![]() These spheres make ductile iron stiffer, stronger and more shock-resistant than gray 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. Gray iron serves well in machinery applications because of its fatigue resistance.ĭuctile iron. Typical gray iron applications include automotive engine blocks, gears, flywheels, brake discs and drums, and machine bases. 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. The test bar’s cross-section usually matches or is related to a particularly critical section of the finished part. It is also specified by cross section and minimum strength of a test bar. Class 20, for example, specifies a minimum tensile strength of 20,000 psi. Gray iron is specified by a two-digit designation. 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. 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. ![]() 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. Another important characteristic of gray iron, particularly for precision machinery, is its ability to damp vibration. 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. But its impact strength is below that of most other cast ferrous metals. These carbon flakes increase the metal’s strength, especially its compressive strength which is three to five times as great as its tensile strength. This iron alloy contains so much carbon it precipitates out in the form of graphite flakes. When tempered, martensite provides machinability with maximum strength and good wear resistance. This adds a martensite structure to the alloy. For example, the precipitation of carbon (as graphite) during provides excellent machinability even when it is boosting at wear-resistance, damps vibrations and helps lubricate wearing surfaces.Ĭast irons can also be flame-hardened, induction-hardened and furnace-heated, then quenched with oil. The key to cast iron’s range of properties is its carbon content. READ MORE: Investment or Sand Casting? Which is Right for Your Application? Cast irons, however, do not have the ductility to be rolled or forged. And because lower-density graphite does not form when cast irons solidify, they can be cast in complex shapes. Molten iron is also more fluid than molten steel and reacts less with molding materials. For example, their melting temperatures are appreciably lower than those of steel. The high carbon and silicon content in cast irons make them well suited for casting. This has let metallurgists develop cast irons with a range of properties. ![]() Cast irons must also contain silicon, usually from 1 to 3% so, they are actually iron-carbon-silicon alloys.Īlthough cast iron is often considered a simple metal to produce and specify, its metallurgy is more complex than that of steel and most other metals. Cast irons, by definition, solidify as heterogeneous alloys and are made up of than one constituent. ![]() 2% is the dividing line below that, the alloy (steel) solidifies as a single-phase material-i.e., austenite-and all the carbon is in the austenite. By comparison, steels are less than 2% carbon and usually less than 1% carbon. 15, 2002.Ĭast iron, by definition, is any iron alloy with more than 2% carbon and uses that carbon as the main alloying additive. ![]()
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