Learn All About the Cast Iron Process
Cast iron is an alloy of iron made by combining certain percentages of carbon with iron. Carbon usually makes up about 1.8 – 4 percent by weight, silicon making 1 – 3 percent by weight and iron making up the rest.
These are melted together and poured out into a mould to cool. The resulting iron usually takes the shape of the mould.
Many of our everyday tools such as many parts of our vehicles are made from cast iron.
Cast iron is not as hard as steel but it is very resistant to abrasion and indentation. Hence, a cheaper alternative to steel.
It is also ductile and elastic. Thus it can be deformed without breaking and even return to its original form. Another plus side of using cast iron is that scrap metals are recycled.
Cast iron is quite old with the oldest pieces of work done using cast iron dates as far as the 5th century, in China. It was a vital part of their tools, weapons, ploughs, and pots. The Chinese made cast iron by keeping the hot cast in an oxidizing environment for a week or more. This allowed the iron to burn off carbon at the surface.
The Chinese also used the sand casting method as early as 645 BCE. Here, sand is used as a mould. The methods of casting have evolved although the main concept remains the same. Today, many people still use the sand casting method.
Types of Cast Iron
Gray cast iron: Grey cast iron is the most common type of cast iron. It has many small fractures within its structure from the addition of graphite to the iron. These fractures give the iron a grey colour hence the name grey cast iron. Grey cast iron has similar strength as steel but cannot absorb as much shock as steel would.
White cast iron: Cementite, a mineral that is part of the white cast iron makes it appear white when viewed through the many small cracks in it. White cast iron can withstand abrasion. Ductile cast iron: Ductile cast iron has a higher carbon content making it soft and ductile. Other compounds such as magnesium and cerium are added in trace amounts when making it. Malleable cast iron: White cast iron when subjected to heat treatment processes would create a type of cast iron that is malleable. Malleable cast iron has good strength and is flexible.
How is casting done today?
Iron exists in nature with other minerals mainly as oxides called iron ore. These ores contain a combination of iron with other minerals. The iron ore when extracted through mining, goes through a refining process to remove the debris with it, most of it being rock. After the extraction, the iron is taken to a blast furnace and heated to produce pig iron. This pig iron is then smelted at a very high temperature. Other scrap metals and alloys are added to the mixture and allowed to melt together. This liquid mixture of metals is then poured into a mould where it cools and gets solidified as it cools.
Cast iron is often made using the sand casting method.
Design: Designing is a planning stage where you draft out what is to be done and have an idea of what the finished work should look like. The design of the object is often dependent on what function the object to be cast would serve. This planning is important so the patterns to be made are perfect fits.
Pattern Making: A pattern is needed to mirror what you want to make. These patterns are made to be exactly the shape of what the item should be when it is finished. Lots of calculations are involved in getting the pattern in the right dimension and specifications Patterns for iron casting could be made out of wood, metal, plaster, or plastic. Patterns are made to allow for contractions that would occur when the metal is cooling. So many patterns are slightly larger than the actual object to be created. The advantage of a well-done pattern is that it can be re-used even for a long time.
Mould making: Since casting is done mostly in sand, the mould made is created in sand. The sand used here is often mixed with clay or resins. A moulding box often made out of wood called a casting flask is used. This casting flask is made in two parts; a top portion called a cope and a bottom part called a drag. When very large pieces are to be made, the casting flask is literally created in the sand on the foundry floor. This casting flask is then filled with sand. After filling the casting flask with the sand, the pattern is then placed in the sand. When the pattern is removed, it creates a hollow space in the sand with the shape to be created. The mould can be dried to remove any excess water from the sand. If not done, hot metal could cause a steam explosion damaging the mould or cause injuries.
There is usually a channel made for the molten iron to flow through and get to the mould.
Core making: Making hollow pieces, you would need to make a core. Cores are made from moulding sand to the desired shape. These moulded cores are inserted into the casting box after the patterns have been removed.
Casting: With the mould set, the iron is heated, melted, and poured at about 1350 degrees centigrade. After the iron has cooled, both the mould and the core are broken to release the casted work. These sands can be reused again. Excess from the process is cleaned up through a process called fettling. The excess iron can be remelted and reused again. This works to put the finishing touch on the casted work. Surface finishing of casted works could be through several methods such as electroplating, porcelain enamelling, painting, etc.
Depending on what was adding in the melting phase, different types of cast iron with specific properties are created.
Iron made through the casting method has several uses depending on which type it is. Some are used in making heavy equipment such as crushers, railroad brakes, hydraulic components etc. Other more malleable forms are used for delicate materials such as electrical components. We use our own method at UK Pavement Light Manufacturing Limited.