Question
Why are manhole covers finished of cast iron as same are through of ductile iron?

Answer
Usually, manholes covers are made of cast iron. Though, in viewpoint of pipe preservation, frequent opening of manhole covers have to be approved out. Consequently, it poses potential safety hazard to workers through lifting-up procedure of manhole covers since cast iron manhole covers are incredibly heavy to regular workers. As a result, research has been conducted and ductile iron is measured as a better choice than cast iron as it be able to oppose same traffic loads with lesser self-weight. Furthermore, as ductile iron is less brittle than cast iron, usual cast iron manhole covers are additional susceptible to harm and thus need higher preservation cost.

Though, ductile iron manhole covers do endure from some demerits. For case, owing to their relative small self-weight, vehicles transient over these manhole covers would lead to movement of covers and generate unpleasant noises. To solve this problem, in its place of increasing self-weight of elastic iron manhole covers which similarly causes safety trouble to workers throughout regular upholding, covers can be designed to be attached to the manhole frames which hold them in firm position.