Classification and Basic Metallurgy of Cast Iron

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Classification and Basic Metallurgy of Cast Iron

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Classification and Basic Metallurgy of Cast Iron

Messaggioda Aldebaran » 27/04/2010, 16:18

Classification
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 (Fe3C 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)
With the advent of metallography, and as the body of knowledge pertinent to cast iron increased, other classifications
based on microstructural features became possible:
· Graphite shape: Lamellar (flake) graphite (FG), spheroidal (nodular) graphite (SG), compacted
(vermicular) graphite (CG), and temper graphite (TG); temper graphite results from a solid-state
reaction (malleabilization)
· Matrix: Ferritic, pearlitic, austenitic, martensitic, bainitic (austempered)
This classification is seldom used by the floor foundryman. The most widely used terminology is the commercial one. A
first division can be made in two categories:
· Common cast irons: For general-purpose applications, they are unalloyed or low alloy
· Special cast irons: For special applications, generally high alloy.
Special cast irons differ from the common cast irons mainly in the higher content of alloying elements (>3%), which
promote microstructures having special properties for elevated-temperature applications, corrosion resistance, and wear
resistance.
Principles of the Metallurgy of Cast Iron
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 decarburized 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 microhardness 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 carbon content
The following sections in this article discuss some of the basic principles of cast iron metallurgy. More detailed
descriptions of the metallurgy of cast irons are available in separate articles in this Volume describing certain types of cast
irons. The Section "Ferrous Casting Alloys" in Casting, Volume 15 of ASM Handbook, formerly 9th Edition Metals
Handbook, also contains more detailed descriptions on the metallurgy of cast irons.
Gray Iron (Flake Graphite Iron)
The composition of gray iron must be selected in such a way as to satisfy three basic structural requirements:
· The required graphite shape and distribution
· The carbide-free (chill-free) structure
· The required matrix
For common cast iron, the main elements of the chemical composition are carbon and silicon. Figure 3 shows the range of
carbon and silicon for common cast irons as compared with steel. It is apparent that irons have carbon in excess of the
maximum solubility of carbon in austenite, which is shown by the lower dashed line. A 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 combined influence of carbon and silicon on the structure is usually taken into account by the carbon equivalent
(CE):
CE = % C + 0.3(% Si)
+ 0.33(% P) - 0.027(% Mn) + 0.4(% S) (Eq 1)
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.
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 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 an FeS-free structure and maximum amount of ferrite is:
% Mn = 1.7(% S) + 0.15 (Eq 2)
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.
Both major and minor elements have a direct influence on the morphology of flake graphite.
Type A graphite is found in inoculated irons cooled with moderate rates. In general,
it is associated with the best mechanical properties, and cast irons with this type of graphite exhibit moderate
undercooling during solidification . Type B graphite is found in irons of near-eutectic composition, solidifying on
a limited number of nuclei. Large eutectic cell size and low undercoolings are common in cast irons exhibiting this type
of graphite. Type C graphite occurs in hypereutectic irons as a result of solidification with minimum undercooling. Type
D graphite is found in hypoeutectic or eutectic irons solidified at rather high cooling rates, while type E graphite is
characteristic for strongly hypoeutectic irons. Types D and E are both associated with high undercoolings during
solidification. Not only graphite shape but also graphite size is important, because it is directly related to strength .
In general, alloying elements can be classified into three categories. Each is discussed below.
Silicon and aluminum increase the graphitization potential for both the eutectic and eutectoid transformations and
increase the number of graphite particles. They form solid 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. Because 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 α Fe 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
both Gr and Fe3C (mottled structure), which will have lower strength but higher hardness.
The influence of composition and cooling rate on tensile strength can be estimated using (Ref 3):
TS = 162.37 + 16.61/D - 21.78(% C)
-61.29(% Si) - 10.59 (% Mn - 1.7% S)
+ 13.80(% Cr) + 2.05(% Ni) + 30.66(% Cu)
+ 39.75(% Mo) + 14.16 (% Si)2
-26.25(% Cu)2 - 23.83 (% Mo)2
(Eq 3)
where D is the bar diameter (in inches). Equation 3 is valid for bar diameters of 20 to 50 mm (0.78 to 2 in.)
The cooling rate, like the chemical composition, can significantly influence the as-cast structure and therefore the
mechanical properties. The cooling rate of a casting is primarily a function of its section size. The dependence of structure
and properties on section size is termed section sensitivity. Increasing the cooling rate will:
· Refine both graphite size and matrix structure; this will result in increased strength and hardness
· Increase the chilling tendency; this may result in higher hardness, but will decrease the strength
Consequently, composition must be tailored in such a way as to provide the correct graphitization potential for a given
cooling rate. For a given chemical composition and as the section thickness increases, the graphite becomes coarser, and
the pearlite/ferrite ratio decreases, which results in lower strength and hardness . Higher carbon equivalent has
similar effects.
The liquid treatment of cast iron is of paramount importance in the processing of this alloy because it can
dramatically change the nucleation and growth conditions during solidification. As a result, graphite morphology, and
therefore properties, can be significantly affected. In gray iron practice, the liquid treatment used is termed inoculation
and consists of minute additions of minor elements before pouring. Typically, ferrosilicon with additions of aluminum
and calcium, or proprietary alloys are used as inoculants. The main effects of inoculation are:
· An increased graphitization potential because of decreased undercooling during solidification; as a
result of this, the chilling tendency is diminished, and graphite shape changes from type D or E to type
A.
· A finer structure, that is, higher number of eutectic cells, with a subsequent increase in strength inoculation improves tensile strength. This influence is more pronounced for low-CE cast irons.

