Secondary carbide precipitation in an 18 wt%Cr-1 wt% Mo white iron

Abstract

High chromium (18%) white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The austenite is destabilized by a hightemperature heat treatment which precipitates chromium-rich secondary carbides. In the as-cast condition the eutectic M7Ca3 carbides are surrounded by a thin layer of martensite and in some instances an adjacent thicker layer of lath martensite. The initial secondary carbide precipitation occurs on sub-grain boundaries during cooling of the as-cast alloy. After a short time (0.25 h) at the destabilization temperature of 1273 K, cuboidal M23C6 precipitates within the austenite matrix with the cube-cube orientation relationship. After the normal period of 4 h at 1273 K, there is a mixture of M23C6 and M7C3 secondary carbides and the austenite is sufficiently depleted in chromium and carbon to transform substantially to martensite on cooling to room temperature.