alloy white iron

Abstract

An unusual microstructure of an as-cast 30 wt% Cr and 2.26 wt% C iron has been examined by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the microstructure varied with positions in the cast bar. In the upper part of the cast bar where the cooling rate was faster, the matrix was essentially austenite with some patches of ferrite-plus-precipitated carbides as dendritic regions. Whereas, the central and the lower parts of the cast bar where cooling rates were slower, there was less austenite with greater amount of ferritic zones. The microstructure of core regions at the centre of the dendritic ferrite-plus-precipitated carbides zones contained interconnected carbides that are believed to be the product of a peritectic reaction. TEM examination confirmed that these peritectic carbides were M7C3 type. Bainite and martensite were also observed in the transition zones close to the ferrite-plus-precipitated carbides zones. These were believed to result from solid-state decomposition of the dendritic austenite in the later stage of cooling.