Advantages of ductile iron manhole cover

Why ductile iron manhole cover is better than gray iron manhole cover?

The advantages of ductile iron manhole cover:

a. High strength. 
b. Better toughness. 
c. Better Elongation, it is 7% more than the gray iron.
d. Anti-corrosion resistance. The life is 2 times longer than gray iron material and 5 times longer than common steel pipe. 
e.Low-cost (compared with steel). The casting is more efficient and the cost of machining for ductile iron is less.

In most municipal applications, such as water, salt water, steam, the corrosion resistance and oxidation resistance of cast iron are more than steel. Because of the spherical graphite cast iron microstructure, ductile iron is better than steel in reduced vibration capacity.