Cast iron is an iron alloy consisting of 2-4% carbon, as well as varying amounts of silicon and manganese, and small quantities of impurities like sulfur and phosphorus. It is created by reducing iron ore in a blast furnace. The molten iron is then poured and solidified into rough ingots known as pigs. These pigs are later melted again, along with scrap metal and additional alloying elements, in cupola furnaces. The resulting molten iron is poured into molds to manufacture a wide range of products.
The production of cast iron dates back to ancient times. The Chinese were producing it as early as the 6th century BCE, while production began in Europe around the 14th century. It was introduced to England around 1500, and the first ironworks in America were established in Virginia’s James River in 1619. During the 18th and 19th centuries, cast iron became a more affordable engineering material compared to wrought iron, as it did not require extensive refinement and hammering. However, it was more brittle and had lower tensile strength.
Nonetheless, its ability to bear heavy loads made it crucial for early structural metal applications, including some of the first skyscrapers. In the 20th century, steel replaced cast iron in construction, but cast iron still finds numerous industrial uses today.
Ultimate strength value of Cast Iron
- Tensile Strength – 100 to 200 MPa
- Compressive Strength – 400 to 1000 MPa
- Shear Strength – 120 MPa
Types of Cast Iron
Cast Iron are of the following types:
Grey Cast Iron
Grey cast iron is a specific type of iron used in castings. It gets its name from its grey color, which is a result of the graphite flake structure formed during the cooling process. This graphite comes from the carbon content in the material. The presence of free graphite gives grey cast iron its lubricative properties, making it suitable for applications requiring a gliding effect. The properties of grey cast iron depend on the materials and casting methods and casting patterns employed. Under a microscope, one can observe the small black graphite flakes that give the material its grey appearance. The size and shape of these flakes influence the mechanical properties of grey cast iron.
The popularity of grey cast iron components is due to the fact that grey cast iron is one of the cheapest types of iron castings that can be produced and has acceptable ductility, tensile strength, yield strength, and impact strength for most applications. Cast iron also has an excellent ability to dampen vibrations, making it ideal for machine bases and many enclosure applications. Grey cast iron has high thermal conductivity and conducts heat more easily through the metal. Grey cast iron parts are often used in machine tools.
Chemical Composition of Grey Cast Iron
Sulphur: 0.02-0.15% and remaining percentage is Iron.
Applications of Grey Cast Iron
- Automobile bodies
- Cylinder blocks
- Heads, housings
- Pipes and fittings
- Agricultural equipment
White Cast Iron
Its white color is due to the fact that it contains no graphite and all the carbon is in the form of carbide known as cementite, the hardest component of iron. White cast iron has high tensile strength and low compressive strength. Because of its hardness, it cannot be processed with ordinary cutting tools and requires grinding as a molding process. White cast iron can be cast against a mold or made by regulation analysis. This mold is used when products such as car wheels, grain crusher rollers, jaw crusher plates, etc. require a hard, wear-resistant surface.
Chemical Composition of White Cast Iron
Manganese: Less than 0.40%
Phosphorous: Less than 0.2%
Sulphur: Less than 0.12% and remaining percentage is Iron.
Application of white cast iron
- Mining Crusher parts
- Roller Crusher
- Oil sand Application
- Lifting Bars
- Slurry Pumps
- Cement Mixers
- Pipe Fittings.
Malleable Cast Iron
The malleable cast iron is a cast iron-carbon alloy that solidifies in the as-cast condition in a graphite-free structure, i.e. total C% is present in the form of cementite (Fe3C). Malleable Cast Iron is less brittle & more ductile as compared to grey cast iron. The tensile strength of the malleable cast iron is usually higher than that of grey cast iron and has excellent machining qualities. Malleable cast iron is preferable for the machine parts in which a degree of accuracy is required.
In order to obtain a malleable iron casting, it the first cast into molds of white cast iron. A suitable heat treatment like annealing is used which then separates the bound carbon in white cast iron into graphite nodules.
Whiteheart Process of Malleable Solid Iron
White iron castings are packed in iron or steel cases surrounded by a mixture of new and used hematite ore. The box is slowly heated to a temperature of 900-950°C and held at this temperature for several days. During this time some of the carbon from the casting is oxidized and the remaining carbon is distributed as small patches throughout the structure. A warming process is followed by a cooling process, which lasts for several days. The result of this heat treatment is a casting that is strong and withstands heat treatment without breakage. This process is known as Whiteheart Process.
