Compacted Graphite Iron

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Compacted Graphite Iron

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Compacted Graphite Iron

Messaggioda Aldebaran » 03/05/2010, 21:04

Introduction
COMPACTED GRAPHITE (CG) cast iron is also referred to as vermicular graphite, upgraded, or semiductile
cast iron (Ref 1). It has been inadvertently manufactured in the past in the process of producing ductile iron, as
a result of undertreatment with magnesium or cerium. Since 1965, after R.D. Schelleng obtained a patent for its
production, CG iron has occupied its rightful place in the family of cast irons.
The graphite morphology of CG iron is rather complex. A typical scanning electron microscope
photomicrograph of a compacted graphite particle etched out of the matrix is shown . It is seen that
compacted graphite appears in clusters that are interconnected within the eutectic cells. Classical optical
metallography exhibits graphite that is similar to type IV ASTM A 247 graphite (see the article
"Classification and Basic Metallurgy of Cast Iron" in this Volume). Compacted graphite appears as thicker,
shorter-flake graphite. In general, an acceptable CG iron is one in which at least 80% of the graphite is
compacted graphite, there is a maximum of 20% spheroidal graphite (SG), and there is no flake graphite (FG).
This graphite morphology allows better use of the matrix, yielding higher strength and ductility than flake
graphite cast iron. Similarities between the solidification patterns of flake and compacted graphite iron explain
the good castability of the latter, compared to ductile iron (ductile iron, which is also termed nodular iron or
spheroidal graphite iron, is called SG iron in this article). Also the interconnected graphite provides better
thermal conductivity and damping capacity than spheroidal graphite.
Reference
1. E. Nechtelberger, H. Puhr, J.B. von Nesselrode, and A. Nakayasu, Paper 1 presented at the 49th International
Foundry Congress, International Committee of Foundry Technical Associations, Chicago, 1982
Compacted Graphite Iron
Doru M. Stefanescu, The University of Alabama
Chemical Composition
The range of acceptable carbon and silicon contents for the production of CG iron is rather wide, as shown in
Nevertheless, the optimum carbon equivalent (CE) must be selected as a function of section thickness, in
order to avoid carbon flotation when too high a CE is used, or excessive chilling tendency, when too low a CE
is used. The manganese content can vary between 0.1 and 0.6%, depending on whether a ferritic or a pearlitic
structure is desired. Phosphorus content should be kept below 0.06% in order to obtain maximum ductility from
the matrix. The initial sulfur level should be below 0.025%, although techniques for producing CG iron from
base irons with higher sulfur levels are now available The change in graphite morphology from the flake graphite in the base iron to the compacted graphite in the
final iron is achieved by liquid treatment with different minor elements. These elements may include one or
more of the following: magnesium, rare earths (cerium, lanthanum, praseodymium, and so on), calcium,
titanium, and aluminum.
Compacted graphite iron has a strong ferritization tendency. Copper, tin, molybdenum, and even aluminum can
be used to increase the pearlite/ferrite ratio. Again, the optimum amounts of these elements for a particular
matrix structure are largely a function of section size.
References cited in this section
1. E. Nechtelberger, H. Puhr, J.B. von Nesselrode, and A. Nakayasu, Paper 1 presented at the 49th International
Foundry Congress, International Committee of Foundry Technical Associations, Chicago, 1982
2. H.H. Cornell and C.R. Loper, Jr., Trans. AFS, Vol 93, 1985, p 435
3. R. Elliott, Cast Iron Technology, Butterworths, 1988
Compacted Graphite Iron
Doru M. Stefanescu, The University of Alabama
Castability
The fluidity of cast iron is a function of its pouring temperature, composition, and eutectic morphology. A
higher temperature and higher CE result in better fluidity. Everything else being equal, the fluidity of CG iron is
intermediate between that of FG (highest) and SG (lowest) iron (Ref 1). However, because CG iron has a higher
strength than FG iron for the same CE, high-CE compositions of CG iron can be used for the pouring of thin
castings.
