Manhole cover comparison by grey iron casting and ductile iron casting
Differences between grey ion manhole cover and ductile iron manhole cover
Traditionally, manholes covers are made of grey iron. However, because of pipe maintenance, frequent opening of cast iron manhole covers has to be carried out. Therefore, it takes potential difficulty to the workers during the lifting-up process of manhole covers because grey iron manhole covers are very heavy to workers. Consequently, research has been conducted and ductile iron is considered as a better choice than grey iron because it can resist the same traffic loads with lower self-weight. Moreover, as ductile iron is less brittle than grey iron, the traditional grey iron manhole covers are easier to be damaged and thus requires higher maintenance cost.
However, ductile iron manhole covers do suffer from some defects. For instance, owing to their relative low self-weight, vehicles passing over these manhole covers would lead to the movement of covers and generate unpleasant noise. To solve this problem, instead of increasing the self-weight of ductile iron manhole covers which similarly causes safety problems to workers during regular maintenance, the covers can be designed to be attached to the manhole frames which hold them in firm position.