Manhole keys are usually made from a single piece of malleable iron or from carbon steel; some are zinc plated. Plastic keys are also available.
Malleable iron
Malleable iron is very strong and has a high melting point, so is used to make things that require strength. It is often used for small castings that require strength and ductility.
Carbon steel
Carbon steel is a strong alloy. Manhole keys that are made from this material are heavy-duty and are ideal for medium to heavy weight manhole covers. The keys are usually zinc-plated to prevent corrosion.
Plastic
Some light-duty lifting keys are made from plastic. They are manufactured from thermoplastic, a strong plastic material suitable for light-duty lifting, where there is only one key way.
