Alloy Cast Irons(1)

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Alloy Cast Irons(1)

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Alloy Cast Irons(1)

Messaggioda Aldebaran » 04/05/2010, 10:29

Introduction
ALLOY CAST IRONS are considered to be those casting alloys based on the iron-carbon-silicon system that
contain one or more alloying elements intentionally added to enhance one or more useful properties. The
addition to the ladle of small amounts of substances (such as ferrosilicon, cerium, or magnesium) that are used
to control the size, shape, and/or distribution of graphite particles is termed inoculation rather than alloying.
The quantities of material used for inoculation neither change the basic composition of the solidified iron nor
alter the properties of individual constituents. Alloying elements, including silicon when it exceeds about 3%,
are usually added to increase the strength, hardness, hardenability, or corrosion resistance of the basic iron and
are often added in quantities sufficient to affect the occurrence, properties, or distribution of constituents in the
microstructure.
In gray and ductile irons, small amounts of alloying elements such as chromium, molybdenum, or nickel are
used primarily to achieve high strength or to ensure the attainment of a specified minimum strength in heavy
sections. Otherwise, alloying elements are used almost exclusively to enhance resistance to abrasive wear or
chemical corrosion or to extend service life at elevated temperatures.
The strengthening effects of the various alloying elements in gray and ductile irons are dealt with in the articles
"Gray Iron" and "Ductile Iron" in this Volume. This article discusses abrasion-resistant chilled and white irons,
high-alloy corrosion-resistant irons, and medium-alloy and high-alloy heat-resistant gray and ductile irons.
Table 1 lists approximate ranges of alloy content for various types of alloy cast irons covered in this article.
Individual alloys within each type are made to compositions in which the actual ranges of one or more of the
alloying elements span only a portion of the listed ranges; the listed ranges serve only to identify the types of
alloys used in specific kinds of applications.
Alloy Cast Irons
Revised by Richard B. Gundlach, Climax Research Services; and Douglas V. Doane, Consulting Metallurgist
Classification of Alloy Cast Irons
Alloy cast irons can be classified as white cast irons, corrosion-resistant cast irons, and heat-resistant cast irons.
White cast irons, so named because of their characteristically white fracture surfaces, do not have any
graphite in their microstructures. Instead, the carbon is present in the form of carbides, chiefly of the types Fe3C
and Cr7C3. Often, complex carbides such as (Fe,Cr)3C and (Cr,Fe)7C3, or those containing other carbideforming
elements, are also present.
White cast irons are usually very hard, which is the single property most responsible for their excellent
resistance to abrasive wear. White iron can be produced either throughout the section (chiefly by adjusting the
composition) or only partly inward from the surface (chiefly by casting against a chill). The latter iron is
sometimes referred to as chilled iron to distinguish it from iron that is white throughout.
Chilled iron castings are produced by casting the molten metal against a metal or graphite chill, resulting in a
surface virtually free from graphitic carbon. In the production of chilled iron, the composition is selected so that
only the surfaces cast against the chill will be free from graphitic carbon . The more slowly cooled
portions of the casting will be gray or mottled iron. The depth and hardness of the chilled portion can be
controlled by adjusting the composition of the metal, the extent of inoculation, and the pouring temperature.
White iron is a cast iron virtually free from graphitic carbon because of selected chemical composition. The
composition is chosen so that, for the desired section size, graphite does not form as the casting solidifies. The
hardness of white iron castings can be controlled by further adjustment of composition.
The main difference in microstructure between chilled iron and white iron is that chilled iron is fine grained and
exhibits directionality perpendicular to the chilled face, while white iron is ordinarily coarse grained, randomly
oriented, and white throughout, even in relatively heavy sections. (Fine-grain white iron can be produced by
casting a white iron composition against a chill.) This difference reflects the effect of composition difference
between the two types of abrasion-resistant iron. Chilled iron is directional only because the casting, made of a
composition that is ordinarily gray, has been cooled through the eutectic temperature so rapidly at one or more
faces that the iron solidified white, growing inward from the chilled face. White iron, on the other hand, has a
composition so low in carbon equivalent or so rich in alloy content that gray iron cannot be produced even at
the relatively low rates of cooling that exist in the center of the heaviest section of the casting.
