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 concern.
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
Recommended practice for annealing ductile iron castings is given below for different
alloy contents and for castings with and without eutectic carbides:
- 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.
- 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.
- 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%
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 exceeding 0.05%.
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
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
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
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 tempering temperatures.
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 baths.
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 austenite transformation.
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.