Table 2. Property Comparisons for Ductile Iron Grades (ASTM A536) Grade
Heat
60-40-18 65-45-12 80-50-06 100-70-03 120-90-02
Treatment 1 2 2 3 4
Tensile Strength Yield Strength
60,000 psi (413 MPa) 40,000 psi (276 MPa) 65,000 psi (448 MPa) 45,000 psi (310 MPa) 80,000 psi (551 MPa) 55,000 psi (379 MPa) 100,000 psi (689 MPa) 70,000 psi (482 MPa) 120,000 psi (827 MPa) 90,000 psi (620 MPa)
noise and vibration, making it suit- able for counterweights, machine tool bases, power transmission parts, and heavy duty engine blocks (Table 4). “If you bang steel or ductile iron,
you get a ping,” explained Tom Kay- ser, sales and marketing manager for gray iron caster Osco Industries, Ports- mouth, Ohio. “If you bang gray iron, you get a thunk. There is little vibra- tion. It’s why gray iron is still used in automobiles for vibration dampening.” Gray iron’s “thunk” can be at-
tributed to its microstructure, which consists of graphite fl akes, as op- posed to ductile iron’s spheroidal nodules. According to Peaslee, the flakes cross over grain boundar- ies and touch each other, leaving little space in the microstructure through which vibrations or sound can travel. Compacted graphite iron, which features a mixture of fl akes and nodules, can offer dampening characteristics close to that of gray iron, with strength properties close to that of ductile iron (Table 5). The combination has the interest of en- gine makers like Cummins, but CGI still is a more expensive material and brings with it other challenges. “The surprising thing about CGI is that machining is a signifi cant cost,” Walling said. “The challenges of ma- chining can cause you to stay away from CGI.”
Break But Won’t Bend Gray iron’s graphite fl akes also aid
in machining because it causes the material to be self-lubricating. In the fi eld, the lubrication allows parts to grind against each other without caus- ing friction on the surfaces. “As parts rub against each other,
they wear in, so they actually become less resistant,” Kayser said. “This works especially well for the compres- sor industry.” Gray iron also features good dimen- sional stability and thermal conductivity
SEPTEMBER/OCTOBER 2010
% Elongation (min. 2 in.)
18 12 6 3 2
Brinell
Hardness 130-170 150-220 170-250 241-300 240-300
(Table 1). Dimensional stability makes it suitable for applications where parts must retain their position accurately. In these instances, the bendability of ductile iron would work as a detriment to the end-use.
Poisson’s Ratio
0.28 0.28 0.28 0.28 0.28
Tensile Elastic Modulus
24.5 Msi (169 GPa) 24.5 Msi (169 GPa) 24.5 Msi (169 GPa) 25.5 Msi (176 GPa) 25.5 Msi (176 Gpa)
“In some cases, the engineers don’t
want the casting to bend; it needs to break,” Kayser said. “For instance, with a shaft going through a piece of equipment, if the casting bends, the shaft would come out of line. In com-
Fig. 1. Gray iron casting production has steadily decreased in the last few decades as many applications have been converted to ductile iron and aluminum.
Fig 2. Ductile iron casting production has increased steadily, reaching 4.1 million tons in 2008.
METAL CASTING DESIGN AND PURCHASING 29
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