Cast Irons
What is cast iron? Cast iron is a casting that is an alloy of Iron and carbon. Carbon sequestration varies between 2.5 and 4.5%. It also contains small amounts of manganese, phosphorus, sulfur and silicon. This last element is the most important.
Cast irons make excellent cast alloys and have a wide range of strength and hardness. However, they are also easy to use in the machine. They show great resistance against abrasion, scratching and oxidation if they are used with alloy.
Their widespread use is due to their lower cost and machinability. Despite the competition created by new materials, cast irons have proven to be economical and suitable materials for thousands of engineering applications.
Why Is Cast Iron Needed?
The raw iron obtained in the blast furnace cannot be used directly due to the substances in its internal structure. The most important substance in its internal structure that prevents its use is the carbon element in excess. Raw iron, which is a blast furnace product, has 2.5-4.5% carbon in its internal structure. The excess of carbon causes the raw iron to be brittle and hard. Reducing this carbon to acceptable levels is a prerequisite for the use of iron in industry.
There are two main production areas where the raw iron produced in the blast furnace is used by reducing the carbon ratio. One of them is steel production and the other is its use as cast iron.
How Are Cast Irons Made?
If the raw iron produced in the blast furnace is to be used as cast iron, it must be subjected to some processes. These processes are generally done in places called cupola furnaces. In cupola furnaces, the carbon in the raw iron internal structure is reduced to between 1.7 and 3.5. On the other hand, it is made with the addition of scrap steel alloys, which makes the cast iron economical.
Cast iron production with carbon reduction is not qualified to meet the needs of the industry. Therefore, by adding other additive elements, cast irons that can be used for different purposes are produced.
Advantages And Weaknesses Of Cast Iron
The general properties of cast iron are as follows;
1. Melting at low temperatures,
2. Low cost,
3. Suitability for casting,
4. High compressive strength,
5. Wear resistance.
Those gathered in 5 items above should be perceived as the advantages of cast irons. Besides, it is a negative feature that cast irons are not suitable for forging.
The compressive stress of cast iron is good, but the tensile stress is low. Unless specially alloyed, it cannot be forged at any temperature and cannot be properly machined.
Very low carbon tensile cast iron is called secondary steel or low alloy steel. because it is close to steel rather than iron in terms of its mechanical and physical properties.
Cast iron is very fluid in the molten state and can easily be poured into even the most complex molds.
While steels can be formed hot and some types cold, the inability of cast iron to be forged even hot is due to its lack of plastic deformation as it transitions from liquid to solid. Cast irons go from a liquid molten state to a solid state in an instant. Cast irons are liquid above 1147°C and solid at lower temperatures. Due to these properties, the plastic consistency required for forging does not occur.
Cast Iron Types
Classification of cast irons is generally made according to the fracture surface of the material. Types of cast iron are given below.
White Cast Irons: Almost all of the carbon in the alloy is in the form of iron carbide (cementite). This item is hard and crunchy.
Gray Cast Irons: Gives a gray fracture surface. This item is soft. Most of the carbon in its structure is in the form of soft graphite. The melting temperature is higher than the white cast types. It is the most used type of cast iron.
Tempered Cast Iron (Malleble cast iron): It is between the above two main types in terms of fracture surface and properties. Tempered cast irons have some ductility because the carbon is spherical, not in the form of leaves.
Ductile Cast Iron: Also known as ductile cast iron. It is a fairly new construction tool, developed in England by the British Cast Iron Research Association. It is obtained by treating molten iron with cerium or magnesium. By making the graphite flakes in its structure spherical, the values of tensile, yield stress and unit elongation have been increased.
