Gray cast iron and ductile cast iron basically differ in the type or structure of the carbon contained in them. The making of cast iron is principally, liquefaction of iron by heating, followed by cooling and hence solidification. In case of gray cast iron, the making involves the addition of carbon in the heated beds where the liquefaction of base iron takes place. Whereas, for ductile cast iron, carbon as well as magnesium are added in the hot beds. These added components bind with the solid (cast iron) which is formed thereafter.

1. Difference between gray cast iron and ductile cast iron
Cast ironisbasicallya ferrousalloymade frompigiron, containing 2 to 4 percent carbon. It was first
produced by the Chinese, dating back to the 6th century BC. It was then re-invented by the
Europeans in the 14th
century AD, with a renewed enthusiasm, for they found it to be uniquely
useful.Apparently,the castironhas causedan evolution, with the discovery of its hidden purposes
and novelty.Overtime,there have beenmany forms of cast iron brought into existence and usage.
Today, when we look at the very objects we use, quite a few essentials are made out of cast iron.
And this cast iron varies from type to type, depending upon the metallurgical properties each of
them hold.
Gray cast iron and ductile cast iron are two most important types of cast iron.
What makes a gray cast iron and a ductile cast iron?
Gray cast ironand ductile castironbasicallydifferinthe type orstructure of the carbon contained in
them.The makingof cast iron isprincipally,liquefactionof iron by heating, followed by cooling and
hence solidification. In case of gray cast iron, the making involves the addition of carbon in the
heatedbedswhere the liquefactionof base iron takes place. Whereas, for ductile cast iron, carbon
as well asmagnesiumare addedinthe hotbeds.These added components bind with the solid (cast
iron) which is formed thereafter.
Short notes on ductile cast iron and gray cast iron
The usage of ductile iron, otherwise known as nodular or graphite iron, underwent a drastic
development over a period of 10 years since early 1950s. Till date, it has been used in several
engineeringapplicationsforcommercial purposes. A varied combination of physical properties has
rendered ductile iron, its uniqueness. The spheroidal molecules present in the micro structure of
ductile ironisbecause of the addition of a specified quantity of magnesium to the hot molten iron
duringitsproduction.The presence of otherelementsinthis molten base iron is strongly restricted
since it may interfere the internal structuring of the end product. Thus, the continuity of the
moleculesismaintained. Also, the magnesium added, reacts with the sulphur and oxygen content
which is already existing in the molten iron. This defines the formation of the rigid graphite
structure. It is hence implicit that ductile iron contains a high amount of Carbon.
Added to these components, it also contains a high amount of Silicon. The spheroids of graphite
molecules have only a considerable influence on the metal’s mechanical properties. Similar to
malleable iron,ductileironalsohasa linearstress-strainrelationalongwithgood yield strength and
ductility. Hence, castings are made in a variety of forms and sizes.
There are different grades of ductile iron which are produced by monitoring and controlling the
matrix of graphite present in it. These can be conceived by either cast or appropriate thermal
treatment. There are also high alloy ductile irons manufactured to cater to special industry needs.
Gray cast iron isknownfor its unique ability that makes it suitable for niche engineering products.
Since most iron castings are of gray cast iron, cast iron simply refers to the gray iron. During its
manufacture,the carbonin the molten metal (base iron) parts away while cooling takes place. The
segregated carbon content transforms into graphite as sharp edged flakes. This renders the micro
structure of gray iron its singularity. The graphite keeps growing within the molten alloy thereby
formingthe flakes.These graphite flakes are adjoined with one another within every eutectic cell.
During breakage, the fractures happen at the graphite flakes and hence causes the gray coloured