Corrosion-resistant irons derive their resistance to chemical attack chiefly from their high alloy content.
Depending on which of three alloying elements--silicon, chromium, or nickel--dominates the composition, a
corrosion-resistant iron can be ferritic, pearlitic, martensitic, or austenitic in its microstructure. Depending on
composition, cooling rate, and inoculation practice, a corrosion-resistant iron can be white, gray, or nodular in
both form and distribution of carbon.
Heat-resistant irons combine resistance to high-temperature oxidation and scaling with resistance to
softening or microstructural degradation. Resistance to scaling depends chiefly on high alloy content, and
resistance to softening depends on the initial microstructure plus the stability of the carbon-containing phase.
Heat-resistant irons are usually ferritic or austenitic as-cast; carbon exists predominantly as graphite, either in
flake or spherulitic form, which subdivides heat-resistant irons into either gray or ductile irons. There are also
ferritic and austenitic white iron grades, although they are less frequently used and have no American Society
for Testing and Materials (ASTM) designations.
Alloy Cast Irons
Revised by Richard B. Gundlach, Climax Research Services; and Douglas V. Doane, Consulting Metallurgist
Effects of Alloying Elements
In most cast irons, it is the interaction among alloying elements (including carbon and silicon) that has the
greatest effect on properties. This influence is exerted largely by effects on the amount and shape of graphitic
carbon present in the casting. For example, in low-alloy cast irons, depth of chill or the tendency of the iron to
be white as-cast depends greatly on the carbon equivalent, the silicon in the composition, and the state of
inoculation. The addition of other elements can only modify the basic tendency established by the carbonsilicon
relationship.
On the other hand, abrasion-resistant white cast irons are specifically alloyed with chromium to produce fully
carbidic irons. One of the benefits of chromium is that it causes carbide, rather than graphite, to be the stable
carbon-rich eutectic phase upon solidification. At higher chromium contents (10% or more), M7C3 carbide
becomes the stable carbon-rich phase of the eutectic reaction.
In general, only small amounts of alloying elements are needed to improve depth of chill, hardness, and
strength. Typical effects on depth of chill are given in for the alloying elements commonly used in low to
moderately alloyed cast irons. High alloy contents are needed for the most significant improvements in abrasion
resistance, corrosion resistance, or elevated-temperature properties.
Alloying elements such as nickel, chromium, and molybdenum are used, singly or in combination, to provide
specific improvements in properties compared to unalloyed irons. Because the use of such elements means
higher cost, the improvement in service performance must be sufficient to justify the increased cost.
Carbon. In chilled irons, the depth of chill decreases (Fig. 2c), and the hardness of the chilled zone increases,
with increasing carbon content. Carbon also increases the hardness of white irons. Low-carbon white irons (
2.50% C) have a hardness of about 375 HB (Fig. 3), while white irons with fairly high total carbon (>3.50% C)
have a hardness as high as 600 HB. In unalloyed white irons, high total carbon is essential for high hardness
and maximum wear resistance. Carbon decreases transverse breaking strength (Fig. 4) and increases brittleness.
It also increases the tendency for graphite to form during solidification, especially when the silicon content is
also high. As a result, it is very important to keep the silicon content low in high-carbon white irons. The
normal range of carbon content for unalloyed or low-alloy white irons is about 2.2 to 3.6%. For high-chromium
white irons, the normal range is from about 2.2% to the carbon content of the eutectic composition, which is
about 3.5% for a 15% Cr iron and about 2.7% for a 27% Cr iron.
The carbon content of gray and ductile alloy irons is generally somewhat higher than that of a white iron of
similar alloy content. In addition, the silicon content is usually higher, so that graphite will be formed upon
solidification.
Silicon is present in all cast irons. In alloy cast irons, as in other types, silicon is the chief factor that determines
the carbon content of the eutectic. Increasing the silicon content lowers the carbon content of the eutectic and
promotes the formation of graphite upon solidification. Therefore, the silicon content is the principal factor
controlling the depth of chill in unalloyed or low-chromium chilled and white irons.