Blackheart Process of Malleable Solid Iron
The castings used are free of carbon and sulfur. They are packed in a neutral substance like sand and the reduction of sulfur helps speed up the process. The castings are heated to a temperature of 850-900°C and held at this temperature for 3-4 days. In contrast to the White Heart process, this process turns carbon into globules. Castings made with this process are more malleable.
Types of Malleable Cast Iron
Black malleable iron
It is obtained after annealing in an inert atmosphere and exhibits black fracture. The microstructure developed in the casting should have a matrix of ferrite with essentially tempered carbon and no flake graphite.
Pearlitic malleable iron
It is obtained after heat treatment and has a homogeneous matrix consisting essentially of pearlite or other transformation products of austenite. Graphite is in the form of lumps of charcoal. The microstructure must not contain flake graphite.
White malleable iron
It is obtained after annealing in a decarburizing atmosphere and has a silvery grey fracture surface with a dark grey to the core of black. The microstructure that develops in a cut depends on the size of the cut. Small section castings are mainly ferrite with some perlite.
Chemical Composition of Malleable Cast Iron
Sulphur: 0.02-0.20% and remaining percentage is Iron.
- Hubs of wagon wheels
- Small fittings for railway rolling stock
- Brake supports
- Parts of agricultural machinery
- Pipe fittings
- Door hinges
Nodular Graphite Cast Iron
This type of cast iron is obtained by adding a small amount of magnesium (0.1-0.8%) to the molten metal. The addition of magnesium causes the graphite to take the form of small nodules or spheres instead of the usual angular flakes. It has excellent fluidity, castability, tensile strength, toughness, wear resistance, pressure resistance, weldability, and machinability. Commonly used for castings that require impact resistance and good machinability.
Chemical Composition of Nodular Graphite Cast Iron
Manganese: Less than 0.3%
Phosphorous: Less than 0.03%
Sulphur: Less than 0.020% and remaining percentage is Iron.
- Hydraulic cylinders
- Cylinder heads
- Rolling mill rolls
- Centrifugal casting products
- Valves & Fittings
- IC Engines
- Steel Mill Rolls
Chilled Cast Iron
White cast iron is made by quenching hot metal. Cast iron produced using quenching is called chilled cast iron. All castings are skin cooled by the contact of molten iron with cold sand in the mold. However, in most castings, this hardness penetrates to a very shallow depth (less than 1mm). Castings can be intentionally cooled or inadvertently cooled to a great depth. Targeted cooling is achieved by inserting iron or steel inserts (tremblers) into the mold. When the molten metal comes in contact with the mold, its heat is easily dissipated and a hard surface is formed. Radiators are used on all surfaces of castings that are resistant to wear and friction.
Chemical Composition of Chilled Cast Iron
Chromium: 0.75%, and remaining percentage is Iron.
- Air Blast machine
- Used for Surface preparation
Mottled Cast Iron
A product between gray cast iron and white cast iron in composition, color, and general properties is mottled cast iron. This is obtained in castings with specific wear surfaces that are cooled.
Chemical Composition of Mottled Cast Iron
Combined Carbon: 1.75%
Iron: 93.5%, remaining slag.
- Manhole covers
- Fire Plugs
Alloy Cast Iron
Cast Iron containing Silicon, Manganese, Sulphur, and Phosphorous is called Plain Cast Iron. Alloyed cast iron is created by adding sufficient amounts of alloying elements such as nickel, chromium, molybdenum, copper, and manganese. These alloying elements increase strength and lead to improved properties. Alloyed cast iron has special properties such as increased strength, high wear resistance, corrosion resistance, and heat resistance. Alloy cast iron is widely used in automotive parts such as gears, cylinders, pistons, piston rings, crankcases, crankshafts, camshafts, sprockets, wheels, pulleys, brake drums, shoes, crusher and grinder parts, etc.
Chemical Composition of Alloy Cast Iron
Phosphorous: 0.05-0.25%, and remaining percentage is Iron.
- Brake Drums
- Automotive applications
In conclusion, cast iron is a versatile material with a rich history. Its alloy composition, including carbon, silicon, and manganese, contributes to its strength and durability. From its early uses in construction to its industrial applications today, cast iron continues to play a vital role in various sectors, showcasing its enduring value.