Tensile Properties: Gray iron’s tensile strength, yield strength, duc- tility and modulus of elasticity can be established by a conventional test, as specified by the standard ASTM A-48. Although yield strength and ductility may be measured, they are seldom determined or specified. Its modulus of elasticity is not constant, as in the case of steel, and varies with the class of iron and type of graphite loading. Te tensile strength of gray iron is influenced by both the normal ele- ments present in plain iron, such as carbon, silicon, phosphorus, sulfur and manganese, and the presence of alloy- ing additions and trace elements. Ductile iron’s commonly estab- lished tensile properties are tensile strength, yield strength and percent elongation. Te minimums for these properties typically are established by the specification or implied by specifying the hardness of the casting. Because of the nominal and consis- tent influence of spheroidal graphite, the tensile properties and the Brinell hardness of ductile iron are well corre- lated. Te relationship between tensile properties and hardness depends on microstructure. Ductile iron combines the pro-
cessing advantages of gray iron (low melting point, good fluidity and cast- ability, and ready machinability) with many of the engineering advantages of steel (high strength, ductility and wear resistance), which allows for higher material properties as tensile and yield strength than gray iron. Table 4 shows the properties for various grades of ductile iron.
Damping Capacity: The rela-
tive ability of a material to absorb vibration is evaluated as its damping capacity. The quelling of vibration by converting the mechanical energy into heat can be important in struc- tures and devices with moving parts. Components made of materials with a high damping capacity can reduce noise such as chatter, ringing and squealing, and minimize the level of applied stresses. Because it can cause unsatisfactory operation or even fail- ure, vibration can be a critical factor
Sept/Oct 2014 | METAL CASTING DESIGN & PURCHASING | 43
Table 4. Compasition Range for Un-Alloyed Cast Iron Iron Family Carbon Gray
Ductile
2.5-4.2 3-4
1.8-3
in the operation of machinery. Gray iron exhibits an exception-
ally high damping capacity. For this reason, it is ideally suited for machine bases and supports, engine cylinder blocks and braking compo-
Table 5. Damping Capacity of Gray Iron Material
White Iron
Malleable Iron Ductile Iron
Gray Iron, Fine Flake
d x 10-4* 2–4 8–15 5–20
20–100
Gray Iron, Coarse Flake 100–500 Eutectoid Steel Armco Iron Aluminum
4 5
0.4
*Natural log of the ratio of successive amplitude
0.15-1 0.1-1
Silicon Manganese Sulfur 1-3
0.02-0.25
Phosphorus 0.021
0.01-0.03 0.01-0.1
nents. The damping capacity of gray iron is considerably greater than that of steel or other kinds of iron. The graphite flakes in gray iron have less influence on its compression proper- ties than on tension properties. Gray iron’s compressive strength is typi- cally three to four times more than its tensile strength. Te damping capacity of ductile
iron is well below that of gray iron. (See Table 5.) For example, if gray iron and ductile iron have a similar composition, the relative damping capacity of gray iron is 1.0, while ductile iron is 0.14. Still, ductile iron compares favorably versus steel alloys in terms of damping capacity, espe- cially in automotive applications such as shaft gears where this increased capacity can reduce noise.
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