Malleable Iron

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Malleable Iron

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Malleable Iron

Messaggioda Aldebaran » 03/05/2010, 21:17

Introduction
MALLEABLE IRON is a type of cast iron that has most of its carbon in the form of irregularly shaped graphite
nodules instead of flakes, as in gray iron, or small graphite spherulites, as in ductile iron. Malleable iron is
produced by first casting the iron as a white iron and then heat treating the white cast iron to convert the iron
carbide into the irregularly shaped nodules of graphite. This form of graphite in malleable iron is called temper
carbon because it is formed in the solid state during heat treatment.
Malleable iron, like ductile iron, possesses considerable ductility and toughness because of its combination of
nodular graphite and a low-carbon metallic matrix. Consequently, malleable iron and ductile iron are suitable
for some of the same applications requiring good ductility and toughness, with the choice between malleable
and ductile iron based on economy and availability rather than properties. However, because solidification of
white iron throughout a section is essential in the production of malleable iron, ductile iron has a clear
advantage when the section is too thick to permit solidification as white iron. Malleable iron castings are
produced in section thicknesses ranging from about 1.5 to 100 mm ( 1
16
to 4 in.) and in weights from less than
0.03 to 180 kg ( 1
16
to 400 lb) or more.
Ductile iron also has clear advantages over malleable iron when low solidification shrinkage is needed. In other
applications, however, malleable iron has a distinct advantage over ductile iron. Malleable iron is preferred in
the following applications:
· Thin-section casting
· Parts that are to be pierced, coined, or cold formed
· Parts requiring maximum machinability
· Parts that must retain good impact resistance at low temperatures
· Parts requiring wear resistance (martensitic malleable iron only)
Malleable iron (and ductile iron as well) also exhibits high resistance to corrosion, excellent machinability,
good magnetic permeability, and low magnetic retention for magnetic clutches and brakes. The good fatigue
strength and damping capacity of malleable iron are also useful for long service in highly stressed parts.
Malleable Iron
Metallurgical Factors
Although variations in heat treatment can produce malleable irons with different matrix microstructures (that is,
ferritic, tempered pearlitic, tempered martensitic, or bainitic microstructures), the common feature of all
malleable irons is the presence of uniformly dispersed and irregularly shaped graphite nodules in a given matrix
microstructure. These graphite nodules, known as temper carbon, are formed by annealing white cast iron at
temperatures that allow the decomposition of cementite (iron carbide) and the subsequent precipitation of
temper carbon.
The desired formation of temper carbon in malleable irons has two basic requirements. First, graphite should
not form during the solidification of the white cast iron, and second, graphite must also be readily formed
during the annealing heat treatment. These two metallurgical requirements influence the useful compositions of
malleable irons and the melting, solidification, and annealing procedures (see the article"Classification and
Basic Metallurgy of Cast Iron" in this Volume for an introduction to the metallurgy of malleable iron).
Metallurgical control is based on the following criteria:
· Produce solidified white iron throughout the section thickness
· Anneal on an established time-temperature cycle set to minimum values in the interest of economy
· Produce the desired graphite distribution (nodule count) upon annealing
Changes in melting practice or composition that would satisfy the first requirement listed above are generally
opposed to satisfaction of the second and third, while attempts to improve annealability beyond a certain point
may result in an unacceptable tendency for the as-cast iron to be mottled instead of white.
Composition. Because of the two metallurgical requirements described above, malleable irons involve a
limited range of chemical composition and the restricted use of alloys. The chemical composition of malleable
iron generally conforms to the ranges given in Table 1. Small amounts of chromium (0.01 to 0.03%), boron
(0.0020%), copper ( 1.0%), nickel (0.5 to 0.8%), and molybdenum (0.35 to 0.5%) are also sometimes present.
The common elements in malleable iron are generally controlled within about ±0.05 to ±0.15%. A limiting
minimum carbon content is required in the interest of mechanical quality and annealability because decreasing
carbon content reduces the fluidity of the molten iron, increases shrinkage during solidification, and reduces
annealability. A limiting maximum carbon content is imposed by the requirement that the casting be white ascast.
