Ductile Iron(part one)

[Aldebaran]

Introduction
DUCTILE CAST IRON, previously known as nodular iron or spheroidal-graphite (SG) cast iron (the
international term is ductile iron), is cast iron in which the graphite is present as tiny spheres (nodules)
. In ductile iron, eutectic graphite separates from the molten iron during solidification in a manner similar to
that in which eutectic graphite separates in gray cast iron. However, because of additives introduced in the
molten iron before casting, the graphite grows as spheres, rather than as flakes of any of the forms characteristic
of gray iron. Cast iron containing spheroidal graphite is much stronger and has higher elongation than gray iron
or malleable iron. It may be considered as a natural composite in which the spheroidal graphite imparts unique
properties to ductile iron.

The relatively high strength and toughness of ductile iron give it an advantage over gray iron or malleable iron
in many structural applications. Also, because ductile iron does not require heat treatment to produce graphite
nodules (as does malleable iron to produce temper-carbon nodules), it can compete with malleable iron even
though it requires a treatment and inoculation process. The mold yield is normally higher than with malleable
iron. Ductile iron can be produced to x-ray standards because porosity stays in the thermal center. Malleable
iron cannot tolerate porosity because voids migrate to the surface of hot spots such as fillets and appears as
cracks.
Typically, the composition of unalloyed ductile iron differs from that of gray iron or malleable iron .
The raw materials used for ductile iron must be of higher purity. All cast irons can be melted in cupolas, electric
arc furnaces, or induction furnaces. Ductile iron, as a liquid, has high fluidity, excellent castability, but high
surface tension. The sands and molding equipment used for ductile iron must provide rigid molds of high
density and good heat transfer.
The formation of graphite during solidification causes an attendant increase in volume, which can counteract
the loss in volume due to the liquid-to-solid phase change in the metallic constituent. Ductile iron castings
typically require only minimal use of risers (reservoirs in the mold that feed molten metal into the mold cavity
to compensate for liquid contraction during cooling and solidification). Gray irons often do not require risers to
ensure shrinkage-free castings. On the other hand, steels and malleable iron generally require heavy risering.
Thus, the mold yield of ductile iron castings (the ratio of the weight of usable castings to the weight of metal
poured) is much higher than that of either steel castings or malleable iron castings, but not as high as that of
gray iron. There are some cases of ductile iron castings being made without risers.
Often designers must compensate for the shrinkage of cast iron during both solidification and subsequent
cooling to room temperature by making patterns with dimensions larger than those desired in the finished
castings. Typically, ductile iron requires less compensation than any other cast ferrous metal. The allowances in
patternmaker rules (shrink rules) are usually:
Type of cast metal Shrinkage
allowance, %
Ductile iron 0-0.7
Gray iron 1.0
Malleable iron 1.0
Austenitic alloy 1.3-1.5
White iron 2.0
Carbon steel 2.0
Alloy steel 2.5
Shrinkage allowance can vary somewhat from the percentages given above, and often different percentages
must be used for different directions in one casting because of the influence of the solidification pattern on the
amount of contraction that takes place in different directions. Shrinkage is volumetric, and the ratio of
dimensions to volume influences each dimension. As ductile iron approaches a condition of shrinkage porosity,
the graphite nodules tend to become aligned and can result in lower fatigue strength.
Most ductile iron castings are used as-cast, but in some foundries, some castings are heat treated before being
shipped. Heat treatment varies according to the desired effect on properties. Any heat treatment, with the
exception of austempering, reduces fatigue properties. Holding at subcritical (705 °C, or 1300 °F) temperature
for no more than 4 h improves fracture resistance. Heating castings above 790 °C (1450 °F) followed by fast
cooling (oil quench or air quench) significantly reduces fatigue strength and above-room-temperature fracture
resistance. Ferritizing by heating to 900 °C (1650 °F) and slow cooling also reduces fatigue strength and aboveroom-
temperature fracture resistance. Heating to above the critical temperature also reduces the combined
carbon content of quenched and tempered microstructures and produces lower tensile strength and wear
resistance than the same hardness produced as-cast. Some castings may be given hardening treatments (either
localized surface or through hardened) that produce bainitic or martensitic matrices.
