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Gray iron, orgrey cast iron, is a type ofcast iron that has agraphitic microstructure. It is named after the gray color of thefracture 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 thestiffness of the component is more important than itstensile strength, such asinternal combustion enginecylinder blocks,pump housings, valve bodies, electrical boxes, and decorativecastings. Grey cast iron's highthermal conductivity andspecific heat capacity are often exploited to makecast iron cookware anddisc brake rotors.[3]
Its former widespread use[clarify] on brakes infreight trains has been greatly reduced in theEuropean Union over concerns regardingnoise pollution.[4][5][6][7]Deutsche Bahn for example had replaced grey iron brakes on 53,000 of itsfreight cars (85% of their fleet) with newer, quieter models by 2019—in part to comply with a law that came into force in December 2020.[8][9][10]
A typical chemical composition to obtain a graphitic microstructure is 2.5 to 4.0%carbon and 1 to 3%silicon by weight. Graphite may occupy 6 to 10% of the volume of grey iron. Silicon is important for making grey iron as opposed towhite cast iron, because silicon is agraphite stabilizing element in cast iron, which means it helps the alloy produce graphite instead ofiron carbides; at 3% silicon almost no carbon is held in chemical form as iron carbide. Another factor affectinggraphitization is the solidification rate; the slower the rate, the greater the time for the carbon to diffuse and accumulate into graphite. A moderate cooling rate forms a morepearlitic matrix, while a fast cooling rate forms a moreferritic matrix. To achieve a fully ferritic matrix the alloy must beannealed.[1][11] Rapid cooling partly or completely suppresses graphitization and leads to the formation ofcementite, which is calledwhite iron.[12]
The graphite takes on the shape of a three-dimensional flake. In two dimensions, as a polished surface, 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 at which stresses concentrate and it therefore behaves in abrittle manner.[12][13] 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 often used as the base for machine tool mountings.
In the United States, the most commonly used classification for gray iron isASTM International standardA48.[2] This orders gray iron intoclasses which correspond with its minimumtensile 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 highcarbon equivalent and a ferrite matrix. Higher strength gray irons, up to class 40, have lower carbon equivalents and apearlite matrix. Gray iron above class 40 requires alloying to providesolid solution strengthening, andheat treating is used to modify the matrix. Class 80 is the highest class available, but it is extremely brittle.[12]ASTM A247 is also commonly used to describe the graphite structure. Other ASTM standards that deal with gray iron includeASTM A126,ASTM A278, andASTM A319.[2]
In the automotive industry, theSAE International (SAE) standardSAE J431 is used to designategrades instead of classes. These grades are a measure of the tensile strength-to-Brinell hardness ratio.[2] The variation of thetensile modulus of elasticity of the various grades is a reflection of the percentage of graphite in the material as such material has neither strength nor stiffness and the space occupied by graphite acts like a void, thereby creating a spongy material.
| Class | Tensile strength (ksi) | Compressive strength (ksi) | Tensile modulus, E (Mpsi) |
|---|---|---|---|
| 20 | 22 | 83 | 10 |
| 30 | 31 | 109 | 14 |
| 40 | 57 | 140 | 18 |
| 60 | 62.5 | 187.5 | 21 |
| 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 | |||
Gray iron is a commonengineering alloy because of its relatively low cost and goodmachinability, which results from the graphite lubricating the cut and breaking up the chips. It also has goodgalling andwear resistance because the graphite flakes self-lubricate. The graphite also gives gray iron an excellentdamping capacity because it absorbs the energy and converts it into heat.[3] Grey iron cannot be worked (forged, extruded, rolled etc.) even at temperature.
| Materials | Damping capacity† |
|---|---|
| Gray iron (highcarbon 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 | 0.47 |
| †Natural log of the ratio of successive amplitudes | |
Gray iron also experiences less solidificationshrinkage than other cast irons that do not form a graphite microstructure. The silicon promotes goodcorrosion resistance and increased fluidity when casting.[12] Gray iron is generally considered easy to weld.[16] Compared to the more modern iron alloys, gray iron has a low tensile strength andductility; therefore, itsimpact andshock resistance is almost non-existent.[16]
