What is Cast Iron?
Cast iron is a group of iron-carbon alloys with a carbon content of more than 2 to 4 percent. In addition, varying amounts of silicon from 1 to 3% by weight and manganese as well as traces of impurities such as sulfur and phosphorus. Cast iron is made by reducing Iron Ore in a Blast Furnace.
The liquid iron is poured or cast and hardened into crude ingots called pigs, and the pigs are then remelted along with scrap and alloying elements in cupola furnaces and recast into molds to produce a variety of products.
The alloying ingredients affect its color when breaks: white cast iron has carbide impurities that allow cracks to pass easily, gray cast iron has graphite flakes that deflect a passing crack and initiate countless new cracks as the material breaks, and ductile cast iron has spheroidal graphite “nodules” that stop the crack prevent further progress.
With the exception of malleable iron, cast iron tends to be brittle. With its relatively low melting point, good fluidity, castability, excellent machinability, deformation resistance, and wear resistance, cast iron has become an engineering material with a wide range of applications.
what is cast iron made of?
Cast iron is made from pig iron, which is the product of melting iron ore in a blast furnace. 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, and carbon (coke), and taking various steps to remove undesirable contaminants.
Phosphorus and sulfur may be burnt out of the molten iron, but this also burns out the carbon, which must be replaced. Depending on the application, carbon and silicon content are adjusted to the desired levels, which may be anywhere from 2–3.5% and 1–3%, respectively.
If desired, other elements are then added to the melt before the final form is produced by casting.
Cast iron is sometimes melted in a special type of blast furnace known as a cupola, but in modern applications, it is more often melted in electric induction furnaces or electric arc furnaces. After melting is complete, the molten cast iron is poured into a holding furnace or ladle.
What is the Difference Between Cast Iron & Steel?
The main difference between the two elements is that steel is produced from iron ore and scrap metals, and is called an alloy of iron, with controlled carbon. Whereas around 4% of carbon in iron makes it cast iron, and less than 2% of carbon makes it steel.
Cast iron is cheaper than most steel. Also, the cast iron melting temperature is lower than that of steel, but it has high compressive strength, high hardness, and high wear resistance. Therefore, the important difference between steel and cast iron is that steel is ductile and malleable, whereas cast iron is hardened and has high compressive strength.
As another important difference between steel and cast iron, we can say that carbon in steel is in the form of iron carbide while cast iron has carbon as graphite or iron carbide or both. In addition, cast iron has excellent fluidity, with no steel.
Properties of Cast iron
A few common mechanical properties for cast iron include:
- Hardness. Cast iron is hard and it can be hardened by heating and sudden cooling. This makes it quite durable. Mild steel can be hardened and tempered by using relevant processes.
- Toughness. Material’s ability to absorb energy
- Ductility. Material’s ability to deform without fracture
- Elasticity. Material’s ability to return to its original dimensions after it has been deformed
- Malleability. Material’s ability to deform under compression without rupturing
- Tensile strength. The greatest longitudinal stress a material can bear without tearing apart
- Fatigue strength. The highest stress that a material can withstand for a given number of cycles without breaking
- Melting Point. Cast iron has a lower melting point (12000C) as compared to the melting point of mild steel which lies in the range of 13000C and 14000C.
- Castability. Cast iron is easier to work with when it comes to casting shapes out of the material. Due to the extra carbon present in cast iron, it’s molten form is more fluid and this makes it easier to cast the material into complex shapes.
- Machinability. Cast iron are almost elastic up to ultimate tensile strength and produce discontinuous chips which break away from the sample easily. This helps to improve the cutting ability. Due to this, cast iron is the preferred material when it comes to high machinability and strength.
Composition of Cast iron
Cast iron, an alloy of iron that contains 2 to 4 percent carbon, along with varying amounts of silicon and manganese and traces of impurities such as sulfur and phosphorus. It is made by reducing iron ore in a blast furnace.
Cast iron also contains a small number of impurities such as silicon, sulfur, manganese and phosphorus, copper, nickel, chromium, which affect its properties on a small or large scale. The effect of these properties is as follows.
- Silicon: In cast iron, silicon particles have up to 4 percent presence. It promotes graphite formation which makes it soft and easily machinable and Increases hardenability and electrical resistivity.
- Sulfur: It is present up to 0.1 percent. It makes cast iron hard and brittle.
- Manganese: It promotes the carbide formation of cast iron which makes it elite, and hard and increases resistance to wear and abrasions. It is present up to 0.75 percent.
- Chromium: It also promotes carbide formation which Increases hardenability, wear resistance, corrosion, and oxidation resistance.
- Nickle: Increases tensile strength.
- Tungsten: It increases hot hardness and hot strength.