In recent years, considerable interest has been shown in the property improvements obtained in ductile iron by
austempering heat treatments. Austempered ductile iron (ADI) has a matrix that is a combination of acicular
(bainitic) ferrite and stabilized austenite. As the matrix structure is progressively varied from ferrite to ferrite
plus pearlite to pearlite to bainite and finally to martensite, hardness, strength, and wear resistance increase, but
impact resistance, ductility, and machinability decrease. Overall, however, this structure results in an
exceptional combination of strength, ductility, and wear resistance. Some of the applications for austempered ductile iron include:
· Gears (including side and timing gears)
· Wear-resistant parts
· High-fatigue strength applications
· High-impact strength applications
· Automotive crankshafts
· Chain sprockets
· Refrigeration compressor crankshafts
· Universal joints
· Chain links
· Dolly wheels
Ductile iron can be alloyed with small amounts of nickel, molybdenum, or copper to improve its strength and
hardenability. The addition of molybdenum is done with caution because of the tendency for intercellular
segregation. Larger amounts of silicon, chromium, nickel, or copper can be added for improved resistance to
corrosion, oxidation, or abrasion, or for high-temperature applications.
Specifications
Most of the specifications for standard grades of ductile iron are based on properties; that is, strength and/or
hardness is specified for each grade of ductile iron, and composition is either loosely specified or made
subordinate to mechanical properties. the ASTM system
for designating the grade of ductile iron incorporates the numbers indicating tensile strength in ksi, yield
strength in ksi, and elongation in percent. This system makes it easy to specify nonstandard grades that meet the
general requirements of ASTM A 536. For example, grade 80-60-03 (80 ksi, or 552 MPa, minimum tensile
strength, 60 ksi, or 414 MPa, yield strength, and 3% elongation) is widely used in applications for which
relatively high ductility is not important. Grades 65-45-12 and 60-40-18 are used in areas requiring high
ductility and impact resistance. Grades 60-42-10 and 70-50-05 are for special applications such as annealed
pipe or as-cast pipe fittings. Grades other than those listed in ASTM A 536 or mentioned above can be made to
the general requirements of A 536, but with the mechanical properties specified by mutual agreement between
purchaser and producer.
The Society of Automotive Engineers (SAE) uses a method of specifying iron for castings produced in larger
quantities that is based on the microstructure and Brinell hardness of the metal in the castings themselves. Both
ASTM and SAE specifications are standards for tensile properties and hardness. The tensile properties are
quasi-static and may not indicate the dynamic properties, such as impact or fatigue strength.
The International System of grade designation (ISO 1083) uses the tensile strength value, in MPa, and
elongation percentage.
Ductile Iron Applications
Ductile iron castings are used for many structural applications, particularly those requiring strength and
toughness combined with good machinability and low cost. The selection of casting, instead of mechanical
fabrication, as the production process often allows the designer to:
· Use to best advantage the combination of properties that is unique to ductile iron
· Combine several functions (or component shapes) in a single integrated configuration
· Realize the economic advantages inherent in casting, which is the simplest and most direct of the
various production processes
Casting is like forming with a homogenous liquid that can flow smoothly into a wide variety of thicknesses,
shapes, and contours. The material is consistent piece to piece, day after day because it is made from a pattern
that has very low wear and dimensional change and that is produced to a specification. The microstructure is
uniform, with neither directional flow lines nor variations from weld junctions or heat-affected zones. Any
porosity is predictable and remains in the thermal center. Machining costs are low because there is less material
to remove and to dispose of, castings are easier to machine, and cutting tools are subjected to less tool wear.
Properties unique to ductile iron include the ease of heat treating because the free carbon in the matrix can be
redissolved to any desired level for hardness and strength control. Free carbon can be selectively hardened by
flame, induction, laser, or electron beam. A 650 °C (1200 °F) anneal for 3 h can produce high toughness at low
temperatures. Ductile iron can be austempered to high tensile strength, high fatigue strength, high toughness,
and excellent wear resistance. Second, lower density makes ductile iron weigh 10% less than steel for the same
section size. Third, the graphite content provides damping properties for quiet running gears. Also, the low
coefficient of friction produces more efficient gear boxes. Furthermore, ductile iron has less tendency to gear
seizures from the loss of lubricant.
Experience has proved that ductile iron works in applications in which experience and handbook data say it
should not. This is because data in the literature do not describe the true properties of ductile iron, whereas it is
very easy to make a prototype casting and try it in the field. This practice has resulted in very large cost savings
and superior performance compared to the material it replaces. A ductile iron casting can be poured and shipped
the same day. As-cast ductile iron castings are consistent in dimensions and weight because there is no
distortion or growth due to heat treatment.
The automotive and agricultural industries are the major users of ductile iron castings. Almost 3 × 106 tonnes
(2.9 × 106 t, or 3.2 × 106 tons) of ductile iron castings were produced in the United States in 1988, the majority
of components being for automotive applications. Because of economic advantages and high reliability, ductile
iron is used for such critical automotive parts as crankshafts, front wheel spindle supports, complex shapes of
steering knuckles, disk brake calipers, engine connecting rods, idler arms, wheel hubs, truck axles, suspension
system parts, power transmission yokes, high-temperature applications for turbo housings and manifolds, and
high-security valves for many various applications. It can be rolled or spun into a desired shape or coined to an
exact dimension.
