PRODUCT INNOVATIONS New Product
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Austempering Gray Iron Aids in Noise Damping, Wear Resistance In addition to the well-known austem-
pered ductile iron (ADI), austempered gray iron (AGI) increasingly has found applications in agricultural equipment and component applications. AGI combines good wear resistance and noise damping at a total manufacturing cost less than ADI, steel or aluminum. This makes it a good material/process combination for applications that require a low cost, complex shape, and good strength and wear resistance, where impact and cyclic stresses are not signifi cant. According to Applied Process Inc.,
Livonia, Mich., which provides austem- pering services, austempering offers manufacturers numerous opportunities to make their iron components tougher, stronger, lighter, quieter and more wear resistant. Because austempering is an isothermal process, it offers several ad- vantages over conventional quenching and tempering and other methods of martensitic hardening. Martensitic trans- formation takes place when a part’s local material temperature drops below the martensite-start temperature. Therefore, the transformation takes place at differ- ent times in sections of differing section modulus. This can result in inconsistent dimensional response and micro- and macro-cracking. However, since the formation of bainite and ausferrite occur uniformly throughout the part over many minutes or hours, austempered components exhibit consistent dimensional response and no cracking. AGI provides the same excellent wear resistance as its aus-
Fig. 1. The tensile strength of gray iron classes 20, 30 and 40 are shown as-cast and austempered at 700F (371C), 600F (316C) and 500F (260C).
Fig. 2. This graph shows the damping coeffi cient of three classes of gray iron both as-cast and austempered at 700F (371C), 600F (316C) and 500F (260C).
ferritic cousin ADI and exhibits higher strength than as-cast gray iron. Fig. 1 shows the tensile strength array of Class 20, 30 and 40 gray iron as-cast and austempered at 700F (371C), 600F (316C) and 500F (260C). AGI’s most salient feature is its ability to dampen noise due
to the combination of an ausferritic matrix and large graphite fl akes. As the austempering temperature is decreased, the strength of the AGI increases, as does the damping coeffi cient (Fig. 2). The graphite fl akes also limit the strength of AGI, acting as angular voids in the metal matrix and allowing maximum strengths no higher than approximately 450 MPa. The complex harvester machine cam in Fig. 3 shows the advantage of AGI’s good castability and machinability. The critical shape of the cam is maintained during the austemper- ing process, and the ausferrite matrix provides good wear resistance for durability and elevated noise damping. Visit
www.appliedprocess.com for more information.
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Fig. 3. This large austempered gray iron cam wheel is used in a harvesting machine.
MODERN CASTING / September 2010
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