The range in silicon content is limited to ensure proper annealing during a short-cycle high-production
annealing process and to avoid the formation of primary graphite (known as mottle) during solidification of the
white iron. Manganese and sulfur contents are balanced to ensure that all sulfur is combined with manganese
and that only a safe, minimum quantity of excess manganese is present in the iron. An excess of either sulfur or
manganese will retard annealing in the second stage and therefore increase annealing costs. The chromium
content is kept low because of the carbide-stabilizing effect of this element and because it retards both the firststage
and second-stage annealing reactions.
A mixture of gray iron and white iron in variable proportions that produces a mottled (speckled) appearance is
particularly damaging to the mechanical properties of the annealed casting, whether ferritic or pearlitic
malleable iron. Primary control of mottle is achieved by maintaining a balance of carbon and silicon contents.
Because economy and castability are enhanced when the carbon and silicon contents of the base iron are in the
higher proportions of their respective ranges, some malleable iron foundries produce iron with carbon and
silicon contents at levels that might produce mottle and then add a balanced, mild carbide stabilizer to prevent
mottle during casting. Bismuth and boron in balanced amounts accomplish this control. A typical addition is
0.01% Bi (as metal) and 0.001% B (as ferroboron). Bismuth retards graphitization during solidification; small
amounts of boron have little effect on graphitizing tendency during solidification, but accelerate carbide
decomposition during annealing. The balanced addition of bismuth and boron permits the production of heavier
sections for a given base iron or the utilization of a higher-carbon higher-silicon base iron for a given section
thickness.
Tellurium can be added in amounts from 0.0005 to 0.001% to suppress mottle. Tellurium is a much stronger
carbide stabilizer than bismuth during solidification, but also strongly retards annealing if the residual exceeds
0.003%. Less than 0.003% residual tellurium has little effect on annealing, but has a significant influence on
mottle control. Tellurium is more effective if added together with copper or bismuth.
Residual boron should not exceed 0.0035% in order to avoid module alignment and carbide formation. Also,
the addition of 0.005% Al to the pouring ladle significantly improves annealability without promoting mottle.
Melting Practices. (Ref 2). The iron for most present-day malleable iron is melted in coreless induction
furnaces rather than the previous air furnace, cupola-air furnace, or cupola-electric furnace systems. The sulfur
and nitrogen contents of the charge carbon used in melting must be high enough to provide 0.07 to 0.09% S and
80 to 120 ppm N in the iron. The sulfur reduces the surface tension and improves fluidity. The nitrogen
increases the tensile strength without impairing elongation and toughness. Long holding periods in the molten
state in the furnace and excessive superheating temperatures should be avoided, because they give rise to an
unsatisfactory solidification structure, which in turn results in unsatisfactory heat-treated structures (Ref 2).
Melting can be accomplished by batch cold melting or by duplexing. Cold melting is done in coreless or
channel-type induction furnaces, electric arc furnaces, or cupola furnaces. In duplexing, the iron is melted in a
cupola or electric arc furnace, and the molten metal is transferred to a coreless or channel-type induction
furnace for holding and pouring. Charge materials (foundry returns, steel scrap, ferroalloys, and, except in
cupola melting, carbon) are carefully selected, and the melting operation is well controlled to produce metal
having the desired composition and properties. Minor corrections in composition and pouring temperature are
made in the second stage of duplex melting, but most of the process control is done in the primary melting
furnace (Ref 2).
Molds are produced in green sand, silicate CO2 bonded sand, or resin-bonded sand (shell molds) on equipment
ranging from highly mechanized or automated machines to that required for floor or hand molding methods,
depending on the size and number of castings to be produced. In general, the technology of molding and
pouring malleable iron is similar to that used to produce gray iron.
Solidification. Molten iron produced under properly controlled melting conditions solidifies with all carbon in
the combined form, producing the white iron structure fundamental to the manufacture of either ferritic or
pearlitic malleable iron . The base iron must contain balanced quantities of carbon and silicon to
simultaneously provide castability, white iron in even the thickest sections of the castings, and annealability;
therefore, precise metallurgical control is necessary for quality production. Thick metal sections cool slowly
during solidification and tend to graphitize, producing mottled or gray iron. This is undesirable, because the
graphite formed in mottled iron or rapidly cooled gray iron is generally of the type D configuration, a flake
form in a dense, lacy structure, which is particularly damaging to the strength, ductility, and stiffness
characteristics of both ferritic and pearlitic malleable iron.
