M3C and M7C3 Carbide Precipitation in Modified H11 Tool Steel

Abstract

Recent modifications in chemical composition have been applied commercially to high alloy tool steels, using different combinations of Cr, Si and Mo contents. Several reports have been published in the literature about the effects of such modifications on mechanical properties and tool performance, but only a few of these studies were concerned with the effects on secondary carbide formation. In previous papers, improvements in toughness and tempering resistance that were found in a 5% Cr tool steel (type H11 with lower Si contents) have been attributed to particular distributions of Cr-rich M7C3particles. Although M7C3carbides have been studied extensively in low alloy steels, some important differences have now been observed by the present authors for high alloy tool steels, especially regarding the effects of Si and Cr. The present work is concerned with the formation of Cr-rich M7C3as well as Fe-rich M3C particles in modified H11 tool steels, discussing the precipitation sequence and particle distributions developed during tempering within the martensite microstructure. By means of transmission electron microscopy, the effect of Si on M3C cementite formation has been found to be responsible for a substantial change in the distribution of the M7C3carbide phase, leading to a concentration of these particles at high energy interfaces in interlath and interpackage regions.