Grey Iron and Ductile Iron are two different types of Cast Irons with differences in the type or structure of the carbon present in them.
Cast Iron: Cast Iron is an Iron alloy prepared by heating the Iron till it liquefies and then adding the alloy component. It is then poured into a mould where it cools down and solidifies.
The Differences Between Grey Iron And Ductile Iron Are:
- Process of Formation: Both Grey Iron and Ductile Iron are prepared by adding carbon in the hot beds where they are liquefied but in the case of Ductile Iron, magnesium is also added.
- Structure of Graphite: The graphite found in Grey Iron has a rod-like or a random flaky pattern while the graphite found in Ductile Iron is shaped like nodules or spheres.
- Ductility: Due to the nodular or spherical shape of the graphite in Ductile Iron, its molecules have lesser discontinuities and give rise to the property of Ductility. Ductility is the ability of a metal to be shaped into wires. The A395 is the most Ductile of the Ductile Irons and has a Ductility of 18 to 30 percent elongation. Grey Iron does not have any recognizable elongation.
- Tensile and Yield Strength: The Tensile Strength of Grey Iron is approximately 20,000 psi (pound per square inch) and its Yield Strength is not measurable. In contrast, the minimum Tensile Strength of Ductile Iron is 60,000 psi and its minimum Yield Strength is 40,000 psi.
- Resistance to Impact: Ductile Iron is more resistant to impacts than Grey Iron. Ductile Iron Casting can resist a minimum of 7 foot-pounds of impact while Grey Iron can survive a maximum of only 2 foot-pounds of impact. This higher resistance is also due to the nodular or the spherical shape of the Graphite in Ductile Irons.
- Thermal Conductivity: The thermal conductivity of Grey Iron is 46 Watts per meter while the thermal conductivity of Ductile Iron is 36 Watts per meter. Due to this lesser thermal conductivity, Ductile Iron is used in the manufacture of gas cylinders while Grey Iron is used in making utensils.
- Vibration: Grey Iron dampens vibrations more effectively than Ductile Iron. Thus, Grey Iron is used in the machineries like the milling machines etc.
Ductile Iron, owing to its better strength and Ductility, is considered the upgraded version of Grey Iron Casting. However, it has not been able to completely replace Grey Iron as Grey Iron is still preferred for the manufacture of lot of tools and machineries.