Solidification structure of Hypereutectic cast chromium alloy

The whole contour of eutectic structure in hypereutectic chromium cast iron is very similar to that of Hypoeutectic and eutectic chromium white cast iron. The difference is that the primary phase is no longer austenite, but hexagonal primary carbide. Eutectic carbides grow continuously on primary carbides, and there is some connection between them, but there is no relationship between eutectic carbides and primary carbides in some sections. Because the chromium alloy with high C content is hypereutectic structure, that is, the primary carbide is surrounded by eutectic structure, the lamellar eutectic structure will prevent the crack from extending along the grain boundary, so the crack can be reduced. Chromium cast iron with hypereutectic composition is usually the preferred alloy for many surfacing applications because of its high C content, more hard carbides in hypereutectic and eutectic, and most of them are (Fe, Cr) 7C3 type with the highest hardness.

However, hypereutectic high chromium cast iron is generally not popular with the producers. The main reason is that coarse eutectic carbides and large primary carbides lead to more casting waste and higher scrap rate. When the carbon equivalent (CE) of chromium alloys exceeds the eutectic point, massive primary carbides will appear, and the volume fraction of carbides will increase significantly, while the volume fraction of matrix will decrease. These primary (Fe, Cr) 7C3 carbides will also agglomerate to a certain extent, which is not conducive to the improvement of the mechanical properties of cast iron, and will lead to a sharp decline in the impact toughness of the alloy, so that the strength of hypereutectic high chromium cast iron will be reduced. If hypereutectic high chromium cast iron wants to obtain high volume fraction of carbides as well as impact toughness and wear resistance, it needs to obtain fine primary carbides in the structure.

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