SECTION VII. HEAT TREATMENT
Normalize and Temper
Quench and Temper
Scaling, Growth and Distortion
One reason for the phenomenal growth in the use of Ductile Iron castings is the high
ratio of performance to cost that they offer the designer and end user. This high value
results from many factors, one of which is the control of microstructure and properties
that can be achieved in the as-cast condition, enabling a high percentage of ferritic and
pearlitic Ductile Iron castings to be produced without the extra cost of heat treatment.
To obtain the advantage of producing high quality castings as-cast requires the use of
consistent charge materials and the implementation of consistent and effective practices
for melting, holding, treating, inoculation and cooling in the mold. By following these
practices, especially the use of high purity charges and good inoculation, castings
can be produced as-cast essentially free of carbides and with pearlite contents less than
10%, in section sizes as low as 0.150 in. (3.8 mm).
However heat treatment is a valuable and versatile tool for extending
both the consistency and range of properties of Ductile Iron castings beyond the limits of
those produced in the as-cast condition. Thus, to fully utilize the potential of
Ductile Iron castings, the designer should be aware of the wide range of heat treatments
available for Ductile Iron, and its response to these heat treatments.
Ductile Iron castings may be heat treated to:
increase toughness and ductility,
increase strength and wear resistance,
increase corrosion resistance,
stabilize the microstructure, to minimize growth,
equalize properties in castings with widely varying section sizes,
improve consistency of properties,
improve machinability, and
relieve internal stresses.
This Section deals with heat treating conventional Ductile Iron.
Austempering heat treatments, and the heat treatment of alloy Ductile Irons, are discussed
in Sections IV, and Section
Although Ductile Iron and steel are superficially similar
metallurgically, the high carbon and silicon levels in Ductile Iron result in important
differences in their response to heat treatment. The higher carbon levels in Ductile Iron
increase hardenability, permitting heavier sections to be heat treated with lower
requirements for expensive alloying or severe quenching media. These higher carbon levels
can also cause quench cracking due to the formation of higher carbon martensite, and/or
the retention of metastable austenite. These undesirable phenomena make the control of
composition, austenitizing temperature and quenching conditions more critical in Ductile
Iron. Silicon also exerts a strong influence on the response of Ductile Iron to heat
treatment. The higher the silicon content, the lower the solubility of carbon in austenite
and the more readily carbon is precipitated as graphite during slow cooling to produce a
Although remaining unchanged in shape, the graphite spheroids in
Ductile Iron play a critical role in heat treatment, acting as both a source and sink for
carbon. When heated into the austenite temperature range, carbon readily diffuses from the
spheroids to saturate the austenite matrix. On slow cooling the carbon returns to the
graphite "sinks", reducing the carbon content of the austenite. This
availability of excess carbon and the ability to transfer it between the matrix and the
nodules makes Ductile Iron easier to heat treat and increases the range of properties that
can be obtained by heat treatment.
All Ductile Iron heat treatments,
apart from stress relief, tempering and subcritical annealing, involve
heating the casting to a temperature above the critical temperature
range (Figure 7.1). In ferrous heat
treatment, the critical temperature (Al) is the temperature
above which the austenite phase is stable. Unlike steels, which have a
constant critical temperature (eutectoid temperature), Figure
7.2, Ductile Irons are ternary, iron-carbon-silicon alloys in which
the critical temperature varies with both carbon and silicon contents. Figure
7.3 shows the effect of carbon on this ternary phase diagram at the
2 % silicon level. Figure 7.4 shows
the effect of silicon on the critical temperatures for typical cast
irons. This relationship, the desired carbon content in the austenite
and the need to dissolve carbides, are the primary determinants of the
correct austenitizing temperature for Ductile Iron.
The most simple and economic form
of heat treatment is the controlled shakeout of the castings from the
mold. By removing the castings from the mold above the critical
temperature, the rate of cooling can be increased, favoring the
formation of pearlite with a resultant increase in casting hardness and
strength (Figure 7.5). If the alloy
content is sufficiently high, castings with bainitic structures can also
be produced by this method. Hardening castings through early shakeout
requires extremely close control of shakeout times and casting
composition and immediate stress relief of complex castings to avoid the
detrimental effects of internal stresses.
