Material types of graphite crucibles and characteristics of graphite crucibles

1. Material types of graphite crucibles

The material of graphite crucible can be roughly divided into artificial graphite, graphite-clay material and graphite-silicon carbide material.

Artificial graphite crucible is made of graphite electrode material.

Graphite-clay crucible is mainly composed of natural flake graphite (crystal graphite) and clay. It is now only used for high temperature melting of cast iron, cast steel and small amounts of precious metals.

Graphite-silicon carbide crucible is mainly composed of natural flake graphite and silicon-carbide. Crucibles made of this material are used in most of the melting of non-ferrous metals such as copper alloys and aluminum alloys. In 1955, Nippon Crucible Co., Ltd. first developed and produced graphite-silicon carbide crucibles in Japan. Compared with graphite-clay material, this material crucible gradually replaced it because of its excellent durability. At present, the demand for graphite-silicon carbide crucibles has accounted for about 97% of the whole.

2. Characteristics of graphite crucibles

Graphite crucibles have taken the best possible measures in terms of raw materials, manufacturing, etc. to meet the use in cruel conditions. The main features are listed as follows:

2.1. High thermal conductivity: The melting time is shortened due to the use of graphite and other raw materials with high thermal conductivity;

2.2. Thermal shock resistance: strong thermal shock resistance, not easy to break for rapid cooling and rapid heating;

2.3. High heat resistance: high refractoriness, can withstand high temperature of 1200 ~ 1600 ℃;

2.4. Corrosion resistance: strong corrosion resistance to molten soup;

2.5. Mechanical impact resistance: It has a certain strength against mechanical impact (when the molten material is put in, etc.);

2.6. Oxidation resistance: Graphite is easily oxidized at high temperature in the oxidizing mist, and the oxidation consumption is less due to the anti-oxidation treatment;

2.7. Blocking resistance: Because graphite is not easy to stick to the molten soup, the penetration and adhesion of the molten soup are less;

2.8. Very little metal pollution: Because there is no impurity mixed into the molten soup, there is very little metal pollution;

2.9. Influence resistance to slag collector (slag remover).

3. Shape and capacity of graphite crucible

In Japan, the shape and capacity of the crucible were defined in the Japanese Industrial Standards (JIS) published in 1950. This is the JIS regulation for clay-graphite crucibles, and crucibles made of graphite-silicon carbide are also manufactured in accordance with this regulation.

Regarding the capacity, it is represented by the "#" specified in JIS, and 1# is defined as the capacity to melt 1KG brass. From this, it can be estimated that the molten copper in the #300 graphite crucible is about 300KG. According to the specific gravity difference of the aluminum alloy, the molten aluminum in the same graphite crucible is about 100KG.

Shapes can be roughly divided into:

3.1. Standard type: The ratio of outer diameter to height is about 0.7 ~ 0.8 : 1. The capacity is from #8 to #4500.

3.2. Long type: It is more slender than the standard type, and the ratio of outer diameter to height is about 0.6 ~ 0.7 : 1. There are #15～#620 in capacity.

3.3. Bowl type: the outer diameter is relatively large, and the ratio of diameter to height is about 1:1. The capacity is #250～#5200. The largest crucible can melt about 1.5 tons of aluminum alloy at one time.

3.4. With mouth type: It is similar to long type connected to the molten soup outflowing groove. The capacity is from #50 to #1800.

3.5. Electric furnace type: cylindrical and relatively slender, the ratio of outer diameter to height is about 0.6: 1. The capacity is from #60 to #5000.

4. The use of graphite crucible

The use of crucibles is most suitable for the melting of various types of alloys in small quantities. That is to say, if you want to change the type of alloy, you only need to exchange the graphite crucible. Other melting methods, such as reverberatory furnaces and non-crucible induction furnaces, are suitable for mass melting of a single alloy. If the type of melting alloy is changed, metal contamination will occur if the refractory lining is not replaced.

The following describes crucibles for melting copper alloys and aluminum alloys, which account for most of the graphite crucibles used.

4.1. Crucible for copper alloy melting

What is important for copper and copper alloys is rapid (rapid) melting. Because copper is easily oxidized at high temperature, if the time is prolonged, and the molten soup is oxidized, which will cause the occurrence of poor casting quality. Of course, the adjustment of the combustion gas mist of the crucible furnace is also important. From the perspective of the shape of the graphite crucible, the long shape has good heat absorption and can shorten the melting time. However, considering that the mouth is small and the long shape with a certain depth is not beneficial when scooping out the molten soup with a spoon from the fixed land. Therefore, standard crucibles that can satisfy both melting time and workability are widely used.

In recent years, with the continuous improvement of the environmental and equipment of the foundry and the continuous improvement of the degree of mechanization, the hydraulic tilting crucible furnace has also been gradually adopted. Especially in high-temperature melting and copper alloy foundries with large specific gravity, the popularity of tilting crucible furnaces is relatively high.

Electric furnace type crucibles are used in high (medium) frequency induction furnaces and low frequency induction furnaces, and dry refractory materials are filled between the inner wall of the furnace and the crucible. For high-frequency induction furnaces, graphite crucibles of about #100～#1500 are generally used, and for low-frequency induction furnaces, large crucibles above #1000 are used.

For copper alloy crucibles, special attention should be paid to the corrosion resistance of the molten soup at high temperatures of 1100°C to 1250°C and the thermal shock resistance in the high temperature mist in the furnace during the production process.

4.2. Crucibles for melting light metal alloys

In the melting of light metals, the same high proportion is the melting of aluminum alloy die-casting and aluminum alloy castings, and a part is the melting of zinc alloy.

When using a crucible to melt aluminum alloys, there are many melting methods. There is a single-furnace melting method in which cold materials are put in and the molten soup is used up at one time; there is also a melting and heat preservation method in which the molten soup is scooped out and the cold material is replenished at an appropriate time; There are also various methods such as receiving the molten soup after melting in the centralized melting furnace and only keeping it warm. Single furnace melting and melting and heat preservation method, suitable for aluminum alloy castings. Because of the absorption of H2 and the mixing of oxides, aluminum alloy castings need to use a relatively large spoon to scoop out the molten soup quietly, so use a standard crucible or bowl-shaped crucible with a large mouth. The method of receiving molten soup for heat preservation is generally more common in die casting. Since a relatively large spoon is used, large bowl-shaped crucibles are often required.

Tilting crucible furnaces are generally used in centralized melting. In addition, for the recycling and melting of chip waste, #3300 graphite crucibles are also widely used.

In the past, most of the aluminum alloys for die casting were melted using cast iron pots. Since more than 80% of the die-casting castings were auto parts with high quality requirements, they were gradually replaced by graphite crucibles. The iron component is easy to melt in the aluminum alloy. After repeated melting and use of the return material, the iron component is accumulated, which will affect the quality of the casting. Therefore, graphite crucibles that do not contaminate metals are widely used. At the same time, the durability of graphite crucibles has also been significantly improved, adapting to meet the requirements of users, and the development of large-scale crucibles has also progressed, thereby promoting the increase in the market demand for graphite crucibles.

Aluminum alloy is melted at a relatively low temperature of 700℃～750℃, and it is also the temperature field where graphite is easily oxidized. Therefore, special emphasis is placed on oxidation resistance for batching and production. At the same time, the influence of using a slag remover containing fluoride or the like was also noticed.