High-carbon steel comes in quite a few forms, each of which could have a different microstructure and characteristics that would make it better for a certain application than another. They do, however, have a very similar chemical composition. The most common high-carbon steel forms are described below.
Hot-rolled: The hot-rolling process is done near the recrystallization temperature, something that creates finer grains in the steel’s microstructure that are more prone to dislocation, making this form less strong than others. It also has fewer residual internal stresses which makes it less hard than its cold-rolled counterpart, and, since it shrinks as it cools, its dimensions are rather hard to control. It is, however, a more affordable option thanks to its low energy needs, so it’s used in applications that aren’t too strict on tolerances, like construction and rail.
Cold-rolled: This form of steel, although less ductile than hot-rolled, is harder with a better surface finish, and more dimensional accuracy. In the cold rolling process, which is done at room temperature, the metal’s grains are elongated to strain-harden the material. Stressing and relieving the steel before use is a must otherwise it could warp. This type is used a lot to make electric motors, water heaters, frying pans, and pressure vessels.
Tempered: Tempering involves reheating the steel to just below its eutectoid point, allowing the carbon to dissolve in the lattice structure, and then quenching it to trap the carbon. Its altered crystal lattice, martensite, makes the steel stronger and harder, which is why you’ll find tempered steel in a lot of construction equipment and tools.
