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Grey iron

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Grey iron

Gray iron, or grey iron, is a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite.[1] It is the most common cast iron and the most widely used cast material based on weight.[2]

It is used for housings where tensile strength is non-critical, such as internal combustion engine cylinder blocks, pump housings, valve bodies, electrical boxes, and decorative castings. Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors.[3]

Structure

A typical chemical composition to obtain a graphitic microstructure is 2.5 to 4.0% carbon and 1 to 3% silicon. Silicon is important to making grey iron as opposed to white cast iron, because silicon is a graphite stabilizing element in cast iron, which means it helps the alloy produce graphite instead of iron carbides. Another factor affecting graphitization is the solidification rate; the slower the rate, the greater the tendency for graphite to form. A moderate cooling rate forms a more pearlitic matrix, while a slow cooling rate forms a more ferritic matrix. To achieve a fully ferritic matrix the alloy must be annealed.[1][4] Rapid cooling partly or completely suppresses graphitization and leads to formation of cementite, which is called white iron.[5]

The graphite takes on the shape of a three dimensional flake. In two dimensions, as a polished surface will appear under a microscope, the graphite flakes appear as fine lines. The graphite has no appreciable strength, so they can be treated as voids. The tips of the flakes act as preexisting notches; therefore, it is brittle.[5][6] The presence of graphite flakes makes the Grey Iron easily machinable as they tend to crack easily across the graphite flakes.Grey iron also has very good damping capacity and hence it is mostly used as the base for machine tool mountings.

Classifications

In the United States, the most commonly used classification for gray iron is ASTM International standard A48.[2] This classifies gray iron into classes which corresponds with its minimum tensile strength in thousands of pounds per square inch (ksi); e.g. class 20 gray iron has a minimum tensile strength of 20,000 psi (140 MPa). Class 20 has a high carbon equivalent and a ferrite matrix. Higher strength gray irons, up to class 40, have lower carbon equivalents and a pearlite matrix. Gray iron above class 40 requires alloying to provide solid solution strengthening, and heat treating is used to modify the matrix. Class 80 is the highest class available, but it is extremely brittle.[5] ASTM A247 is also commonly used to describe the graphite structure. Other ASTM standards that deal with gray iron include ASTM A126, ASTM A278, and ASTM A319.[2]

In the automotive industry the SAE International (SAE) standard SAE J431 is used to designate grades instead of classes. These grades are a measure of the tensile strength-to-Brinell hardness ratio.[2]

Properties of ASTM A48 classes of gray iron[7]
Class Tensile
strength [ksi]
Compressive
strength [ksi]
Tensile modulus
(E) [106 psi]
20 22 33 10
30 31 109 14
40 57 140 18
60 62.5 187.5 21
Properties of SAE J431 grades of gray iron[7]
Grade Brinell hardness t/h Description
G1800 120–187 135 Ferritic-pearlitic
G2500 170–229 135 Pearlitic-ferritic
G3000 187–241 150 Pearlitic
G3500 207–255 165 Pearlitic
G4000 217–269 175 Pearlitic
t/h = tensile strength/hardness

Advantages and disadvantages

Gray iron is a common engineering alloy because of its relatively low cost and good machinability, which results from the graphite lubricating the cut and breaking up the chips. It also has good galling and wear resistance because the graphite flakes self lubricate. The graphite also gives gray iron an excellent damping capacity because it absorbs the energy.[3]

Relative damping capacity of various metals
Materials Damping capacity
Gray iron (high carbon equivalent) 100–500
Gray iron (low carbon equivalent) 20–100
Ductile iron 5–20
Malleable iron 8–15
White iron 2–4
Steel 4
Aluminum 0.465654
Natural log of the ratio of successive amplitudes


Gray iron also experiences less solidification shrinkage than other cast irons that do not form a graphite microstructure. The silicon promotes good corrosion resistance and increase fluidity when casting.[5] Gray iron is generally considered easy to weld.[8] Compared to the more modern iron alloys, gray iron has a low tensile strength and ductility; therefore, its impact and shock resistance is almost non-existent.[8]

See also

Notes

References

Further reading

  • fr:Fonte (métallurgie)#Fonte grise
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