We, at VR Foundries, are one of the major casting manufacturers in Coimbatore and casting companies in Coimbatore, manufacturing cast and machine components, parts and accessories for the auto industry. Not surprisingly, many auto components are made of iron castings. However, cast iron itself has different grades and forms. Our specialist team of engineers have a proclivity towards the two types of cast iron: ductile iron and grey iron. Whilst both can be employed across the automotive industry, their physical characteristics may vary, with each having different engineering capabilities and applications. This blog will enumerate the differences between ductile and grey iron along those lines.

Grey Iron

We have already seen the history of Grey iron castings, and the role played by it in the development of the industry, beginning from the 19th century, in our previous blogs. The name ‘Grey Iron’ is derived from the grey colour of fracture created when the material is cracked. With a metal composition of 1-3 per cent silicon and 2.5-4 per cent carbon, grey cast iron comprises a graphite microstructure like those of the flaked type. Grey iron exhibits excellent machinability, achieves near-net manufacturing, then machining for outstanding surface preparation and mass reduction. Some of the industry products that are made of grey iron include valves, diesel engine parts, valves, compressors and more. This is due to the fact that grey iron is really cost-effective, as a replacement for steel. Some of the other characteristics of grey iron include extreme strength and durability.  

Further, grey iron is easily machinable. Cutting it and at greater speeds accelerate the process of engineering. The resultant turnaround times are reflected in the costs. Grey iron can also be made use of in components subjected to higher temperatures due to its outstanding thermal properties. Grey iron’s highlight is perhaps its vibration damping capacity. Or in other words, it can fully absorb vibrations, thereby turning out to be useful in the manufacture of brake and engine components.

Whilst not particularly known for its impact, grey iron definitely exhibits high vibration levels. Grey Iron has the ability to withstand vibrations, therefore accounting for an extremely low elongation value. This is the reason why it is a popular material for making machinery bases and boiler parts, in addition to equipment frames. No wonder, it is still the most popular material for passenger, heavy-duty and passenger vehicle sectors.

Ductile Iron

Ductile iron is a new kid in the block. Much has been already said about the history of ductile iron or SG iron castings, and how it has pivoted the growth of the engineering sector in our previous blogs. Ductile Iron is also known by other names such as Spheroidal Graphite Iron, Nodular Cast Iron, Graphite Cast Iron or GS Iron.

As for its chemical composition, Ductile Iron has magnesium and cerium among others, with a high amount of carbon content between 3.2 per cent and 3.6 per cent, and is usually considered ideal for mechanical and engineering applications. It is not as easily fracturable as grey irons, due to the graphite nodules’ spherical nature, whilst production is more specific to a spheroidising agent form to convert flake graphite into sphere-like nodules, bottom-up

Ductile Iron is characterised by incredible tensile strength in fact, better than other types of cast iron. The tensile strength of the strongest type (ASTM A536) is 827 N/mm2, whilst the toughest grey iron (SAE G4000) comes to 276 N/mm2. These numbers are indications that ductile iron isn’t going to fracture simply on an impact.

Higher yield strength indicates it is capable of withstanding greater stresses before it changes shape. This means it is comparatively more ductile than a material like aluminium. If flexibility is everything, however, ductile iron can go through a number of deformation modes without cracking. 

Compared to grey iron, ductile is proven to display greater elongation values. Simply put, it has the ability to increase in length when subjected to tension without failing, a measure of its robustness. Ferritic irons marked by high ductility will be able to achieve an elongation of 18-30 per cent. Previously seen as a material that could alternate malleable iron, ductile iron became a bit expensive and inconsistent when subjected to different heat treatments. However, ductile is the ideal replacement for steel, and a notch better than grey iron in that.  

The durability of ductile iron can be attributed to its microstructure comprising microscopic graphite spheres and a ferrous matrix, unlike the graphite flakes, seen earlier, which can result in higher stress concentrations. Ductile Iron have different grades and usually, cast iron is employed for various applications in machine frames, pumps and valves. This makes it the easy-to-go for all applications that require robust resistance. Though ductile iron is a tad costlier than grey iron, it is simpler to cast compared to thin steel.

There is every reason why even after material science disrupting innovation, iron continues to play a role in the manufacture of suspension, braking and engine components and so on. The important among these is, of course, its reliability. Mankind over several centuries has relied on iron for its withstanding ability to stress and impact. Such reliability is imperative wherever safety is involved.  Crucially, nevertheless, sturdiness cannot replace versatility. Indeed, application-wise, iron can be cast into different forms and shapes with complex sand cores and moulds. While materials like steel, carbon fibre and magnesium present their own advantages, there is always a demand for cast iron components and variants for grey and ductile iron in particular.