Effect of niobium on primary carbides of hypereutectic high chromium cast iron

Abstract

The effect of niobium on the primary M₇C₃ carbides of a hypereutectic high chromium cast iron containing 4·0 wt-%C and 20·0 wt-%Cr was studied by means of optical microscopy, scanning electron microscope, X-ray diffraction, water quenching, and energy dispersive X-ray spectrometry. The primary carbides were refined as niobium content increased. Theoretical calculation and the experimental results indicate that NbC particles form before the precipitation of primary carbides when niobium is added to the alloy. The first precipitated NbC particles lead to a reduction of the hypereutectic degree and the solidification temperature range of the alloy and, therefore, lead to finer hypereutectic primary carbides. The first precipitated NbC particles can act as heterogeneous nuclei of the primary M₇C₃ carbides, which also results in the refinement of the primary carbides.