Austenitizing Ductile Cast Iron
The usual objective of austenitizing is to produce an austenitic
matrix with as uniform carbon content as possible prior to
thermal processing. For a typical hypereutectic ductile cast
iron, an upper critical temperature must be exceeded so that the
austenitizing temperature is in two-phase (austenite and
graphite) field. This temperature varies with alloy content.
The "equilibrium" austenite carbon content in equilibrium with
graphite increases with an increase in austenitizing
temperature. This ability to select (within limits) the matrix
austenite carbon content makes austenitizing temperature control
important in processes that depend on carbon in the matrix to
drive a reaction. This is particularly true in structures to be
austempered, in which the hardenability (or austemperability)
depends to a significant degree on matrix carbon content. In
general, alloy content, the original microstructure, and the
section size determine the time required for austenitizing. The
sections to follow on annealing, normalizing, quenching and
tempering, and austempering discuss austenitizing when it is of
Annealing Ductile Cast Iron
When maximum ductility and good machinability are desired and
high strength is not required, ductile iron castings are
generally given a full ferritizing anneal. The microstructure is
thus converted to ferrite, and the excess carbon is deposited on
the existing nodules. This treatment produces ASTM grade
60-40-18. Amounts of manganese, phosphorus, and alloying
elements such as chromium and molybdenum should be as low as
possible if superior machinability is desired because these
elements retard the annealing process.
Recommended practice for annealing ductile iron castings is
given below for different alloy contents and for castings with
and without eutectic carbides:
1. Full anneal for unalloyed 2 to 3% Si iron with no eutectic
carbide: Heat and hold at 870 to 900°C (1600 to 1650°F) for 1 h
per inch of section. Furnace cool at 55°C/h (100°F/h) to 345°C
(650°F). Air cool.
2. Full anneal with carbides present: Heat and hold at 900 to
925°C (1650 to 1700°F) for 2 h minimum, longer for heavier
sections. Furnace cool at 110°C/h (200°F/h) to 700°C (1300°F).
Hold 2 h at 700°C (1300°F). Furnace cool at 55°C/h (100°F/h) to
345°C (650°F). Air cool.
3. Subcritical anneal to convert pearlite to ferrite: Heat and
hold at 705 to 720°C (1300 to 1330°F), 1 h per inch of section.
Furnace cool at 55°C/h (100°F/h) to 345°C (650°F). Air cool.
When alloys are present, controlled cooling times through the
critical temperature range down to 400°C (750°F) must be reduced
to below 55°C/h (100°F/h).
However, certain carbide-forming elements, mainly chromium, form
primary carbides that are very difficult, if not impossible, to
decompose. For example, the presence of 0.25% Cr results in
primary intercellular carbides that cannot be broken down in 2
to 20 h heat treatments at 925°C (1700°F). The resulting matrix
after pearlite breakdown is carbides in ferrite with only 5%
elongation. Other examples of carbide stabilizers are molybdenum
contents greater than 0.3%, and vanadium and tungsten contents
Hardenability of Ductile Cast Iron
The hardenability of ductile cast iron is an important parameter
for determining the response of a specific iron to normalizing,
quenching and tempering, or austempering.
Hardenability is normally measured by the Jominy test, in which
a standard-sized bar (1 inch diameter by 4 inch in length) is
austenitized and water quenched from one end. The variation in
cooling rate results in micro-structural variations, giving
hardness changes that are measured and recorded.
The higher matrix carbon content resulting from the higher
austenitizing temperature results in an increased hardenability
(the Jominy curve is shifted to larger distances from the
quenched end) and a greater maximum hardness.
The purpose of adding alloy elements to ductile cast irons is to
increase hardenability. Manganese and molybdenum are much more
effective in increasing hardenablitty, per weight percent added,
than nickel or copper. However, as is the case with steel,
combinations of nickel and molybdenum, or copper and molybdenum,
or copper, nickel, and manganese are more effective than the
separate elements. Thus heavy-section castings that require
through hardening or austempering usually contain combinations
of these elements. Silicon, apart from its effect on matrix
carbon content, does not have a large effect on hardenability.
Normalizing Ductile Cast Iron
Normalizing (air cooling following austenitizing) can result in
a considerable improvement in tensile strength and may be used
in the production of ductile iron of ASTM type 100-70-03.
The microstructure obtained by normalizing depends on the
composition of the castings and the cooling rate. The
composition of the casting dictates its hardenability that is,
the relative position of the fields in the time-temperature CCT
diagram. The cooling rate depends on the mass of the casting,
but it also may be influenced by the temperature and movement of
the surrounding air, during cooling.
Normalizing generally produces a homogeneous structure of fine
pearlite, if the iron is not too high in silicon content and has
at least a moderate manganese content (0.3 to 0.5% or higher).
Heavier castings that require normalizing usually contain
alloying elements such as nickel, molybdenum, and additional
manganese, for higher hardenability to ensure the development of
a fully pearlitic structure after normalizing. Lighter castings
made of alloyed iron may be martensitic or may contain an
acicular structure after normalizing.
The normalizing temperature is usually between 870 and 940°C
(1600 and 1725°F). The standard time at temperature of 1 h per
inch of section thickness or 1 h minimum is usually
satisfactory. Longer times may be required for alloys containing
elements that retard carbon diffusion in the austenite. For
example, tin and antimony segregate to the nodules, effectively
preventing the solution of carbon from the nodule sites.
