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Gray iron is the most commonly used cast iron alloy. When gray iron is broken apart, it may reveal the gray color from which it takes its name. This color is a result of the presence of graphite in the alloy. The presence of graphite is the result of particular ratios of carbon and silicon in the alloy, which helps to facilitate the formation of what is known as a graphitic microstructure. Gray iron is a relatively inexpensive alloy that is widely used in the casting of everything from engine blocks to cookware.
To create gray iron, the alloy must contain around 2.5 percent to 4 percent carbon and 1 percent to 3 percent silicon. The presence of carbon and silicon in these ratios allows the alloy to form the graphitic microstructure for which it is known. Graphite is a hexagonally crystallized allotrope of carbon, so the presence of carbon is necessary for graphite to form at all. However, silicon in the alloy is necessary to act as a graphite stabilizing agent. An alloy with a lower silicon content may allow iron carbides to form rather than graphite, especially if the alloy is cooled quickly, resulting instead in what is known as white iron.
Gray iron has less tensile strength than steel, though its compressive strength is similar. It is less expensive to produce than steel, which makes it useful in applications where tensile strength isn't critically important. This has led to gray iron being the most commonly used cast iron alloy.
The presence of graphite in the alloy contributes to it being easily machinable. This is because of the lubricating nature of the graphite flakes present in the alloy. The graphite may also assist in resistance to wear and galling, while the presence of silicon can assist in corrosion resistance.
There are several different grades of gray iron, each of which has a different tensile and compressive strength. In the United States, these grades are defined by numbers that correspond to the amount of tensile strength the material has. Thus, gray iron of grade 20 would have to have a minimum tensile strength of 20 kilo-pound per square inch (KSI), or 20,000 pound-force per square inch (PSI) (about 6,894 kilopascals). Compressive strength increases along with tensile strength, so a higher-grade alloy will also have a higher compressive strength than a lower-grade alloy. Higher-grade alloys contain less carbon than lower-grade alloys and also tend to be more brittle.