Selecting the Right Cast Iron

Types of Cast Iron

There are several types of cast iron. Four of the most common are:

Gray iron. This iron alloy contains so much carbon it precipitates out in the form of graphite flakes. These carbon flakes increase the metal’s strength, especially its compressive strength which is three to five times as great as its tensile strength. But its impact strength is below that of most other cast ferrous metals.

Gray iron has no distinct yield point (as defined by classical formulas), so it should not be used when  permanent and predictable plastic deformation is preferred over catastrophic failure. Another important characteristic of gray iron, particularly for precision machinery, is its ability to damp vibration. Its damping capability is a function of the amount and type of graphite flakes; as the amount of graphite decreases, so too does the damping capacity.

Gray irons resist wear and even the softer grades perform well under certain borderline lubrication conditions, such as in the upper cylinder walls of internal combustion engines. To increase gray iron’s hardness, which is advantageous if a part will be exposed to abrasive wear, technicians can add alloying elements, or use special foundry techniques or heat treatments.

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Gray iron is specified by a two-digit designation. Class 20, for example, specifies a minimum tensile strength of 20,000 psi. It is also specified by cross section and minimum strength of a test bar. The test bar’s cross-section usually matches or is related to a particularly critical section of the finished part. This second specification is necessary because gray iron’s strength is highly sensitive to cross section, with smaller cross sections cooling faster and creating stronger parts.

Typical gray iron applications include automotive engine blocks, gears, flywheels, brake discs and drums, and machine bases. Gray iron serves well in machinery applications because of its fatigue resistance.

Ductile iron. Ductile iron contains trace amounts of magnesium which reacts with sulfur and oxygen in the molten iron and leads to carbon precipitating out as small spheres of graphite. These spheres make ductile iron stiffer, stronger and more shock-resistant than gray iron. Metallurgists make different grades of ductile iron by fine-tuning the iron’s crystalline structure around the graphite, which can be done before casting by formulation or after casting by applying heat treatments.

Adding magnesium to the alloy ends up making ductile iron stronger and more shock-resistant than gray iron. It also has a higher modulus of elasticity, but its damping capacity and thermal conductivity are lower than those of gray iron. By weight, ductile iron parts are more expensive than gray iron parts. But finished parts are stronger with better impact resistance and overall part costs can be about the same.

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A three-part designation lets designers specify ductile iron with defined characteristics. The designation for one typical alloy, 60-40-18, for example, specifies a minimum tensile strength of 60,000 psi, a minimum yield strength of 40,000 psi and 18% elongation in 2 in.

Ductile iron is used in applications such as crankshafts because it is easy to machine, and it has high fatigue strength and modulus of elasticity. It is used for heavy-duty gears due to its high yield strength and wear resistance. And it can be found in door hinges for cars, thanks to its ductility.