Shrinkage Characteristics. With CG irons, obtaining sound castings free from external and internal shrinkage
porosity is easier than with SG irons and slightly more difficult than with FG irons. This is because the
tendency for mold wall movement also lies between that of SG and FG irons.
In relative numbers, solidification expansion has been found to be 4.4 for SG iron and 1 to 1.8 for CG iron if
FG iron is 1 (Ref 4). Because of the rather low shrinkage of CG iron, it can sometimes be cast riserless.
Expensive pattern changes are therefore not necessary when converting from gray iron to CG iron because the
same gating and risering techniques can be applied.
Chilling Tendency. Although many believe that the chilling tendency of CG iron is also intermediate between
that of FG (lowest) and SG (highest) irons, this is not true.
References cited in this section
1. E. Nechtelberger, H. Puhr, J.B. von Nesselrode, and A. Nakayasu, Paper 1 presented at the 49th International
Foundry Congress, International Committee of Foundry Technical Associations, Chicago, 1982
4. D.M. Stefanescu, I. Dinescu, S. Craciun, and M. Popescu, "Production of Vermicular Graphite Cast Irons by
Operative Control and Correction of Graphite Shape," Paper 37 presented at the 46th International Foundry
Congress, Madrid, 1979
5. D.M. Stefanescu, F. Martinez, and I.G. Chen, Trans. AFS, Vol 91, 1983, p 205
Compacted Graphite Iron
Doru M. Stefanescu, The University of Alabama
Mechanical Properties at Room Temperature
The in-service behavior of many structural parts is a function not only of their mechanical strength, but also of
their deformation properties. Thus it is not surprising to find that many castings fail not because of insufficient
strength, but because of a low capacity for deformation. This is especially true under conditions of rapid
loading and/or thermal stress. Particularly sensitive to such loading are casting zones that include some defects
or abrupt changes in section thickness. The elongation values of about 1% obtainable with high-strength gray
iron are insufficient for certain types of applications such as diesel cylinder heads (Ref 6). Compacted graphite
irons have strength properties close to those of SG irons, at considerably higher elongations than those of FG
iron, and with intermediate thermal conductivities. Consequently, they can successfully outperform other cast
irons in a number of applications.
The main factors affecting the mechanical properties of CG irons both at room temperatures and at elevated
temperatures are:
· Composition
· Structure (nodularity and matrix)
· Section size
In turn, the structure is heavily influenced by processing variables such as the type of raw materials,
preprocessing of the melt (superheating temperature, holding time, desulfurization), and liquid treatment
(graphite compaction and postinoculation).
Compacted graphite irons exhibit linear elasticity for both pearlitic and ferritic matrices, but to a lower limit of
proportionality than does SG iron. The ratio of yield strength to tensile strength ranges from 0.72 to
0.82, which is higher than that for SG iron of the same composition. This makes possible a higher loading
capacity. The limit of proportionality is 125 MPa (18 ksi) for both ferritic and pearlitic CG irons. It is slightly
lower than that of SG iron. This can be explained by the higher notching effect from the sharper-edged
morphology of the compacted graphite compared to spheroidal graphite. Consequently, for the same stress,
plastic deformation occurs sooner around the graphite in CG iron than in SG iron, causing earlier divergence
from proportionality (Ref 1).
As the hardness increases, tensile strength increases but elongation decreases, this being the effect of a higher
pearlite/ferrite ratio . The ratio of tensile strength to Brinell hardness is somewhat higher for CG iron
than for FG iron. In general, CG iron has lower hardness than an FG iron of equivalent strength because of the
higher amount of ferrite in the structure. For the same elongation, CG iron has considerably less yield strength
than SG iron.
Effect of Composition. It has been demonstrated that the tensile properties of CG irons are much less sensitive
to variations in carbon equivalent than are those of FG irons. Even at CE near the eutectic value of 4.3, both
pearlitic and ferritic CG irons have higher strengths than does low-CE, high-duty, unalloyed FG cast iron.
