Silicon nitride tools for hot rolling of high-alloyed steel and superalloy wires – load analysis and first practical tests

Abstract

Abstract For hot rolling wires of high-alloyed steels or superalloys tools are nowadays made of cemented carbides. In service they suffer from roughening of the surfaces and severe wear, which deteriorates the surface quality of the wires and restricts the lifetime of the tool. Due to their high hardness and good high-temperature properties, improvements of tool behaviour can be expected by the use of ceramic. In this paper the suitability of silicon nitride as material for rolls is investigated. The thermal and mechanical loads in silicon nitride rolls during the hot rolling of steel and superalloy wires are analysed. Although the working temperature can be up to 1100 °C the tensile thermal stresses in the rolls reach only a few percent of the materials strength. But mechanical stresses due to contact stresses may become severe. When rolling wires of superalloys tensile contact stresses in the rolls can reach up to 600 MPa – about 60 % of the characteristic bending strength of the silicon nitride material. Experiments in the rolling mill of Boehler in Kapfenberg confirm these theoretical findings. When rolling high-speed tool steels the silicon nitride rolls were superior to the common hard metal rolls. But when rolling superalloys cracks in the rolls arise. For less demanding applications (driving rollers, guiding rollers) silicon nitride rolls are still routinely used by Boehler in Kapfenberg. In summary, silicon nitride ceramics are well suited as tool material for rolling steel wires, if the rolls are properly manufactured and used. For rolling superalloy wires the ceramic material is at its limit, and a safe operation can only be expected for rolls with a material-based design.