Tensile Properties of Austempered Ductile Iron
Metal, to be useful, must be strong without being stiff, and changeable without being weak. Tensile strength measures how well metal stands up to stress without cracking, and ductility tells us how easily metal can be drawn out into useful shapes, such as wire. Iron from the ground is brittle and must be heated and cooled to achieve a useful form.
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The Process
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The higher you heat iron, the better you straighten out its atomic lattice and the stronger the iron will be, once it cools down. Heating and cooling iron to harden the metal has been done for centuries. Austempering is a modern method for hardening iron to make it more ductile, uniformly hard and better shock-resistant (or with greater tensile strength). Cast iron from the foundry is plunged into a hot-salt bath. The iron enters the bath in a form known as austenite; it emerges as bainite, still iron but with its molecules rearranged. Precision devices control how hot to maintain the quench bath, and how long to quench the metal, the two key factors affecting the final product's tensile strength.
Tensile Properties
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ASTM International sets the standards used by industry to benchmark an array of products, including austempered ductile iron. The lowest of five grades of this ductile iron has a tensile strength of 130,000 lbs. per square inch; the highest, 230,000 lbs. Tensile strength is a metal's breaking point. A related yardstick is yield strength -- how much weight the metal will take before it begins to deform. Yield strength for austempered ductile iron, lowest to highest grade, is 90,000 to 185,000 lbs. per square inch.
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Ductility
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Iron isn't copper or gold, but it is still somewhat ductile, a measure of a metal's plasticity. (On the other hand, it's a lot cheaper than those other metals.) Metallurgy is all about tradeoffs -- greater tensile strength, yield strength and hardness, less stretchiness. Even the lowest ASTM grade of austempered ductile iron has an elongation factor of only 9 percent, and the highest grade drops to 1 percent. Those numbers are sufficient to handle the expansion and contraction of iron in ordinary use, but you won't see any austempered ductile iron jewelry at Christmastime.
Uses
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Machinery shafts, gears, clamps and brackets are some ways that the three lowest grades of austempered ductile iron are used, when manufacturers are looking for a lower-cost alternative to aluminum or steel. These grades can still be machined after they're heat treated. The top two grades are so hard they must be machined in advance, then heat treated. They're used widely in construction and farming for digging equipment and machine tools because the top grades resist abrasion.
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