2. appearance.The graphite flakes,alsoattribute tothe physical andmechanical properties of the gray
iron. The size, composition and distribution of the flakes and the relative strength of the metal
matrix contribute to these properties. The carbon and silicon content of the alloy, and the rate of
cooling are the influencing factors in the composition of graphite flakes. If the cooling rate is slow
while the carbon and silicon contents are high, larger flakes are produced. This adds to the softer
matrix structure and lowerstrengthof the iron.Some unique characteristics like high machinability
and extreme wear resistance at hardness levels are due to the presence of graphite flakes.
Now, let us have a closer look on the differences between these two types of cast irons.
 Physical properties
Physical propertiesof alloysare fundamental inmetallurgy.They define every kind of alloy.
When it comes to gray cast iron, it is found to be of much less tensile strength and yield
strength than that of the ductile cast iron. The tensile strength of gray iron is about 20000
psi per square inch but its yield strength is not measurable, while ductile iron has a high
tensile strength of about 60000 psi and a minimum yield strength of about 40000 psi. Also,
ductile ironhasa higherrate of elongationthangrayiron.The elongation rate of gray iron is
so low that it does not meet the basic requirements of elongation. Yet, gray iron is said to
have a higher abrasion resistance and cast ability than ductile iron. As a matter of fact,
tensile strength is fundamental for application of any alloy. In that per se, ductile iron is
better than the gray and hence it has a higher longevity and reliability.
 Graphite structure
The micro structure of graphite component in gray iron is somewhat like a rod or a random
flaky pattern. Whereas, ductile iron constitutes graphite in a shape similar to nodules or
spheres. This difference attributes to the physical properties of both the types. In case of
gray iron,the flakesof graphite leadtohigherstressthanthat causedbythe graphite ballsin
ductile iron.Thisevidencesthe lowertensilestrengthandhigherelongationstrength in gray
iron.
 Ductility
The molecular structure of ductile iron is such that, it adds to the quality of ductility. The
graphite spheresenhance the continuity in its molecular structure and hence the ability to
be drawn intowires.Since gray iron has lesser elongation strength, it is not ductile enough
to be drawn into metal wires.
 Impact resistance
Ductile ironismore impactresistantwhencompared to gray iron. Its ability to resist impact
is such that, it can bear a minimum of 7 foot-pounds of impact whereas gray iron can
withstandonlyupto2 foot-poundsof impact.This shows how high the physical strength of
ductile iron is. Again, this ability is attributed to the molecular structure of ductile iron.
 Thermal conductivity
The thermal conductivity of a material defines its ability to conduct or carry heat. It is an
importantproperty for metals, since they are widely applied for conduction purposes. It is
measuredinwattspermetre.Forgray iron,its thermal conductivity is at about 46 watts per
metre while for ductile iron it is about 36 watts per square metre. The less thermal
conductivity of ductile cast iron makes it feasible to be used in the production of gas
cylinders. On the other hand, gray cast iron is preferred for manufacturing utensils.

3.  Resonance and vibration
Use of metalsisthe main cause of noise pollutioninandaroundindustries. It is advisable to
use metalswhichcan dampenvibrations caused by the machinery. Gray iron, being loosely
structuredinternally, can absorb vibrations much easily and effectively when compared to
ductile iron. Hence it is used widely in the manufacture of machinery and parts like the
milling machines etc.When it comes to shock resistance, ductile iron is more resistant to
shocks than gray iron.
 Draw resistance
The draw resistance of a metal alloydecidesthe extent up to which it can be flexibly drawn
into wires with least deformation of its microstructure. For ductile cast iron, the graphite
spheroidsprotectthe internal matrix stability from deformation. Hence it is more suited to
be drawn intowires.The draw resistance of ductile cast iron ranges between 60 and 100 kg
per square millimetre, while that of gray iron lies between 18 and 30 kg per square
millimetre.
 Production and processing
Ductile iron is apparently complex to produce when compared to gray iron. It involves the
applicationof a nodulizer, also called as the “spheroidizing agent”, so as to accomplish the
nodulizingandinoculationprocesses.Thiscomplexproductionprocess increases the cost of
ductile iron. The production cost, also depends upon the type and price of the nodulizers
used.
 Choice of iron based on the type of application
The type of iron casting is chosen according to the application. For the manufacturing of
parts whichrequire considerabletensilestrength,grayiron will do well. And the cost is also
substantially cut down. Whereas, for products like parts of machinery and automobile,
ductile iron is best suited, as they require highly tough material.
 Applications
Both,ductile andgray iron,have specificapplicationsforparticularpurposes. Some of them
are briefed here.
The spheroidsof ductile castironguarantee the efficiencyof loadbearingupto 100 percent.
Thismakesit suitable formanufacturing the gratings of manholes and rainwater gullies. Its
high machinability, greater fatigue strength and excellent wear resistance makes it best
suited for the manufacture of crankshafts and the like. Its high ductility is utilized for the
making of automobile hinges. It is also used for manufacturing punching tools, body
housings, engines and drive components, turbo charger housings, diffusors, in plant cum
machine constructions,pipes,pipeline networks,fittingsandvalveslikeshut-off gate valves,
butterfly valves, hydrants and so on.
Whereas, for gray cast iron, it has a completely different set of particular applications.
Typically,grayironisusedingears androtor basedproducts. It isusedin the manufacture of
heavy components such as machine supporting elements, gear wheels and boxes, pinch
rolls, belt pulleys, body housings and posts, motor and cylinder blocks which are used in
combustion engines, brake discs, brake cylinders, furnace parts, pipes and pipe fittings,
flywheels, furnace parts, machine bases and similar components used in rotation based
machinery applications.

4.  Cost differences
Owingtoits highstrengthand niche engineering properties, ductile cast iron is way dearer
in cost when compared to gray cast iron.
The overall comparisonwouldgive you a fair idea about which of the two will suffice your industry
requirements. You can now choose the one that fits in your budget while satisfying your needs.
Author Bio: VR Foundries is one of the best casting foundries in Coimbatore, India. The
manufacturer provides high quality SG(Ductile) & grey iron casting products for various industries
such as automobiles, pump manufacturers, textiles, hydraulics and more.