What type of iron should fittings used in underground steel piping be made of?

Fittings used in underground steel piping should be made of malleable iron due to its excellent strength and ductility. Malleable iron is specifically designed to withstand the stresses and potential corrosion encountered in underground environments, making it suitable for applications involving liquid storage and transportation.

In contrast, cast iron, while strong, is more brittle and may not handle the pressures and environmental conditions as effectively. Carbon steel, although also strong, can be prone to corrosion if not properly protected, which is critical in underground applications where moisture and soil contact are factors. Commercial steel can refer to a variety of steel types, but does not specifically provide the necessary properties for underground storage applications like malleable iron does. Thus, malleable iron is the preferred choice to ensure durability and safety in underground piping systems.