What Are the Causes of Banding in Medium Carbon Alloy Steel?
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Carbon Steel
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Carbon alloy steel is essentially iron that has carbon added to it to make steel that is harder, stronger, and more resistant to elemental forces than pure iron. While carbon steel is very common and used for many different applications, it can become brittle, which means it can be cracked or broken under extreme force more easily than other types of steel alloys. Also, carbon steel does not bond easily to other metals, making it difficult to weld different alloys to carbon steel.
There are a couple different kinds of carbon steel, but the "average" and most common version is known as plain carbon steel, which has less than 1 percent of carbon. Banding can occur in nearly any type of carbon steel, but is generally most common in low-carbon or "mild" steels, those that are less than 0.30 percent carbon and are reserved for welding and manufacturing work. Medium carbon steels are made with 0.30 to 0.45 percent carbon, making them stronger but more difficult to weld.
Banding
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Banding, or the formation of discolored bands across the steel with greater concentrations of other alloy substances, can make the metal weaker and more prone to cracking, not to mention affect the look of the metal significantly. Carbon steel must be heated to very high temperatures to properly fuse the carbon and the iron. Sometimes, as the metal is cooling, an event known as "ferrite banding" occurs, which can be thought of like the growth of "crystals" within the metal. Like crystal growth, those bands are made from accumulating material being drawn together due to similar properties---in this case, the carbon attracts other carbon molecules.
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Banding Causes
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The most important stage of the manufacturing process is the cooling. If the carbon steel is cooled rapidly, the carbon has little time to migrate and little banding occurs. If the steel has a slow cool-down time, then the carbon can move more easily and banding is more likely. Reheating steel and allowing it to cool slowly often causes banding to occur, for similar reasons.
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