Effects of secondary carbides in new hot work tool steels

Abstract

Hot-work steels are used for tools which shape metal products into its final form by plastic deformation at high temperatures. Tool microstructures therefore are composed of quenched and tempered martensite, and required properties are mainly governed by the high-temperature resistance to tempering. In the case of some particular applications where moulds or blanks are subject to crack formation, toughness properties are also important, depending upon final hardness and secondary hardening events controlled by the tempering temperature. This project will investigate the properties of hot work steels based upon the traditional H specifications, but after some specific modifications in chemical composition are introduced with respect to Si, P, Cr and Mo contents. During industrial development, Si and P have shown important effects on temper brittleness at high temperatures, which will be evaluated in the present project. Higher Mo and lower Cr contents will also be investigated because Mo tends to increase the tempering resistance by forming secondary carbides of improved stability while, at the same time, lower Cr contents were shown to magnify this effect of Mo. Mechanical properties will be determined as a function of tempering temperature, by measuring the loss of high-temperature hardness as a function of time, and by applying impact as well as creep tests. All the results obtained by mechanical testing will be correlated with microstructural observation, mainly relying upon transmission electron microscopy in order to analyze secondary carbides and, if possible, the interphase segregation of P. (AU)