In high-alloy white irons, silicon has a negative effect on hardenability; that is, it tends to promote pearlite
formation in martensitic irons. However, when sufficient amounts of pearlitic-suppressing elements such as
molybdenum, nickel, manganese, and chromium are present, increasing the silicon contest raises the Ms
temperature of the alloy, thus tending to increase both the amount of martensite and the final hardness.
The silicon content of chilled and white irons is usually between 0.3 and 2.2%. In martensitic nickel-chromium
white irons, the desired silicon content is usually 0.4 to 0.9%. It is necessary to select carefully the charge
constituents when melting a martensitic iron so that excessive silicon content is avoided. In particular, it is
necessary to give special attention to the silicon content of the ferrochromium used in the furnace charge.
Silicon additions of 3.5 to 7% improve high-temperature properties by raising the eutectoid transformation
temperature. Elevated levels of silicon
also reduce the rates of scaling and growth by forming a tight, adhering oxide scale. This occurs at silicon
contents above 3.5% in ferritic irons and above 5% in 36% Ni austenitic irons. Additions of 14 to 17% (often
accompanied by additions of about 5% Cr and 1% Mo) yield cast iron that is very resistant to corrosive acids,
although resistance varies somewhat with acid concentration.
High-silicon irons (14 to 17%) are difficult to cast and are virtually unmachinable. High-silicon irons have
particularly low resistance to mechanical and thermal shock at room temperature or moderately elevated
temperature. However, above about 260 °C (500 °F), the shock resistance exceeds that of ordinary gray iron.
Manganese and sulfur should be considered together in their effects on gray or white iron. Alone, either
manganese or sulfur increases the depth of chill, but when one is present, addition of the other decreases the
depth of chill until the residual concentration has been neutralized by the formation of manganese sulfide.
Generally, sulfur is the residual element, and excess manganese can be used to increase chill depth and
hardness, as shown in Fig. 2(d). Furthermore, because it promotes the formation of finer and harder pearlite,
manganese is often preferred for decreasing or preventing mottling in heavy-section castings.
Manganese, in excess of the amount needed to scavenge sulfur, mildly suppresses pearlite formation. It is also a
relatively strong austenite stabilizer and is normally kept below about 0.7% in martensitic white irons. In some
pearlitic or ferritic alloy cast irons, up to about 1.5% Mn can be used to help ensure that specified strength
levels are obtained. When manganese content exceeds about 1.5%, the strength and toughness of martensitic
irons begin to drop. Abrasion resistance also drops, mainly because of austenite retention. Molten iron with a
high manganese content tends to attack furnace and ladle refractories. Consequently, the use of manganese is
limited in cast irons, even though it is one of the least expensive alloying elements.
The normal sulfur contents of alloy cast irons are neutralized by manganese, but the sulfur content is kept low
in most alloy cast irons. In abrasion-resistant cast irons, the sulfur content should be as low as is commercially
feasible, because several investigations have shown that sulfides in the microstructure degrade abrasion
resistance. A sulfur content of 0.03% appears to be the maximum that can be tolerated when optimum abrasion
resistance is desired.
Phosphorus is a mild graphitizer in unalloyed irons; it mildly reduces chill depth in chilled irons (Fig. 2c). In
alloyed irons, the effects of phosphorus are somewhat obscure. There is some evidence that it reduces the
toughness of martensitic white irons. The effect, if any, on abrasion resistance has not been conclusively
proved. In heavy-section castings made from molybdenum-containing irons, high phosphorus contents are
considered detrimental because they neutralize part of the deep-hardening effect of the molybdenum. It is
considered desirable to keep the phosphorus content of alloy cast irons below about 0.3%, and some
specifications call for less than 0.1%. In cast irons for high-temperature or chemical service, it is customary to
keep the phosphorus content below 0.15%.
