5.7.1 Graphite Products for continuous casting
The continuous casting method can save the ingot casting and ingot opening process, and directly extrude or calender the rough plate. This continuous casting method has been widely used in the production of steel plates and steel bars; it is also widely used in plates and bars of aluminum and other non-ferrous metals. For more than 50 years, tubing has been produced for the same purpose as brass production. Because continuous casting can simplify the process, improve the product qualification rate, and the product structure is uniform. Continuous casting is almost always used for plastically working materials of non-ferrous metals. The properties of graphite for continuous casting are shown in Table 5-43. The graphite crystallizer for continuous casting is shown in Figure 5-44.
For nozzles and mandrels used in continuous casting, the following characteristics are required:
(1) Good thermal conductivity.
(2) High thermal stability and thermal shock resistance.
(3) Good lubricity.
(4) Do not infiltrate with molten metal.
(5) It does not react with cast metal and does not affect the quality of the metal.
(6) Easy to process under strict tolerance requirements.
In general, graphite materials can almost meet the above conditions. Especially the isotropic graphite material with micro powder structure has high mechanical strength, uniform, and dense structure, no large pores and the expansion coefficient can be adjusted within a considerable range. It is a very good crystallization agent for metal continuous casting. Material.
Now the continuous casting nozzle made of graphite is not only suitable for cast steel but also suitable for copper, bronze, brass, phosphor bronze, white copper, aluminum alloy, cast iron, etc. During the production process, attention should be paid to the management of cooling water to eliminate gas to prevent Surface deformation.
Graphite material characteristics for continuous casting, bulk density 1.70-1.90g/cm3, bending strength 40~, tensile strength 25~, thermal conductivity 110~150W/(m K) round rod diameter φ, length, the round tube is divided into φ390 /φ and φ370/φ. The vertical gusset is, and the horizontal plate is. The pouring speed is different according to the size of the pouring plate, which is 0.7~4m/min respectively. Continuous casting is divided into the horizontal continuous casting method and the vertical continuous casting method (see Figure 5-45). The horizontal continuous casting method is simple and easy to produce high-quality bars. The schematic diagram of the hard mold part of the horizontal continuous casting device is omitted. Figure 5-46 is a schematic diagram of the crystallizer part of the vertical continuous casting equipment for the production of non-metallic pipes.
5.7.2 Forming
5.7.2.1 Die casting mold
The die-casting method was mostly used for non-ferrous metals before, but in 1950, the United States succeeded in die-casting steel wheels with graphite dies. It is said that 1000 of the graphite cast [type] have been produced. The life of a wheel. The technology has been further improved, and the production of stainless steel slabs and billets by die-casting has been successful.
Die casting can produce uniform and defect-free castings with high yield, no defects such as central deformation or shrinkage cavity segregation that are inevitable in ordinary castings, high dimensional accuracy of castings, and good internal quality of castings. Dense artificial graphite with high mechanical strength is required for casting molds of zinc alloy or copper alloy die castings such as automotive parts.
Figure 5-47 is a schematic diagram of the die-casting hub manufacturing device and the billet manufacturing device.
5.7.2.2 Molds for centrifugal casting
The centrifugal casting method is to inject molten steel into the graphite mold while rotating, relying on centrifugal force to make the molten steel adhere to the inner wall of the mold for casting and solidification. Therefore, hollow castings such as cylinders and tubes or most round castings are suitable for one-time molding casting. With this method, impurities such as gas and slag in molten steel will be concentrated on the inner surface of the casting due to different densities.
5.7.3 Hot pressing mold
Artificial graphite hot pressing molds have the following advantages in cemented carbide pressure sintering: First, if the pressing temperature is raised to 1350~1450°C, the required unit pressure can be reduced to 6.7-(that is, 1/10 of the cold pressing pressure at the same time ); Second, pressurization and heating are carried out in the same process, and a dense sintered body can be obtained in a short time of sintering, which greatly reduces the cost. Under such high temperatures and high-pressure conditions, the advantages of using artificial graphite materials are unmatched by other materials. The structure of the cemented carbide hot die-casting mold is shown in Figure 5-48.
In recent years, artificial graphite hot stamping dies have also achieved good results in the production of boron nitride tools. When the heating temperature reaches about 1780°C, the unit pressure is 56~. The upper and lower punches, modules, gaskets, and heaters of the hot pressing die are all made of artificial graphite, and its construction principle is similar to that in Figure 5-47. However, because hot pressing requires high temperature and high pressure, the upper and lower punches and the female mold sleeve should be made of high-strength, high-density artificial graphite materials, such as M205.
With the emergence of new ceramics, artificial graphite materials are also used in forming molds. Due to the small linear expansion coefficient of artificial graphite materials, the products produced with it have high shape and dimensional stability. In addition, because the artificial graphite material has a large thermal conductivity and good thermal shock resistance, it is also conducive to the rapid cooling and rapid heating of the mold. The high-temperature strength of artificial graphite materials is a necessary condition for hot-pressing molds. Artificial graphite hot pressing molds have also been successfully used in the shaping of diamond grinding wheels and diamond drill bits.
In recent years, with the development of hot pressing technology, the requirements for molding conditions are developing in the direction of high temperature and high pressure, which puts forward higher requirements for the artificial graphite materials used in the manufacture of molds. The newly developed M205 artificial graphite material that can be used as a hot pressing mold in my country has a compressive strength of more than 100%. At present, the flexural strength of graphite materials used as hot-pressing dies in some countries can reach up to. Carbon fiber composites have higher strength, but their use is limited due to issues such as price.
When designing the artificial graphite mold for hot pressing, the force on the cylindrical mold is:
In the formula, a is the inner diameter of the mold; is the outer diameter of the mold; he is the internal pressure.
The ratio of outer diameter to inner diameter is generally 3-4. Considering safety issues, as mechanical carbon, b/a can be 5-10. When the pressure is not high, considering that the hot stamping die is a consumable, to reduce the cost, b/a can also adopt a lower value L5~2. When using artificial graphite hot stamping dies, creep needs to consider the artificial graphite material. Technically, around 2400°C is defined as the operating temperature limit, since graphite materials tend to creep to some extent above this temperature. Some users found that the graphite mold will also have a certain degree of creep below 2400 ℃, although it is small, it should be checked frequently. According to experience, longer service life can be obtained below 1200°C.
5.7.4 Other molds
5.7.4.1 Glass forming molds
Charcoal has been used as a mold for glass shaping since ancient times. For example, the handmade wooden fixing board is based on the principle that when it comes into contact with high-temperature glass, the surface is carbonized to prevent it from melting with the glass. Carbon graphite materials also have good chemical stability, are not easily infiltrated by molten glass, and will not change the composition of the glass; graphite materials also have good thermal shock resistance and small size changes with temperature, so in recent years, in the glass manufacturing process, has become an indispensable mold material, and can be used to make various molds such as glass tubes, elbows, and funnels. Shaped glass bottle.
5.7.4.2 Sintering Mold and Others
Utilizing the characteristics of extremely small thermal deformation of artificial graphite materials, sintering molds and brackets for transistors can be manufactured, which has been widely used and has become an indispensable material for the development of the semiconductor industry.
In addition, artificial graphite is also used in casting molds for cast iron, durable casting molds for various non-ferrous metals, casting molds for cast steel, casting molds for heat-resistant metals (titanium, aluminum, pliers, etc.), and welding guide rails. Thermite casting mold, diamond tool sintering mold, etc.
The shape and size of artificial graphite molds are often selected by users, and carbon graphite product factories use large blanks for machining according to requirements.