The mechanical properties of alumi- num casting alloys are obtainable only if the chemical and heat treating specifications are followed carefully. It should be noted that the properties obtained from one particular combination of casting alloy, foundry practice and thermal treatment may not necessarily be identical to those achieved with the same alloy in a different foundry or with a different thermal treating source. In all aluminum casting alloys, the percentages of alloying elements and impurities must be controlled carefully. If they are not, characteristics such as soundness, machinability, corrosion resistance and conductivity are affected adversely. Read more
Compacted graphite iron is a fairly recent addition to the family of commercially produced aluminum casting irons. Its characteristics are intermediate to those of the gray and ductile irons. As in gray iron, the graphite in CG iron is in the form of interconnected flakes. This facilitates the production of sound castings, especially those of complex shape or with intricately cored passages. However, the relatively short span and blunted edges of CG graphite provides improved strength, some ductility, and a better machined finish than gray iron. Read more
Ductile iron is characterized by having all of its graphite occur in microscopic spheroids. Although this graphite constitutes about 10% by volume of ductile iron, its compact spherical shape minimizes the effect on mechanical properties. The graphite in commercially produced ductile iron is not always in perfect spheres. It can occur in a somewhat irregular form, but if it is still chunky as Type II in ASTM Standard A247, the properties of the iron will be similar to cast iron with spheroidal graphite. Of course, further degradation can influence mechanical properties. The shape of the graphite is established when the metal solidifies, and it cannot be changed in any way except by remelting the metal. Read more
Microscopically, all gray irons contain flake graphite dispersed in a silicon-iron matrix. How much graphite is present, the length of the flakes and how they are distributed in the matrix directly influence the properties of the iron.
The basic strength and hardness of the iron is provided by the metallic matrix in which the graphite occurs. The properties of the metallic matrix can range from those of a soft, low carbon steel to those of hardened, high carbon steel. The matrix can be entirely ferrite for maximum machinability but the iron will have reduced wear resistance and strength. An entirely pearlitic matrix is characteristic of high strength gray irons, and many castings are produced with a matrix microstructure of both ferrite and pearlite to obtain intermediate hardness and strength. Alloy additions and/or heat treatment can be used to produce gray iron with very fine pearlite or with an acicular matrix structure. Read more
The term “cast iron” designates an entire family of metals with a wide variety of properties. It is a generic term like steel which also designates a family of metals. Steels and cast irons are both primarily iron with carbon as the main alloying element. Steels contain less than 2% and usually less than 1% carbon, while all cast irons contain more than 2% carbon. About 2% is the maximum carbon content at which iron can solidify as a single phase alloy with all of the carbon in solution in austenite. Thus, the cast irons by definition solidify as heterogeneous alloys and always have more than one constituent in their microstructure. Read more