Heat treatment can considerably alter the matrix structure, although graphite shape and size remain basically
unaffected. A rather low proportion of the total gray iron produced is heat treated. Common heat treatment may consist of
stress relieving or of annealing to decrease hardness.
Ductile Iron (Spheroidal Graphite Iron)
Composition. The main effects of chemical composition 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.

As mentioned previously, 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 minor elements that adversely affect graphite shape are said to degenerate graphite shape.
The generic influence of various elements on graphite shape is given in Table 4. 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, Mgresid, 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, Sin, and the recovery of magnesium, η, in the particular process used.

The presence of antispheroidizing (deleterious) 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.
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 used in the processing of 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 ferritic 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 Irons
Compacted graphite irons have a graphite shape intermediate between spheroidal and flake. Typically, compacted
graphite looks like type IV graphite . Consequently, most of the properties of CG irons lie in between those of
gray and ductile iron.
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 more 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 standpoint 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 (Fig. 18). 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.
Liquid treatment can have two stages, as for ductile iron. Modification can be achieved with magnesium, Mg + Ti, Ce
+ Ca, and so on. Inoculation must be kept at a low level to avoid excessive nodularity.
Heat treatment is not common for CG irons.
Malleable Irons
Malleable cast irons differ from the types of irons previously discussed in that they have an initial as-cast white structure,
that is, a structure consisting of iron carbides in a pearlitic matrix. This white structure is then heat treated (annealing at
800 to 970 °C, or 1470 to 1780 °F), which results in the decomposition of Fe3C and the formation of temper graphite. The
basic solid state reaction is:
Fe3C ®γ+ Gr (Eq 5)
The final structure consists of graphite and pearlite, pearlite and ferrite, or ferrite. The structure of the matrix is a function
of the cooling rate after annealing. Most of the malleable iron is produced by this technique and is called blackheart
malleable iron. Some malleable iron is produced in Europe by decarburization of the white as-cast iron, and it is called
whiteheart malleable iron.
The composition of malleable irons must be selected in such a way as to produce a white as-cast structure and to allow
for fast annealing times. Although higher carbon and silicon reduce the
heat treatment time, they must be limited to ensure a graphite-free structure upon solidification. Both tensile strength and
elongation decrease with higher carbon equivalent. Nevertheless, it is not enough to control the carbon equivalent. The
annealing time depends on the number of graphite nuclei available for graphitization, which in turn depends on, among
other factors, the C/Si ratio. A lower C/Si ratio (that is, a higher silicon content for a constant carbon
equivalent) results in a higher temper graphite count. This in turn translates into shorter annealing times.
Manganese content and the Mn/S ratio must be closely controlled. In general, a lower manganese content is used when
ferritic rather than pearlitic structures are desired. The correct Mn/S ratio can be calculated with Eq 2. . Under the line described by Eq 2, all sulfur is stoichiometrically tied to manganese as MnS. The excess
manganese is dissolved in the ferrite. In the range delimited by the lines given by Eq 2 and the line Mn/S = 1, a mixed
sulfide, (Mn,Fe)S, is formed. For Mn/S ratios smaller than 1, pure FeS is also formed. It is assumed that the degree of
compacting of temper graphite depends on the type of sulfides occurring in the iron . When FeS is predominant,
