Cast irons are alloys of iron, carbon, and silicon in which more carbon is
present than can be retained in solid solution in austenite at the eutectic
temperature. In gray cast iron, the carbon that exceeds the solubility in
austenite precipitates as flake graphite.
Gray irons usually contain 2.5 to 4% C, 1 to 3% Si, and additions of
manganese, depending on the desired microstructure (as low as 0.1% Mn
in ferritic gray irons and as high as 1.2% in pearlitics). Sulphur and
phosphorus are also present in small amounts as residual impurities.
For common cast iron, the main elements of the chemical composition
are carbon and silicon. High carbon content increases the amount of
graphite or Fe3C. High carbon and silicon contents increase the
graphitization potential of the iron as well as its castability.
The composition of gray iron must be selected in such a way
to satisfy three basic structural requirements:
- The required graphite shape and distribution
- The carbide-free (chill-free) structure
- The required matrix
The combined influence of carbon and silicon on the structure
is usually taken into account by the carbon equivalent (CE):
CE = %C + 0.3x(%Si) + 0.33x(%P) - 0.027x(%Mn) + 0.4x(%S)
Although increasing the carbon and silicon contents improves the
graphitization potential and therefore decreases the chilling tendency,
the strength is adversely affected. This is due to ferrite promotion and
the coarsening of pearlite.
The manganese content varies as a function of the desired matrix. Typically,
it can be as low as 0.1% for ferritic irons and as high as 1.2% for
pearlitic irons, because manganese is a strong pearlite promoter.
The effect of sulfur must be balanced by the effect of manganese. Without
manganese in the iron, undesired iron sulfide (FeS) will form at grain
boundaries. If the sulfur content is balanced by manganese, manganese
sulfide (MnS) will form, which is harmless because it is distributed
within the grains. The optimum ratio between manganese and sulfur for
a FeS-free structure and maximum amount of ferrite is:
%Mn = 1.7x(%S) + 0.15
Other minor elements, such as aluminum, antimony, arsenic, bismuth,
lead, magnesium, cerium, and calcium, can significantly alter both
the graphite morphology and the microstructure of the matrix.
In general, alloying elements can be classified into three categories.
Silicon and aluminum increase the graphitization potential for both the
eutectic and eutectoid transformations and increase the number of graphite
particles. They form colloid solutions in the matrix. Because they
increase the ferrite/pearlite ratio, they lower strength and hardness.
Nickel, copper, and tin increase the graphitization potential during
the eutectic transformation, but decrease it during the eutectoid
transformation, thus raising the pearlite/ferrite ratio. This second
effect is due to the retardation of carbon diffusion. These elements
form solid solution in the matrix. Since they increase the amount of
pearlite, they raise strength and hardness.
Chromium, molybdenum, tungsten, and vanadium decrease the graphitization
potential at both stages. Thus, they increase the amount of carbides and
pearlite. They concentrate in principal in the carbides, forming
(FeX)nC-type carbides, but also alloy the aFe solid solution. As long as
carbide formation does not occur, these elements increase strength and
hardness. Above a certain level, any of these elements will determine
the solidification of a structure with Fe3C (mottled structure), which
will have lower strength but higher hardness.
Generally, it can be assumed that the following properties of gray
cast irons increase with increasing tensile strength from class 20
to class 60:
- All strengths, including strength at elevated temperature
- Ability to be machined to a fine finish
- Modulus of elasticity
- Wear resistance.
Successful production of a gray iron casting depends on the fluidity
of the molten metal and on the cooling rate, which is influenced by the
minimum section thickness and on section thickness variations.
On the other hand, the following properties decrease with increasing
tensile strength, so that low-strength irons often perform better than
high-strength irons when these properties are important:
- Resistance to thermal shock
- Damping capacity
- Ability to be cast in thin sections.
Casting design is often described in terms of section sensitivity. This
is an attempt to correlate properties in critical sections of the casting
with the combined effects of composition and cooling rate. All these
factors are interrelated and may be condensed into a single term,
castability, which for gray iron may be defined as the minimum section
thickness that can be produced in a mold, cavity with given volume/area
ratio and mechanical properties consistent with the type of iron being
Scrap losses resulting from missruns, cold shuts, and round corners
are often attributed to the lack of fluidity of the metal being poured.
Mold conditions, pouring rate, and other process variables being equal,
the fluidity of commercial gray irons depends primarily on the amount
of superheat above the freezing temperature (liquidus). As the total
carbon content decreases, the liquidus temperature increases, and the
fluidity at a given pouring temperature therefore decreases. Fluidity
is commonly measured as the length of flow into a spiral-type fluidity
The significance of the relationships between fluidity, carbon content,
and pouring temperature becomes apparent when it is realized that the
gradation in strength in the ASTM classification of gray iron is due
in large part to differences in carbon content (~3.60 to 3.80% for
class 20; ~2.70 to 2.95% for class 60). The fluidity of these irons
thus resolves into a measure of the practical limits of maximum
pouring temperature as opposed to the liquidus of the iron being poured.
The usual microstructure of gray iron is a matrix of pearlite with
graphite flakes dispersed throughout. Foundry practice can be varied
so that nucleation and growth of graphite flakes occur in a pattern
that enhances the desired properties. The amount, size, and distribution
of graphite are important. Cooling that is too rapid may produce so-called
chilled iron, in which the excess carbon is found in the form of massive
carbides. Cooling at intermediate rates can produce mottled iron, in which
carbon is present in the form of both primary cementite (iron carbide)
Flake graphite is one of seven types (shapes or forms) of graphite
established in ASTM A 247. Flake graphite is subdivided into five types
(patterns), which are designated by the letters A through E. Graphite
size is established by comparison with an ASTM size chart, which shows
the typical appearances of flakes of eight different sizes at l00x
Type A flake graphite (random orientation) is preferred for most
applications. In the intermediate flake sizes, type A flake graphite
is superior to other types in certain wear applications such as the
cylinders of internal combustion engines.
Type B flake graphite (rosette pattern) is typical of fairly rapid
cooling, such as is common with moderately thin sections (about 10 mm)
and along the surfaces of thicker sections, and sometimes results from
The large flakes of type C flake graphite are formed in hypereutectic
irons. These large flakes enhance resistance to thermal shock by
increasing thermal conductivity and decreasing elastic modulus. On
the other hand, large flakes are not conducive to good surface
finishes on machined parts or to high strength or good impact resistance.
The small, randomly oriented interdendritic flakes in type D flake
graphite promote a fine machined finish by minimizing surface pitting,
but it is difficult to obtain a pearlitic matrix with this type of
graphite. Type D flake graphite may be formed near rapidly cooled
surfaces or in thin sections. Frequently, such graphite is surrounded
by a ferrite matrix, resulting m soft spots in the casting.
Type E flake graphite is an interdendritic form, which has a preferred
rather than a random orientation. Unlike type D graphite, type E graphite
can be associated with a pearlitic matrix and thus can produce a casting
whose wear properties are as good as those of a casting containing only
type A graphite in a pearluic matrix. There are, of course, many
applications in which flake type has no significance as long as the
mechanical property requirements are met.
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