Plain carbon steels are made only of iron, carbon, and undesired elements. They fall into three general groups. Plain carbon steel with 0.05 to 0.2% carbon does not harden when cooled quickly. Welding it is simple, so it is used for shipbuilding, boilers, pipes, fence wire and other purposes where low cost is important. Plain steels are used for springs, gears, and engine parts. Plain carbon steel with 0.45 to 0.8% carbon is used for very hard items such as shears and machine tools.