Table 3. Property Comparisons for Gray Iron Classes Property
Class 25 (as-cast)
Brinell Hardness Tensile Strength
Modulus of Elasticity Tensile Poisson’s Ratio
Compression Poisson’s Ratio Compression-to-Tensile Strength Ratio 187
29.9 ksi (206 MPa) 16.6 Msi (114 GPa) 0.29 0.27 3.68
MPa); however, its modulus is 20% lower than steel with a comparable strength. Compacted Graphite Iron (CGI)—
In CGI, graphite locally occurs as interconnected blunt flakes. This graphite structure and the re- sulting properties are interme- diate between gray and ductile irons. The compacted graphite shape also is called quasi-flake, aggregated flake, semi-nodular and vermicular graphite. White iron—White iron is hard and
essentially free of graphite. The metal so- lidifies with a compound called cementite, which is a phase that dominates the mi- crostructure and properties of white iron. The carbides are in a matrix that may be pearlitic, ferritic, austenitic, martensitic or any combination thereof. Malleable iron—In malleable iron,
the graphite occurs as irregularly shaped nodules called temper carbon because it is formed in the solid state during heat treat- ment. The iron is cast as a white iron of a suitable chemical composition to respond to the malleabilizing heat treatment. Alloyed iron—This classification in-
cludes gray irons, ductile irons and white irons that have more than 3% alloying elements (nickel, chromium, molybde- num, silicon or copper). Malleable irons are not heavily alloyed because many of the alloying elements interfere with the graphite-forming process that occurs during heat treatment.
Specification Sources There are a number of standard
specifications for several cast iron products, such as pipes and fittings. But due to the important influence of the individual design on the characteristics of a casting, requirements (actual strength, dimensional tolerances and surface finish) cannot be stated in a general specification. Table 2 gives iron specifications, char-
acteristics and applications. There are no generally accepted standards for the surface finish, machining allowances or dimensional tolerances. Although some production metalcasting facilities have established guidelines for their own
16 Metal Casting Design & PurChasing
capability in dimensional control, these controls are typically established through concurrent engineering based on the requirements of the application.
Material Properties vs. Casting Processes
The properties of all metals are in-
fluenced by the manner in which they solidify and cool. The individual design of a casting (the molding process, the way the
Class 30 (as-cast)
207
33.7 ksi (232 MPa) 17.0 Msi (117 GPa) 0.19 0.28 3.84
(annealed) 109
20.6 ksi (142 MPa) 14.5 Msi (100 GPa) 0.21 0.26 4.05
Class 30
Class 35 (as-cast)
212
34.8 ksi (240 MPa) 18.0 Msi (124 GPa) 0.22 0.28 3.63
Class 40 (as-cast)
235
41.9 ksi (289 MPa) 18.2 Msi (126 GPa) 0.24 0.23 3.71
With the capability of drastically reducing weight, CGI is finding its way into many applications. This 2.7L V6 diesel engine is in high-volume production.
molten metal is introduced into the cavity and the pouring temperature) determines the rate of cooling in the various parts of a casting. The cooling rate in any particular section factors heavily into the mechanical properties of the iron. Therefore, a series of standard test bars of increasing size has been established specifically for gray iron.
One of these bars should be selected to determine the cooling rate in the critical sections of the casting.
Types of Specifications When mechanical properties are im-
portant, the most common procedure to qualify iron is to use a standard test bar poured separately with the specified lot of general engineered castings. Most specifications of the American Society for Testing and Materials (ASTM) apply this method to qualify the iron used to pour the castings. The actual properties of the metal in the casting will depend upon its characteristics and the cooling rate of the metal in its various sections. Ductile iron has some variation in
properties when not heat treated; how- ever, a single size test bar generally is satisfactory (except for large castings). Malleable iron is not poured into heavy sections, and because all malleable iron castings are heat treated, a single size test bar will work. For some applications, the finished component is tested in the manner in which it will be used. For ex- ample, pressure-containing parts can be 100% hydraulically proof tested. The acceptance of
Ductile iron combines the pro- cess advantages of gray iron (low melting point, good fluid-
ity, good castability and ready machinability) with high ductility.
Shown above is a 40-lb. rear chute pivot for concrete mixer trucks.
2010 Casting sourCe DireCtory
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