the castings is based on characteristics that a metalcasting facility can evaluate and control during production. Some specifi cations limit the amount of certain elements because they can affect proper- ties or characteristics of the iron that are not readily apparent or measured. For example, the alloy content of iron can infl uence its corrosion resistance or its properties at elevated temperatures. With the exceptions of castings that have special performance applications, such as use at elevated temperatures, the most economical approach is to let the metalcasting facility recommend an iron composition that provides the desired properties.
Gray Iron Properties
Gray iron’s high damping capacity, combined with its excellent machinability and high hardness, is unique to this mate- rial and makes it ideally suited for machine bases and supports, engine cylinder blocks and brake components. Excessive vibration causes inaccuracies in precision machinery and excessive wear on gear teeth and bear- ings. The damping capacity of gray iron is considerably greater than that of steel and other iron types. For ex- ample, if gray
iron, CGI and duc- osit
tile iron have a similar composition, the relative damping capacity of gray iron is 1, CGI is 0.35 and
ductile iron is 0.14. The damping capacity of gray iron is about 20-25 times higher than steel. For comparison, aluminum’s damping capacity is one-tenth that of steel. The graphite fl akes in gray iron have
less infl uence on its compression proper- ties than on tension properties. A compari- son of compression and tension properties is shown in Table 3. Gray iron’s compressive strength is
typically three to four times more than its tensile strength. The 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 tension properties.
60-40-18 65-45-12 80-50-06 100-70-03 120-90-02
Treatment 1 2 2 3 4
Heat Strength Tensile
Poisson’s ratio remains constant at 0.25 over a large compressive stress range and increases at higher stress levels. To classify gray iron in accordance to its
tensile strength, ASTM Standard A48 and Society of Automotive Engineers (SAE) Standard J431 provide the best details. The two specifi cations approach the task from different standpoints, but the concept es- sentially remains the same. For example, the number in a Class 30 gray iron refers to the minimum tensile strength in ksi. In ASTM A48, a standard size test bar is added to the class. Class 30A indicates that the iron must have a minimum 30 ksi (207 MPa) tensile strength in an “A” bar (0.875- in. as-cast diameter).
Table 4. Property Comparisons for Ductile Iron Grades (ASTM A536) Grade
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)
2010 Casting sourCe DireCtory Yield
Cast in austempered ductile iron, this seed boot (below) weighs 15% less than the original steel weldment. The seed boot, visible at the bottom of the above photo, is designed to stand up to extremely rough terrain.
In SAE Standard J431, tensile strength is not required, but hard- ness and a minimum tensile strength to hardness ratio are required. The class then is identifi ed as a grade. A Class 30B iron for ASTM A48 would be comparable to a grade G3000 in SAE Standard J431. The other gray iron specifi cations build off of these two primary specifi cations.
Ductile Iron Properties Five grades of ductile iron are classifi ed
by their tensile properties in ASTM Standard A536 (Table 4). SAE Standard J434c (for automotive castings and similar applica- tions) identifi es these fi ve grades of ductile iron only by Brinell hardness. However, the appropriate microstructure for the indicated hardness also is a requirement. Specifi cations for specialty applications not only defi ne tensile properties but also have composition limits. In the ASTM specifi cation, the focus
% Elongation Brinell (min. 2 in.) Hardness
18 12 6 3 2
130-170 150-220 170-250 241-300 240-300
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)
Metal Casting Design & PurChasing 17
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