metals that include gray, ductile, aus- tempered ductile, carbidic austempered ductile, compacted graphite, malleable, white and alloyed irons. Because of their excellent proper-
I
ties and castability, gray and duc- tile irons are the dominant metals produced in metalcasting facilities today. Gray cast irons are the pre- ferred material when the designer is seeking low cost complex geom- etries with strength and high density. For applications in which thermal conductivity is needed, compacted graphite iron (CGI) is an ideal candi- date. For higher strength applications, ductile iron can be used. Austemper- ing of ductile iron creates additional opportunities by further improving strength, fatigue resistance, and wear resistance. Carbidic austempered duc- tile iron (CADI) is the next genera- tion of austempered ductile iron ADI with higher wear resistance, making it suitable for use as digger teeth, ripper
Iron Alloys
ron castings are produced by a variety of molding methods and are available with a wide range of properties. Cast iron is a generic term that designates a family of
points, wear plates, rasp bars, cutters and crushers.
Properties Te basic strength and hardness of
all iron alloys is provided by the metal- lic structures containing graphite. Te properties of the iron matrix can range from those of soft, low-carbon steel (18 ksi/124 MPa) to those of hardened, high-carbon steel (230 ksi/1,586 MPa). Te modulus of elasticity varies with the class of iron and the shape (spheric- ity) and volume fraction of the graphite phase (percent free carbon). Because of their relatively high
silicon content, cast irons inherently resist oxidation and corrosion by developing a tightly adhering oxide and subscale to repel further attack. Iron castings are used in applications where this resistance provides long life. Resistance to heat, oxidation and corrosion are appreciably enhanced with alloyed irons. Properties of the cast iron family
can be adjusted over a wide range and enhanced by heat treatment. Anneal- ing produces a matrix of soft machin- able ferrite. In limited situations, this
annealing can be accomplished at sub- critical temperatures. Heating above critical temperatures takes the carbon from the graphite and places it in the matrix. This engineered material can be through-hardened and tempered using conventional heat treating or surface hardening (Table 1). These adjustments create the different mem- bers of the cast iron family.
Types Gray iron—Flake graphite provides
gray iron with unique properties (such as excellent machinability) at hardness levels that produce superior wear-resis- tant characteristics, the ability to resist galling and excellent vibration damping. Te damping capacity of gray iron is considerably greater than that of steel and other iron types. Gray iron’s compressive strength is
typically three to four times more than its tensile strength (Table 2). Te lack of graphite-associated volume changes allows for a similar Poisson’s ratio (the ratio of the relative contraction strain to the relative extension strain) to other engineering metals but different ten- sion properties. Poisson’s ratio remains
The Agri-Speed Hitch helps farmers avoid the dangerous area between a tractor and trailer. The new cast ductile iron hitch (bottom right) is more versatile than the previous steel weldment version (top right) and can be used with both larger and smaller machinery.
2012 Casting sourCe DireCtory Metal Casting Design & PurChasing 15
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