The difference between grey cast iron, often referred to as cast iron, and ductile iron is the addition of trace elements. Grey iron has a random flake graphite formation.
In ductile iron, the addition of a trace element, typically magnesium, results in round graphite structures rather than flakes.
This results in a material with superior mechanical properties to grey cast iron and similar corrosion and wear resistance.
Both cast iron to EN 1561 and ductile iron to EN 1563 are acceptable materials to use for valves.
valves comply with relevant design standards which specify certain allowable cast and ductile iron grades to meet those standards and the requirements of the Pressure Equipment Directive (PED).
The selection of materials can also relate to manufacturing processes, valve size and pressure rating.
Ref - cranefs