Compacted (Vermicular) Graphite Iron Castings

Background

Compacted Graphite Iron was first patented at about the same time as ductile iron in the late 1940's. At the time it was viewed more as a curiosity rather than a production material, but its unique properties has led it to be used in many applications which are unsuitable for grey or ductile iron.

Compacted (vermicular) Graphite Iron is a form of cast iron that
mixes some of the beneficial properties of grey iron and ductile
iron to produce a material that is neither a grey iron or a fully
ductile iron but can be described as a "half way house" between
the two. In grey iron the free graphite forms as thin flakes
which run through the ferrite/perlite matrix, hence the
alternative name of flake graphite iron.

These flakes give excellent heat transfer characteristics particularly
with repeated heating and cooling cycles. However, because the flakes end in sharp points which act as stress raisers and crack propagation sites, flake graphite iron is a brittle material, excellent under compression but of limited use in tension or under shock loading. In ductile cast iron the graphite forms in small spheres (giving the alternative name of spheroidal graphite iron) which act as crack arresters thus giving rise to a ductile material.

Ductile iron is therefore an excellent material under both compression and tension but because it doesn’t have a lattice work of heat conducting graphite running through it lacks the heat transfer properties of a flake graphite iron. Chemically, flake graphite and ductile iron are very similar, the difference in graphite shape being achieved by treating the molten iron with magnesium so that the graphite solidifies as spheres rather than flakes.

In Compacted Graphite Iron the graphite forms a flake with rounded ends which look "worm like" under a microscope. This achieves two things. Firstly it removes the sharp stress and crack propagation points of the normal graphite flake leading to a ductile rather than a brittle material and secondly, because there is still a lattice like structure of graphite running through the matrix, it retains heat transfer properties similar to a grey iron.

The graphite form is achieved by starting the treatment process for ductile iron but then halting the reaction with a titanium addition so that only a limited number of spheres fully form, ideally about 5% to 10%. The end result is a cast iron with excellent heat transfer properties coupled with good mechanical strength in compression and tension.

 

The first image above, a microstructure, shows the "worm" shaped free graphite along with a few spheres of graphite which are always present in CGI. The second image, a deep etched micrograph, shows the continuous nature of the graphite which gives the excellent heat transfer properties.

Mechanical Properties of the CGI Grades in ISO 16112

Mechanical and Physical Properties of CGI in comparison to conventional grey cast iron at 20°C

We make CGI to ISO 16112:2006. However, due to the international nature of the material, buyers may find other descriptions and standards on drawings which can cause confusion. All of the following terms can be used to describe this material:-

CGI
GJV
VG
JV
GGV
Compacted Graphite Iron
Vermicular Cast Iron

There are also other standards, usually national or industry specific, which can be used to specify CGI such as ASTM A842-85, and SAE J1887 JUL2002. This list is not exhaustive and if you have a casting that you think is made in CGI and you’re not sure, ring us and we will try to determine if it is.

The first commercial application for compacted graphite iron was for the brake discs for high speed rail trains. More recently compacted graphite iron has been used for diesel engine blocks. It has proven to be useful in the manufacture of V topology diesel engines where the loading on the block is very high between the cylinder banks, and for heavy goods vehicles which use diesel engines with high combustion pressures. It is also used for turbo housings and exhaust manifolds, in the latter case to reduce corrosion. Because of the increased tensile strength of compacted graphite iron compared to grey iron, thinner castings can be used thus reducing weight.

Please browse our website for more information about Compacted (Vermicular) Graphite Iron Castings from Durham Foundry and our ability to manufacture Engineering Cast Iron Castings then contact us on 0114 249 4977 or e-mail us on castings@durhamfoundry.com.