- Molybdenum: Increases hardenability.
- Vanadium: Increases hardenability and hot hardness.
- Aluminum: Works as a deoxidizer in steel.
- Titanium: Works as a deoxidizer in steel.
- Niobium: It reduces hardenability and increases ductility, which results in increased impact strength.
- Cobalt: It reduces hardenability and resists softening at elevated temperatures.
Types of Cast Iron
There are four basic types of cast iron:
- White iron.
- Gray iron.
- Ductile iron.
- Malleable iron.
1. Gray Cast Iron
The most common type, gray iron, has a graphite microstructure made up of many small fractures. It is called “grey iron” because the presence of these small cracks creates the appearance of a gray color.
During the production of gray iron, the cracks open up and reveal the grey-colored graphite beneath the surface. Gray iron is not as strong as steel, nor can it absorb the same amount of impact as steel. Gray cast iron offers a similar compressive strength as steel. As a result, it has become a popular metal choice for applications where compressive strength is important.
Characterized with graphite in the microstructure, giving:
- Good machinability
- Good resistance to wear and galling
2. White Cast Iron
White iron, while not as common as gray iron, is another type worth mentioning. It gets its namesake from its off-white color, which is the result of iron compounds known as cementite. Like gray cast iron, white cast iron has many small cracks.
The difference is that white cast iron features cementite below its surface, while gray cast iron has graphite below its surface. The graphite produces a gray color appearance while the cementite produces a white color appearance. White cast iron is hard and offers excellent abrasion resistance.
Characterized by the prevalence of carbides, impacting:
- High compressive strength
- Good resistance to wear
3. Ductile Cast Iron
Ductile iron, also known as nodular iron, is a type of soft, ductile, high-carbon iron alloy. It is typically made with traces of other compounds, including magnesium and cerium.
When these trace compounds are added, they inhibit the rate at which graphite grows, keeping the metal soft and ductile. The ductile iron was invented in the early to mid-1940s.
Gray iron with small amounts of magnesium and cesium nodules the graphite, resulting
- High Strength
- High Ductility
4. Malleable Cast Iron
Finally, a malleable iron that is easy to “work on”. It is typically made by heat treatment processes on white cast iron. The white cast iron is heated for up to two days and then cooled. Once completed, malleable iron can be bent and manipulated to achieve unique shapes and sizes.
5. Mottled Cast Iron
Cast iron has an equal amount of free carbon and carbide known as mottled cast iron. It has intermediate properties and color of Gray cast iron and white cast iron.
6. Chilled Cast Iron
If the white cast iron quickly cooled from the molten stage, the iron produced is known as chilled cast iron.
7. Alloy Cast Iron
Alloy cast iron is produced by adding some alloying elements in cast iron like nickel, chromium, copper, etc. It has increased properties according to the alloying element. This cast iron is produced to get the desired properties of cast iron.
Use of Cast Iron
Cast iron can be used for many types of materials and for making different tools etc.
- Grey cast iron: Engine cylinder blocks, flywheels, gearbox cases, machine-tool bases.
- White cast iron: Bearing surfaces.
- Ductile cast iron: Gears, camshafts, crankshafts, piston ring.
- Many types of sanitary fittings like sewer pipes, manholes, water pipes, and cisterns are manufactured using cast iron.
- Column base and metal columns can be made using cast iron
- Casting mold is used for making lamp posts, metal staircases, gates,s, etc.
- Carriage wheels and rail chairs are made from cast iron.
- Various types of agriculture equipment and implements can be made from it.
- Various machinery parts can be made from cast iron
- It is used in making automotive parts
- It is used in making pots pans and utensils
- It is used in making anchors for ships
Advantages of cast iron
- It has good casting properties
- It has good Sensibility
- It has excellent resistance to wear
- It has good machinability.
- It has very low-notch sensitivity
- It has a Low-stress concentration
- It bears Low cost
- It has Durability
- It has Resistance to deformation
- It has three to five times higher compression strength than steel.
- It has excellent anti-vibration (or damping) properties so it is used to make machine frames.
- It has constant mechanical properties between 20 and 350 ° C.
- It is available in large quantities, hence produced on a mass scale. Tools required for the casting process are relatively cheap and inexpensive. This results in a low cost of its products.
- It can be given any complex shape and size without using costly machining operations
Disadvantages of cast iron
- It is Prone to rust
- It has a poor tensile strength
- It has a high weight-to-strength ratio
- It has High brittleness
- It has poor impact resistance
- Compared to steel it has poor machinability
- Its parts are section sensitive; this is due to the slow cooling of thick sections.
- The failure of its parts is sudden and in total, it does not reflect the yield point.
- It is Non-machinable (white cast iron).