The cast iron pipe industry is another major user of ductile iron.
Austempered ductile iron has resulted in many new applications for ductile iron. It is a high-strength, wearresistant,
heat-treated material. It has more than double the strength of conventional ductile iron for a given
level of ductility . Austempered ductile iron gets its remarkable properties from a special austempering
heat treatment. The resultant strength properties can be varied by controlling the heat treat cycle, which is
described in this article in the section "Heat Treatment." In the austempering process, good-quality ductile iron
can be transformed into a superior engineering material. It cannot transform poor-quality iron into a goodquality
material.
Metallurgical Control in Ductile Iron Production
Greater metallurgical and process control is required in the production of ductile iron than in the production of
other cast irons. Frequent chemical, mechanical, and metallurgical testing is needed to ensure that the required
quality is maintained and that specifications are met.
The manufacture of high-quality ductile iron begins with the careful selection of charge materials that will give
a relatively pure cast iron, free of the undesirable residual elements sometimes found in other cast irons.
Carbon, manganese, silicon, phosphorus, and sulfur must be held at specified levels. Magnesium, cerium, and
certain other elements must be controlled to attain the desired graphite shape and offset the deleterious effects
of elements such as antimony, lead, titanium, tellurium, bismuth, and zirconium, which interfere with the
nodulizing process and must be either eliminated or restricted to very low concentrations and neutralized by
additions of cerium and/or rare earth elements. Alloying elements such as chromium, nickel, molybdenum,
copper, vanadium, and boron act as carbide formers, as pearlite stabilizers, or as ferrite promoters. Alloys are
controlled to the extent needed to obtain the required mechanical properties and/or microstructure in the critical
section(s) of the casting.
A reduction of the sulfur content in the base iron to below 0.02% is necessary prior to the nodulizing process
(except with certain pure magnesium treatment processes); this can be accomplished by basic melting alone or
by desulfurization of the base metal (more commonly used today) before the magnesium-nodulizing alloy is
added. If base iron sulfur is not reduced, excessive amounts of costly nodulizing alloys are required, and greater
amounts of slag and dross are generated.
Graphite Shape and Distribution
There are three major types of nodulizing agents, all of which contain magnesium:
· Unalloyed magnesium metal
· Magnesium-containing ferrosilicons
· Nickel-base nodulizers
Unalloyed magnesium metal has been added to molten iron as wire, ingots, or pellets; as briquets in
combination with sponge iron; as pellets in combination with granular lime; or in the cellular pores of
metallurgical coke.
Magnesium-Containing Alloys. The method of introducing magnesium alloys has varied from an open-ladle
or covered-ladle method (in which the alloy is placed at the bottom of a ladle that has a height-to-diameter ratio
of between 2:1 and 1.5:1 and iron is poured rapidly over the alloy) to a plunging method, or to a pressurecontainer
method in which unalloyed magnesium is placed inside a container holding molten iron and the
container is rotated so that the iron flows over the magnesium. The magnesium metal can also be plunged to the
bottom of the iron in a pressure ladle. In all cases, magnesium is vaporized, and the vapors travel through the
molten iron, lowering the sulfur content and promoting the formation of spheroidal graphite.
Nickel-Base Nodulizers. A nickel alloy with 14 to 16% Mg can be added to the ladle during filling or by
plunging. The reaction is spectacular, but not violent, and a very consistent recovery is obtained. A
disadvantage lies in the accompanying increase in nickel and the cost of the alloy. Other nickel alloys
containing much lower magnesium contents (down to 4%) have also been used and involve a much quieter
reaction.
Ferrosilicon-based inoculant is usually added to the nodulized iron when it is transferred to the pouring
ladle. This produces a high nodule count and a matrix of preferred microstructure. With the proper control of
shakeout time and temperature, certain ductile iron castings can meet grade specifications as-cast, without
further heat treatment. Hardness uniformity is attained by the addition of pearlite-forming elements such as
copper.
Testing and Inspection
Various tests are used to control the processing of ductile iron. The first is the analysis of raw materials and of
the molten metal, both before and after the nodulizing treatment. Rapid thermal-arrest methods are used to
confirm carbon, silicon, and carbon equivalence (CE) in the molten iron. Carbon equivalence is not used to
allow broader carbon or silicon ranges; these elements still have definite individual ranges that must be held.