Normalizing is sometimes followed by tempering to attain the
desired hardness and relieve residual stresses that develop upon
air cooling when various parts of a casting, with different
section sizes, cool at different rates. Tempering after
normalizing is also used to obtain high toughness and impact
resistance. The effect of tempering on hardness and tensile
properties depends on the composition of the iron and the
hardness level obtained in normalizing. Tempering usually
consists of reheating to temperatures of 425 to 650°C (800 to
1200°F) and holding at the desired temperature for 1 h per inch
of cross section. These temperatures are varied within the above
range to meet specification limits.
Quenching and Tempering Ductile Cast Iron
An austenitizing temperature of 845 to 925°C (1550 to 1700°F) is
normally used for austenitizing commercial castings prior to
quenching and tempering. Oil is preferred as a quenching medium
to minimize stresses and quench cracking, but water or brine may
be used for simple shapes. Complicated castings may have to be
oil quenched at 80 to 100°C (180 to 210°F) to avoid cracks.
The influence of the austenitizing temperature on the hardness
of water-quenched cubes of ductile iron shows that the highest
range of hardness (55 to 57 HRC) was obtained with austenitizing
temperatures between 845 and 870°C (1550 and 1600°F). At
temperatures above 870°C, the higher matrix carbon content
resulted in a greater percentage of retained austenite and
therefore a lower hardness.
Castings should be tempered immediately after quenching to
relieve quenching stresses. Tempered hardness depends on
as-quenched hardness level, alloy content, and tempering
temperature, as well as time. Tempering in the range from 425 to
600°C (800 to 1100°F) results in a decrease in hardness, the
magnitude of which depends upon alloy content, initial hardness,
and time. Vickers hardness of quenched ductile iron alloys
change with tempering temperature and time.
Tempering ductile iron is a two-stage process. The first
involves the precipitation of carbides similar to the process in
steels. The second stage (usually shown by the drop in hardness
at longer times) involves nucleation and the growth of small,
secondary graphite nodules at the expense of the carbides. The
drop in hardness accompanying secondary graphitization produces
a corresponding reduction in tensile and fatigue strength as
well. Because alloy content affects the rate of secondary
graphitization, each alloy will have a unique range of useful
Austempering Ductile Cast Iron
When optimum strength and ductility are required, the heat
treater has the opportunity to produce an austempered structure
of austenite and ferrite. The austempered matrix is responsible
for a significantly better tensile strength-to-ductility ratio
than is possible with any other grade of ductile cast iron. The
production of these desirable properties requires careful
attention to section size and the time-temperature exposure
during austenitizing and austempering.
Section Size and Alloying. As section size increases, the rate
of temperature change between the austenitizing temperature and
austempering temperature decreases. Quenching and austempering
techniques include the hot-oil quench (up to 240°C, or 460°F,
only), nitrate/nitrite sail quenches, fluidized-bed method (for
thin, small parts only), and, in tool-type applications, lead
In order to avoid high-temperature reaction products (such as
pearlite in larger section sizes), salt bath quench severities
can be increased with water additions or with alloying elements
(such as copper, nickel, manganese, or molybdenum) that enhance
pearlite hardenability. It is important to understand that these
alloying elements tend to segregate during solidification so
that a nonuniform distribution exists throughout the matrix.
This has a potentially detrimental effect on the austempering
reaction and therefore on mechanical properties. Ductility and
impact toughness are the most severely affected.
Manganese and molybdenum have the most powerful effect upon
pearlite hardenability but will also segregate and freeze into
intercellular regions of the casting to promote iron or alloy
carbides. While nickel and copper do not affect hardenability
nearly as much, they segregate to graphite nodule sites and do
not form detrimental carbides. Combinations of these elements,
which segregate in opposite fashions, are selected for their
synergistic effect on hardenability.
Austenitizing Temperature and Time. Usual schematic phase
diagram shows that as austenitizing temperature increases, so
does the matrix carbon content; the actual matrix carbon content
depends in a complex way on the alloy elements present, their
amount, and their location (segregation) within the matrix.
The most important determinant of matrix carbon content in
ductile irons is the silicon content; as silicon content
increases for a given austenitizing temperature, the carbon
content in the matrix decreases. Austenitizing temperatures
between 845 and 925°C (1550 and 1700°F) are normal, and
austenitizing times of approximately 2 h have been shown to be
sufficient to recarburize the matrix fully. Austenitizing
temperature, through its effect upon matrix carbon, has a
significant effect on hardenability. The higher austenitizing
temperature with its higher carbon content promotes increased
hardenability, which causes a slower rate of isothermal
Austempering Temperature and Time. The austempering temperature
is the primary determinant of the final microstructure and
therefore the hardness and strength of the austempered product.
As the austempering temperature increases, the strength and
impact toughness vary.
The attainment of maximum ductility at any given austempering
temperature is a sensitive function of time. The initial
increase in elongation occurs as stage I and elongation
progresses to completion, at which point the fraction of
austenite is a maximum. Further austempering merely serves to
reduce ductility as the stage II reaction causes decomposition
to the equilibrium bainite product. Typical austempering times
vary from 1 to 4 h.
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