After it solidifies and cools, the metal is in a white iron state, and gates, sprues, and feeders can be removed
easily from the castings by impact, This operation, called spruing, is generally performed manually with a
hammer because the diversity of castings produced in the foundry makes the mechanization or automation of
spruing very difficult. After spruing, the castings proceed to heat treatment, while gates and risers are returned
to the melting department for reprocessing.
First-Stage Anneal. Malleable iron castings are produced from the white iron by an annealing process that
converts primary carbides into temper carbon. This initial anneal is then followed by additional heat treatments
that produce the desired matrix microstructures. This section focuses on the initial (first-stage) anneal that
produces the temper carbon in blackheart malleable iron. The additional heat treatments used to produce the
desired matrix microstructure are discussed in the sections relating to ferritic, pearlitic, or martensitic
microstructures.
During the first-stage annealing cycle, the carbon that exists in combined form, either as massive carbides or as
a microconstituent in pearlite, is converted into nodules of graphite (temper carbon). The rate of annealing of a
hard iron casting depends on chemical composition, nucleation tendency (discussed in the section "Control of
Nodule Count" in this article), and annealing temperature. With the proper balance of boron content and
graphitic materials in the charge, the optimum number and distribution of graphite nuclei are developed in the
early portions of first-stage annealing, and growth of the temper carbon particles proceeds rapidly at any
annealing temperature. An optimum iron will anneal completely through the first-stage reaction in
approximately 3 1
2
h at 940 °C (1720 °F). Irons with lower silicon contents or less-than-optimum nodule counts
may require as much as 20 h for completion of first-stage annealing.
The temperature of first-stage annealing exercises considerable influence on the rate of annealing and the
number of graphite particles produced. Increasing the annealing temperature accelerates the rate of
decomposition of primary carbide and produces more graphite particles per unit volume. However, high firststage
annealing temperatures can result in excessive distortion of castings during annealing, which leads to
straightening of the casting after heat treatment. Annealing temperatures are adjusted to provide maximum
practical annealing rates and minimum distortion and are therefore controlled within the range of 900 to 970 °C
(1650 to 1780 °F). Lower temperatures result in excessively long annealing times, while higher temperatures
produce excessive distortion.
Annealing is done in high-production controlled-atmosphere continuous furnaces or batch-type furnaces,
depending on production requirements. The furnace atmosphere for producing malleable iron in continuous
furnaces is controlled so that the ratio of CO to CO2 is between 1:1 and 20:1. In addition, any sources of water
vapor or hydrogen are eliminated; the presence of hydrogen is thought to retard annealing, and it produces
excessive decarburization of casting surfaces. Proper control of the gas atmosphere is important for avoiding an
undesirable surface structure. A high ratio of CO to CO2 retains a high level of combined carbon on the surface
of the casting and produces a pearlitic rim, or picture frame, on a ferritic malleable iron part. A low ratio of CO
to CO2 permits excessive decarburization, which forms a ferritic skin on the casting with an underlying rim of
pearlite. The latter condition is produced when a significant portion of the subsurface metal is decarburized to
the degree that no temper carbon nodules can be developed during first-stage annealing. When this occurs, the
dissolved carbon cannot precipitate from the austenite, except as the cementite plates in pearlite.
Control of Nodule Count. Proper annealing in short-term cycles and the attainment of high levels of casting
quality require that controlled distribution of graphite particles be obtained during first-stage heat treatment.
With low nodule count (few graphite particles per unit area or volume), mechanical properties are reduced from
optimum, and second-stage annealing time is unnecessarily long because of long diffusion distances. Excessive
nodule count is also undesirable, because graphite particles may become aligned in a configuration
corresponding to the boundaries of the original primary cementite. In martensitic malleable iron, very high
nodule counts are sometimes associated with low hardenability and nonuniform tempering. Generally, a nodule
count of 80 to 150 discrete graphites particles per square millimeter of a photomicrograph magnified at 100×
appears to be optimum. This produces random particle distribution, with short distances between the graphite
particles.