Silicon content is determined by thermoelectric, spectrometric, and wet chemical analysis. Chill tests are used
to control nucleation, indicate a carbide-forming tendency, and alert production control in the event that silicon
has been omitted. The temperature of the molten iron in the furnace is measured by immersion thermocouple
prior to the nodulizing process. It is also measured in the pouring ladle. Weight control is exercised for the
amount of metal being treated, the amount of alloys being added, and the amount of metal being inoculated.
Time control is used to ensure that all treated metal is poured within a given period, beginning with the
nodulization treatment and including the inoculation. Minimum pouring temperatures are established, and any
iron that does not meet the minimum standard is not allowed to be poured into molds.
A standard test coupon for microscopic examination must be poured from each batch of metal with the same
iron that is poured into the last mold. This coupon is described in ASTM A 395. One ear of the test coupon is
removed and polished to reveal graphite shape and distribution, as well as matrix structure. These
characteristics are evaluated by comparison with standard ASTM/AFS photomicrographs, and acceptance or
rejection of castings is based on this comparison.
Tensile-test specimens are machined from separately cast keel blocks, Y blocks, or modified keel blocks, as
described in ASTM A 395. If the terms of purchase require tensile specimens to be taken from castings, the part
drawing must identify the area of the casting and the size of the test specimen. A tensile specimen should never
be cut from the centerline of a round section, but rather from the midradius or as near the surface as possible.
These terms must be mutually acceptable to producer and purchaser.
The hardness testing of production castings is also used to evaluate conformance to specified properties. Some
standard specifications, such as SAE J434b. A comparison of
tensile properties with hardness of as-cast ductile iron and pearlitic malleable iron is shown in Fig. 5. This
shows that the tensile strength of as-cast ductile iron is significantly higher than that of pearlitic malleable iron
at all hardnesses. The elongation is higher than that of oil-quenched pearlitic malleable iron. Air-quenched
pearlitic malleable iron has a higher elongation than oil-quenched iron because the air-quenched iron has a
ferrite ring around the temper-carbon nodules. As oil-quenched pearlitic malleable iron is tempered to a lower
hardness, the elongation increases. This is also true in the case of fatigue strength, as shown in Fig. 6, in which
the endurance limit of 217 HB oil-quenched pearlitic malleable iron is higher than that of 255 HB. The yield
strength of as-cast ductile iron is lower than oil-quenched and tempered pearlitic malleable iron when the
hardness is above 217 HB. This is because the ferrite ring around the graphite nodules shows yielding at 0.2%
offset. The yield strength of as-cast ductile iron can be increased to as high or higher than the yield strength of
oil-quenched and tempered pearlitic malleable iron, and the hardness can be as high, but the tensile strength and
elongation will be significantly lower.
When ductile
iron is heated to 900 °C (1650 °F), oil quenched, and tempered, the peak of combined carbon between graphite
nodules is reduced and spread out to the nodules, thereby eliminating the ring of ferrite. A loss in wear
resistance is experienced, because of the lower combined carbon content at the same hardness as the as-cast
iron. The producer and the purchaser must agree on a suitable location, usually indicated on the part drawing,
for hardness testing. The preferred method for ductile iron hardness testing is the Brinell method. Surface
preparation must be done carefully, and the casting must be cool before it is indented with the 10-mm tungsten
carbide ball in a regularly calibrated machine. A proving ring is used to check the machine load of 30 kN (3000
kgf), and a stage micrometer is used to check the calibration of the Brinell scope. A hardness value may be
listed as a Brinell indentation diameter (BID) or Brinell hardness number (HB). Other methods of measuring
hardness with small indentors, such as the Rockwell method, have variations due to the small area of
indentation that contains soft graphite nodules. Electromagnetic techniques may be used for the hardness testing
of as-cast simple shapes, such as crankshafts.
Verification of graphite nodularity in castings can be obtained using ultrasonic methods that measure the
velocity of sound through the section being tested. Several companies have installed on-line systems to verify
nodularity, particularly in automotive components such as brake calipers and steering knuckles.
Heat Treatment
The heat treatment of ductile iron castings produces a significant difference in mechanical properties from ascast
ductile iron. The beneficial result of heat treating ductile iron is the increase in impact resistance if the
temperature is restricted to 705 °C (1300 °F) and the time is limited to 4 h. Most other forms of heat treatment,
with the exception of austempering, cause a loss in fatigue strength, wear resistance, and room-temperature
impact resistance, compared to as-cast ductile iron of the same hardness.
Stress Relieving. Occasionally ductile iron castings of large or nonuniform cross section are stress relieved at
540 to 595 °C (1000 to 1100 °F), which reduces warping and distortion during subsequent machining.
Mechanical properties are reduced if the temperature selected causes a reduction in hardness.