Although increasing the silicon content decreases the pearlite to ferrite ratio in the as-cast state, both the
strength and hardness of as-cast and annealed CG irons improve. This is because of the hardening of ferrite by
silicon. For the same reasons, elongation in the annealed condition decreases, but increases for the as-cast state
(Ref 10). Although increasing the phosphorus content slightly improves strength, a maximum of 0.04% P is
desirable to avoid lower ductility and impact strength.
The pearlite/ferrite ratio, and thus the strength and hardness of CG irons, can be increased by the use of a
number of alloying elements such as copper, nickel, molybdenum, tin, manganese, arsenic, vanadium, and
aluminum (Ref 6, 14). The effect of copper and molybdenum on the tensile properties of CG irons is shown in
. After annealing to a fully ferritic structure, it is possible to increase the yield point of CG iron by 24%
when using 1.5% Ni (Table 3). This is because of the strengthening of the solid solution by nickel (Ref 6). The
reader is cautioned, however, that additions of copper, nickel, and molybdenum may increase nodularity (Ref 6).
In order to compare the quality of different types of irons, several quality indexes can be used, such as the
product of tensile strength and elongation (TS × El) or the ratio of tensile strength to Brinell hardness (TS/HB).
Higher values of these indexes will characterize a better iron. Using the data given in Ref 10, some typical
values were calculated for these indexes for unalloyed CG irons. Figure 10 compares the TS × El product and
the TS/HB ratio for unalloyed and aluminum-alloyed CG irons. It can be seen that when 2% Si is replaced by
2% Al, a much better quality CG iron is produced (Ref 11).
Effect of Section Size. Like all other irons, CG irons are rather sensitive to the influence of cooling rate, that
is, to section size, because it affects both the pearlite/ferrite ratio and graphite morphology. As mentioned
before, a higher cooling rate promotes more pearlite and increased nodularity. A typical example of the
influence of section size on the microstructure of CG iron is provided in Fig. 12. Although CG iron is less
section sensitive than FG iron, the influence of cooling rate may be
quite significant .When the section size decreases until it is below 10 mm (0.4 in.), the
tendency to increased nodularity and for higher chilling must be considered. This is particularly true for
overtreated irons. While it is possible to eliminate the carbides that result from chilling by heat treatment, it is
impossible to change the graphite shape, which remains spheroidal, with the associated consequences. Other
factors influencing the cooling of castings, such as shakeout temperature, can also influence properties.
Compressive Properties.
It can be seen that an elastic behavior occurs up to a compression stress of 200 MPa (30 ksi). Some
compressive properties of the 179 HB as-cast ferritic CG iron in Table 2 are compared with those of SG iron in
Table 5. It can be seen that the 0.1% proof stress in compression for CG iron is 76 MPa (11 ksi) higher than the
0.1% proof stress in tension, while for SG iron the difference is only 23 MPa (3.3 ksi). Compressive strengths
up to 1400 MPa (203 ksi) have been reported for ferritic annealed CG irons (Ref. 8).
Shear Properties. For a pearlitic CG iron, the shear strength on 20 mm (0.8 in.) diam specimens was
measured at 365 MPa (53 ksi), with a shear-to-tensile strength ratio of 0.97 (Ref 17). Ratios of 0.90 for SG iron
and of 1.1 to 1.2 for FG iron have been reported. Materials exhibiting some ductility have ratios lower than 1.0
(Ref 8).
Modulus of Elasticity. As is evident , CG irons exhibit a clear zone of proportionality, both
in tension and in compression. Typical values for both static and dynamic (resonance frequency metho. Dynamic tests give slightly higher numbers. In general, the moduli of
elasticity for CG iron are similar to those of high-strength FG irons and can even be higher as nodularity
increases.
The elasticity modulus measured by the tangent method depends on the level of stress, a A
comparison of the stress dependency of the elasticity modulus for different types of cast irons is shown in
. Poisson's ratios of 0.27 to 0.28 have been reported for CG irons (Ref 8).
Impact Properties. While SG iron exhibits substantially greater toughness at low pearlite contents, pearlitic
CG irons have impact strengths equivalent to those of SG irons . Charpy impact energy measurements
at 21 °C (70 °F) and -41 °C (-42 °F) showed that CG irons produced from an SG-base iron absorbed greater
energy than those made from gray iron-base iron (Ref 10). This is attributed to the solute hardening effects of
tramp elements in the gray iron.
