Carbon steel is composed of iron and 0.12 – 2.00 percent carbon. The broader definition includes alloy steels, which can also contain up to 10.5 percent alloy content. Even within the confines of under two percentage points of carbon, there is huge variance in physical characteristics—especially hardness.

When people talk about carbon steel, they are usually referring to the high carbon steel used in knives and tools. High carbon steels are very hard, which makes them good at resisting abrasion and retaining shape. They can withstand significant force before deforming. Unfortunately, hard metals are also brittle: when placed under extreme tensile stress, high carbon steels are more likely to crack than bend.

Low carbon steels are more common than high carbon because of (1) lower production costs, (2) greater ductility, and (3) ease of use in manufacturing. Low carbon steels tend to deform under stress instead of breaking, and that ductility makes low carbon steels easy to machine and weld. They are frequently used in automobile body panels, bolts, fixtures, seamless tubes, and steel plate.