Ductile steel wheels are made of a special iron alloy that utilizes graphite nodules as opposed to the graphite flakes found in standard gray cast iron. This makes them more malleable, flexible, and much less brittle. As a result, they are more durable, capable of handling heavier loads, and much more impact resistant than virtually any other wheel. Under most factory conditions it is all but impossible to destroy these wheels. Ductile iron wheels are usually used in demanding industrial applications where heavy loads and abusive conditions are prevalent. They are only recommended for steel and hardened concrete floors.