Mechanical Properties of Cast Iron

Cast iron is an alloy of carbon (over 2%) and other elements. The rigidity and dampening factor of cast iron make it an excellent material for machine tool frames and parts.

There are many ways of classification cast irons. Some of the common classifications are based on the appearance of their fracture surface, their microstructure or their material properties. The other common method is to classify cast iron by type - gray iron, ductile cast iron and mottled cast iron. This classification describes the color / appearance of their fractured surfaces. Mottled iron is a mixed appearance of gray and white.

Ductile cast iron (also known as nodular iron and spheroidal graphite cast iron) has wide variety of properties that depend on the amount of carbon and other material added to it.

Here are a few common mechanical properties of cast iron:

This is the ability of a material to withstand an external pulling force. It is thus the maximum stress reached during a tension test. Tensile strength is important in the use of brittle materials (cast iron) more than ductile materials (ductile cast iron). In ductile materials the ultimate measured strength commonly will be reached before fracture, with a lower stress level recorded at the point of fracture. Yield point is the point when the material is unable to cope with the applied external force, and gives away. Two important points to note are:

Elongation is a measurement of ductility. It is a measurement of the strain side of the graph and is usually measured as a percentage of permanent growth over a set length of the test specimen at fracture.

Fracture Toughness (applicable especially for gray cast iron)

The fracture toughness reflects the highest load intensity that can be resisted by a gray iron component with known pit dimensions. Because of its peculiar properties, fracture tests are ideal to measure the resistance of the gray cast iron to crack propagation and provide a useful estimate of the residual material strength.

Graphitisation (specific to gray cast iron)

'Graphitisation' describes the network of graphite flakes that remain behind when the iron in the cast has been leached away by the corrosion.

These tests are used as an approximate measure of strength and sometimes as an indication of relative machinability. The standard hardness strength is the Brinell Hardness test.

Bend test is a measurement of a material’s ductile properties. Ductile materials will bend without breaking or cracking, brittle materials will crack or fracture during a bend test. A common bend test is to deform sample 180 degrees around a mandrel of a set radius to determine if the material can resist the bending without cracking or fracture.

Impact testing measures an object's ability to resist high-rate loading. Although not a direct measurement of toughness, impact testing does represent a material’s relative toughness. Impact resistance can be one of the most difficult properties to quantify. The ability to quantify this property is a great advantage in product liability and safety. Ductile iron has a greater resistance to impacts, and is able to resist impact than gray cast iron.

The most common impact test is the Charpy method.

Fatigue strength is the highest stress that a material can withstand for a given number of cycles without breaking. The aim of the test is to produce repeated loading and unloading in tension, compression, bending, torsion or combinations of these stresses. The fatigue strength of cast iron (ductile or cast) is typically affected by environmental factors such as corrosion.

There are many other forms of mechanical properties that can be tested, but we have covered above the most important tests in the context of various cast irons.

All the mechanical properties mentioned above affect the quality of the cast iron (ductile, gray), so it is important to choose a manufacturer that meticulously undertakes these states. Only a few cast iron manufacturers in India have these facilities, so if you want to outsource ductile cast irons or any machined component in general to a manufacturer in India, evaluate their merits carefully.