The colder a metal is when rolled or hammered, the greater the resulting hardness. The depth of the hardness depends upon the depth to which the rolling or hammering pressure penetrates.

There is extensive use for steel rods and bars which have hard, smooth surfaces, both for the hardness and for the smoothness. These resist wear and take an excellent polish. They also show smoothly when nickel-plated. Also, a hot-rolled rod or bar cannot be rolled to exact dimensions, as cooling after rolling decreases its cross-section area an uncertain amount. To produce small steel bars and rods which are hard, smooth, and of exact dimensions in cross-section, cold rolling is resorted to. The metal is first rolled hot to near the finished size, pickled to remove the oxidized coating and is then passed many times between chilled rolls. Rods up to 5 inches in diameter are rolled in this way, and the resistance to twisting, due to so hard a surface, makes this product useful as shafting for transmitting power in shops.