very compacted, nodular temper graphite forms, but some undissolved Fe3C may persist in the structure, resulting in
lower elongations. When MnS is predominant, although the graphite is less compacted, elongation is higher because of
the completely Fe3C-free structure.
Cooling Rate. Like all other irons, malleable irons are sensitive to cooling rate. Nevertheless, because the final structure
is the result of a solid-state reaction, they are the least section sensitive irons.
The liquid treatment of malleable iron increases the number of nuclei available for the solid-state graphitization
reaction. This can be achieved in two different ways, as follows:
· By adding elements that increase undercooling during solidification. Typical elements in this category
are magnesium, cerium, bismuth, and tellurium. Higher undercooling results in finer structure, which in
turn means more γ-Fe3C interface. Because graphite nucleates at the γ-Fe3C interface, this means more
nucleation sites for graphite. Higher undercooling during solidification also prevents the formation of
unwanted eutectic graphite
· By adding nitrite-forming elements to the melt. Typical elements in this category are aluminum, boron,
titanium, and zirconium
The heat treatment of malleable iron determines the final structure of this iron. It has two basic stages. In the first
stage, the iron carbide is decomposed in austenite and graphite (Eq 5). In the second stage, the austenite is transformed
into pearlite, ferrite, or a mixture of the two. Although there are some compositional differences between ferritic and
pearlitic irons, the main difference is in the heat treatment cycle. When ferritic structures are to be produced, cooling rates
in the range of 3 to 10 °C/h (5 to 18 °F/h) are required through the eutectoid transformation in the second stage. This is
necessary to allow for a complete austenite-to-ferrite reaction. The goal of
the treatment is to achieve a eutectoid transformation according to the austenite-to-pearlite reaction. In some limited
cases, quenching-tempering treatments are used for malleable irons.
Special Cast Irons
Special cast irons, as previously discussed, are alloy irons that take advantage of the radical changes in structure produced
by rather large amounts of alloying elements. Abrasion resistance can be improved by increasing hardness, which in turn
can be achieved by either increasing the amount of carbides and their hardness or by producing a martensitic structure.
The least expensive material is white iron with a pearlitic matrix. Additions of 3 to 5% Ni and 1.5 to 2.5% Cr result in
irons with (FeCr)3C carbides and an as-cast martensitic matrix. Additions of 11 to 35% Cr produce (CrFe)7C3 carbides,
which are harder than the iron carbides. Additions of 4 to 16% Mn will result in a structure consisting of (FeMn)3C,
martensite, and work-hardenable austenite.
Heat resistance depends on the stability of the microstructure. Irons used for these applications may have a ferritic
structure with graphite (5% Si), a ferritic structure with stable carbides (11 to 28% Cr), or a stable austenitic structure
with either spheroidal or flake graphite (18% Ni, 5% Si). For corrosion resistance, irons with high chromium (up to 28%),
nickel (up to 18%), and silicon (up to 15%) are used.
References cited in this section:
1. R. Elliot, Cast Iron Technology, Butterworths, 1988
2. C.F. Walton and T.J. Opar, Ed., Iron Castings Handbook, Iron Castings Society, 1981
3. C.E. Bates, AFS Trans., Vol 94, 1986, p 889
4. R. Barton, B.C.I.R.A.J., No. 5, 1961, p 668
5. R.W. Lindsay and A. Shames, AFS Trans., Vol 60, 1952, p 650
6. H. Morrogh, AFS Trans., Vol 60, 1952, p 439
7. D.M. Stefanescu, AFS Int. Cast Met. J., June 1981, p 23
8. J.F. Janowak and R.B. Gundlach, AFS Trans., Vol 91, 1983, p 377
9. G.F. Sergeant and E.R. Evans, Br. Foundryman, May 1978, p 115
10. D.M. Stefanescu, Metalurgia, No. 7, 1967, p 368
11. K. Roesch, Stahl Eisen, No. 24, 1957, p 1747
13. K.M. Ankab, O.E. Shulte, and P.N. Bidulia, Isvestia Vishih Utchebnik Zavedenia-Tchornaia, Metallurghia,
No. 5, 1966, p 168
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