Vacuum heat treatment of soft magnetic materials
The raw materials for soft magnetic alloys are mainly rods, strips and plates, and the parts are cut, punched, stamped or rolled in a cold process. During the process, the original grain structure of the material will be destroyed, resulting in a decrease in the permeability of the material, and the processing stresses generated during the process will lead to dimensional instability of the part. Therefore, parts made of soft magnetic alloys must be annealed after processing and forming to regain regular equiaxed grain structure and reduce the content of impurities in the material to restore and improve the magnetic properties of the material, while eliminating processing stresses and stabilizing the size of the part. Depending on the material and use requirements, soft magnetic alloy parts can be annealed at high temperature (1150-1250 ℃) and medium temperature annealing (800-850 ℃). For high permeability materials, Permalloy is usually annealed at high temperatures, sometimes with magnetic field annealing to further improve the permeability [ 1 ]; while for transformers, inductors or motors, stacked or rolled silicon steel sheets (usually called cores) are annealed at medium temperatures [ 2 ]. Before the vacuum furnace was available, many manufacturers used a well-type external heating atmosphere furnace with nitrogen or hydrogen protection, which had obvious drawbacks, especially when nitrogen was used as the protective atmosphere. First of all, when using nitrogen protection, the purity of the atmosphere obtained by nitrogen replacement in the furnace is not sufficient, and the flow of nitrogen has a large impact on the surface quality of the workpiece. This is due to the existence of areas of poor nitrogen flow in the furnace and slight oxidation of some workpieces. Secondly, the temperature uniformity of the external heating furnace is poor, and the performance of the treated parts varies greatly and the qualification rate is low. In addition, in terms of energy consumption, the thicker lining of the external-heat furnace consumes more power on the lining during the heating process, resulting in low thermal efficiency; the heating process requires constant charging of high-purity nitrogen, resulting in high nitrogen consumption; and the low service life of the heat-resistant steel tank at high temperatures, resulting in high operating costs for the external-heat furnace. If hydrogen is used as the protective atmosphere, the performance and surface quality of the workpiece can be improved by the strong decarburization and reduction effect of hydrogen, but the problems of uneven temperature and high operating costs are still inevitable. The use of endothermic vacuum annealing furnaces is of great importance for the annealing of soft magnetic alloy parts. The endothermic vacuum furnace has obvious advantages in terms of workpiece processing results, operating costs, productivity and production environment.