In ductile iron the free graphite is in the form of discrete nodules
or spheroids. The matrix may be ferritic, pearlitic or a mixture of
the two. Other bainitic and martensitic structures may be formed by
heat treatment processes such as quenching and tempering and ‘austempering’.
The discrete form of the graphite nodules in comparison to the
planes of weakness of the graphite flakes in grey iron means that
the properties of ductile iron are determined more by the matrix of
the material than the form of the graphite. Thus ductile iron has
higher strengths, greater elongation and better resistance to impact
than grey iron.
In the as cast condition a range of properties from high ductility
to comparatively high strengths can be produced by control of the
composition and production process. This range of properties may be
extended by alloy adjustments and subsequent heat treatment
including surface hardening and through hardening by quench and
Whilst the production of ductile iron is more involved than grey
iron, it is still possible to produce complex shapes which are more
easily machined than steel.
The wide range of properties mean that the various grades of ductile
iron can be used in a variety of applications.
Ductile iron castings have production and machining cost advantages
over steel fabrications, forgings and castings within the
limitations of the ductility and impact properties and have strength
to weight advantages over grey iron castings where breakage is a
Some specific applications that have been successful are:
●Automobile crankshafts and camshafts replacing steel forgings.
●Gear rings and drive rings replacing fabrications, castings and
●Shear pin housings and drive couplings replacing steel castings.
●Main shafts and rotors for machinery drives.
●Brackets replacing fabrications.
●Thin section castings replacing fabrications for covers, delivery
chutes, housings and other machinery components .
●Gear wheels produced in ductile iron and hardened or austempered
give greater freedom of design than steel forgings.
●Rolls for pelleting presses produced in Austempered Ductile Iron as
a competitor to tool steel.
●Moulds for aluminium ingots cast in ductile iron have superior
cooling characteristics and life than the normal Hematite grey iron.
Steel vs Ductile Iron = No Contest !
Whether specifying a new casting, forging or fabrication or
improving an existing one, the potential benefits of Ductile Iron
over Steel are clear :
●Improved strength to weight ratio.
●Better surface definition and finish.
●Easier to machine and a reduced machining allowance.
●Reduced component weight (Ductile Iron is approx. 10% lighter than
●Reduced Component Cost.
Austempered Ductile Iron (ADI)
ADI is a high strength, wear resistant material produced by heat
treating (Austempering) a high quality Ductile Iron. The desired
strength / wear characteristics are achieved by a combination of the
micro structure achieved by the foundry and the subsequent control
of the three stage heat treatment process.
T.H. Dick & Co. Ltd have over 20 years experience in the production
of ADI for a wide range of applications.
Compacted Graphite Iron (CGI)
Compacted Graphite irons have mechanical and physical properties
between those of grey and ductile irons. The material has higher
strength than grey iron and better thermal conductivity than ductile
The matrix can be ferritic, pearlitic or a mixture as with grey iron
and ductile iron. The graphite is in the form of relatively short
thick flakes with rounded ends and undulating surfaces. In compacted
graphite the graphite does not have the same weakening effect as
flake graphite in grey iron, but it is still continuous and gives
greater thermal conductivity than the discrete graphite nodules in
Compacted graphite is an intermediate form with degrees of
compaction. This ranges from slight, where the graphite is difficult
to distinguish from the structure of ordinary flake, to high, where
the particles of graphite become very short, thick and irregular
with bulbous ends. The production of compacted graphite requires
careful control to achieve a required degree of compaction in a
given section of a component.
The current British Standard for Compacted Graphite Irons is ISO
16112 . The mechanical and physical properties are between those of
grey iron and ductile iron: Typically, tensile strength is 360 N/mm2
and 1 - 5% Elongation, depending upon the matrix structure.
Compared to grey irons, compacted graphite irons have higher thermal
conductivity, greater resistance to growth and scaling and better
thermal fatigue resistance to cracking and crazing at higher
Compacted graphite cast iron has been successfully used for ingot
moulds, bottle moulds, automobile exhaust manifolds and brake
components. The use of these irons is limited by the need to develop
and maintain production conditions to ensure the required structure
and by the difficulty in achieving that structure in components of
varying sections. Compacted graphite iron is not readily applicable
to one-off and jobbing production.