The term Cast Iron can be described as a large family of ferrous alloy. These alloys contain 2-4% carbon, along with silicon, manganese. Cast irons are manufactured and produced in specified shapes for processing by machining, heat treating, or assembly. Sometimes, to meet specific requirement it can be forged or rolled. Casts iron are produced by melting pig iron and then combining it with steel or scrap iron.
Normally, cast iron contains silicon up to 3%. Though this composition may vary while making special compositions, such as:
· Silal: Silicon up to 6%
· Duriron: Silicon up to 12%
One can purchase cast iron in several commercial grades, such as gray iron, chilled iron, mottled iron, white iron, malleable iron, ductile iron, spheroidal graphite iron, nodular iron, and austenitic cast iron.
Production Process
Cast iron is produced by melting pig iron. Often, this melting is done along with scrap steel and scrap iron. It also involves a several steps which lead to the removal of undesirable elements (phosphorus and sulfur).
· The main objective of this processing is to reduce carbon and silicon content to the desired levels. Then, the addition of other elements to the melt takes place.
· A small blast furnace known as a cupola is used to melt iron.
· After melting is complete, the molten iron is removed from the forehearth of the blast furnace.
· The final form is made by casting.
During solidification, the major proportion of the carbon precipitates in the form of graphite or cementite. When solidification is just complete, the precipitated phase is embedded in a matrix of austenite that has an equilibrium carbon concentration of about 2 wt%. On further cooling, the carbon concentration of the austenite decreases as more cementite or graphite precipitates from solid solution. For conventional cast irons, the austenite then decomposes into pearlite at the eutectoid temperature. However, in grey cast irons, if the cooling rate through the eutectoid temperature is sufficiently slow, then a completely ferritic matrix is obtained with the excess carbon being deposited on the already existing graphite.
Types of Cast Irons
Given below is a list of different types of cast irons and their properties:
· White Cast Iron: White cast iron is hard to be machined as it's tough and brittle.
· Grey Cast Iron: Due to the presence of microstructure of graphite in transformed-austenite and cementite matrix, grey cast irons are softer.
· Spheroidal Graphite Cast Iron: The chemical composition of the cast iron is similar to that of the grey cast iron but with 0.05 wt% of magnesium.
· Ductile Cast Iron: One of the most popular alloys used in casting. It has variety of applications including automobile components, industrial machinery, wind turbine electrical energy generation, valves, air conditioning machinery, lawn and garden equipment and agricultural products.
· Spheroidal Graphite Cast Iron: Spheroidal graphite cast iron is known for its excellent toughness and has many applications, e.g. crankshafts.
Cast Iron Alloy Castings
Cast iron alloys have found wide usage in casting applications for their characteristics, such as strength, good strength to weigh ratio, economical price, and availability in abundance, capability to produce complex geometries. The cast iron castings are used in a multitude of applications, such as automotive, agriculture, appliance, and more.
Advantages in Casting:
· A family of materials which is capable to meet a variety of engineering and manufacturing application requirements (the "family", includes - gray iron, ductile iron, compacted graphite iron, malleable iron, and white iron).
· Capability of casting with inserts of other materials.
· Capability to manufacture and design highly complex geometries and sections in various sizes.
· Possibility to cast intricate shapes as well as very thin to very thick section sizes.
· Good strength to weight ratio.
· Generally economical than competing materials and relatively low cost per unit of strength than other materials.
· Superior damping capacity, especially in Gray Irons.
· Capability of redesigning and combining two or more parts from other materials into a single casting, thereby reducing assembly time and cost.
· Different types of casting processes for low, medium or high production.
· Variety of iron castings can be used without heat treatment (as cast) however, in case if required, they can be heat treated to enhance the overall properties or specific properties such as surface hardness.
