Tribological behaviour of hot rolling rolls

The study of wear mechanisms represents a very important task for the development of roll materials, with improved hot tribological properties. In the present paper the wear behaviour of different materials, i.e., high alloy steels (high speed steels (HSS)) and cast irons (high chromium irons (HiCr) and indefinite chill irons (IC)), produced by centrifugal casting, has been comparatively evaluated by means of a rolling sliding hot wear test. A disc specimen of C40 plain carbon steel, induction heated up to 700 °C, is allowed to rotate against a high alloy steel/iron at 200 rpm, corresponding to a slip rate of 28%, for a total period of time comprised between 1 and 3 h. A maximum contact hertzian stress of 300 MPa was imposed. The wear rates have been evaluated by weight measurements of the specimen, before and after each test period. The surface roughness was also determined as representative for the roll profile retention during service. The friction was continuously monitored. The wear mechanism is given by the combination of abrasion and oxidation. High speed steels show remarkable lower wear rates than high chromium and indefinite chill irons, because of their higher hot hardness. The retention of high hardness at elevated temperature is very important to reduce abrasive wear and also to support the protective oxide layer which develops on the surface of the roll material. As a general rule the roughness at the end of test tends to increase with increasing wear rate, as demonstrated by the relatively soft indefinite chill iron. However, the oxidation resistance also influences the final roughness, higher roughness values being observed for poorly oxidized materials. A simplified model explaining this experimental evidence is proposed.