Annealing. Full ferritizing annealing is used to remove carbides or stabilized pearlite in order to meet ASTM
grade 60-40-18 requirements. This heat treatment usually involves heating to 900 °C (1650 °F), holding at
temperature long enough to dissolve the carbides (possibly up to 3 h), then slow cooling at 85 °C/h (150 °F/h)
to 705 °C (1300 °F) and still-air cooling to room temperature. This hypercritical anneal lowers the nil-ductility
transition temperature (NDTT) to improve low-temperature fracture resistance, but results in a lower uppershelf
impact energy and a significant reduction in fatigue strength. In some cases, it may reduce the tensile
and/or yield strength to levels below ASTM standards. The nil-ductility transition temperature is the
temperature at which the material transforms from a partially ductile to a fully brittle material with no ductility.
Subcritical annealing produces either ASTM grade 60-40-18 or grade 65-45-12 for applications requiring high
toughness and ductility. It produces greater low-temperature fracture resistance than as-cast ductile iron used
for greater low-temperature fracture resistance than as-cast ductile iron used for steering knuckle applications.
Normalizing, quenching, and tempering produces ASTM grade 100-70-03, which is widely used for
applications requiring good strength, wear resistance, and good response to localized hardening. Castings are
heated to 900 °C (1650 °F), held at temperature for 3 h (allowing 1 h/25 mm, or 1 h/in., of section size to reach
temperature), then air blasted or oil quenched, followed by tempering at 540 °C (1000 °F) or up to 675 °C
(1250 °F), depending on the required final hardness. The yield strength may be higher than as-cast iron for the
same hardness, but the ultimate tensile strength and elongation will be lower. The wear resistance at 228 HB
(4.1 BID) will be lower than as-cast iron of the same hardness. At 255 HB (3.8 BID), the short-time fatigue
strength (350,000 cycles) of D7003 oil quenched and tempered (OQT) may appear to be slightly higher, but the
long life (106 cycles) will be significantly lower than that of as-cast iron. At 217 HB (4.1 BID), both the shorttime
and long-time fatigue strength are significantly lower. This is shown in Fig. 5. The impact resistance of
oil-quenched and tempered ductile iron is lower than that of as-cast iron at 50 °C (125 °F) and above, but higher
at temperatures under 45 °C (110 °F).
Martensitic ductile iron ASTM grade 120-90-02 is produced by heating to 900 °C (1650 °F), holding to
homogenize, quenching in agitated oil, and tempering at 510 to 565 °C (950 to 1050 °F).
Austempered ductile iron requires a two-stage heat treatment. The first stage, austenitizing, requires heating
to and holding at about 900 °C (1650 °F). This is followed by the second stage, which requires quenching and
isothermally holding at the required austempering temperature, usually in a salt bath.
Typically, austempered ductile iron is produced by heating the castings in a controlled atmosphere to an
austenitizing temperature between 815 to 925 °C (1500 to 1700 °F). The castings are held at temperature for a long enough time to saturate the austenite with carbon in solution. The castings are then cooled at a rate
sufficiently fast enough to avoid the formation of pearlite and other high-temperature transformation products
to the appropriate transformation temperature (this may vary from 230 to 400 °C, or 450 to 750 °F, depending
on the hardness and strength required). The castings are held at the selected transformation temperature for a
long enough time to produce the desired properties. Austempered ductile iron transformed in the 370 °C (700
°F) range exhibits high ductility and impact resistance at a tensile strength of about 1035 MPa (150 ksi). When
transformed at 260 °C (500 °F), it exhibits wear resistance comparable to case hardened steel and tensile
strength in excess of 1380 MPa (200 ksi).
Thus, the selection of the austempering temperature and time of holding is critical. In general, austempering in
the 240 to 270 °C (465 to 520 °F) range provides a component that has maximum strength but limited ductility,
while austempering in the 360 to 380 °C (680 to 715 °F) range yields a component that exhibits maximum
ductility and toughness, in combination with relatively high strength, albeit lower than that obtained at the
lower austempering temperature.
Surface Hardening
Ductile iron may be surface hardened to as high as 60 HRC which may have a microhardness of 62 HRC
because the cone indentor averages the soft graphite nodules with the matrix hardness resulting in a lower
reading than the actual metallic components of the matrix. The hardened zone produces a highly wear-resistant
surface layer backed up by a core of tougher metal. Flame or induction methods can be used to heat the surface
layer to about 900 °C (1650 °F) for a few seconds, after which the heated surface is quenched in a spray of
water (which often contains a few percent of a water-soluble quenching aid). Surface hardening is most
successful when the matrix is fully pearlitic and thus ASTM grades 100-7-03 and 120-90-02 respond best.
