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Summary Knowledge of Compacted Graphite Iron, CGI Properties

 

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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