The paper reports on investigations into the erosion, microstructural features, and material removal mechanisms of AISI H13 core boxes eroded by two types of core sand (silica and chromite). Different heat treatment operations are carried out, namely martempering, carbonitriding, quenching, and tempering, in order to vary hardness, microstructure, and surface morphology. Scanning electron microscopy inspections show that erosion is not only a function of surface hardness of the target material but also of its microstructure. Erosion of martensitic microstructure, consisting of very fine carbides with a uniform martensitic substructure, is much less severe than erosion of a material characterized by dispersed coarse carbide. Hardness effects on the erosion process are discussed in terms of material removal modes.