However, as-cast hardnesses as low as 217 HB (4.1 BID) can be successfully surface hardened. Laser and
electron-beam methods are also used for surface hardening.
References cited in this section
2. Lyle Jenkins, Ductile Iron--An Engineering Asset, in Proceedings of the First International Conference on
Austempered Ductile Iron: Your Means to Improved Performance, Productivity, and Cost, American Society
for Metals, 1984
3. R.B. Gundlach and J.F. Janowak, A Review of Austempered Ductile Iron Metallurgy, in Proceedings of the
First International Conference on Austempered Ductile Iron: Your Means to Improved Performance,
Productivity, and Cost, American Society for Metals, 1984
4. P.A. Blackmore and R.A. Harding, The Effects of Metallurgical Process Variables on the Properties of
Austempered Ductile Irons, in Proceedings of the Fist International Conference on Austempered Ductile
Irons: Your Means to Improved Performance, Productivity, and Cost, American Society for Metals, 1984
Mechanical Properties
Most of the standard specifications for ductile iron require minimum strength and ductility, as determined by
the use of separately cast, standard ASTM test bars described in ASTM A 395. The various specification limits
have been established by the evaluation of the results from thousands of these test bars. The properties of test
bars are useful approximations of the properties of finished castings. Test bar properties also make it possible to
compare the metal from many different batches without having to account for the variations due to differences
in the shapes being cast or in the production practices used in different foundries.
Test bars are machined from keel blocks, Y blocks, or modified keel blocks (see ASTM A 395 for details and
dimensions). These test blocks are designed for ideal feeding from heavy molten metal heads over the mold and
for controlled cooling at optimum rates. In practice, these characteristics may not be economically feasible or
may be impossible to achieve because of the configuration of the casting. As a result, the properties of actual
production castings may differ from those of test bars cast from the same heat of molten metal, a fact that is
sometimes overlooked. Test bar properties are the most informative when their relationships to the properties of
production castings have been previously established by the testing of bars machined from castings, by the
selective overloading of castings, by stress analysis, or by field testing. Test bars cast to near testing dimensions
may be attached to the casting and removed for machining and testing. Dynamic properties such as impact
resistance and fatigue strength may be tested by casting dynamic tear and reverse-bending paddle bar
specimens. Figure 10 shows the brittle fracture appearance of a dynamic tear specimen.
Effect of Composition. The properties of ductile iron depend first on composition. Composition should be
uniform within each casting and among all castings poured from the same melt. Many elements influence
casting properties, but those of greatest importance are the elements that exert a powerful influence on matrix
structure or on the shape and distribution of graphite nodules.
Carbon influences the fluidity of the molten iron and the shrinkage characteristics of the cast metal. Excess
carbon not in solution, but in suspension, reduces fluidity. The volume of graphite is 3.5 times the volume of
iron. As ductile iron solidifies, the carbon in solution precipitates out as graphite and causes an expansion of the
iron, which can offset the shrinkage of the iron as it cools from liquid to solid. The amount of carbon needed to
offset shrinkage and porosity is indicated in the following formula:
% C + 1
7
% Si ≥3.9% (Eq 1)
Carbon contents greater than this amount begin to decrease fatigue strength and impact strength before the
effect is noticed on tensile strength. The size and the number of graphite nodules formed during solidification
are influenced by the amount of carbon, the number of graphite nuclei, and the choice of inoculation practice.
Normal graphite-containing ductile iron has 10% less weight than steel of the same section size. The graphite
also provides lubricity for sliding friction, and the low coefficient of friction permits more efficiently running
gears, which, furthermore, will not seize if a loss of lubricant is experienced in service. The graphite also
produces ADI gears that are silent in operation. Graphite in the structure can provide good machinability of a
ferritic material and then be available to redissolve into solution by heat treating to produce high strength and
wear resistance.
The relationship between carbon content and silicon content in terms of the carbon equivalent, CE, is
% %
3
CE = C + Si (Eq 2)
A chemical analysis for carbon content should be done prior to any formation of graphite. After graphite
formation, the ability to obtain an accurate carbon content is limited because of the loss of carbon in sample
preparation or, in some cases, because of segregation.
