The general smelting process involves several key steps to ensure the production of high-quality alloy. First, based on the specified alloy grades in casting technical requirements, the chemical composition range is determined, and the appropriate composition is selected. Next, the ingredients are calculated considering the element's burning rate and composition requirements. The charge materials are then prepared, ensuring they are clean, free from rust, and preheated before feeding into the furnace.
Before melting, the equipment, paint, and preheating are inspected to prevent contamination from gases, inclusions, and harmful elements. The feeding sequence typically starts with return charge, followed by intermediate alloys and metal materials. Low-melting-point metals like magnesium are added after the charge is melted to avoid oxidation. The alloy should be melted quickly to minimize suction and oxidation, and if necessary, a covering agent is used to protect the molten metal.
After melting, refining treatment is carried out to purify the alloy liquid, and the refining effect is tested. Further processing may be required to improve the microstructure and performance of the alloy. Temperature adjustment and casting follow, with some alloys requiring stirring before pouring to prevent density segregation.
Different metals require specific smelting methods and equipment. For example, steel is melted using converters, open hearth furnaces, electric arc furnaces, or induction furnaces. Cast iron is usually melted in cupola furnaces, while non-ferrous metals such as aluminum and copper alloys are often melted in specialized furnaces like the Komatsu A furnace.
Aluminum alloy smelting is particularly challenging due to its low melting point, susceptibility to oxidation, and the evaporation of low-boiling elements like Mg and Zn. Smelting must be done in an environment isolated from air and gas. Common equipment includes coke-heated furnaces, electric heating mandarins, and graphite crucibles. The smelting process involves careful ingredient calculation, preheating, and the use of refining agents like hexachloroethane for degassing and removing impurities.
Refining is crucial to remove gases and inclusions. Hexachloroethane reacts with the molten aluminum to form bubbles that carry hydrogen and alumina inclusions to the surface, where they can be removed. Proper ventilation is essential during this process. After degassing, the alloy is left to settle, and the gas content is checked by observing the surface for bubbles.
For certain alloys, modifiers are added to improve mechanical properties. These are typically introduced at specific temperatures and concentrations. Quality control measures include checking the chemical composition, ingot thickness, and surface conditions. Any defects require remelting or further analysis.
Preventing blowholes in castings involves managing moisture in raw materials, controlling smelting time and temperature, and optimizing the mold’s gas permeability. Sand cores must be properly vented to allow gas escape, and the sand mixture should have controlled moisture levels. Additionally, cold irons can be used to enhance solidification and improve exhaust capabilities.
Overall, the goal of alloy smelting is to produce a molten metal that meets strict quality standards. This requires attention to every step, from material preparation to final casting, ensuring minimal defects and optimal performance.
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