Austenitizing is the process of
holding the Ductile Iron casting above the critical temperature for a
sufficient period of time to ensure that the matrix is fully transformed
to austenite. The austenitizing temperature, along with the silicon
content, determines the carbon content of the austenite. Both
austenitizing time and temperature depend on the microstructure and
composition of the as-cast material. In order to break down primary
carbides, austenitizing temperatures in the range 1650-1750oF
(900-940oC) are normally used, with times ranging from one to
three hours. High silicon content and high nodule count reduce breakdown
times, while the presence of carbide stabilizers such as chromium,
vanadium and molybdenum require substantially longer times. Pearlite
decomposition occurs much more rapidly and at lower temperatures than
carbide breakdown. This breakdown is enhanced by high silicon and high
modularity and retarded by pearlite stabilizing elements such as
manganese, copper, tin, antimony and arsenic. The segregation of
manganese and chromium to cell boundaries can result in the incomplete
dissolution of both pearlite and carbides and the resulting impairment
of mechanical properties.
Annealing softens Ductile Iron by
producing a carbide-free, fully ferritic matrix. Table
7.1 describes recommended practices for annealing Ductile Iron.
These procedures range from a low temperature or sub-critical anneal
used to ferritize carbide-free castings, to two-stage and high
temperature anneals designed to break down carbides. The primary purpose
of annealing, or ferritizing, Ductile Iron is the production of castings
with maximum ductility and toughness, reduced strength and hardness,
improved machinability and uniform properties. Figure
3.17 (Section 3) shows that
annealing castings with different levels of copper and tin has reduced
strength and hardness, increased elongation, and generally eliminated
the variations in as-cast properties produced by the different alloy
levels (Figure 3.16). Figures
and Table 3.4 illustrate the
effects of both standard and subcritical annealing on the fracture
toughness of Ductile Iron.
7.1 Recommended practices for annealing Ductile Iron castings.
absence of carbides.
To obtain Grades 60-45-12,
(720 to 730C)
per inch of
cool (100F or 55C per hour) to 650F (345C). Air Cool.
absence of carbides.
To obtain Grade 60-40-18
with max. low temperature
870 to 900C
equalize at control temperature
cool (100F or 55C per hour) to 650F
(345C). Air Cool.
presence of carbides. To obtain Grades 60-45-12,
900 to 925C
@200F (95C) per
hour to 1300F (700C).
@ 100F (55C) per
hour to 650F (345C)
presence of carbides.
To obtain Grades 60-45-12,
60-40-18 where repid cooling
870 to 900C
of cross section
to 1250 to
1300F (675 to 700C).
Reheat to 1350F
(730 C). 2 hours per
inch of cross section. Air cool.
Temperature of castings.
(b) Slow cooling from 1000 to 650F (540 to 315C) is to mimimize
Normalizing involves the austenitizing of a Ductile Iron casting,
followed by cooling in air through the critical temperature. An as-cast
Ductile Iron casting is normalized in order to: break down carbides,
increase hardness and strength, and produce more uniform properties (see
Figures 3.16 and 3.18).
Normalizing should be carried out at an austenitizing temperature
approximately 100oC (212oF) above the critical
temperature range. Typically, austenitizing temperatures in the range
1600-1650oF (875-900oC) and holding times of one
hour, plus one hour per inch of casting thickness, are adequate to
produce a fully austenitic structure in unalloyed castings relatively
free of carbide. The cooling rate should be sufficiently rapid to
suppress ferrite formation and produce a fully pearlitic structure.
Depending on casting section size and alloy content, adequate cooling
rates can be achieved in still air, or large fans may be required. If
fan cooling cannot produce the desired pearlitic structure, the castings
should be alloyed with pearlite stabilizing elements such as copper,
tin, nickel or antimony. Figure 7.6
illustrates the effect of alloy content and section size on the hardness
of normalized Ductile Iron. Step normalizing, which employs a second,
lower temperature stage prior to air cooling, can be used to provide the
improved matrix control required for the production the pearlitic/ferritic
grades of Ductile Iron.
