Gray iron or grey iron was the original “cast iron”. It is an alloy of carbon, silicon, and iron, containing from 1.7 to 4.5% C and 1 to 3% Si. It is relatively easy and inexpensive to make. Compared to the more modern engineered irons, gray iron has a lower tensile strength and lower ductility. 

Ductile iron, also called ductile cast iron, spheroidal graphite iron, or nodular cast iron, is a type of cast iron invented in 1943 by Keith Millis. While most varieties of cast iron are brittle, ductile iron is much more flexible and elastic, due to its nodular graphite inclusions.