Chromium has three major uses in cast irons:
· To form carbides
· To impact corrosion resistance
· To stabilize the structure for high-temperature applications
Small amounts of chromium are routinely added to stabilize pearlite in gray iron, to control chill depth in
chilled iron, or to ensure a graphite-free structure in white iron containing less than 1% Si. At such low
percentages, usually no greater than 2 to 3%, chromium has little or no effect on hardenability, chiefly because
most of the chromium is tied up in carbides. However, chromium does influence the fineness and hardness of
pearlite and tends to increase the amount and hardness of the eutectic carbides. Consequently, chromium is
often added to gray iron to ensure that strength requirements can be met, particularly in heavy sections. On
occasion, it can be added to ductile iron for the same purpose. Also, relatively low percentages of chromium are
used to improve the hardness and abrasion resistance of pearlitic white cast irons.
When the chromium content of cast iron is greater than about 10%, eutectic carbides of the M7C3 type are
formed, rather than the M3C type that predominates at lower chromium contents. More significantly, however,
the higher chromium content causes a change in solidification pattern to a structure in which the M7C3 carbides
are surrounded by a matrix of austenite or its transformation products. At lower chromium contents, the M3C
carbide forms the matrix. Because of the solidification characteristics, hypoeutectic irons containing M7C3
carbides are normally stronger and tougher than irons containing M3C carbides.
The relatively good abrasion resistance, toughness, and corrosion resistance found in high-chromium white
irons have led to the development of a series of commercial martensitic or austenitic white irons containing 12
to 28% Cr. Because much of the chromium in these irons is present in combined form as carbides, chromium is
much less effective than molybdenum, nickel, manganese, or copper in suppressing the eutectoid
transformation to pearlite and therefore has a lesser effect on hardenability than it has in steels. Martensitic
white irons usually contain one or more of the elements molybdenum, nickel, manganese, and copper to give
the required hardenability. These elements ensure that martensite will form upon cooling from above the upper
transformation temperature either while the casting is cooling in the mold or during subsequent heat treatment.
It is difficult to maintain low silicon content in high-chromium irons because of the silicon introduced by highcarbon
ferrochrome and other sources. Low silicon content is advantageous in that it provides for ready
response to annealing and yields high hardness when the alloy is air quenched from high temperatures. High
silicon content lessens response to this type of heat treatment. Although high-chromium white irons are
sometimes used as-cast, their optimum properties are obtained in the heat-treated condition.
For developing resistance to the softening effect of heat and for protection against oxidation, chromium is the
most effective element. It stabilizes iron carbide and therefore prevents the breakdown of carbide at elevated
temperatures; 1% Cr give adequate protection against oxidation up to about 760 °C (1400 °F) in many
applications. For temperatures of 760 °C (1400 °F) and above, chromium contents up to 5.5% are found in
austenitic ductile irons for added oxidation resistance. For long-term oxidation resistance at elevated
temperatures, white cast irons having chromium contents of 15 to 35% are employed. This percentage of
chromium suppresses the formation of graphite and makes the alloy solidify as white cast iron.
High levels of chromium stabilize the ferrite phase up to the melting point; typical high-chromium ferritic irons
contain 30 to 35% Cr. Austenitic grades of high-chromium irons, which have significantly higher strength at
elevated temperatures, contain 10 to 15% Ni, along with 15 to 30% Cr.
Nickel is almost entirely distributed in the austenitic phase or its transformation products. Like silicon, nickel
promotes graphite formation, and in white and chilled irons, this effect is usually balanced by the addition of
about one part chromium for every three parts nickel in the composition. If fully white castings are desired, the
amount of chromium can be increased. Some low- and medium-alloy cast irons have a ratio as low as one part
chromium to 1.3 parts nickel. In high-chromium irons, the nickel content may be as high as 15% to stabilize the
austenite phase.
When added to low-chromium white iron in amounts up to about 2.5%, nickel produces a harder and finer
pearlite in the structure, which improves its abrasion resistance. Nickel in somewhat larger amounts--up to
about 4.5%--is needed to completely suppress pearlite formation, thus ensuring that a martensitic iron results
when the castings cool in their molds. This latter practice forms the basis for production of the Ni-Hard cast
irons (which are usually identified in standard specification as nickel-chromium martensitic irons). With small
castings such as grinding balls, which can be shaken out of the molds while still hot, air cooling from the
shakeout temperature will produce the desired martensitic structure even when the nickel content is as low as
2.7%. On the other hand, an excessively high nickel content (more than about 6.5%) will so stabilize the
austenite that little martensite, if any, can be formed in castings of any size. Appreciable amounts of retained
austenite in Ni-Hard cast irons can be transformed to martensite by refrigerating the castings at -55 to -75 °C (-
70 to -100 °F) or by using special tempering treatments.