Maximum hardness in Ductile Iron
castings is obtained by austenitizing, followed by quenching
sufficiently rapidly to suppress the formation of both ferrite and
pearlite, to produce a metastable austenite which transforms to
martensite at lower temperature. As-quenched hardness depends on the
carbon content of the martensite and the volume fraction of martensite
in the matrix. In conjunction with the silicon content, the
austenitizing temperature determines the carbon content of the
austenite. For a silicon content of approximately 2.5%, an austenitizing
temperature of 1650oF (900oC) will result in the
optimum carbon content and maximum hardness (Figure
7.7). Lower temperatures, 1475-1550oF (800-845oC),
will produce a low carbon austenite which, on cooling, will transform to
a softer martensite.
The formation of low
carbon martensite will cause reduced distortion and cracking in complex
castings during quenching and, when tempered , low carbon martensite has
toughness superior to both tempered high carbon martensite and
normalized microstructures (see Figure
3.44, Section III). Higher
austenitizing temperatures increase the carbon content of the austenite
but the bulk hardness is reduced due to retained austenite and a lower
resultant martensite content. Regardless of the austenitizing and
quenching conditions, quenched Ductile Iron castings must be tempered
before use to eliminate internal stresses, control strength and hardness
and provide adequate ductility.
Hardenability is a measure of how
rapidly the Ductile Iron casting must be cooled in order to suppress the
ferrite and pearlite transformations and produce a martensitic, bainitic
or austempered matrix. Hardenability is an important property of any
casting that is to be quench hardened because it determines the depth to
which a fully or partially martensitic matrix can be produced and the
severity of quench required to harden castings of different section
size. The effects of various alloying elements on the hardenability of
Ductile Iron are illustrated in Figure 7.8.
To calculate the hardenability of a casting the absolute hardenability
(DA), based on the carbon content, is first determined. The ideal
critical diameter (DI) is then calculated by multiplying DA by the
multiplying factors determined from Figure
7.8 for each alloying element. For example, a Ductile Iron of the
|Total Carbon, %
the ideal critical diameter would be
calculated as follows:
DI = DA
x (MFSi) x (MFMN) x (MFP) x (MFNi)
= 3.45 inches (88 mm).
Thus, for the composition
used in this example, a 3.45 in. (88 mm) diameter bar, when quenched in
water, will have a matrix containing 50/o martensite at
the bar center.
Alloying elements for
quenched and tempered Ductile Iron should not be selected on the basis
of hardenability alone. Chromium, which is extremely effective in
promoting hardenability, is very detrimental to Ductile Iron quality
because it increases the formation of carbides in the as-cast state.
Manganese not only promotes the formation of carbides but also retards
the tempering process. Thus, for both metallurgical and economic
reasons, alloying elements should be selected carefully and used at the
lowest levels which provide the desired hardenability.
TTT (time, temperature,
transformation) diagrams are also useful in selecting heat treatment
practices for Ductile Irons. Figure 7.9
shows a typical TTT diagram for a low silicon gray iron. Each cooling
path in this Figure defines the time-temperature cooling relationship
required to produce a specific microstructure. The position of the
transformation zone on the TTT diagram, defined by start and finish
curves, determines the rate and extent of cooling required to avoid
certain transformations and promote others. To ensure that a quenched
component is entirely martensitic, the slowest cooling rate must be
sufficiently fast to avoid the "nose" of the transformation
Each composition of iron
has a unique TTT diagram, with the location of the transformation zone
controlled by the composition (Figure 7.
10). In this Figure the influence of molybdenum on the various
transformations reveals why it has a high hardenability multiplying
factor (Figure 7.8). Increasing
molybdenum content shifts the transformation zones to the right,
allowing complete transformation to martensite at the slower cooling
rates found in larger casting section sizes. Knowledge of the many TTT
diagrams published for Ductile Iron enables the foundry and heat treater
to select appropriate alloy contents and quenching conditions to produce
suitably hardened castings.
The quenching medium and the degree of agitation in the quench bath
are important variables that can be used to ensure that a suitable
microstructure is produced by the quenching process. Common quench
media, in order of increasing severity are oil, water and brine.
Agitation of the quenching bath may be required to increase both quench
severity and the uniformity of cooling in complex castings or batches of
castings. To minimize internal stresses, distortion and cracking,
especially in complex castings, the least severe quenching medium that
produces the desired microstructure should be selected. As the required
severity of quenching increases, it becomes increasingly important to
temper the castings immediately after quenching.
