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Annealing of Large Silicon Steel Laminated Cores and Silicon Steel Strip Wound Cores
Silicon steel is steel with a silicon content of 0.8% to 4.8%. In the transformer manufacturing industry, a large number of iron cores made of silicon steel (usually containing 2.8% to 4.8% of silicon) are used as the magnetic conductive components of the transformer. When the transformer is working, the power loss caused by the heating of the iron core is called “iron loss”, which is one of the important indicators of the transformer. The heating of the iron core is mainly caused by the reluctance of the iron core itself and the “eddy current” generated under the alternating magnetic field. The purpose of laminating or winding the iron core is to reduce the influence of “eddy current”, and the magnetic resistance of the iron core depends on the magnetic permeability of the iron core. In the process of cold working such as punching, shearing or winding of silicon steel sheets, the original grain structure of the material will be destroyed, the magnetic orientation of the material will be changed, and the no-load loss of the transformer will be increased by 5% to 30%. Therefore, the ideal must be regained by annealing. The fibrous structure restores the original magnetic properties of the material.
After the transformer core is laminated or wound, it needs to be clamped by a clamp and then loaded into the furnace for processing. Usually, the weight of an iron core ranges from 200 to 2000kg, and each furnace needs to process multiple iron cores to reduce production costs. Therefore, the vacuum furnace used for iron core annealing needs to have a large furnace capacity.
Due to the large volume of the transformer core, the large weight and the large section size, the design of the annealing furnace and the preparation of the annealing process have some special points compared with the conventional furnace. Due to the large cross-sectional size of the iron core, the thermal conductivity is poor, and the volume of the vacuum furnace is also large, so the overall heat storage is large. In terms of heating, in addition to the consideration of heating power, a heater with a large radiation area and high efficiency should be used. The vacuum furnace usually used for annealing treatment has no special requirements for cooling speed, but from the perspective of improving production efficiency, the core annealing furnace should have a certain cooling capacity.
In the annealing process, the basic principle of the heating stage is to ensure the penetration, that is, to ensure that the core temperature of the workpiece is consistent with the surface temperature. Vacuum heating is used below 300 °C, the main purpose is to remove most of the impurity gas through baking and evacuation. From 300 to 450 ℃, nitrogen filling is used to protect the heating, increase the heating speed, and reduce the temperature difference between the inside and outside of the workpiece. After 450 ℃, vacuum heating is adopted, the heating speed is controlled, and the heat preservation platform is set to ensure the uniform heating of the workpiece as a whole. After the temperature is kept at 810°C, it enters the controlled cooling stage. At this time, the atomic diffusion ability inside the material is strong, and the grain repair begins, and the lattice distortion is gradually eliminated. After the temperature drops to 550°C, high-purity nitrogen can be charged to accelerate the cooling process, and the time to turn on the cooling fan can be determined according to production needs.
After the silicon steel core is vacuum annealed by the above process, all the indicators meet or exceed the national standard, the surface maintains the original quality of the silicon steel sheet, the performance of the same furnace is completely consistent, and there is no rework or scrapping. In addition, compared with the external heating annealing furnace, the internal heating vacuum furnace has higher thermal efficiency, and the heat-resistant steel fixture of the iron core will not be lost due to oxidation, which greatly reduces the production cost and has obvious economic benefits.
Equipment selection: The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.