What are the quality requirements for crucibles used in medium frequency induction heating furnace?

As we all know, when metal is smelted in a medium frequency induction heating furnace, a crucible is required. But what kind of crucible is better? Today, Zhengzhou Gou's technical staff will talk about the quality requirements for crucibles used in medium frequency induction heating furnaces.

Quality requirements for crucibles used in medium frequency induction heating furnace

1.Refractoriness

Crucible products should have high refractoriness to avoid the premature emergence of liquid phase inside the crucible material, causing melting loss of the crucible and lowering the maximum allowable working temperature of the crucible. Generally speaking, the refractoriness of crucibles for steelmaking is 1700°C, that for smelting nickel-based alloys is 1600°C, and that for smelting cast iron is 1500°C.

2.Physical and chemical stability

The refractory materials used to make crucibles are required to have excellent resistance to physical and chemical reactions.

3. Resistant to chemical attack by slag

The greater the difference between the alkalinity level of the crucible material and the alkalinity composition of the slag, the more susceptible the crucible is to erosion. Most crucible damage is related to chemical attack. Currently, it can only be controlled by adjusting the fluidity of the slag.

4. Resistant to chemical attack by molten steel

Elements in the molten steel such as C, Al, Ti, B, Re, etc. will react with the crucible material under the action of high temperature and vacuum, causing the crucible to be corroded. The stability of refractory materials under vacuum has a great relationship with the purity of their materials. The higher the purity, the smaller the weight loss.。

5. Thermal shock stability

During use, the crucible undergoes heating and cooling periodically, resulting in volume expansion and contraction. At this time, compressive stress will occur inside the crucible, which will cause cracks under the repeated action of stress. If the crack develops, the crucible will be destroyed.

Factors that affect the resistance of crucible products to rapid cooling and rapid heating include the ratio of crucible sand materials and the density of the knots. Expansion coefficient of refractory materials, etc. In order to extend the life of the crucible, the crucible products are required to have good thermal shock stability.

6.High temperature strength

During the smelting process, the crucible itself is subjected to various forces, including: the impact of the charge on the crucible, the static pressure of the molten steel, the force during electromagnetic stirring motion, the stress of the temperature difference between the inside and outside of the crucible, and the internal stress caused by rapid cooling and rapid heating changes. wait. Due to the action of these forces, the crucible must have a certain strength at high temperatures and room temperature to prevent cracking. The high temperature strength of the crucible is related to the type of refractory material, molding pressure, sintering process, sand particle size ratio and other factors. Therefore, measures must be taken from all aspects to increase the high temperature strength of the crucible to ensure the normal use of the crucible.

7.Insulation performance

Since there is a certain voltage difference between the molten steel and the induction coil (usually tens to hundreds of volts), the crucible material must have certain insulation properties to avoid breakdown. However, the insulation resistance of the crucible material cannot be too large, otherwise it will affect the electrical efficiency of the furnace. Because part of the magnetic energy will be lost when the magnetic field lines pass through the crucible wall, the smaller the resistance of the crucible wall, the smaller the loss when the magnetic field lines pass through. Therefore, on the premise of ensuring insulation, the specific resistivity of the crucible material should be as small as possible, so that it can not only ensure insulation but also meet the requirements of improving electrical efficiency.

The specific resistivity of commonly used crucible materials decreases with increasing temperature. The specific resistivity value in the steelmaking temperature range is 102~104 ohm.cm. The purity of the crucible material has the most obvious impact on insulation and specific resistivity. Among them, ferromagnetic substances such as Fe3O4, Fe2O3, etc. can significantly reduce the insulation and specific resistivity. Other ingredients that form low melting point compounds also serve the same purpose. In order to ensure that the crucible works safely at high temperatures, the requirements for insulation performance should be given top priority, and the electrical efficiency can be achieved by adjusting the thickness of the crucible wall to achieve the desired effect. Therefore, when actually selecting the crucible material, its purity is required to be as high as possible to ensure that the crucible has good insulation properties at high temperatures.

8. Thermal conductivity

During the smelting process, the temperature difference between the inside and outside of the crucible is very large, with the maximum temperature reaching 1400~1600°C. According to heat balance calculation, about 10~15% of the heat is lost outward from the crucible wall. Reducing this heat loss can improve the resistivity. Therefore, the crucible material is required to have high insulation properties, that is, the thermal conductivity of the crucible material should be low. Commonly used crucible materials have a thermal conductivity of about 1 to 2 kcal/m at smelting temperature. Refractory materials with too high thermal conductivity are not suitable as crucible materials due to large thermal conduction losses.

9. No pollution, no pollution and low cost.

The crucible material will evaporate under the action of high temperature and high vacuum. The volatile matter should not pollute the environment and be harmless to human health. Some crucible materials, such as beryllium oxide, can produce beryllium-containing dust or beryllium vapor which are harmful to health when used. Sometimes some refractory materials contain radioactive elements, which can pollute the environment and affect health.

The crucible is a consumable item and accounts for a certain proportion of the production cost. When selecting crucible materials for production, it is necessary to choose refractory materials that meet quality requirements and are cheap.