Iron Alloys


The American Foundry Society Technical Dept., Schaumburg, Illinois George M. Goodrich, Professional Metallurgical Services, Buchanan, Michigan


I


ron castings are produced by a va- riety of molding methods and are available with a wide range of prop- erties. Cast iron is a generic term that designates a family of metals.


To achieve the best casting for a particular application at the lowest cost consistent with the component’s requirements, it is necessary to have an understanding of the six types of cast iron: • gray iron; • ductile iron; • compacted graphite iron (CGI); • malleable iron; • white iron; • alloyed iron. Table 1 lists the typical composition


ranges for common elements in five of the six generic types of cast iron. The classifica- tion for alloyed irons has a wide range of base compositions with major additions of other elements, such as nickel, chromium, molybdenum or copper. The basic strength and hardness of all iron


alloys is provided by the metallic structures containing graphite. The properties of the


Table 1. Composition Range for Un-Alloyed Cast Irons (in %) Iron Family Gray


Ductile CGI


Malleable White


Carbon 2.5-4.2 3-4


2.5-4


2.2-2.8 1.8-3.6


Silicon 1-3


1.8-3 1.5-3


1.2-1.9 0.5-2


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). The modulus of elasticity varies with the class of iron, shape (sphericity) and vol- ume 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 adher- ing 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 appre- ciably enhanced with alloyed irons. Properties of the cast iron family can be adjusted over a wide range and enhanced


Manganese 0.15-1 0.1-1


0.10-1


0.15-1.2 0.15-0.8


Sulfur


0.02-0.25 0.01-0.03 0.01-0.03 0.02-0.2 0.02-0.2


Phosphorus 0.02-1


0.01-0.1 0.01-0.1 0.02-0.2 0.02-0.2


by heat treatment. Annealing produces a matrix of soft machinable ferrite. In limited situations, this annealing can be accomplished at sub-critical temperatures. Heating above this critical temperature 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. These adjustments create the different members of the cast iron family. Gray iron—Flake graphite provides


gray iron with unique properties (such as excellent machinability) at hardness levels that produce superior wear-resistant characteristics, the ability to resist galling


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.


14 Metal Casting Design & PurChasing 2010 Casting sourCe DireCtory


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