Temper carbon is formed predominantly at the interface between primary carbide and saturated austenite at the
first-stage annealing temperature, with growth around the nuclei taking place by a reaction involving diffusion
and carbide decomposition. Although new nuclei undoubtedly form at the interfaces during holding at the firststage
annealing temperature, nucleation and graphitization are accelerated by the presence of nuclei that are
created by appropriate melting practice. High silicon and carbon contents promote nucleation and
graphitization, but these elements must be restricted to certain maximum levels because of the necessity that the
iron solidify white.
References cited in this section
1. C.F. Walton and T.J. Opar, Ed., Iron Castings Handbook, Iron Castings Society, 1981, p 297-321
2. L. Jenkins, Malleable Cast Iron, in Encyclopedia of Materials Science and Engineering, Vol 4, M.B. Bever,
Ed., MIT Press, 1986, p 2725-2729
Malleable Iron
Types and Properties of Malleable Iron
There are two basic types of malleable iron: blackheart and whiteheart. Blackheart malleable iron is the only
type produced in North America and is the most widely used throughout the world. Whiteheart malleable iron is
the older type and is essentially decarburized throughout in an extended heat treatment of white iron. This
article considers only the blackheart type.
Malleable iron, like medium-carbon steel, can be heat treated to produce a wide variety of mechanical
properties. The different grades and mechanical properties are essentially the result of the matrix
microstructure, which may be a matrix of ferrite, pearlite, tempered pearlite, bainite, tempered martensite, or a
combination of these (all containing nodules of temper carbon). This matrix microstructure is the dominant
factor influencing the mechanical properties. Other less significant factors include nodular count and the
amount and compactness of the graphite (Ref 1). A higher nodular count may slightly decrease the tensile and
yield strengths (Ref 3) as well as ductility (Ref 4). More graphite or a less compact form of graphite also tends
to decrease strength (Ref 1).
The different microstructures of malleable irons are determined and controlled by variations in heat treatment
and/or composition.
Because the mechanical properties of malleable iron are dominated by matrix microstructure, the mechanical
properties may relate quite well to the relative hardness levels of different matrix microstructures.This general
effect of microstructure on malleable irons is similar to that of many other steels and irons. The softer ferritic
matrix provides maximum ductility with lower strength, while increasing the amount of pearlite increases
hardness and strength but decreases ductility. Martensite provides further increases in hardness and strength but
with additional decreases in ductility.
The mechanical properties of pearlitic and martensitic malleable irons are closely related to hardness, as
discussed in "Mechanical Properties" in the section "Pearlitic and Martensitic Malleable Irons" in this article.
Therefore, grades of malleable irons are dependably specified by hardness and microstructure in ASTM A 602
and SAE J158. Malleable irons are also classified according to microstructure and minimum tensile
properties .
Ferritic Malleable Iron
. A satisfactory structure consists of temper
carbon in a matrix of ferrite. There should be no flake graphite and essentially no combined carbon in ferritic
malleable iron. Because ferritic malleable iron consists of only ferrite and temper carbon, the properties of
ferritic malleable castings depend on the quantity, size, shape, and distribution of temper carbon and on the
composition of the ferrite.
Heat Treatment. Ferritic malleable iron requires a two-stage annealing cycle. The first stage converts primary
carbides to temper carbon, and the second stage converts the carbon dissolved in austenite at the first-stage
annealing temperature to temper carbon and ferrite.
After first-stage annealing, the castings are cooled as rapidly as practical to 740 to 760 °C (1360 to 1400 °F) in
preparation for second-stage annealing. The fast cooling step requires 1 to 6 h, depending on the equipment
used. Castings are then cooled slowly at a rate of about 3 to 10 °C (5 to 20 °F) per hour. During cooling, the
carbon dissolved in the austenite is converted to graphite and deposited on the existing particles of temper
carbon. This results in a fully ferritic matrix.
Composites. Fully annealed ferritic malleable iron castings contain 2.00 to 2.70% graphite carbon by weight,
which is equivalent to about 6 to 8% by volume. Because the graphite carbon contributes nothing to the
strength of the castings, those with the lesser amount of graphite are somewhat stronger and more ductile than
those containing the greater amount (assuming equal size and distribution of graphite particles). Elements such
as silicon and manganese in solid solution in the ferritic matrix contribute to the strength and reduce the
elongation of the ferrite. Therefore, by varying base metal composition, slightly different strength levels can be
obtained in a fully annealed ferritic product.