Silicon (Ref 6) is a powerful graphitizing agent. Within the normal composition limits, increasing amounts of
silicon promote structures that have progressively greater amounts of ferrite; furthermore, silicon contributes to
the solution strengthening and hardness of ferrite. Increasing the amount of ferrite reduces the yield strength
and tensile strength, but increases the elongation and impact strength. The ferrite envelope surrounding the
graphite nodule in pearlitic ductile iron reduces the indicated yield strength, but increases elongation, impact
strength, and fatigue strength. Silicon reduces the impact strength of ferritic ductile iron both as-cast and
subcritically annealed. To provide maximum resistance to fracture from room temperature down to -40 °C (-40
°F), silicon must be kept below 2.75% if the phosphorus content is below 0.02%. If the phosphorus content is
0.05%, the silicon content should be limited to 2.55%. High-temperature applications such as turbocharger
housings require silicon contents from 3.75 to 4.25% and molybdenum contents up to 0.70%. This combination
provides high-temperature oxidation resistance and dimensional stability. Even higher silicon contents are used
for abrasion resistance.
Manganese (Ref 7). Among the alloying elements commonly used to improve the mechanical properties of
ductile iron, manganese acts as a pearlite stabilizer and increases strength, but reduces ductility and
machinability. It also promotes segregation to the cell boundaries and must be limited when making
austempered ductile iron.
Nickel (Ref 7) is frequently used to increase strength by promoting the formation of fine pearlite and to
increase hardenability, especially for surface-hardening applications or for producing austempered ductile iron.
Copper (Ref 7) is used as a pearlite former for high strength with good toughness and machinability.
Molybdenum (Ref 7) is used to stabilize the structure at elevated temperatures. It is also used to add
hardenability to heavy sections in producing austempered ductile iron. The amount must be controlled because
of the tendency of molybdenum to segregate to the cell boundaries as stable carbides.
Effect of Graphite Shape. The presence of graphite in ductile cast iron in the shape of spheroidal nodules
instead of sharp flakes such as those found in gray cast iron is caused by the addition of magnesium (or
magnesium and cerium) to the molten iron, resulting in a fivefold to sevenfold increase in the strength of the
cast metal. Shapes that are intermediate between a true nodular form
and a flake form yield mechanical properties that are inferior
to those of ductile iron with a true nodular graphite. The size and uniformity of distribution of graphite nodules
also influence properties, but to a lesser degree than graphite shape. Small, numerous nodules are usually
accompanied by high tensile properties and tend to reduce the likelihood of the formation of chilled iron in thin
sections or at edges. An optimum nodule density exists. Excessive nodules may weaken a casting to such a
degree that it may not withstand the rigors of its intended application. Each material must be evaluated for its
specific application.
A ductile iron composition can be converted to a compacted graphite iron composition. Minor alterations in
composition produce a controlled microstructure consisting of more than 5% spheroidal and less than 20%
spheroidal graphite, with the remainder of the graphite being a compacted, blunt, vermicular shape. This then
becomes compacted graphite (CG) iron, which has lower strength than standard ductile iron and higher strength
than gray iron, but which has better thermal transfer characteristics and less tendency for porosity than standard
ductile iron.
Effect of Section Size. The variable chiefly affected by section size is the cooling rate. It, in turn, affects both
the size of the graphite nodules and the microstructure of the matrix. The heavier the section, the more slowly it
cools, and therefore the larger and fewer are the graphite nodules that form during solidification. When casting
ductile iron in sections greater than about 64 mm (2 1
2
in.), there is a possibility of producing centerline or
inverse chill in the last section to solidify. This is usually controlled by inoculation techniques.
The structure of the matrix is essentially determined by the cooling rate through the eutectoid temperature
range, although the specific effects of cooling rate are modified by the presence of alloying elements, as
discussed previously in the section "Effect of Composition." Slow cooling rates prevalent in heavy sections
promote the transformation to ferrite. If a pearlitic matrix is desired, pearlite formers (such as copper) are added
to the molten iron. It is important that castings be allowed sufficient cooling time in the mold to allow the
lamellar pearlite to be tempered in order to break up the plates partially and be rendered machinable at
hardnesses up to 321 HB (3.40 BID). Insufficient cooling time may produce very fine pearlite, which reduces
machinability. As-cast ductile iron anneals itself in the mold. Without a pearlite former, castings with variations
in section thickness will have variations in hardness. Bainite and martensite are not found in as-cast structures
because they are formed by heat treatment. Rapid cooling of thin sections may produce acicular structures,
which are usually some form of combined carbon (carbides).
Bainitic ferrite and retained austenite are the main matrix constituents in austempered ductile iron. In order to
minimize the harmful effects of segregation in medium-and thick-section ADI castings, the graphite nodule
count must be maintained as high as possible, ideally at a level greater than 150 nodules/mm2 (9.7 × 104
nodules/in.2).
Tensile Properties. The tensile properties of one heat of ductile iron heat treated to strength levels
approximately equivalent to four standard ductile irons are given in Table 7. These values are not necessarily
the average property values that can be expected for metal produced as-cast to the indicated grades. As-cast
tensile and elongation values are higher than heat treated values; however, the yield strength values may be
lower.