The results from dynamic tear tests were similar, although greater temperature dependence was observed. A
comparison of the dynamic tear energies of CG cast irons is presented in Fig. 18. It is noted that significant
differences in the values obtained occur in the ferritic condition, but that equivalent values are obtained when
the matrix structure is primarily pearlitic.
Studies on crack initiation and growth under impact loading conditions showed that, in general, the initiation of
matrix cracking was preceded by graphite fracture at the graphite-matrix interface, or through the graphite, or
both. The most dominant form of graphite fracture appeared to be that occurring along the boundaries between
graphite crystallites (Ref 18). Matrix cracks were usually initiated in the ferrite by transgranular cleavage
(graphite was nearly always surrounded by ferrite), although in some instances intergranular ferrite fracture
appeared to be the initiating mechanism. Matrix crack propagation generally occurred by a brittle cleavage
mechanism, transgranular in ferrite, and interlamellar in pearlite. In general, the impact resistance of CG irons
increases with carbon equivalent and decreases with phosphorous or increasing pearlite. Cerium-treated CG irons seem to exhibit a higher impact energy than magnesiumtitanium-
treated irons. It is thought that this may be attributed to TiC and TiCN inclusions present in the matrix
of magnesium-titanium-treated CG irons (Ref 6).
Fatigue Strength. Because the notching effect of graphite in CG iron is considerably lower than that in FG
irons, it is expected that CG iron will have higher fatigue strengths than FG iron . The as-cast ferritic (>95% ferrite) CG with a hardness of 150
HB had the highest fatigue strength and the highest fatigue-endurance ratio (fatigue strength/tensile strength). It is
evident that pearlitic structures, higher nodularity, and unnotched samples resulted in better fatigue strength.
The fatigue-endurance ratio was 0.46 for a ferritic matrix, 0.45 for a pearlitic matrix, and 0.44 for a pearlitic
higher-nodularity CG iron (Ref 17). With fatigue notch factors (ratio of unnotched to notched fatigue strength)
of 1.71 to 1.79, CG iron is almost as notch sensitive as SG iron (>1.85). Gray iron is considerably less notch
sensitive, with a notch factor of less than 1.5 (Ref 6).
Statistical analysis of a number of experimental data allowed the calculation of a relationship between fatigue
strength (FS) and tensile strength (TS) of CG irons (Ref 6):
FS (in MPa) = (0.63 - 0.00041 · TS)
·TS (in MPa) (Eq 1)
References cited in this section
1. E. Nechtelberger, H. Puhr, J.B. von Nesselrode, and A. Nakayasu, Paper 1 presented at the 49th
International Foundry Congress, International Committee of Foundry Technical Associations, Chicago,
1982
6. E. Nechtelberger, The Properties of Cast Iron up to 500 °C, Technicopy Ltd., 1980
7. J. Sissener, W. Thury, R. Hummer, and E. Nechtelberger, AFS Cast Met. Res. J., 1972, p 178
8. C.F. Walton and T.J. Opar, Ed., Iron Castings Handbook, Iron Casting Society Inc., 1981
9. G.F. Sergeant and E.R. Evans, The British Foundryman, May 1978, p 115
10. K.P. Cooper and C.R. Loper, Jr., Trans. AFS, Vol 86, 1978, p 241
11. F. Martinez and D.M. Stefanescu, Trans. AFS, Vol 91, 1983, p 593
13. J. Fowler, D.M. Stefanescu, and T. Prucha, Trans. AFS, Vol 92, 1984, p 361
14. R.B. Gundlach, Trans. AFS, Vol 86, 1978, p 551
15. Spravotchnik po Tchugunomu Ljitiu (Cast Iron Handbook), 3rd ed., Mashinostrojenie, 1978
16. K.H. Riemer, Giesserei, Vol 63 (No. 10), 1976, p 285
17. K.B. Palmer, BCIRA J., Report 1213, Jan 1976, p 31
18. A.F. Heiber, Trans. AFS, Vol 87, 1979, p 569
Compacted Graphite Iron
Doru M. Stefanescu, The University of Alabama
Elevated-Temperature Properties
Tensile Properties. The variation of tensile properties with temperature for CG iron produced with ceriummischmetal
treatment alloys is similar to that typical for SG iron , but the values are somewhat lower
(Ref 19). .