The mechanical properties that are most important for design purposes are tensile strength, yield strength,
modulus of elasticity, fatigue strength, and impact strength. Hardness can be considered an approximate
indicator that the ferritizing anneal was complete. The hardness of ferritic malleable iron almost always ranges
from 110 to 156 HB and is influenced by the total carbon and silicon contents.
The tensile properties of ferritic malleable iron are usually measured on unmachined test bars. These
properties are listed in Table 2.
The fatigue limit of unnotched ferritic malleable iron is about 50 or 60% of the tensile strengt. Also plots the fatigue properties with notched specimens. Notch radius
generally has little effect on fatigue strength, but fatigue strength decreases with increasing notch depth The modulus of elasticity in tension is about 170 GPa (25 × 106 psi). The modulus in compression ranges
from 150 to 170 GPa (22 × 106 to 25 × 106 psi); in torsion, from 65 to 75 GPa (9.5 × 106 to 11 × 106 psi).
Fracture Toughness. Because brittle fractures are most likely to occur at high strain rates, at low
temperatures, and with a high restraint on metal deformation, notch tests such as the Charpy V-notch test are
conducted over a range of test temperatures to establish the toughness behavior and the temperature range of
transition from ductile to a brittle fracture. Figure 5 illustrates the behavior of ferritic malleable iron and several
types of pearlitic malleable iron in the Charpy V-notch test. This shows that ferritic malleable iron has a higher
upper shelf energy and a lower transition temperature to a brittle fracture than pearlitic malleable iron.
Welding and Brazing. Welding of ferritic malleable iron almost always produces brittle white iron in the weld
zone and the portion of the heat-affected zone immediately adjacent to the weld zone. During welding, temper
carbon is dissolved, and upon cooling it is reprecipitated as carbide rather than graphite. In some cases, welding
with a cast iron electrode may produce a brittle gray iron weld zone. The loss of ductility due to welding may
not be serious in some applications. However, welding is usually not recommended unless the castings are
subsequently annealed to convert the carbide to temper carbon and ferrite. Ferritic malleable iron can be fusion
welded to steel without subsequent annealing if a completely decarburized zone as deep as the normal heataffected
zone is produced at the faying surface of the malleable iron part before welding. Silver brazing and tinlead
soldering can be satisfactorily used.
Pearlitic and Martensitic Malleable Iron
Pearlitic and martensitic-pearlitic malleable irons can be produced with a wide variety of mechanical properties,
depending on heat treatment, alloying, and melting practices. The lower-strength pearlitic malleable irons are
often produced by air cooling the casting after the first-stage anneal, while the higher-strength (pearliticmartensitic)
malleable irons are made by liquid quenching after the first-stage anneal. These two methods are
discussed in the sections "Heat Treatment for Pearlitic Malleable Irons" and "Heat Treatment for Pearlitic-
Martensitic Malleable Irons" in this article.
Given suitable heat treatment facilities, air cooling or liquid quenching after the first-stage anneal is generally
the most economical heat treatment for producing pearlitic or martensitic-pearlitic malleable irons, respectively.
Otherwise, ferritic iron produced from two-stage annealing is reheated to the austenite temperature and then
quenched. This method is discussed in the section "Rehardened-and-Tempered Malleable Iron" in this article.
Finally, the lower-strength pearlitic malleable irons can also be produced by alloying and a two-stage annealing
process. The last method involves alloying during the melting process so that the carbides dissolved in the
austenite do not decompose during cooling from the first-stage annealing temperature.
Heat Treatment for Pearlitic Malleable Irons. In the production of pearlitic malleable iron, the first-stage
anneal is identical to that used for ferritic malleable iron. After this, however, the process changes. Some
foundries then slowly cool the castings to about 870 °C (1600 °F). During cooling, the combined carbon
content of the austenite is reduced to about 0.75%, and the castings are then air cooled. Air cooling is
accelerated by an air blast to avoid the formation of ferrite envelopes around the temper carbon particles (bull'seye
structure) and to produce a fine pearlitic matrix (Fig. 8). The castings are then tempered to specification, or
they are reheated to reaustenitize at about 870 °C (1600 °F), oil quenched, and tempered to specification. Large
foundries usually eliminate the reaustenitizing step and quench the castings in oil directly from the first-stage
annealing furnace after stabilizing the temperature at 845 to 870 °C (1550 to 1600 °F).
