Historically, the first classification of cast iron was
based on its fracture. Two types of iron were initially
recognized:
- White iron: Exhibits a white, crystalline fracture
surface because fracture occurs along the iron carbide
plates; it is the result of metastable solidification
(Fe-C eutectic)
- Gray iron: Exhibits a gray fracture surface because
fracture occurs along the graphite plates (flakes); it
is the result of stable solidification (Gr eutectic)
Special cast irons differ from the common cast irons
mainly in the higher content of alloying elements which
promote microstructures having special properties for
elevated-temperature applications, corrosion resistance,
and wear resistance.
The goal of the metallurgist is to design a process that
will produce a structure that will yield the expected
mechanical properties. This requires knowledge of the
structure-properties correlation for the particular
alloy under consideration as well as of the factors
affecting the structure.
When discussing the metallurgy of cast iron, the main
factors of influence on the structure that one needs to
address are:
- Chemical composition
- Cooling rate
- Liquid treatment
- Heat treatment.
In addition, the following aspects of combined carbon
in cast irons should also be considered:
- In the original cooling or through subsequent heat
treatment, a matrix can be internally decarbonized or
carburized by depositing graphite on existing sites or
by dissolving carbon from them.
- Depending on the silicon content and the cooling rate,
the pearlite in iron can vary in carbon content. This is a
ternary system, and the carbon content of pearlite can be
as low as 0.50% with 2.5% Si.
- The conventionally measured hardness of graphitic
irons is influenced by the graphite, especially in gray
iron. Martensite micro hardness may be as high as 66 HRC,
but measures as low as 54 HRC conventionally in gray iron
(58 HRC in ductile).
- The critical temperature of iron is influenced (raised)
by silicon content, not by carbon content.
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 manganese content varies as a function of the desired
matrix. Typically, it can be as low as 0.1% for ferrule
irons and as high as 1.2% for pearlitic irons, because
manganese is a strong pearlite promoter.
From the minor elements, phosphorus and sulfur are the most
common and are always present in the composition. They can
be as high as 0.15% for low-quality iron and are
considerably less for high-quality iron, such as
ductile iron or compacted graphite iron.
The main effects of chemical composition to nodular
(ductile) iron are similar to those described for gray
iron, with quantitative differences in the extent of
these effects and qualitative differences in the
influence on graphite morphology. The carbon equivalent
has only a mild influence on the properties and structure
of ductile iron, because it affects graphite shape
considerably less than in the case of gray iron.
Nevertheless, to prevent excessive shrinkage, high chilling
tendency, graphite flotation or a high impact transition
temperature, optimum amounts of carbon and silicon must
be selected. Minor elements can significantly alter the
structure in terms of graphite morphology, chilling
tendency, and matrix structure. Minor elements can
promote the spheroidization of graphite or can have an
adverse effect on graphite shape.
The general influence of various elements on graphite shape.
The elements in the first group - the spheroidizing elements
- can change graphite shape from flake through compacted to
spheroidal. The most widely used element for the production
of spheroidal graphite is magnesium. The amount of residual
magnesium required to produce spheroidal graphite is
generally 0.03 to 0.05%. The precise level depends on
the cooling rate. A higher cooling rate requires less
magnesium. The amount of magnesium to be added in the iron
is a function of the initial sulfur level. A residual
magnesium level that is too low results in insufficient
nodularity. This in turn results in a deterioration of
the mechanical properties of the iron. If the magnesium
content is too high, carbides are promoted.
The presence of antispheroidizing minor elements may result
in graphite shape deterioration, up to complete graphite
degeneration. Therefore, upper limits are set on the amount
of deleterious elements to be accepted in the composition
of cast iron. These values can be influenced by the
combination of various elements and by the presence of
rare earths in the composition. Furthermore, some of
these elements can be deliberately added during liquid
processing in order to increase nodule count.
Alloying elements have in principle the same influence
on structure and properties as for gray iron. Because
better graphite morphology allows more efficient use of
the mechanical properties of the matrix, alloying is
more common in ductile iron than in gray iron.
Cooling Rate. When changing the cooling rate,
effects similar to those discussed for gray iron also
occur in ductile iron, but the section sensitivity of
ductile iron is lower. This is because spheroidal
graphite is less affected by cooling rate than flake
graphite.
The liquid treatment of ductile iron is more complex
than that of gray iron. The two stages for the liquid
treatment of ductile iron are:
- Modification, which consists of magnesium or
magnesium alloy treatment of the melt, with the
purpose of changing graphite shape from flake to
spheroidal.
- Inoculation (normally, postinoculation that is,
after the magnesium treatment) to increase the nodule
count. Increasing the nodule count is an important goal,
because a higher nodule count is associated with less
chilling tendency and a higher as-cast ferrite/pearlite
ratio.
Heat treatment is extensively applied on ductile iron
because better advantage can be taken of the matrix
structure than for gray iron. The heat treatments usually
applied are as follows:
- Stress relieving
- Annealing to produce a feritic matrix
- Normalizing to produce a pearlitic matrix
- Hardening to produce tempering structures
- Austempering to produce a ferritic bainite.
The advantage of austempering is that it results in
ductile irons with twice the tensile strength for the
same toughness. Compacted graphite (CG) irons have a
graphite shape intermediate between spheroidal and
flake. Typically, compacted graphite looks like type
IV graphite.
The chemical composition effects are similar to those
described for ductile iron. Carbon equivalent influences
strength less obviously than for the case of gray iron,
but than for ductile iron. The graphite shape is
controlled, as in the case of ductile iron, through
the content of minor elements. When the goal is to
produce compacted graphite, it is easier from the stand
point of controlling the structure to combine
spheroidizing (magnesium, calcium, and/ or rare earths)
and antispheroidizing (titanium and/or aluminum)
elements.
The cooling rate affects properties less for gray iron
but more for ductile iron. In other words, CG iron is
less section sensitive than gray iron. However, high
cooling rates are to be avoided because of the high
propensity of CG iron for chilling and high nodule count
in thin sections.
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) and
graphite. Very slow cooling of irons that contain
large percentages silicon and carbon is likely to
produce considerable ferrite and pearlite throughout
the matrix, together with coarse graphite flakes.
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 100x magnification.
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 poor
inoculation.
The large flakes of type C flake graphite are typical
of kish graphite that is 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 in 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 6 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 pearlitic matrix.
Solidification of Gray Iron. In a hypereutectic
gray iron, solidification begins with the precipitation
of kish graphite in the melt. Kish grows as large,
straight, undistorted flakes or as very thick, lumpy
flakes that tend to rise to the surface of the melt
because of their Sow relative density. When the
temperature has been lowered sufficiently, the remaining
liquid solidifies as a eutectic structure of austenite
and graphite. Generally, eutectic graphite is finer
than kish graphite.