As expected, a slight increase in nodularity led to higher tensile strength values at all temperatures.

Growth and Scaling. Tests conducted for 32 weeks in air have shown that at 500 °C (930 °F) the growth and
scaling of CG iron was not significantly different from that exhibited by FG irons of similar composition.
However, at 600 °C (1110 °F), the growth of CG irons was less than that of FG iron, and scaling resistance was
superior.
In other oxidation studies of cast irons conducted at 600 °C (1110 °F), it was concluded that weight gains due to
oxidation are 10 to 15% higher for CG irons than for SG irons, but 30 to 60% lower for CG irons than for FG
irons (Ref 21).
Thermal Fatigue. When castings are used in an environment where frequent changes in temperature occur, or
where temperature differences are imposed on a part, thermal stresses occur in castings and may result in elastic
and plastic strains and finally in crack formation. The casting can thus be destroyed as a result of thermal
fatigue. Changes in microstructure, associated with stress-including volume changes, as well as surface and
internal oxidation, may also be associated with temperature difference induced stresses.
The interpretation of thermal fatigue tests is complicated by the many different test methods employed by
various investigators. The two widely accepted methods are constrained thermal fatigue and finned-disk
thermal shock tests (Ref 22, 23).
In the constrained thermal fatigue test, a specimen is mounted between two
stationary plates that are held rigid by two columns, heated by high frequency (450 kHz) induction current, and
cooled by conduction of heat to water-cooled grips . The thermal stress that develops in the test
specimen is monitored by a load cell installed in one of the grips holding the specimen. During thermal cycling,
compressive stresses develop upon heating, and tensile stresses develop upon cooling. As thermal cycling
continues, the specimen accumulates fatigue damage in a fashion similar to that in mechanical fatigue testing;
ultimately, the specimen fails by fatigue. Initially the specimen develops compressive stress upon heating due
to constrained thermal expansion . Some yielding and stress relaxation occur during holding at 540 °C
(1000 °F), and upon subsequent cooling the specimen develops residual tensile stress. During subsequent thermal cycling, the maximum compressive stress that has developed upon heating decreases continuously, and
the maximum tensile stress upon cooling increases, as shown for six different irons .
Experimental results point to higher thermal fatigue for CG iron than for FG iron and also indicate the
beneficial effect of molybdenum. In fact, regression analysis of experimental results indicates that the main
factors influencing thermal fatigue are tensile strength (TS) and molybdenum content:
log N = 0.934 + 0.026 · TS + 0.861 · Mo (Eq 2)
where N is the number of thermal cycles to failure, tensile strength is in kps per square inch (ksi), and
molybdenum is in percent.
In the finned-disk thermal shock test, the specimen is cycled between a moderatetemperature
environment and a high-temperature environment, which causes thermal expansion and
contraction. . Because in this type of test thermal
conductivity plays a significant role, FG iron showed much greater resistance to cracking than did CG iron.
Major cracking occurred in less than 200 cycles in all CG iron specimens, while the unalloyed FG iron
developed minor cracking after 500 cycles and major cracking after 775 cycles. The alloyed FG iron, because
of its higher elevated-temperature strength, did not show any sign of cracking even after 2000 cycles (Ref 23).
The CG iron containing more ferrite had a slightly better thermal fatigue resistance than the CG iron with less
ferrite.
In general, for good resistance to thermal fatigue, cast irons must have high thermal conductivity; low modulus
of elasticity; high strength at room and elevated temperatures; and, for use above 500 to 550 °C (930 to 1020
°F), resistance to oxidation and structural change. The relative ranking of irons varies with test conditions.