In some
instances, the ranges of expected strength and ductility overlap those for the next higher or lower grade.
As shown in Table 7, the modulus of elasticity in tension lies in the range of 162 to 170 GPa (23.5 × 106 to 24.5
× 106 psi) and does not vary greatly with grade. This value in tension should not be used in design for cantilever
or three-point beam or torsion loading because the deflection is greater, and a value of 142 GPa (20.5 × 106 psi)
should be used. Compressive Properties. The 0.20% offset yield strength of ductile iron in compression is generally reported
to be 1.0 to 1.2 times the 0.2% offset yield strength in tension. The compressive properties shown in Table 7
were determined using specimens from the same single heat of ductile iron described in the section above.
Torsional Properties. Few data are available on the ultimate shear strength of ductile iron because it is very
difficult to obtain accurate shear data on materials that exhibit some ductility. It is generally agreed that the
ultimate shear strength of ductile cast iron is about 0.9 to 1.0 times the ultimate tensile strength. Table 7 gives
data for shear strength and for 0.0375% offset yield strength in torsion for a single heat of ductile iron heat
treated to strength levels approximately equivalent to those of four standard ductile irons.
Damping Capacity. The average damping capacity of ductile iron in the hardness range of 156 to 241 HB is
about 6.6 times that of SAE 1018 and about 0.12 times that of ASTM class 30 gray cast iron. These data were
obtained from resonant-frequency measurements:
Austenitic ductile iron has a lower damping capacity than unalloyed ferritic ductile iron.
Impact Properties. Dynamic tear energy data for ferritic ductile iron are shown in Fig. 13. This figure
compares the effect of heat treatment. Subcritical anneal was 3 h at 700 °C (1290 °F). Full ferritization was
obtained by heating to 955 °C (1750 °F) and slow cooling in the same manner as for the ferritic malleable iron
heat treatment. This data shows that ferritizing at temperatures above the critical temperature results in lower
upper-shelf energy. It also reduces the fatigue strength of ductile iron, compared to a subcritical anneal. Figure
7 shows that heating to 900 °C (1650 °F) and then oil quenching and tempering reduces the upper-shelf energy,
but raises the low-temperature energy to fracture.
Fracture Toughness. Certain lower-strength grades of ductile iron do not fracture in a brittle manner when
tested under nominal plane-strain conditions in a standard fracture toughness test. This behavior is contrary to
the basic tenets of fracture mechanics and has been attributed to localized deformation in the ferrite envelope
surrounding each graphite nodule. In the low-strength ductile irons, plane-strain conditions are established only
at temperatures low enough to embrittle the ferrite. Otherwise, an increase in the size of the fracture toughness
test specimens does not provide the degree of mechanical constraint necessary to obtain a valid measurement of
KIc.
Fatigue Strength.
The absolute value of the as-cast endurance limit at any strength level
depends on the surface or near-surface characteristics associated with a specific casting process. Shot blast
cleaning contributes up to a 30% increase in the fatigue strength of the sand cast surface. This is controlled by
the size of the shot, the size of the load, and the length of the cycle. The effect of shot blasting (shot peening)
can be monitored by attaching an Almen block and strip to a sample casting and including it with the load. This
should be done for any change in process.
Ductile iron is cycle frequency sensitive. The cycle frequency used in testing should not exceed that which the
part would experience in service.
Endurance ratio is defined as endurance limit divided by tensile strength. Because the endurance ratio of ductile
iron declines as tensile strength increases, regardless of matrix structure, there may be little value in specifying
a higher-strength ductile iron for a structure that is prone to fatigue failure; redesigning the structure to reduce
stresses or strains may prove to be a better solution. For tempered martensitic ductile iron, the improvement in
fatigue strength due to an increase in tensile strength is proportionately greater than for a ferritic or pearlitic
grade.
Strain Rate Sensitivity. Ductile iron, like many steels, is strain rate sensitive. In a forming operation such as
coining to a close dimension to remove a machining operation, rolling an idler arm ball socket, or thread
rolling, the rate of material movement should be low enough to avoid cracking. In coining, a hydraulic press
should be used when the operation is carried out at room temperature. When a stroke press is used, the part
must be heated to a high enough temperature to avoid cracking during forming. When high rates of strain must
be tolerated, the section size should be increased to reduce the degree of strain.
The notch sensitivity of ductile iron must be considered when designing crankshafts. The fillet in the
crankpin must be a minimum of 1.65 mm (0.065 in.) thick, in contrast to 1.14 mm (0.045 in.) for the pearlitic
malleable iron it replaces. It will have significantly greater fatigue strength than the part it replaces.