One of the Ni-Hard family of commercial alloy white irons (type IV Ni-Hard) contains 1.0 to 2.2% Si, 5 to 7%
Ni, and 7 to 11% Cr. In the as-cast condition, it has a structure of M7C3 eutectic carbides in a martensitic
matrix. If retained austenite is present, the martensite content and hardness of the alloy can be increased by
refrigeration treatment or by reaustenitizing and air cooling. Ni-Hard IV is often specified for pumps and other
equipment used for handling abrasive slurries because of its combination of relatively good strength, toughness,
and abrasion resistance.
Nickel is used to suppress pearlite formation in large castings of high-chromium white iron (12 to 28% Cr.) The
typical amount of nickel is about 0.2 to 1.5%, and it is usually added in conjunction with molybdenum. Nickel
contents higher than this range tend to excessively stabilize the austenite, leading to austenite retention. Control
of composition is especially important for large castings that are intended to be martensitic, because their size
dictates that they cool slowly regardless of whether they are to be used as-cast or after heat treatment.
Nickel additions of more than 12% are needed for optimum resistance to corrosion or heat. High-nickel gray or
ductile irons usually contain 1 to 6% Cr and may contain as much as 10% Cu. These elements act in
conjunction with the nickel to promote resistance to corrosion and scaling, especially at elevated temperatures.
All types of cast iron with nickel contents above 18% are fully austenitic.
Copper in moderate amounts can be used to suppress pearlite formation in both low- and high-chromium
martensitic white irons. The effect of copper is relatively mild compared to that of nickel, and because of the
limited solubility of copper in austenite, copper additions probably should be limited to about 2.5% or less. This
limitation means that copper cannot completely replace nickel in Ni-Hard-type irons. When added to chilled
iron without chromium, copper narrows the zone of transition from white to gray iron, thus reducing the ratio of
the mottled portion to the clear chilled portion.
Copper is most effective in suppressing pearlite when it is used in conjunction with about 0.5 to 2.0% Mo. The
hardenability of this combination is surprisingly good, which indicates that there is a synergistic effect when
copper and molybdenum are added together to cast iron. Combined additions appear to be particularly effective
in the martensitic high-chromium irons. Here, copper content should be held to 1.2% or less; larger amounts
tend to induce austenite retention.
Copper is used in amounts of about 3 to 10% in some high-nickel gray and ductile irons that are specified for
corrosion or high-temperature service. Here, copper enhances corrosion resistance, particularly resistance to
oxidation or scaling.
Molybdenum in chilled and white iron compositions is distributed between the eutectic carbides and the
matrix. In graphitic irons, its main functions are to promote deep hardening and to improve high-temperature
strength and corrosion resistance.
In chilled iron compositions, molybdenum additions mildly increase depth of chill (they are about one-third as
effective as chromium; see Fig. 2d). The primary purpose of small additions (0.25 to 0.75%) of molybdenum to
chilled iron is to improve the resistance of the chilled face to spalling, pitting, chipping, and heat checking.
Molybdenum hardens and toughens the pearlitic matrix.
Where a martensitic white iron is desired for superior abrasion resistance, additions of 0.5 to 3.0% Mo
effectively suppress pearlite and other high-temperature transformation products . Molybdenum is even
more effective when used in combination with copper, chromium, nickel, or both chromium and nickel.
Molybdenum has an advantage over nickel, copper, and manganese in that it increases depth of hardening
without appreciably overstabilizing austenite, thus preventing the retention of undesirably large amounts of
austenite in the final structure. Figure 6 illustrates the influence of different amounts of molybdenum on the
hardenability of high-chromium white irons and shows that the hardenability (measured as the critical diameter
for air hardening) increases as the ratio of chromium to carbon increases.
The pearlite-suppressing properties of molybdenum have been used to advantage in irons of high chromium
content. White irons with 12 to 18% Cr are used for abrasion-resistant castings. The addition of 1 to 4% Mo is
effective in suppressing pearlite formation, even when the castings are slowly cooled in heavy sections.