When high cooling rates are encountered, experimental data and commercial experience show that thermal
conductivity and a low modulus of elasticity are most important. Consequently, gray irons of high carbon
content (3.6 to 4%) are superior (Ref 22, 23). When intermediate cooling rates exist, ferritic SG and CG irons
have the highest resistance to cracking, but are subject to distortion. When low cooling rates exist, high-strength
pearlitic SG irons or SG irons alloyed with silicon and molybdenum are best with regard to cracking and
distortion .
References cited in this section
6. E. Nechtelberger, The Properties of Cast Iron up to 500 °C, Technicopy Ltd., 1980
9. G.F. Sergeant and E.R. Evans, The British Foundryman, May 1978, p 115
19. K. Hutterbraucker, O. Vohringer, and E. Macherauch, Giessereiforschung, No. 2, 1978, p 39
20. D.M. Stefanescu and G. Niculescu, unpublished research
21. I. Riposan, M. Chisamera, and L. Sofroni, Trans. AFS, Vol 93, 1985, p 35
22. K. Roehrig, Trans. AFS, Vol 86, 1978, p 75
23. Y.J. Park, R.B. Gundlach, R.G. Thomas, and J.F. Janowak, Trans. AFS, Vol 93, 1985, p 415
Compacted Graphite Iron
Doru M. Stefanescu, The University of Alabama
Physical Properties
Thermal conductivity plays a significant role in structural components subjected to thermal stress. The higher
the thermal conductivity, the lower the thermal gradients throughout the casting, and therefore the lower the
thermal stresses. The microstructure of cast iron, and especially graphite morphology, greatly influence thermal
conductivity. Graphite exhibits the highest thermal conductivity of all
the metallographic constituents. The conductivity of graphite parallel to the basal plane is about four times
higher than that perpendicular to its basal plane (Ref 24). Consequently, FG has higher thermal conductivity
than SG. It is therefore expected that FG iron will have higher thermal conductivity than SG iron,
(Ref 26). Not unexpectedly, as the amount of
graphite increases, thermal conductivity is also improved.
The thermal conductivity of ferrite is reduced by dissolved alloying elements. For steel, the conductivity of the
matrix can be calculated by the equation (Ref 6):
λ= λ0 - lnΣC (Eq 3)
where λis the thermal conductivity of alloyed steel, λ0 is the thermal conductivity of unalloyed steel, and ΣC is
the sum of alloying elements in %. This equation can also be used to estimate the influence of various alloying
additions on the conductivity of cast irons.
. From this last table it can be seen that the thermal conductivity of CG iron
is very close to that of gray cast iron and considerably higher than that of SG iron (Ref 1, 9). This behavior is
explained by the fact that much like flake graphite, compacted graphite is interconnected. As for FG irons,
increasing the carbon equivalent results in higher thermal conductivity for CG iron. As the temperature is
increased, the thermal conductivity reaches a maximum at about 200 °C (390 °F), an effect also shown by SG
irons, but not by FG iron .
With results for an ingot mold and bottom plate made from FG iron and a sample of ferritic SG iron (Ref 27).
As previously implied, increased nodularity results in lower thermal conductivity (Ref 1, 28).
Sonic and Ultrasonic Properties. Resonant frequency (sonic testing) and ultrasonic velocity measurements
provide reliable methods for verifying the structure and properties of castings. As shown in Fig. 35, ultrasonic
velocity is directly related to nodularity. Unfortunately it is rather difficult to distinguish between CG and lownodularity
SG irons. Better results seem to be obtained when ultrasonic velocity is related to tensile strength.
Figure 36 shows the correlation between tensile strength and ultrasonic velocity or resonant frequency for test
bars of 30 mm (1.2 in.) diameter.
When these tests are applied to castings, the ultrasonic velocity for CG structures is independent of the shape of
the casting, but should be calibrated for the section thickness. Thus, for 30 mm (1.2 in.) diam bars, the range
associated with CG is between 5.2 and 5.45 km/s, but for very large castings such as ingot molds, the ultrasonic
velocity for good CG structures lies between 4.85 and 5.10 km/s. Sonic testing, on the other hand, must be
calibrated for a particular design of casting, for which examples of satisfactory and unsatisfactory structures
must be previously checked to provide a calibration range (Ref 9).
