Machinery's Handbook, 31st Edition
1480
MATERIALS FOR IRON AND STEEL CASTING METAL CASTING, MOLDING, AND EXTRUSION Material Properties for Iron and Steel Casting
Cast irons and cast steels encompass a large family of ferrous alloys, which, as the name implies, are cast to shape rather than being formed by working in the solid state. In gen- eral, cast irons contain more than 2 percent carbon and from 1 to 3 percent silicon. Varying the balance between carbon and silicon, alloying with different elements, and changing melting, casting, and heat-treating practices can produce a broad range of properties. In most cases, the carbon exists in two forms: free carbon in the form of graphite and com- bined carbon in the form of iron carbide (cementite). Mechanical and physical properties depend strongly on the shape and distribution of the free graphite and the type of matrix surrounding the graphite particles. The four basic types of cast iron are white iron, gray iron, malleable iron, and ductile iron. In addition to these basic types, there are other specific forms of cast iron to which special names have been applied, such as chilled iron, alloy iron, and compacted graphite cast iron. Gray Cast Iron.— Gray cast iron may easily be cast into any desirable form and it may also be machined readily. It usually contains from 1.7 to 4.5 percent carbon, and from 1 to 3 percent silicon. The excess carbon is in the form of graphite flakes, which impart to the material the dark-colored fracture that gives it its name. Gray iron castings are widely used for such applications as machine tools, automotive cylinder blocks, cast-iron pipe and fittings, and agricultural implements. The American National Standard Specifications for Gray Iron Castings—ANSI/ASTM A48-76 groups the castings into two categories. Gray iron castings in Classes 20A, 20B, 20C, 25A, 25B, 25C, 30A, 30B, 30C, 35A, 35B, and 35C are characterized by excellent machinability, high damping capacity, low modulus of elasticity, and comparative ease of manufacture. Castings in Classes 40B, 40C, 45B, 45C, 50B, 50C, 60B, and 60C are usually more difficult to machine, have lower damping capacity, higher modulus of elasticity, and are more difficult to manufacture. The prefix number indicates minimum tensile strength in pounds per square inch, e.g., 20 is 20,000 psi (138 MPa), 25 is 25,000 psi (172 MPa), 30 is 30,000 psi (207 MPa), etc. High-strength iron castings produced by the Meehanite-controlled process may have various combinations of physical properties to meet different requirements. In addition to a number of general engineering types, there are heat-resisting, wear-resisting and corro sion-resisting Meehanite castings. White Cast Iron.— When nearly all of the carbon in a casting is in the combined or ce- mentite form, it is known as white cast iron. It is so named because it has a silvery-white fracture. White cast iron is very hard and also brittle; its ductility is practically zero. Cast ings of this material need particular attention with respect to design since sharp corners and thin sections result in material failures at the foundry. These castings are less re- sistant to impact loading than gray iron castings, but they have a compressive strength that is usually higher than 200,000 psi (1379 MPa) as compared to 65,000–160,000 psi (448–1103 MPa) for gray iron castings. Some white iron castings are used for applications that require maximum wear resistance but most of them are used in the production of malleable iron castings. Chilled Cast Iron.— Many gray iron castings have wear-resisting surfaces of white cast iron. These surfaces are designated by the term “chilled cast iron” since they are produced in molds having metal chills for cooling the molten metal rapidly. This rapid cooling results in the formation of cementite and white cast iron. Alloy Cast Iron.— This term designates castings containing alloying elements such as nickel, chromium, molybdenum, copper, and manganese in sufficient amounts to appre ciably change the physical properties. These elements may be added either to increase the strength or to obtain special properties such as higher wear resistance, corrosion resistance,
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