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
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
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