Hot friction and wear behaviour of high speed steel and high chromium iron for rolls

The wear resistance at high temperature of different high speed steels (HSS) for rolls is evaluated on the basis of a tribological test aimed at reproducing the damage mechanisms occur during hot rolling. The test is positively used to highlight the different wear and friction behaviour of high speed steels and high chromium irons to be used in the early finishing stands of hot strip mill. The microstructure of these materials is given by a tempered martensitic matrix surrounded by an interconnected network of primary and eutectic carbides. In this paper a closer look into the properties of high speed steels is proposed. A wide range of microstructural conditions was obtained by changing the chemical composition and even by tempering the steels at different temperatures, in order to obtain different matrix microhardnesses. The wear behaviour can be explained on the basis of the operating mechanism, given by the combination of abrasion, triboxidation and by adhesion. While abrasion rules the tribological behaviour of HSS, triboxidation becomes more and more decisive in controlling the behaviour of HiCr irons. In these alloys the friction coefficient shows a transition (not observed in HSS), which was ascribed to the transition form a metal-oxide to an oxide-oxide contact, i.e., to the formation of wear protective layers. The tribological behaviour was correlated to the microstructure.