References cited in this section
1. E. Nechtelberger, H. Puhr, J.B. von Nesselrode, and A. Nakayasu, Paper 1 presented at the 49th
International Foundry Congress, International Committee of Foundry Technical Associations, Chicago,
1982
6. E. Nechtelberger, The Properties of Cast Iron up to 500 °C, Technicopy Ltd., 1980
9. G.F. Sergeant and E.R. Evans, The British Foundryman, May 1978, p 115
13. J. Fowler, D.M. Stefanescu, and T. Prucha, Trans. AFS, Vol 92, 1984, p 361
24. E. Mayer-Rassler, Giesserei, Vol 54 (No. 13), 1967, p 348
25. H. Kempers, Giesserei, Vol 53 (No. 1), 1966, p 15
26. K. Lohberg and J. Motz, Giesserei, Vol 44 (No. 11), 1957, p 305
27. P.A. Green and A.J. Thomas, Trans. AFS, Vol 87, 1979, p 569
28. R.W. Monroe and C.E. Bates, Trans. AFS, Vol 93, 1985, p 615
Compacted Graphite Iron
Doru M. Stefanescu, The University of Alabama
Other Properties
Corrosion Resistance. At room temperature, the corrosion rate of CG iron in 5% sulfuric acid is nearly half
that of FG iron but higher than that of SG iron . With increasing temperature, the difference becomes
smaller. The pearlitic matrix has higher corrosion resistance than the ferritic one. As expected, corrosion
accelerates when stress is applied (Ref 29).
Machinability. Standardized machinability tests comparing CG irons with other castings are difficult to find in
the literature.
Nevertheless, in general, from both experimental data and practical experience in machine shops, it can be
concluded that for a given matrix the machinability of CG iron is between that of gray and ductile iron (Ref 1,
9, 21). The CG morphology makes the iron sufficiently brittle for machine swarf to break into small chips, yet
strong enough to prevent the swarf from forming powdery chips. Neither large swarf nor fine, powdery swarf is
ideal for high machinability (Ref 10).
Damping Capacity. The relative damping capacity of various irons, obtained by measuring the relative rates at
which the amplitude of an imposed vibration decreases with time, Ref 17, is:
FG iron:CG iron:SG iron = 1.0:0.6:0.34 (Eq 4)
Apparently, changes in the carbon equivalent or matrix do not significantly influence the damping capacity, but
heavier sections will produce higher damping capacities (Ref 9).
References cited in this section
1. E. Nechtelberger, H. Puhr, J.B. von Nesselrode, and A. Nakayasu, Paper 1 presented at the 49th
International Foundry Congress, International Committee of Foundry Technical Associations, Chicago,
1982
9. G.F. Sergeant and E.R. Evans, The British Foundryman, May 1978, p 115
10. K.P. Cooper and C.R. Loper, Jr., Trans. AFS, Vol 86, 1978, p 241
17. K.B. Palmer, BCIRA J., Report 1213, Jan 1976, p 31
21. I. Riposan, M. Chisamera, and L. Sofroni, Trans. AFS, Vol 93, 1985, p 35
29. A.E. Krivosheev, B.V. Marintchenkov, and N.M. Fettisov, Russ. Casting Prod., 1973, p. 86
Compacted Graphite Iron
Doru M. Stefanescu, The University of Alabama
Applications
The applications of CG irons stem from their relative intermediate position between FG and SG irons.
Compared to FG irons, CG irons have certain advantages:
· Higher tensile strength at the same carbon equivalent, which reduces the need for expensive alloying
elements such as nickel, chromium, copper, and molybdenum
· Higher tensile strength to hardness ratio
· Much higher ductility and toughness, which result in a higher safety margin against fracture
· Lower oxidation and growth at high temperatures
· Less section sensitivity for heavy sections
Compared to SG irons, certain advantages can be claimed for CG irons:
· Lower coefficient of thermal expansion
· Higher thermal conductivity
· Better resistance to thermal shock
· Higher damping capacity
· Better castability, leading to higher casting yield, and the capability for pouring more intricate castings
· Improved machinability
CG iron can be substituted for FG iron in all cases in which the strength of FG iron has become insufficient, but
in which a change to SG iron is undesirable because of the less favorable casting properties of the latter.