Molybdenum can replace some of the nickel in the nickel-chromium type of martensitic white irons. In heavysection
castings in which 4.5% Ni would be used, the addition of 1% Mo permits a reduction of nickel content
to about 3%. In light-section castings of this type, where 3% Ni would normally be used, the addition of 1% Mo
permits a reduction of nickel to 1.5%.
Molybdenum, in quantities of about 1 to 4%, is effective in enhancing corrosion resistance, especially in the
presence of chlorides. In quantities of 1
2
to 2%, molybdenum improves high-temperature strength and creep
resistance in gray and ductile irons with ferritic or austenitic matrices. Figure 7 illustrates the influence of
molybdenum on the strength and creep resistance of high-silicon (4% Si) ferritic ductile iron at 705 °C (1300)F.
Vanadium is a potent carbide stabilizer and increases depth of chill. The magnitude of the increase of depth of
chill depends on the amount of vanadium and the composition of the iron as well as on section size and
conditions of casting. The powerful chilling effect of vanadium in thin sections can be balanced by additions of
nickel or copper, by a large increase in carbon or silicon, or both. In addition to its carbide-stabilizing influence,
vanadium in amounts of 0.10 to 0.50% refines the structure of the chill and minimizes coarse columnar grain
structure. Because of its strong carbide-forming tendency, vanadium is rarely used in gray or ductile irons for
corrosion or elevated-temperature service.
Effects of Inoculants
Certain elements, when added in minute amounts in the pouring ladle, have relatively strong effects on the size,
shape, and distribution of graphite in graphitic cast irons. Other elements are equally powerful in stabilizing
carbides. These elements, called inoculants, appear to act more as catalysts than as participants in the reactions.
The main graphitizing inoculant is ferrosilicon, which is often added in detectable amounts (several kilograms
per tonne) as a final adjustment of carbon equivalent in gray or ductile irons. In ductile irons, it is essential that
the graphite be present in the final structure as nodules (spherulites) rather than as flakes. Magnesium, cerium,
rare-earth elements, and certain proprietary substances are added to the molten iron just before pouring to
induce the graphite to form in nodules of the desired size and distribution.
In white irons, tellurium, bismuth, and sometimes vanadium are the principal carbide-inducing inoculants.
Tellurium is extremely potent; an addition of only about 5 g/t (5 ppm) is often sufficient. Tellurium has one
major drawback. It has been found to cause tellurium halitosis in foundry workers exposed to even minute
traces of its fumes; therefore, its use as an inoculant has been discouraged and sometimes prohibited.
Bismuth, in amounts of 50 to 100 g/t (50 to 100 ppm), effectively suppresses graphite formation in unalloyed or
low-alloy white iron. In particular, bismuth is used in the low-carbon compositions destined for malleabilizing
heat treatment. It has been reported that bismuth produces a fine-grain microstructure free from spiking, a
condition that is sometimes preferred in abrasion-resistant white irons.
Vanadium, in amounts up to 0.5%, is sometimes considered useful as a carbide stabilizer and grain refiner in
white or chilled irons. Nitrogen- and boron-containing ferroalloys have also been used as inoculants with
reported beneficial effects. In general, however, the economic usefulness of inoculants in abrasion-resistant
white irons has been inconsistent and remains unproved. Inoculants other than appropriate graphitizing or
nodularizing agents are used rarely, if ever, in high-alloy corrosion-resistant or heat-resistant irons.
Alloy Cast Irons
Revised by Richard B. Gundlach, Climax Research Services; and Douglas V. Doane, Consulting Metallurgist
Abrasion-Resistant Cast Irons
It should be presumed that parts subjected to abrasion will wear out and will therefore need to be replaced from
time to time. Also, for many applications, there will be one or more types of relatively low-cost material that
will have adequate wear resistance and one or more types of higher-cost material that will have measurably
superior wear resistance. For both situations, the ratio of wear rate to replacement cost should be evaluated; this
ratio can be a very effective means of evaluating the most economical use of materials. It is often more
economical to use a less wear-resistant material and replace it more often. However, in some cases, such as
when frequent occurrences of downtime cannot be tolerated, economy is less important than service life. Total
cost-effectiveness must take into account the actual cost of materials, heat treatment, time for removal of worn
parts and insertion of new parts, and other production time lost.