The rate of cooling after first-stage annealing is important in the formation of a uniform pearlitic matrix in the
air-cooled casting, because slow rates permit partial decomposition of carbon in the immediate vicinity of the
temper carbon nodules, which results in the formation of films of ferrite around the temper carbon (bull's-eye
structure). When the extent of these films becomes excessive, a carbon gradient is developed in the matrix. Air
cooling is usually done at a rate not less than about 80 °C (150 °F) per minute.
Air-quenched malleable iron castings have hardnesses ranging from 269 to 321 HB, depending on casting size
and cooling rate. Such castings can be tempered immediately after air cooling to obtain pearlitic malleable iron
with a hardness of 241 HB or less.
Heat Treatment for Pearlitic-Martensitic Malleable Irons. High-strength malleable iron castings of
uniformly high quality are usually produced by liquid quenching and tempering. The most economical
procedure is direct quenching after first-stage annealing. In this procedure, the castings are cooled in the
furnace to the quenching temperature of 845 to 870 °C (1550 to 1600 °F) and held for 15 to 30 min to
homogenize the matrix. The castings are then quenched in agitated oil to develop a matrix microstructure of
martensite having a hardness of 415 to 601 HB. Finally, the castings are tempered at an appropriate temperature
between 590 and 725 °C (1100 and 1340 °F) to develop the specified mechanical properties. The final
microstructure consists of tempered martensite plus temper carbon . In heavy sections,
higher-temperature transformation products such as fine pearlite are usually present.
Some foundries produce high-strength malleable iron by an alternative procedure in which the castings are
forced-air cooled after first-stage annealing, retaining about 0.75% C as pearlite. The castings are then reheated
at 840 to 870 °C (1545 to 1600 °F) for 15 to 30 min, followed by quenching and tempering as above for the
direct-quench process.
Rehardened-and-tempered malleable iron can also be produced from fully annealed ferritic malleable iron
with a slight variation in the heat treatment used for arrested-annealed (air-quenched) malleable. The matrix of
fully annealed ferritic malleable iron is essentially carbon free, but can be recarburized by heating at 840 to 870
°C (1545 to 1600 °F) for 1 h. In general, the combined carbon content of the matrix produced by this procedure
is slightly lower than that of arrested-annealed pearlitic malleable iron, and the final tempering temperatures
required for the development of specific hardnesses are lower. Rehardened malleable iron made from ferritic
malleable may not be capable of meeting certain specifications.
Tempering times of 2 h or more after either air cooling or liquid quenching are needed for uniformity. In
general, the control of final hardness of the castings is precise, with process limitations approximately the same
as those encountered in the heat treatment of medium- or high-carbon steels. This is particularly true when
specifications require hardnesses of 241 to 321 HB where control limits of ±0.2 mm Brinell diameter can be
maintained with ease. At lower hardnesses, a wider process control limit is required because of certain unique
characteristics of the pearlitic malleable iron microstructure.
The mechanical properties of pearlitic and martensitic malleable iron vary in a substantially linear
relationship with Brinell hardness . In the low-hardness ranges, below about 207 HB, the
properties of air-quenched and tempered pearlitic malleable are essentially the same as those of oil-quenched
tempered martensitic malleable. This is because attaining the low hardnesses requires considerable coarsening
of the matrix carbides and partial second-stage graphitization. Either an air-quenched pearlitic structure or an
oil-quenched martensitic structure can be coarsened and decarburized to meet this hardness requirement.
At higher hardnesses, oil-quenched and tempered malleable iron has higher yield strength and elongation than
air-quenched and tempered malleable iron because of greater uniformity of matrix structure and finer
distribution of carbide particles. Oil-quenched quenched and tempered pearlitic malleable iron is produced
commercially to hardnesses as high as 321 HB, while the maximum hardness for high-production air-quenched
and tempered pearlitic malleable iron is about 255 HB. The lower maximum hardness is applied to the airquenched
material because:
· Hardness upon air quenching normally does not exceed 321 HB and may be as low as 269 HB;
therefore, attempts to temper to a hardness range above 255 HB produce nonuniform hardness and make
the process control limits excessive
· Very little structural alteration occurs during the tempering heat treatment to a higher hardness, and the
resulting structure is more difficult to machine than an oil-quenched and tempered structure at the same
hardness
· There is only a slight improvement in other mechanical properties with increased hardness above 255
HB
Because of these considerations, applications for air-quenched and tempered pearlitic malleable iron are usually
those requiring moderate strength levels, while the higher-strength applications need the oil-quenched and
tempered material.