Tempering reduces the strength and hardness and increases the
ductility, toughness and machinability of quenched or normalized Ductile
Iron. In addition, tempering quenched castings also reduces residual
stresses, decreases the amount of retained austenite, and reduces the
probability of cracking. These changes in properties are achieved by
holding the castings at a temperature that is below the critical
temperature. Tempering is a diffusional process and thus is time and
temperature dependent. Tempering conditions are influenced strongly by
the desired change in properties, the alloy content, the microstructure
being tempered and the nodule count. Low alloy content, martensitic
structures and high nodule count reduce tempering temperatures and/or
times, while high alloy content, a normalized (pearlitic) structure and
low nodule count increase tempering times.
Castings may be tempered after normalizing to provide an optimum
combination of high strength and toughness. This process also provides
the additional advantage of improving the control of properties through
selection of tempering temperature and time.
Quenching and tempering are the standard heat treatments applied to
Ductile Iron castings requiring maximum strength and wear resistance. In
addition to maximizing strength, these treatments can provide close
control of casting properties over a wide range of strength and
ductility, and optimum combinations of strength and toughness (see Figure
3.44). Figure 7.11 illustrates
the wide range of properties of quenched and tempered Ductile Iron
castings that can be obtained through selection of the appropriate
tempering temperature (Figure 7.12).
Temper embrittlement, a type of embrittlement found in certain
quenched and tempered steels, may also occur in similarly treated
Ductile Irons with susceptible compositions. This form of embrittlement,
which does not affect normal tensile properties but causes significant
reductions in fracture toughness, can occur in Ductile Irons containing
high levels of silicon and phosphorus which have been tempered in the
range 650-1100oF (350-600oC) and cooled slowly
after tempering. Although normally associated with tempered martensite,
temper embrittlement can also occur if the matrix is tempered to the
fully ferritic condition. Temper embrittlement can be prevented by
keeping silicon and phosphorus levels as low as possible, adding up to
0.15 per cent molybdenum and avoiding the embrittling heat treating
The formation of secondary graphite during the tempering of
martensitic Ductile Iron can be responsible for both the degradation and
increased variability of mechanical properties. Secondary graphitization
is favoured by high austenitizing and tempering temperatures and high
levels of silicon, copper and nickel. Like temper embrittlement, the use
of small additions of molybdenum can eliminate this problem. To further
prevent its occurrence, the tempering of martensitic Ductile Irons to
hardnesses below 270 BHN, which require high temperature tempering,
should be avoided.
Ductile Iron can be surface hardened by flame or induction heating
of the casting surface layer to about 1650oF (900oC),
followed by a quenching spray. Hardness levels as high as HRC 60 can be
achieved by these procedures, producing a highly wear resistant surface
backed by a tough, ductile core. Pearlitic grades of Ductile Iron, which
have an intimate mixture of lamellar carbide and ferrite, respond most
effectively to surface hardening due to their reduced diffusion
The presence of residual stresses
can be detrimental to both the production and performance of Ductile
Iron castings. If sufficiently severe, residual stresses can cause
castings to distort and crack even during normal handling. Lower
residual stresses can cause the casting to distort during subsequent
heat treatment or machining. Residual stresses can also result in
premature yielding or fracture when the casting is used in an applied
stress environment that should have ensured safe operation.
Both the occurrence and
the effects of residual stresses in castings vary according to the
design of the casting, production procedures, and the end use of the
casting. Large, heavy section, or "chunky" (all dimensions
approximately equal) Ductile Iron castings are usually stress free
as-cast and require no subsequent stress relief. Complex castings with
large variations in section size or constrained thin castings are more
likely to contain residual stresses requiring stress relief. Sand molds
are good insulators and even complex castings may cool sufficiently
slowly to prevent the development of significant residual stresses.
However, the premature "shakeout" of castings from molds can
cause severe residual stresses, in addition to variations in hardness.
Rigid molds and cores may
prevent normal metal contraction during cooling and result in residual
casting stresses. Subsequent processing such as shot peening, welding,
heat treatment or surface hardening, if not performed properly, can
induce significant residual stresses that may become evident during
machining or subsequent use of the casting.