Examples include bed plates for large diesel engines, crankcases, gearbox housings, turbocharger housings,
connecting forks, bearing brackets, pulleys for truck servodrives, sprocket wheels, and eccentric gears.
Because the thermal conductivity of CG iron is higher than that of SG iron, CG iron is preferred for castings
operating at elevated temperature and/or under thermal fatigue conditions. Applications include ingot molds,
crankcases, cylinder heads, exhaust manifolds, and brake disks.
The largest industrial application by weight of CG iron produced is for ingot molds weighing up to 54 Mg (60
tons). According to a number of reports summarized in Ref 30, the life of ingot molds made of CG iron is 20 to
70% longer than the life of those made of FG iron.
In the case of cylinder heads, it was possible to increase engine output by 50% by changing from alloyed FG
iron to ferritic CG iron (Ref 1). The specified minimum values for cylinder heads are 300 MPa (43 ksi) tensile
strength, 240 MPa (35 ksi) yield strength, and 2% elongation.
Modern car and truck engines require that manifolds work at temperature ranges of 500 °C (930 °F). At this
temperature, FG iron manifolds are prone to cracking, while SG iron manifolds tend to warp. CG iron
manifolds warp and oxidize less and thus have a longer life. Other engineering applications are summarized in
Ref 1 and 30.
References cited in this section
1. E. Nechtelberger, H. Puhr, J.B. von Nesselrode, and A. Nakayasu, Paper 1 presented at the 49th
International Foundry Congress, International Committee of Foundry Technical Associations, Chicago,
1982
30. D.M. Stefanescu and C.R. Loper, Jr., Giesserei-Prax., No. 5, 1981, p 74
Compacted Graphite Iron
Doru M. Stefanescu, The University of Alabama
References
1. E. Nechtelberger, H. Puhr, J.B. von Nesselrode, and A. Nakayasu, Paper 1 presented at the 49th
International Foundry Congress, International Committee of Foundry Technical Associations, Chicago,
1982
2. H.H. Cornell and C.R. Loper, Jr., Trans. AFS, Vol 93, 1985, p 435
3. R. Elliott, Cast Iron Technology, Butterworths, 1988
4. D.M. Stefanescu, I. Dinescu, S. Craciun, and M. Popescu, "Production of Vermicular Graphite Cast Irons
by Operative Control and Correction of Graphite Shape," Paper 37 presented at the 46th International
Foundry Congress, Madrid, 1979
5. D.M. Stefanescu, F. Martinez, and I.G. Chen, Trans. AFS, Vol 91, 1983, p 205
6. E. Nechtelberger, The Properties of Cast Iron up to 500 °C, Technicopy Ltd., 1980
7. J. Sissener, W. Thury, R. Hummer, and E. Nechtelberger, AFS Cast Met. Res. J., 1972, p 178
8. C.F. Walton and T.J. Opar, Ed., Iron Castings Handbook, Iron Casting Society Inc., 1981
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20. D.M. Stefanescu and G. Niculescu, unpublished research
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24. E. Mayer-Rassler, Giesserei, Vol 54 (No. 13), 1967, p 348
25. H. Kempers, Giesserei, Vol 53 (No. 1), 1966, p 15
26. K. Lohberg and J. Motz, Giesserei, Vol 44 (No. 11), 1957, p 305
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28. R.W. Monroe and C.E. Bates, Trans. AFS, Vol 93, 1985, p 615
29. A.E. Krivosheev, B.V. Marintchenkov, and N.M. Fettisov, Russ. Casting Prod., 1973, p. 86
30. D.M. Stefanescu and C.R. Loper, Jr., Giesserei-Prax., No. 5, 1981, p 74
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