How is cast iron made from pig iron?

Hello, I'm a metallurgical engineer with extensive experience in the production and processing of various types of iron and steel. It's my pleasure to guide you through the process of making cast iron from pig iron.

The production of cast iron from pig iron is a fascinating process that involves several steps to refine and transform the pig iron into a more useful form. Pig iron, which is the raw iron produced in a blast furnace, contains a high amount of carbon, typically between 3.8% and 4.7%, along with other impurities such as silicon, manganese, phosphorus, and sulfur. To make cast iron, these impurities need to be reduced, and the carbon content needs to be adjusted to a level suitable for casting.

Step 1: Melting the Pig Iron
The process begins with the melting of pig iron. This is done in a cupola furnace or an electric arc furnace. The cupola furnace is a traditional method where coke is burned to provide the heat necessary to melt the pig iron. The electric arc furnace uses electric arcs to generate the heat. The choice between the two depends on various factors such as the scale of production, energy costs, and environmental considerations.

Step 2: Adding Alloying Elements
Once the pig iron is melted, various alloying elements may be added to modify its properties. These elements can include silicon, manganese, and others that are designed to improve the castability, hardness, and strength of the final cast iron product.

Step 3: Desulfurization
Desulfurization is a critical step in the production of cast iron. Sulfur in pig iron can lead to the formation of iron sulfide, which is brittle and can cause problems during casting. To remove sulfur, a desulfurization agent such as magnesium or calcium carbide is added to the molten pig iron. This reacts with the sulfur to form a gas that can be removed from the furnace.

Step 4: Adjusting Carbon Content
The carbon content in pig iron is too high for most casting applications. To reduce the carbon content, a process known as graphitization is carried out. This involves adding elements that promote the formation of graphite, such as magnesium or silicon, which react with the carbon to form graphite nodules. This step is crucial as it affects the mechanical properties and machinability of the cast iron.

**Step 5: Removing Undesirable Contaminants**
After the carbon content is adjusted, the next step is to remove any remaining undesirable contaminants. This can involve adding materials like limestone or other fluxes that react with the impurities to form a slag. The slag is then removed from the molten cast iron, taking the impurities with it.

Step 6: Pouring the Molten Cast Iron
Once the molten cast iron has been refined, it is poured into a holding furnace or ladle. This is done carefully to avoid introducing any new impurities or air bubbles into the cast iron.

Step 7: Casting
Finally, the molten cast iron is poured into molds to create the desired cast iron products. The molds can be made from sand, metal, or other materials, depending on the complexity and size of the products being cast.

Throughout this process, careful control and monitoring are required to ensure that the final cast iron meets the desired specifications. The properties of the cast iron, such as its hardness, strength, and ductility, can be tailored by adjusting the composition and processing parameters.

Now, let's move on to the translation of the above explanation into Chinese.

Cast iron can be made directly from the molten pig iron or by re-melting pig iron, often along with substantial quantities of iron, steel, limestone, carbon (coke) and taking various steps to remove undesirable contaminants. ... After melting is complete, the molten cast iron is poured into a holding furnace or ladle.