Failure analysis of silicon nitride rings for hot rolling of highly alloyed steel and superalloy wires

Rolls for rolling of metal wires are traditionally made from cemented carbides, but they suffer from roughening of the working surfaces and severe wear which restrict the roll lifetimes. Silicon nitride ceramics are feasible alternative materials for rolls (and other components in rolling mills) due to their good combination of properties, e.g. high strength and hardness, excellent high-temperature properties, oxidation resistance and good thermal shock behaviour. Increasingly the switch is being made to such ceramics for extension of life and improvements in surface quality of the rolled product.In a collaborative project between universities, industrial partners and research centres, rolls made of silicon nitride, supported and clamped by steel rings were tested at the rolling mill of Böhler-Edelstahl in Kapfenberg, Austria. Under low and medium hard loading conditions the suitability of these composite rolls has been very satisfactorily demonstrated. However, under very severe loading conditions one pair of silicon nitride rings failed. In this paper, the failure of these ceramic rings is analysed, and suggestions for further improvements are presented.