Study On Compound Modification And Heat Treatment Of High-speed Steel Used For Rolls

The casting high-speed steel (HSS) rolls contain high carbon and a large amount of alloy elements, which results in different kinds of carbides. During centrifugal casting process, the carbides would segregate due to density variation, so the properties of the rolls would become unhomogeneous radially. Meanwhile, some carbides distribute along the boundary of austenite grains while they are precipitating from the liquid phase, which leads to the emergence of continuous network of carbides at room temperature, and this would cause a severe decline of rolls' properties. Thus it is an efficient approach to improve rolls' properties by preventing segregation and breaking the network of carbides distributing along the grain boundaries. And reasonable selection of materials, modification and heat treatment could save a large number of roll materials, reduce production cost of rolling and improve quality and output of rolls.In view of the problem existing in manufacturing technology of centrifugal casting HSS rolls, the effects of chemical composition, modification treatment and heat treatment of HSS on the microstructure and mechanical properties were investigated in this thesis. Due to its high density and severe segregation, the amount of tungsten was reduced in component design and high-speed steel containing high molybdenum and vanadium had been developed. The effect of K/Na, Mo and V compound modification on the microstructure of HSS was investigated utilizing optical microscope and Hi-scope video microscope (HSVM). The results showed that the grain size was refined, the primary austenite became equiaxed and the morphology of carbides was changed greatly after compound modification. The network of eutectic carbides was broken, many massive and granular carbides were produced and distributed homogeneously. The high-speed steel modified by 1.0%K/Na-Mo-V has the best structure.In this thesis, the effects of quenching temperature, cooling pattern, temper temperature and times have also been researched. In conditions of oil cooling, salt bath and air cooling, the hardness of high-speed steel increased along with the increasing of quenching temperature under 1025℃, and the variation above 1075℃had an opposite tendency. In condition of salt bath, the high-speed steel had high hardenability and was not prone to cracking. And the optimal quenching was salt bath hardening from 1050～1075℃. In this condition, the maximum tempering hardness was obtained at 525℃. Second temper was able to satisfy the requirement of HSS roll, while the hardness decreased after third temper. It was found that the optimum heat treatment was salt bath hardening thrice from 1050～1075℃for 50 minutes and tempering twice at 520～530℃for 3 to 5 hours, then air-cooling.The results of mechanical property measurement showed that, after quenching and tempering, the hardness and red-hardness of the high-speed steel modified by 1.0%K/Na-Mo-V changed a little, but the impact toughness and wear resistance were largely improved, and the good performances were gained.