Crucibles for Induction Melting
Classification and Quality Requirements of Crucibles
1. Classification of Crucibles
A crucible is an important component of an induction furnace. In addition to holding molten steel during melting, it also provides heat insulation, electrical insulation and energy transfer.
The material of the crucible must not only meet metallurgical requirements, but also have certain electrical properties. Crucibles used for induction melting are classified according to their material and manufacturing method.
Classification by Material
According to the chemical properties of the refractory materials used to make crucibles, crucibles can be divided into three types: alkaline crucibles, acidic crucibles and neutral crucibles. These three types are used for different purposes.
Alkaline Crucibles
Alkaline crucibles are made from alkaline oxides. The oxides used for making alkaline crucibles include CaO, MgO, ZrO₂, BeO and ThO₂.
Among them, except for MgO, which is widely used, the other oxides are generally limited to small crucibles for experimental use because of their high cost. Alkaline crucibles can be used for melting various steels and alloys, making them the most widely used type of crucible. At present, the largest alkaline crucible has a capacity of 32 tons.
Acidic Crucibles
Acidic crucibles are made from acidic oxides. The only oxide used for making acidic crucibles is SiO₂. Almost all acidic crucibles are made from refractory materials mainly composed of SiO₂.
Acidic crucibles account for a certain proportion in steelmaking production. They are especially used in large induction furnaces with capacities of 5 to 30 tons. For melting cast iron, acidic crucibles are used entirely.
Neutral Crucibles
Neutral crucibles are made from neutral oxides or composite compounds, such as Al₂O₃, MgO·Al₂O₃, ZrO₂·SiO₂ and similar materials. Crucibles made from graphite also belong to this category.
The most widely used neutral crucibles are made from compounds such as Al₂O₃ and MgO·Al₂O₃. They are mostly used for melting high-alloy steel and alloys.
Classification by Manufacturing Method
According to the size of the crucible capacity, the manufacturing methods also differ. Based on the forming method, crucibles can be divided into three types: externally formed prefabricated crucibles, internally formed crucibles and brick-lined crucibles.
Externally Formed Prefabricated Crucibles
In this type of crucible, forming and drying are carried out outside the furnace. The sand material is placed into a mold and pressed into shape.
This method can only be used to produce small-capacity crucibles. At present, in China, crucibles with a capacity below 200 kg can be produced by this method. These crucibles are easy to manage and replace, and are suitable for laboratory use.
Internally Formed Crucibles
Internally formed crucibles are made by tamping the crucible lining inside the furnace using dry ramming methods. At present, most crucibles with capacities from 100 kg to several dozen tons belong to this type.
Brick-Lined Crucibles
Brick-lined crucibles are made by building special refractory bricks and filling materials inside the furnace to form a crucible with a certain capacity.
The capacity of this type of brick-lined crucible is usually above 5 tons. Most of them are built with special refractory bricks such as magnesia bricks. Brick-lined crucibles are easy to manufacture, save materials and are suitable for large crucibles. At present, the largest induction furnace used for steelmaking, with a capacity of 60 tons, adopts a brick-lined acidic crucible.
2. Quality Requirements for Crucibles
According to the working conditions of crucibles during induction melting, there are a series of quality requirements. These requirements must be satisfied through material selection and manufacturing processes, so that an ideal crucible can be obtained, ensuring smooth melting operations and high-quality products.
The specific quality requirements for crucibles include the following five aspects.
Refractoriness
Crucible products should have high refractoriness to prevent the crucible material from forming a liquid phase too early, which would cause crucible melting loss and reduce the maximum allowable working temperature of the crucible.
Different steel grades and melting processes have different requirements for working temperature. For steelmaking crucibles, the required temperature is 1700°C; for melting iron-based alloys, it is 1600°C; and for melting cast iron, it is 1500°C.
The allowable maximum working temperatures of commonly used crucible materials are usually listed according to material type.