Stress relief is achieved by heating the casting to a sufficiently
high temperature that its strength is reduced to the extent that the
residual stress can be relieved by plastic deformation. The extent to
which stresses will be relieved or eliminated is dependent on several
factors, including the initial severity of the residual stresses, the
stress relieving time and temperature, the heating-cooling cycle, and
the composition and microstructure of the casting. Figure
7.13 shows that stress relief is proportional to the level of
initial stress, and that the degree of stress relief is strongly
temperature dependent. After stress relief a uniform rate of cooling
must be maintained throughout the casting to prevent the reintroduction
of stresses. This is normally accomplished by cooling in the furnace
from the stress relieving temperature to approximately 800oF
(430oC). For complex castings, and where the greatest degree
of stress relief is desired, furnace cooling to 300oF (150oC)
is recommended. The heating rate may be as important as the cooling rate
in the prevention of internal stresses, especially for complex or highly
stressed castings. Placing such castings in a hot furnace will result in
differential thermal stresses that could cause distortion during the
subsequent heat treatment.
Growth and Distortion
Scaling, growth and distortion of castings during heat treatment
should be considered in order to minimize the detrimental effects of
these phenomena. Scaling, which increases with time and temperature, can
be eliminated by the use of a controlled atmosphere furnace. An overall
increase in casting dimensions may occur during heat treatment due to
the graphitization of eutectic carbides and the conversion of pearlite
to ferrite. At austenitizing temperatures Ductile Iron castings have
very low strength and will easily sag and distort if not properly
supported. To reduce the risk of distortion, austenitizing time and
temperature should be kept to the minimum required to ensure complete
carbide breakdown and austenitization of the matrix.
J. E. Rehder, "Critical
Temperature Heat Treatment of Cast Irons," Foundry, June, 1965.
L. J. Ebert and J. F. Wallace, "How
Composition Affects the Properties of Ductile Iron," Metal
Progress, December, 1961.
The Iron Castings
Handbook, Iron Castings Society, Inc., 1981.
W. H. Browne and R.J. Christ, "Ferritization
of Ductile Iron", Transactions, American Foundrymen's Society, Vol
74, 1966, pp 371-379.
W. Gruver, "Double Annealing Heat
Treatment and it's Effect on the Impact Transition Temperature of
Ductile Iron," Private Correspondence, 1968.
K. B. Palmer, "Heat treatment of
cast iron - hardening and tempering," BC@ journal, November, 1974.
American Society For Metals, Metals
Handbook, 9th edition, Vol. 4, Metals Park, OH, 1981.
J.W. Boyes and N. Carter,
"Hardenability of nodular cast irons." The British Foundryman,
Sept 1966, pp 379-386.
C. C. Reynolds, N. T. Whittington, and H.
F. Taylor, "Hardenability of Ductile Cast Iron," Transactions,
American Foundrymen's Society, Vol. 63, 1955 pp 116-120.
A. P. Alexander, "Normalized vs.
Quenched and Tempered Nodular Iron," Transactions, American
Foundrymen's Society, Vol. 81, 1973.
C. R. Isleib and R. E. Savage,
"Normalized Alloy Ductile Irons," Transactions, American
Foundrymen's Society, Vol. 65, 1957.
A. P. Alexander, "Normalized vs.
Quenched and Tempered Iron." Transactions, American Foundrymen's
Society, Vol 81, 1973, pp 115-121.
D. R. Askeland and F. Farinez,
"Factors Affecting the Formation of Secondary Graphite in Quenched
and Tempered Ductile Iron" Transactions, American Foundrymen's
Society, Vol 87, 1979, pp 99-106.
K.B. Rundman and T.N. Rouns, "On the
Effects of Molybdenum on the Kinetics of the Secondary Graphitization in
Quenched and Tempered Ductile Irons.", Transactions, American
Foundrymen's Society, Vol 90, 1982, pp 487-497.
J. E. Bevan and W. G. Scholtz,
"Effect of Molybdenum on the Transformation Characteristics and
Properties of High Strength Ductile Irons," Transactions, American
Foundrymen's Society, Vol. 85, 1977.
R. E. Savage, "Heat Treating Ductile
Iron," Steel, November, 1955.
M. M. Hallet and P. D. Wing, "Stress
Relief Heat treatment of Alloy Cast Iron," Foundry Trade journal,
Vol. 56, 1949.
J. H. Schaum, "Stress Relief of Gray
Cast Iron," Transactions, American Foundrymen's Society, Vol. 56,