In general, chilled iron and unalloyed white iron are less expensive than alloy irons; they are also less wear
resistant. However, the abrasion resistance of chilled or unalloyed white iron is entirely adequate for many
applications. It is only when a clear performance advantage can be proved that alloy cast irons will show an
economic advantage over unalloyed irons. For example, in a 1-year test in a mill for grinding cement clinker,
grinding balls made of martensitic nickel-chromium white iron had to be replaced only about one-fifth as often
as forged and hardened alloy steel balls. In another test of various parts in a brick-making plant, martensitic
nickel-chromium white iron was found to last three to four times as long as unalloyed white iron, in terms of
both tonnage handled and lifetime in days. In both cases, martensitic nickel-chromium white iron showed a
clear economic advantage as well as a clear performance advantage over the alternative materials.
Typical Compositions. The first two lines of Table 1 list the composition ranges for the typical commercial
unalloyed and low-alloy grades of white and chilled irons used for abrasion-resistant castings. These are
nominally classed as pearlitic white irons. Historically, most of the early white iron castings produced for
abrasion resistance were cast from low-carbon, 1.0 to 1.6% Si unalloyed compositions, which were also used
for malleable iron castings. As changes have occurred in demand and specific uses, the trend has been to
produce a more abrasion-resistant 2.8 to 3.6% C, low-silicon grade, which is usually alloyed with chromium to
suppress graphite and to increase the fineness and hardness of the pearlite. Other alloying elements such as
nickel, molybdenum, copper, and manganese are used primarily to increase hardenability in order to obtain
austenitic or martensitic structures.
Martensitic white irons have largely displaced pearlitic white irons for making many types of abrasion-resistant
castings, with the possible exception of chilled iron rolls and grinding balls. Although martensitic white irons
cost more than pearlitic irons, their much superior abrasion resistance, combined with the increasing costs of all
castings, makes martensitic alloy white irons economically attractive. The better strength and toughness of
martensitic irons favor their use, and the trend toward replacing cupola melting with electric furnace melting
makes martensitic white irons relatively easy to produce.
The iron alloys of classes II and III are either pearlitic or austenitic as-cast, except in slow-cooling heavy
sections, which may be partially martensitic. The iron alloys of classes II and III are usually heat treated as
described below. There are several situations in which the abrasion resistance of the as-cast austenitic casting is
very good; no heat treatment is applied in such cases.
Heat Treatment. Various high- and low-temperature heat treatments can be used to improve the properties of
white and chilled iron castings. For the unalloyed or low-chromium pearlite white irons, heat treatment is
performed primarily to relieve the internal stresses that develop in the castings as they cool in their molds.
Generally, such heat treatments are used only on large castings such as mill rolls and chilled iron car wheels.
Temperatures up to about 705 °C (1300 °F) can be used without severely reducing abrasion resistance. In some
cases, the castings can be removed from their molds above the pearlitic-formation temperature and can then be
isothermally transformed to pearlite (or to ferrite and carbide) in an annealing furnace. As the tempering or
annealing temperature is increased, the time at temperature must be reduced to prevent graphitization.
Residual stresses in large castings result from volume changes during the transformation of austenite and during
subsequent cooling of the casting to room temperature. Because these volume changes may not occur
simultaneously in each part of the casting, they tend to set up residual stresses, which may be very high and
may therefore cause the casting to crack in the foundry or in service.
The nickel-chromium martensitic white irons, containing up to about 7% Ni and 11% Cr, are usually put into
service after only a low-temperature heat treatment at 230 to 290 °C (450 to 550 °F) to temper the martensite
and to increase toughness. If retained austenite is present and the iron therefore has less than optimum hardness,
a subzero treatment down to liquid nitrogen temperature can be employed to transform much of the retained
austenite to martensite. Subzero treatment substantially raises the hardness, often as much as 100 Brinell points.
