 Gray Irons are a group of cast irons that form flake graphite
during solidification, in contrast to the spheroidal graphite
morphology of ductile irons. The flake graphite in gray irons is
dispersed in a matrix with a microstructure that is determined
by composition and heat treatment. The usual microstructure of
gray iron is a matrix of pearlite with the graphite flakes
dispersed throughout. In terms of composition, 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 pearlitic).
Other alloying elements include nickel, copper, molybdenum, and
chromium.
The heat treatment of gray irons can considerably alter the
matrix microstructure with little or no effect on the size and
shape of the graphite achieved during casting. The matrix
microstructures resulting from heat treatment can vary from
ferrite-pearlite to tempered martensite. However, even though
gray iron can be hardened by quenching from elevated
temperatures, heat treatment is not ordinarily used commercially
to increase the overall strength of gray iron castings because
the strength of the as-cast metal can be increased at less cost
by reducing the silicon and total carbon contents or by adding
alloying elements. The most common heat treatments of gray iron
are annealing and stress relieving.
Chemical composition is another important parameter influencing
the heat treatment of gray cast irons. Silicon, for example,
decreases carbon solubility, increases the diffusion rate of
carbon in austenite, and usually accelerates the various
reactions during heat treating. Silicon also raises the
austenitizing temperature significantly and reduces the combined
carbon content (cementite volume). Manganese, in contrast,
lowers the austenitizing temperature and increases hardenability.
It also increases carbon solubility, slows carbon diffusion in
austenite, and increases the combined carbon content. In
addition, manganese alloys and stabilizes pearlitic carbide and
thus increases the pearlite content.
Annealing
The heat treatment most frequently applied to gray iron, with
the possible exception of stress relieving, is annealing. The
annealing of gray iron consists of heating the iron to a
temperature high enough to soften it and/or to minimize or
eliminate massive eutectic carbides, thereby improving its
machinability. This heat treatment reduces mechanical properties
substantially. It reduces the grade level approximately to the
next lower grade: for example, the properties of a class 40 gray
iron will be diminished to those of a class 30 gray iron. The
degree of reduction of properties depends on the annealing
temperature, the time at temperature, and the alloy composition
of the iron.
Gray iron is commonly subjected to one of three annealing
treatments, each of which involves heating to a different
temperature range. These treatments are ferritizing annealing,
medium (or full) annealing, and graphitizing annealing.
Ferritizing Annealing. For an unalloyed or low-alloy cast
iron of normal composition, when the only result desired is the
conversion of pearlitic carbide to ferrite and graphite for
improved machinability, it is generally unnecessary to heat the
casting to a temperature above the transformation range. Up to
approximately 595°C (1100°F), the effect of short times at
temperature on the structure of gray iron is insignificant. For
most gray irons, a ferritizing annealing temperature between 700
and 760°C (1300 and 1400°F) is recommended.
Medium (full) annealing. It is usually performed at
temperatures between 790 and 900°C (1450 and 1650°F). This
treatment is used when a ferritizing anneal would be ineffective
because of the high alloy content of a particular iron. It is
recommended, however, to test the efficacy of temperatures below
760°C (1400°F) before a higher annealing temperature is adopted
as part of a standard procedure.
Holding times comparable to those used in ferritizing annealing
are usually employed. When the high temperatures of medium
annealing are used, however, the casting must be cooled slowly
through the transformation range, from about 790 to 675°C (1450
to 1250°F).
Graphitizing Annealing. If the microstructure of gray
iron contains massive carbide particles, higher annealing
temperatures are necessary. Graphitizing annealing may simply
serve to convert massive carbide to pearlite and graphite,
although in some applications it may be desired to carry out a
ferritizing annealing treatment to provide maximum machinability.
The production of free carbide that must later be removed by
annealing is, except with pipe and permanent mold castings,
almost always an accident resulting from inadequate inoculation
or the presence of excess carbide formers, which inhibit normal
graphitization; thus, the annealing process is not considered
part of the normal production cycle.
To break down massive carbide with reasonable speed,
temperatures of at least 870°C (1600°F) are required. With each
additional 55°C (100°F) increment in holding temperature, the
rate of carbide decomposition doubles. Consequently, it is
general practice to employ holding temperatures of 900 to 955°C
(1650 to 1750°F).
Normalizing
Gray iron is normalized by being heated to a temperature above
the transformation range, held at this temperature for a period
of about 1 hour per inch of maximum section thickness, and
cooled in still air to room temperature. Normalizing may be used
to enhance mechanical properties, such as hardness and tensile
strength, or to restore as-cast properties that have been
modified by another heating process, such as graphitizing or the
preheating and postheating associated with repair welding.
The temperature range for normalizing gray iron is approximately
885 to 925°C (1625 to 1700°F). Austenitizing temperature has a
marked effect on microstructure and on mechanical properties
such as hardness and tensile strength.
The tensile strength and hardness of a normalized gray iron
casting depend on the following parameters:
1. Combined carbon content
2. Pearlite spacing (distance between cementite plates)
3. Graphite morphology.
The graphite morphology does not change to any significant
extent during normalization, and its effect on hardness and
tensile strength is omitted in this discussion on normalizing.
Combined carbon content is determined by the normalizing (austenitizing)
temperature and the chemical composition of the casting. Higher
normalizing temperatures increase the carbon solubility in
austenite (that is, the cementite volume in the resultant
pearlite). A higher cementite volume, in turn, increases both
the hardness and the tensile strength. The alloy composition of
a gray iron casting also influences carbon solubility in
austenite. Some elements increase carbon solubility, some
decrease it, and others have no effect on it. The carbon content
of the matrix is determined by the combined effects of the
alloying elements.
The other parameter affecting hardness and tensile strength in a
normalized gray iron casting is the pearlite spacing. Pearlite
spacing is determined by the cooling rate of the casting after
austenitization and the alloy composition. Fast cooling results
in small pearlite spacing, higher hardness, and higher tensile
strength. Too high a cooling rate may cause partial or full
martensitic transformation. The addition of alloying elements
may change hardness and tensile strength significantly.
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