Compacted Graphite Iron
Cast irons are differentiated by the shape of the graphite
particles. Grey cast iron is characterized by randomly oriented
graphite flakes, while the graphite in ductile iron exists as
individual spheres. The graphite particles in CGI are randomly
oriented and elongated as in grey iron, but they are shorter,
thicker and have rounded edges.
The difference between the three types of graphite is more
striking when viewed in the three-dimensional deep-etch scanning
electron micrographs.
In comparison to either grey iron or ductile iron, the entangled
compacted graphite 'clusters' interlock themselves into the iron
matrix to provide a strong adhesion. This graphite shape
suppresses crack initiation and propagation and is the source of
both the increased mechanical properties relative to grey iron
and the improved thermal conductivity relative to ductile iron.
CGI Properties
The CGI database defines CGI properties as a function of
microstructure and chemistry for tensile and compressive testing
at room and elevated temperatures, fatigue behaviour, damping
capacity, wear resistance, thermal expansion and thermal
conductivity up to 400ºC. The properties of grey iron, CGI and
ductile iron are summarised in the following overview.
Property |
Grey Iron |
CGI |
Ductile Iron |
Tensile strength (MPa) |
250 |
450 |
750 |
Elastic Modulus (GPa) |
105 |
150 |
160 |
Elongation (%) |
0 |
1.5 |
2.5 |
Endurance Ratios
- Rotating Bending
- Three Point Bending
- Tension Compression |
0.35-0.50
0.55-0.65
0.20-0.30 |
0.45-0.50
0.60-0.70
0.25-0.35 |
0.45-0.50
0.65-0.75
0.65-0.75 |
Thermal Conductivity
(W/m-K) |
48 |
38 |
32 |
CGI Microstructure Evaluation
The graphite microstructure of Compacted Graphite Irons is
expressed in terms of Percent Nodularity. For simultaneous
optimisation of mechanical properties, castability,
machinability and thermal conductivity, the graphite should be
controlled within 0-20% nodularity specification range (more
than 80% of the graphite particles must be in the
compacted/vermicular form) in all performance-critical sections
of the casting. Flake graphite is not permitted.
CGI Standards
Various national and international organizations have developed
and published standards for Compacted Graphite Iron. These
standards specify the CGI grades in terms of the tensile
strength and the graphite microstructure, expressed as Percent
Nodularity. The currently available standards are summarized as
follows:
Summary of CGI Standards
Country Issuing |
Body |
Number |
Year |
Romania |
STAS |
12443-86 |
1986 |
China |
JB |
4403-87 |
1987 |
USA |
ASTM |
A 842-85 |
1997 |
Germany |
VDG |
W 50 |
2002 |
International |
SAE |
J 1887 |
2002 |
International |
ISO |
16112:2006 |
2006 |
Beyond the standards issued by national and international
organizations, several OEMs have also established their own
internal CGI standards.
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