The tensile properties of pearlitic malleable irons are normally measured on machined test bars. These
properties are listed in Table 2.
The compressive strength of malleable irons is seldom determined, because failure in compression seldom
occurs. As a result of the decreased influence of the graphite nodules and the delayed onset of plastic
deformation in compression, compressive yield strengths are characteristically slightly higher than tensile yield
strengths for the same hardness (Ref 1, 7).
Shear and Torsional Strength. The shear strength of ferritic malleable irons is approximately 80% of the
tensile strength, and for pearlitic iron it ranges from 70 to 90% of the tensile strength (Ref 7). The ultimate
torsional strength of ferritic malleable irons is about 90% of the ultimate tensile strength. The yield strength in
torsion is 75 to 80% of the value in tension (Ref 1). Torsional strengths for pearlitic grades are approximately
equal to, or slightly less than, the tensile strength of the material. Yield strengths in torsion vary from 70 to 75%
of the tensile yield strength (Ref 7). The characteristic torsional properties of ferritic and pearlitic malleable
irons are related to hardness, . As expected, the amount of twist before failure decreases
with increasing strength.
Wear Resistance. Because of its structure and hardness, pearlitic and martensitic malleable irons have
excellent wear resistance. In some moving parts where bushings are normally inserted at pivot points, heattreated
malleable iron has proved to be so wear resistant that the bushings have been eliminated. One example
of this is the rocker arm for an overhead-valve automotive engine.
Welding and Brazing. Welding of pearlitic or martensitic malleable iron is difficult because the high
temperatures used can cause the formation of a brittle layer of graphite-free white iron. Pearlitic and martensitic
malleable iron can be successfully welded if the surface to be welded has been heavily decarburized.
Pearlitic or malleable iron can be brazed by various commercial processes. One application is the induction
silver brazing of a pearlitic malleable casting and a steel shaft to form a planetary output shaft for an
automotive transmission. In another automotive application, two steel shafts are induction copper brazed to a
pearlitic malleable iron shifter shaft plate.
Selective Surface Hardening. Pearlitic malleable iron can be surface hardened by either induction heating
and quenching or flame heating and quenching to develop high hardness at the heat-affected surface.
Considerable research has been done to determine the surface-hardening characteristics of pearlitic malleable
and its capability of developing high hardness over relatively narrow surface bands. In general, little difficulty
is encountered in obtaining hardnesses in the range of 55 to 60 HRC, with the depth of penetration being
controlled by the rate of heating and the surface temperature of the part being hardened .
The maximum hardness obtainable in the matrix of a properly hardened pearlitic malleable part is 67 HRC.
However, conventional hardness measurements made on castings show less than 67 HRC because of the
presence of the graphite particles, which are averaged into the hardness. Generally, a casting with a matrix
microhardness of 67 HRC will have about 62 HRC average hardness, as measured with the standard Rockwell
tester. Similarly, a Rockwell or Brinell hardness test on softer structures will show less than matrix
microhardness because of the presence of graphite.
Two examples of automobile production parts hardened by induction heating are rocker arms and clutch hubs.
An example of a flame-hardened pearlitic malleable iron part is a pinion spacer used to support the cup of a
roller bearing. To preclude service failures, the ends of the pinion spacer are flame hardened to a depth of about
2.3 mm ( 3
32
in.).
Malleable iron can be carburized, carbonitrided, or nitrided to produce a surface with improved wear resistance.
In addition, heat treatments such as austempering have been used in specialized applications.
Damping Capacity
The good damping capacity and fatigue strength of malleable irons are useful for long service in highly stressed
parts. The production of high
internal stresses by quenching malleable iron can double the damping capacity, which is then gradually reduced
as tempering relieves residual stresses (Ref 1).
