Coreless induction furnaces have the advantages of high efficiency, energy saving, low pollution, easy adjustment of composition, easy control of atmosphere, strong heating capacity, intermittent operation, etc., and are widely used in steelmaking, casting, non-ferrous metal smelting precision alloy production. Induction furnaces are divided into power frequency induction furnaces (within 50Hz) according to different frequencies; Medium frequency induction furnace (50Hz-10000Hz) and high frequency induction furnace (10000Hz or more). In recent years, with the development of high-power thyristor variable frequency power supply and the improvement of reliability, intermediate frequency furnace gradually replaces the power frequency furnace. Compared with the power frequency furnace, the intermediate frequency furnace has the advantages of high thermal and electrical efficiency, short melting time, low power consumption and easy automation. And the induction furnace to the direction of large capacity, high power, there are higher requirements for refractory materials.
Refractory lining is an important factor that determines the output of induction furnace, casting quality and safety and reliability of production and operation. To obtain refractory lining with good quality and long life, we must first understand the use conditions: (1) the thickness of refractory lining is thin, and the temperature gradient of the lining is large; (2) Electromagnetic stirring metal liquid in the furnace, resulting in mechanical erosion of the lining refractory material; (3) The refractory lining has repeated acute cooling and thermal shock.
Therefore, the selected refractories must have: high enough refractoriness and load softening temperature; Good thermal shock stability; No chemical reaction with metal and slag; Has a certain high temperature mechanical strength; Good insulation and heat insulation; There are better construction, high filling density, easy sintering, easy maintenance; Refractory raw materials are rich in resources and low in price. The development of induction furnace is closely related to the progress of refractory technology, and the design of large-scale power frequency crucible induction furnace is often started from the selection of refractory materials and furnace lining simulation test. In any case, the selection of furnace lining refractories is mainly based on the utilization rate and economy of the furnace. For the sake of tight coupling on the appliance, the thinner the lining thickness is, the better, without affecting the life of the furnace.
1. The structure of coreless induction furnace and the construction method of refractory lining
1) Lining structure
The coil made of copper tube is outside the lining. The traditional lining structure is to lay a layer of mica paper next to the coil, followed by an alarm electrode, followed by a layer of asbestos sheet (cloth), and then a dry ramming lining. In the process of baking and use, the dry ramming lining gradually forms the sintered layer, the transition layer and the unsintered layer, and the reasonable three-layer structure thickness is one of the important factors to determine the life of the lining. In order to meet the bottom-top type furnace removal method, the coil insulation cement appeared, and the cement replaced the mica layer, adding a layer of protective barrier to the coil. The mastic layer is generally thicker than the mica layer, but the thermal conductivity is higher than that of mica, so the lining structure containing mastic is called an improved structure. Composite lining structure is to meet the needs of some metal melting, with pre-fired crucible and filler instead of ramming lining, this structure solves the problem of dry material difficult to sintering, while improving the serious corrosion of some metal molten liquid or slag on the lining.
2) Types of refractory materials for coreless induction furnaces and construction methods of lining