Heat Treating of Gray Irons,Heat Treating of Gray Irons,Heat Treating of Gray Irons
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. When gray iron is quenched and tempered, this is usually done to increase resistance to wear and abrasion by increasing hardness with a structure consisting of graphite embedded in hard martensite. The most
common heat treatments of gray iron are annealing and stress relieving.
Although the size of the graphite flakes in gray irons is unaffected by heat treatment, the size does have a marked
influence on the carbon kinetics during heat treating. In castings with fine graphite flakes, the carbon diffusion paths are
shorter, and ferritization or normalization is achieved in a shorter time than in those castings with large graphite flakes.
Castings with fine graphite flakes not only are easier to heat treat, but also display superior mechanical properties. The
factors that affect the graphite morphology achieved during casting are discussed in Volume 1 of ASM Handbook,
formerly 10th Edition Metals Handbook.
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. Further,
manganese reduces pearlite spacing and generally slows the heat-treating process.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, however. 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. Figure 1 shows the effect of full
annealing on the tensile strength of class 30 gray iron arbitration bars. 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. As the temperature increases above 595 °C (1100 °F),

the rate at which iron carbide decomposes to ferrite plus graphite increases markedly, reaching a maximum at the lower
transformation temperature (about 760 °C, or 1400 °F, for unalloyed or low-alloy iron). This is indicated in Fig. 2, which
shows the structure of unalloyed gray iron in the as-cast condition (Fig. 2a) and after being held for 1 h at 760 °C (1400
°F) per inch of section (Fig. 2b). Heating to a higher temperature for the same period of time may be detrimental to the
annealing process if it causes partial or complete transformation to austenite.

For most gray irons, a ferritizing annealing temperature between 700 and 760 °C (1300 and 1400 °F) is recommended.
The furnace temperature profile must be such that castings are sure to reach the set temperatures. Precise temperatures
within this range are determined by the exact composition of the iron. When machining properties are of primary
importance, it is advisable to anneal several samples at various temperatures between 700 and 760 °C (1300 and 1400 °F)
in order to determine the temperature that yields the lowest final hardness.
The casting must be held at temperature long enough to allow the graphitizing process to proceed to completion. At
temperatures below 700 °C (1300 °F), an excessively long holding time is usually required. At temperatures between 700
and 760 °C (1300 and 1400 °F), holding time varies with chemical composition, and may be as short as 10 min for
unalloyed irons. If an unusually low rate of cooling is used, the time at temperature may be further reduced.
Although the rate of cooling per se is not of great importance to the annealing process, slow cooling is recommended if
the stress relief that automatically occurs during annealing is to be retained as the casting cools to room temperature. A
cooling rate ranging from as high as 110 °C/h (200 °F/h) to 290 °C (550 °F) is satisfactory for all except the most
complex castings.
Medium (full) annealing 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, that the efficacy of temperatures at or below 760 °C (1400 °F) be tested 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). However, at 925 °C (1700 °F) and
above, the phosphide eutectic present in irons containing 0.10% P or more may melt.
The holding time at temperature may vary from a few minutes to several hours. The chill carbide (white iron) in some
high-silicon, high-carbon irons can be eliminated in as little as 15 min at 940 °C (1720 °F). In all applications, unless a
controlled-atmosphere furnace is used, the time at temperature should be as short as possible because at these high
temperatures gray iron is susceptible to scaling if moisture is present in the furnace atmosphere.
The cooling rate chosen depends on the final use of the iron. If the principal object of the treatment is to break down
carbides and it is desired to retain maximum strength and wear resistance, the casting should be air cooled from the
annealing temperature to about 540 °C (1000 °F) to promote the formation of a pearlitic structure. If maximum
machinability is the object, the casting should be furnace cooled to 540 °C (1000 °F), and special care should be exerted
to ensure slow cooling through the transformation range. In both instances, cooling from 540 °C (1000 °F) to about 290
°C (550 °F) at not more than 110 °C/h (200 °F/h) is recommended to minimize residual stresses.
Effect of Alloy Content on lime at Temperature. Certain elements, such as carbon and silicon, accelerate the
decomposition of pearlite and massive carbide at annealing temperatures. Therefore, when these elements are present in
sufficient percentages, the time at annealing temperature may be reduced. In an investigation of the decomposition of
pearlite at various temperatures in irons containing 1.93 and 2.68% Si, it was determined that the pearlite always broke
down more rapidly in the higher-silicon iron and that this iron could be effectively annealed over a greater temperature
range. For example, at an annealing temperature of 750 °C (1380 °F), the complete breakdown of pearlite occurred in the
higher-silicon iron in 10 min, whereas 45 min were required for the lower-silicon iron. This shows the pronounced effect
of silicon as an aid to the diffusion of carbon to the flakes present in the iron.
On the other hand, the pearlite-promoting elements (antimony, tin, vanadium, chromium, manganese, phosphorus, nickel,
and copper) delay pearlite decomposition.

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