Cast Iron Types – Microstructure

Cast Iron is considered the most widely used metal-matrix composite from the 1920s. Follow microstructural development in Cast Iron types to learn about graphite nucleation mechanism, lediburite formation, and eutectic and eutectoid reactions within cast iron.

Cast Iron is being used in a wide area of industry i.e. boilers, water conduit pipes in Oil and Gas Industry. This wide application of cast iron types is mainly due to following three reasons;

1. The ease of Production with Cupola Furnace
2. Lower melting temperature than steel
3. Molten cast iron has excellent fluidity for complex shapes

With understanding of microstructure development, we can estimate cast iron types based on alloying elements, cooling mechanism and post-casting heat treatments. Cast iron types are;

• White Cast Iron
• Grey Cast Iron
• Malleable Cast Iron
  • White Heart Cast Iron
  • Black Heart Cast iron
• Nodular Cast Iron/Ductile Cast Iron
• Chilled Cast Iron
• Alloy Cast Iron

White Cast Iron (cast iron types)

For an understanding of microstructure development, it’s better to follow the topic: Microstructure development in Cast Iron. Within topic, we have mentioned all proceeding in equilibrium cooling due to equilibrium reactions followed in the phase diagram. As we have explained in the topic, as we cross the eutectic solidification line, we have Cementite and Austenite in the microstructure.

From here, equilibrium generates graphite flakes in a matrix with carbon getting decreased in austenite. If the cooling rate is fast enough to prevent nucleation of graphite flakes than austenite with 2wt% carbon gets converted into pearlite and austenite without a graphite flakes formation in a matrix. This results in a microstructure comprising of Pearlite and Cementite called White cast iron.

One unique characteristic of Cast Iron type is that it is only a unique unit of cast iron with carbon present in the form of carbide. In the rest of the cast irons, carbon as a separate identity is present as well.

Why it’s called White cast iron?

White cast iron consists of carbides and pearlite. Carbides are extremely hard and brittle. When a crack appears, due to brittle nature, it flows straight through the material. No secondary or minor cracks appear due to the brittle nature of carbides which results in light reflection from the fracture surface. This results in white cast iron. These carbides are a reason for high compressive strength, high hardness, and high-temperature properties.

Microstructure of White cast Iron

A typical microstructure of White cast iron with composition in Hypo-eutectic region is shown below;

This microstructure is of white cast iron with 3-3.5 wt% C. In this microstructure, we clearly see two phases appearing in the microstructure. While you perform etching of material, the reaction between etchant and one of a phase of cast iron happens. Etching basically indicates chemical removal of material for exposing certain microstructural features. So, here due to etching, microstructure divides into two phases.

The white phase is Cementite and the black phase depicts pearlite. Cementite is Iron carbide extremely resistant against etchants, that’s why no chemical reaction happens in this phase. The matrix of pearlite appears to be black here. Pearlite consists of ferrite and cementite. Etchant reacts with ferrite and causes the removal of ferrite. This pearlite in high-resolution microscope appears as black and white lines but, here, the black phase indicates pearlite. An optical microscope, when light strikes the cast iron etched surface, the light comes up straight after striking cementite and deflects when it comes from pearlite. This causes cementite to appear as white and pearlite as black.

Grey Cast Iron

Grey cast iron or commonly cast iron are used for large number of cast irons having graphite flakes in ferritic or Pearlitic matrix. These cast iron types are developed by equilibrium or slow cooling. Cast iron types of these types normally contain 2.4% to 4% carbon, 1% to 3% silicon, and small amount of magnesium with range in-between 0.1% to 0.2%.

Why it’s called Grey cast iron?

In grey cast iron, graphite flakes are embedded in Pearlitic and ferritic matrix. When crack flows through graphite and crack is initiated, they cause nucleation of countless cracks which distorts the light being reflected. This distorted light generates grey fracture surface. Grey fracture surface makes equilibrium casting grey cast iron.

Role of Inoculants

Basically, graphite flakes are source of brittle fracture due to ineligibility of energy absorption. Strength comes from Pearlitic and ferritic matrix. The graphite flakes are needed to be fine and small to improve strength of cast iron types.

For grey cast iron, ASTM A-48 specification is considered which offers range of cast iron types with tensile strength ranging from 20 to 60 Ksi tensile strength. Above 40 Ksi of tensile strength, cast iron are called high strength iron.

Microstructure of Grey Cast Iron

Microstructure of Grey Cast iron is being shown below;

In this microstructure, flakes of carbon are present in matrix of ferrite. To learn about microstructure development of grey cast iron, Follow cast iron types development.

