Hardness increases due to (Fe, Cr)2C carbide precipitated during natural aging in high chromium cast iron

The effect of natural aging on the microstructure of carbides and martensite in high chromium cast iron subjected to a destabilizing heat treatment was investigated by correlative experimental and theoretical methods. It was observed for the first time that fine η-(Fe, Cr)2C carbides precipitated from the martensite matrix in water-quenched high chromium cast iron during natural room temperature aging, which led to a remarkable hardness increase (by 3.4 HRC). On one hand, the obvious hardness increase was attributed to the fine precipitation of M2C carbides which was observed by transmission electron microscopy. On the other hand, it is also caused by high Cr content in the η-(Fe, Cr)2C carbides, which was confirmed by first-principles calculations.