References cited in this section
1. C.F. Walton and T.J. Opar, Ed., Iron Castings Handbook, Iron Castings Society, 1981, p 297-321
3. D.R. Askeland and R.F. Fleischman, Effect of Nodule Count on the Mechanical Properties of Ferritic
Malleable Iron, Trans. AFS, Vol 86, 1978, p 373-378
4. J. Pelleg, Some Mechanical Properties of Cupola Malleable Iron, Foundry, Oct 1960, p 110-113
5. L.W.L. Smith et al., Properties of Modern Malleable Irons, BCIRA International Center for Cast Metals
Technology, 1987
6. "Standard Specification for Malleable Iron Castings," A 47, Annual Book of ASTM Standards, American
Society for Testing and Materials
7. G.N.J. Gilbert, Engineering Data on Malleable Cast Irons, British Cast Iron Research Association, 1968
8. W.L. Bradley, Fracture Toughness Studies of Gray, Malleable and Ductile Cast Iron, Trans. AFS, Vol 89,
1981, p 837-848
Malleable Iron
Applications
Malleable iron castings are often selected because the material has excellent machinability in addition to
significant ductility. In other applications, malleable iron is chosen because it combines castability with good
toughness and machinability. Malleable iron is often chosen because of shock resistance alone. Tables 3 and 4
list some of the typical applications of malleable iron castings.
The requirement that any iron produced for conversion to malleable iron must solidify white places definite
section thickness limitations on the malleable iron industry. Thick metal sections can be produced by melting a
base iron of low carbon and silicon contents or by alloying the molten iron with a carbide stabilizer. However,
when carbon and silicon are maintained at low levels, difficulty is invariably encountered in annealing, and the
time required to convert primary and pearlitic carbides to temper carbon becomes excessively long. Highproduction
foundries are usually reluctant to produce castings more than about 40 mm (1 1
2
in.) thick. Some
foundries, however, routinely produce castings as thick as 100 mm (4 in.).
After heat treatment, ferritic or pearlitic malleable castings are cleaned by shotblasting, gates are removed by
shearing or grinding, and, where necessary, the castings are coined or punched. Close dimensional tolerances
can be maintained in ferritic malleable iron and in the lower-hardness types of pearlitic malleable iron, both of
which can be easily straightened in dies. The harder pearlitic malleable irons are more difficult to press because
of higher yield strength and a greater tendency toward springback after die pressing. However, even the
highest-strength pearlitic malleable can be straightened to achieve good dimensional tolerances.
Automotive and associated applications of ferritic and pearlitic malleable irons include many essential parts in
vehicle power trains, frames, suspensions, and wheels. A partial list includes differential carriers, differential
cases, bearing caps, steering-gear housings, spring hangers, universal-joint yokes, automatic-transmission parts,
rocker arms, disc brake calipers, wheel hubs, and many other miscellaneous castings.
. Ferritic and pearlitic malleable irons are also used in the railroad industry and in agricultural
equipment, chain links, ordnance material, electrical pole line hardware, hand tools, and other parts requiring
section thicknesses and properties obtainable in these materials.
Malleable Iron
References
1. C.F. Walton and T.J. Opar, Ed., Iron Castings Handbook, Iron Castings Society, 1981, p 297-321
2. L. Jenkins, Malleable Cast Iron, in Encyclopedia of Materials Science and Engineering, Vol 4, M.B. Bever,
Ed., MIT Press, 1986, p 2725-2729
3. D.R. Askeland and R.F. Fleischman, Effect of Nodule Count on the Mechanical Properties of Ferritic
Malleable Iron, Trans. AFS, Vol 86, 1978, p 373-378
4. J. Pelleg, Some Mechanical Properties of Cupola Malleable Iron, Foundry, Oct 1960, p 110-113
5. L.W.L. Smith et al., Properties of Modern Malleable Irons, BCIRA International Center for Cast Metals
Technology, 1987
6. "Standard Specification for Malleable Iron Castings," A 47, Annual Book of ASTM Standards, American
Society for Testing and Materials
7. G.N.J. Gilbert, Engineering Data on Malleable Cast Irons, British Cast Iron Research Association, 1968
8. W.L. Bradley, Fracture Toughness Studies of Gray, Malleable and Ductile Cast Iron, Trans. AFS, Vol 89,
1981, p 837-848
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