Malleable Cast Iron (cast iron types)

In malleable cast iron, graphite particles are present in form irregular shaped nodules rather than flakes. This type of cast iron is developed by heat treatment of white cast iron. Irregular shaped carbon developed in cast iron is called temper carbon because it is developed by heat treatment of iron carbide of white cast iron.

Importance of Malleable Cast Iron types

Iron carbide appears to be metastable state which will be transformed into iron and carbon due to heat treatment. The process of malleableization is performed to convert all combined carbon into Iron and Carbon. This process of malleableization is performed in two annealing cycles i.e. first annealing above 900^o C and second annealing below this temperature. The process of annealing takes almost 72 hours. With long heat treatment cycle, graphite nodules are form which has more ductility than graphite flakes. Higher ductility of graphite nodules and strength of matrix make malleable cast iron better than grey cast iron in terms of strength and ductility.

The microstructure of Malleable cast iron types containing black irregular carbon and white matrix as follows;

Types of Malleable cast iron

After annealing stages, process of cooling can divide malleable iron into three grades;

• Ferritic malleable iron

After annealing, extreme slow cooling results in ferritic matrix with graphite nodules.

• Pearlitic malleable iron

After annealing process, equilibrium cooling results in Pearlitic matrix.

• Martensitic malleable iron

With nodules formation during tempering or heat treatment process, extreme fast cooling results in formation of martensite matrix from Austenitizing temperature, generating martensitic malleable iron.

White Heart Cast Iron

In oxidizing atmosphere, all graphite will be oxidized. With annealing at 900^o C, all iron carbide is decomposed into iron and carbon. This nodular iron from surface will be oxidized by removing carbon from surface.

Black Heart Cast Iron

Cast iron types developed in Cupola contains interspersed graphite nodules with ferritic matrix. This type of cast iron has low strength and ductility and it is used for pipe fittings.

Chilled Cast Iron

During solidification, certain area of grey cast iron is cooled rapidly, this results in white cast iron region within casting of grey cast iron.

Carbon and other alloying elements have a considerable impact on formation of chilled region on surface of chilled cast iron. By adjusting the carbon composition, normal cooling rate on surface of white cast iron is enough to produce white cast iron. Chilled region where white cast iron is formed has certain depth which is dependent of carbon composition. With increase in carbon, tendency to develop chilled region within grey cast iron increases i.e. depth of chilled region increases. Chilled region on surface causes high hardness and abrasion resistance on surface of cast iron. High hardness on surface due to white cast iron and high strength due to grey iron in center of casting is major advantage of chilled cast iron.

The microstructure of chilled cast iron appears to be as follow;

Ductile Cast Iron

Nodular or spheroidal cast iron is used in many industrial applications ranging from agriculture tractors, and oil well pumps. Within Spheroidal cast iron types (SG iron), graphite appears as balls or nodules.  These spherical nodules appear less matrix and they are the main reason for high strength and toughness within spheroidal cast iron.

Microstructural development in Ductile Cast iron

Importance of spheroidal graphite nodules is clear from little introduction. The development of these nodules is very similar to graphite flakes nucleated in grey cast iron. All casting proceedings are similar to grey cast iron with addition of minor alloying elements like Magnesium and Cerium which will reduce surface tension of graphite flake and generating spheroidal graphite.

Spheroidal cast iron can be developed with addition of as low as 0.05% magnesium which will act as nucleating site for graphite. These nucleating sites provide the surface for graphite nucleation. This means, there will be magnesium or cerium particle at center of each nodule around which graphite solidifies.

Microstructure of Nodular cast iron is given below;

This is micrograph of a hypo eutectic SG cast iron specimen with 2.5 – 3.5%C, white region depicts ferrite and lamellar region depicts pearlite and dark black portion shows carbon nodules.

This structure has higher hardness than grey cast iron while higher ductility than white cast iron.

Major breakthrough in SG iron is development of bainitic matrix which is developed by isothermal transformation of austenite.

Alloy Cast Iron (cast iron types)

In alloy cast iron, certain alloying elements are added to improve mechanical and physical properties of cast iron. Alloying elements added are as follows;

Chromium

Chromium is added in cast iron types to develop chromium carbide. This chromium carbide increases strength, depth of chill, and wear and abrasion resistance.

Copper

Copper is one of major alloying element added in cast iron to decompose massive cementite and strengths matrix.

Molybdenum

This alloying element is added in cast iron to improve certain important mechanical properties like fatigue strength, heat resistance, harness and tensile strength.

Nickel

Nickel is added as alloying element to promote Pearlitic formation and giving better properties in cast iron.

Most common alloy cast iron types are divided into three distinct groups as follows;

• High silicon cast iron
• High chromium (Ni-hard) cast iron

• High nickel (Ni-resist) cast iron

Comparison of Cast Iron properties