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     Ductile Iron Properties, Wear Resistance, Noise Reduction and Fatigue Strength

 

Ductile iron grades are characterized by their tensile strength, yield strength, and elongation, but those properties are not always useful in determining the suitability of its use in a specific application. Tensile properties do not provide any information about wear resistance, vibration damping, or fatigue strength, which are important properties when selecting materials for a specific application.

Wear Resistance: Ductile iron can be surface hardened to 60 HRC using conventional heat-treating methods. The wear properties after heat treatment are similar to those of 8620 carburized and hardened steel. Tests reveal that the abrasion resistance of ductile iron as measured by volume loss is less than 8620 steel when it has been quench and tempered to a final hardness of at least 30HRC. The volume loss of austempered ductile iron (ADI) is even less, and does not appear to be influenced by the final hardness. Improved wear resistance results from the presence of graphite nodules, which improves heat transfer and helps to lubricate the sliding wear surfaces.

Noise Reduction: The presence of precipitated graphite in both gray and ductile iron helps dampen vibration in gears, machine tool parts, and hydraulic components. The relative damping capacity of gray iron is as much as 100 times that of steel. Ductile iron has approximately 10 times the damping characteristics of steel.

Noise reduction in automotive balance shaft gears has been reported to be as much as 20 decibels when the same gear machined from gray iron was tested against steel. Ductile iron gears have shown noise reductions to be as much as eight decibels.

Currently, ductile iron is being considered to replace 8620 steel in hydraulic gear pumps in order to lower machining cost and decrease noise.

Fatigue Strength: The fatigue strength of ductile iron using a reverse bending variable speed plate testing machine will be in the range of 30-40 ksi, depending on the exact nature of the test and the grade of ductile iron being tested.

Additional testing recently performed at the University of Dayton Material Testing Lab shows the relative strength of quench and tempered ductile iron and austempered ductile iron compared to 8620 steel. A spur test gear was machined and set in a fixture that simulates the contact points on the gear tooth. The gears were loaded in a tension-tension test to determine the maximum load to failure at 10-million cycles.

Test results show that heat-treated ductile iron gears have approximately 90 percent of the fatigue strength of 8620 carburized and hardened gears. Austempered ductile iron gears have as much as 93 percent of the fatigue strength by comparison.

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