Structure
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’.

Properties
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 tempering.

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.

Applications
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 problem.

Some specific applications that have been successful are:

●Automobile crankshafts and camshafts replacing steel forgings.
●Gear rings and drive rings replacing fabrications, castings and forgings.
●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 Steel)
●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 iron.

Structure

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 ductile iron.

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.

Properties
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 temperatures.

Applications
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.

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