**Introduction**
Nodular grey cast iron, also known as ductile iron or spheroidal graphite iron, is a type of cast iron that has improved mechanical properties compared to grey cast iron. It is obtained by adding a small amount of magnesium to the grey cast iron during the casting process.

**Explanation**
The addition of magnesium to grey cast iron promotes the formation of graphite in a spheroidal or nodular form rather than the flake-like form found in regular grey cast iron. This change in graphite morphology is what gives nodular grey cast iron its improved properties.

**Formation of Nodular Graphite**
When molten grey cast iron solidifies, carbon in the form of graphite precipitates out of the liquid iron. In regular grey cast iron, the graphite forms in flake-like structures, which act as stress raisers and reduce the mechanical properties of the cast iron.

The addition of a small amount of magnesium to the molten iron changes the way the graphite forms. Magnesium reacts with sulfur and oxygen impurities in the iron, forming magnesium sulfide and magnesium oxide. This reaction reduces the availability of these impurities for combining with carbon, which promotes the formation of graphite in a nodular form.

The nodular graphite, also known as spheroidal graphite or nodules, has a spherical or nodular shape with rounded edges. This morphology of graphite improves the mechanical properties of the cast iron, making it more ductile, stronger, and tougher compared to regular grey cast iron.

**Advantages of Nodular Grey Cast Iron**
Nodular grey cast iron has several advantages over regular grey cast iron, including:
1. Improved ductility: The nodular graphite structure allows for greater deformation before fracture, making nodular grey cast iron more ductile.
2. Increased strength: The nodular graphite structure provides improved strength compared to flake graphite found in regular grey cast iron.
3. Enhanced toughness: The rounded nodules of graphite act as crack arresters, absorbing energy and increasing the toughness of the cast iron.
4. Better machinability: The nodular graphite structure reduces tool wear during machining operations, improving the machinability of nodular grey cast iron.

Overall, the addition of magnesium to grey cast iron to obtain nodular grey cast iron is a widely used technique that significantly improves the mechanical properties and performance of the cast iron.