Mechanical Properties of S.G. Cast Iron

Grey cast irons are metal matrix composites with low shrinkage and good castability. For that reason, they find application in products that could be obtained directly by moulding taking advantage of this good castability as is the case of different parts used in the automotive industry. However, their main disadvantage is the notch effect caused by the flakes of graphite. This causes gray cast iron to have poor tensile strength and impact resistance. Spheroidal graphite cast iron (S.G. cast iron) is formed by the addition of heterogeneous nucleating agents, such as magnesium–nickel or magnesium–iron–silicon during the solidification of gray cast iron. The density of these cast irons, which is around 7.1 g/cm3, changes with the increase of carbon content (diminishing) and the high graphitizing degree (diminishing) as well. The spheroidal graphite provides better ductility and castability than gray cast iron.

The ductility and toughness of spheroidal graphite cast irons, despite being lower than in steels, are better in comparison with cast irons of flake graphite. This better aptitude to plastic deformation justifies the name of ductile cast iron, which is usually employed in the spheroidal graphite cast irons designation. The ferritic matrix (how much a ferrous alloy can stretch) can reach 25%, which is why S.G. cast iron is popular choice for machined components that need to have good elongation coefficient in addition to good tensile strength. And unlike gray cast iron, which just crack up, S.G. cast irons are malleable and have a good yield strength.

In a foundry, S.G. cast iron is produced by melting mild steel scrap, coke and other material in an induction furnace. Once the melt is ready, it is inoculated with small addition of magnesium or cerium available in ferro blends. The metal is then poured into moulds, cooled and fettled. The total carbon remains as spheroid in 'as-cast' condition and inhibits the creation of linear cracks. This is why S.G. cast irons can withstand distortion. However, it may further be annealed to achieve the desired properties if needed.

In general, cast irons have a lower melting point than traditional steel because of the greater carbon percentage they have, and this makes them easier to cast than standard steels. Because of its high fluidity when molten, S.G. cast iron can be shaped easily, allowing filling of intricate moulds and forming complex shapes. This is one of the main reasons S.G. iron is used in automobile industry, injection moulding machines, and piping industry. Of course, because of its ductility, S.G. cast iron finds uses in other industrial sectors as well such as agricultural, electrical and mining.

Mechanical Properties of S.G. Cast Iron
In the context of foundries and castings, 'properties' imply 'mechanical properties'. The addition of different metals - even in miniscule quantities - imparts different properties to castings, and S.G. cast iron is no different. The addition of a small amount of cerium or chromium imparts mechanical properties to S.G. castings that are considerably different from simple gray cast iron

1. Tensile strength
The tensile strength of any material indicates how the material will react to forces being applied in tension when it is pulled until broken. A tensile test is a fundamental mechanical test where a carefully prepared specimen is loaded in a very controlled manner while measuring the applied load and the elongation of the specimen over some distance. Tensile tests are used to determine the modulus of elasticity, elastic limit, elongation, proportional limit, reduction in area, tensile strength, yield point, yield strength and other tensile properties. The tensile strength of S.G. cast iron is greater than that of gray cast iron.

2. Yield strength
Yield strength of S.G. cast iron (or any casting, for that matter) is the stress level at which the material being tested begins to deform permanently. It is the stage as which a casting starts to strain plastically. The yield strength is a practical approximation of the elastic limit. Quite often, it is not possible to determine the exact point at which the material suffers permanent deformity, so in practice a proof stress is used. Again, the yield strength of S.G. iron is better than that of gray iron.

3. Elongation
Elongation is defined as the permanent increase in length, expressed as a percentage of a specified gauge length marked in a tensile test bar, which is produced when the bar is tested to failure. Elongation is a very useful property as it is a measure of the ductility of the casting. The higher the elongation, better the malleability. Elongation is usually expressed as a percentage of the original gauge length. Brittle materials such as gray cast iron can fail in tension without any significant elongation, but S.G. or ductile iron is quite malleable.

4. Hardness
Hardness is not exactly a fundamental property of castings, but is a composite property including in some way tensile strength, proportional limit, ductility, work-hardening properties, shear strength, modulus of elasticity, and other properties. The Brinell hardness test is commonly used to determine the hardness of materials like metals and alloys. The test involves applying a known load to the surface of the tested material through a hardened steel ball of known diameter. The diameter of the resulting permanent impression in the tested metal is measured and the Brinell Hardness Number is calculated from that. Hardness basically affects the choice of material wherever machined components are to be used.