Following subzero treatment, the castings are almost always tempered at 230 to 260 °C (450 to 500 °F) The
austenite-martensite microstructures produced in nickel-alloyed irons are often desirable for their intrinsic
toughness.
It is possible to transform additional retained austenite by heat treating nickel-chromium white irons at about
730 °C (1350 °F). Such a treatment decreases matrix carbon and therefore raises the Ms temperature. However,
high-temperature treatments are usually less desirable than subzero treatments because the former are more
costly and more likely to induce cracking due to transformation stresses.
The high-chromium martensitic white irons (>12% Cr) must be subjected to a high-temperature heat treatment
to develop full hardness. They can be annealed to soften them for machining, then hardened to develop the
required abrasion resistance. Because of their high chromium content, there is no likelihood of graphitization
while the castings are held at the reaustenitizing temperature.
The usual reaustenitizing temperature for high-chromium irons ranges from about 955 °C (1750 °F) for a 15Cr-
Mo iron to about 1065 °C (1950 °F) for a 27% Cr iron. An appreciable holding time (3 to 4 h minimum) at
temperature is usually mandatory to permit precipitation of dispersed secondary carbide particles in the
austenite. This lowers the amount of carbon dissolved in the austenite to a level that permits transformation to
martensite during cooling to room temperature. Air quenching is usually used, although small, simply shaped
castings can be quenched in oil or molten salt without producing quench cracks. Following quenching, it is
advisable to stress relieve (temper) the castings at about 205 to 260 °C (400 to 500 °F). Figure 10 is a
continuous-cooling time-temperature-transformation diagram for a typical high-chromium iron designed for use
in moderately heavy sections.
Microstructure. With rapid solidification, such as that which occurs in thin-wall castings or when the iron
solidifies against a chill, the austenite dendrites and eutectic carbides are fine grained, which tends to increase
fracture toughness. In low-chromium white irons, rapid solidification will also reduce any tendency toward
formation of graphite. The presence of graphite severely degrades abrasion resistance. Chills in the mold can be
used to promote directional solidification and therefore reduce shrinkage cavities in the casting.
Certain inoculants, notably bismuth, may beneficially alter the solidification pattern by reducing spiking or by
producing a finer as-cast grain size.
Immediately after solidification, the microstructure of unalloyed or low-chromium white irons consists of
austenite dendrites, containing up to about 2% C, surrounded by M3C carbides . When the chromium
content of the iron exceeds about 7 wt%, the structure contains M7C3 eutectic carbides surrounded by austenite
. This reversal of the continuous phase in the structure tends to increase the fracture toughness of white
irons, but only those irons that have a hypoeutectic or eutectic carbon equivalent. All hypereutectic white irons
are relatively brittle and are seldom used commercially.
After a white iron casting has solidified and begins to cool to room temperature, the carbide phase may
decompose into graphite plus ferrite or austenite. This tendency to form graphite can be suppressed by rapid
cooling or by the addition of carbide-stabilizing alloying elements--usually chromium, although inoculating
with tellurium or bismuth is also very effective. Austenite in the solidified white iron structure normally
undergoes several changes as it cools to ambient temperature. If it is cooled slowly enough, it tends to reject
hypereutectoid carbon, either on existing eutectic carbide particles or as particles, platelets, or spines within the
austenite grains. This precipitation occurs principally between about 1040 and 760 °C (1900 and 1400 °F). The
rate of precipitation depends on both time and temperature.
As the austenite cools further, through the range of 705 to 540 °C (1300 to 1000 °F), it tends to transform to
pearlite. This transformation, however, can be suppressed by rapid cooling and/or by the use of pearlitesuppressing
elements in the iron.
Nickel, manganese, and copper are the principal pearlite-suppressing elements. Chromium does not contribute
significantly to pearlitic suppression (hardenability) in many white irons, because most of the chromium is tied
up in carbides. Molybdenum, a potent carbide former, is also tied up in carbides; however, in high-chromium
irons, there is enough chromium and molybdenum remaining in the matrix to contribute significantly to
hardenability.
Upon cooling below about 540 °C (1000 °F), the austenite may transform to bainite or martensite, thus
producing martensitic white iron, which is currently the most widely used type of abrasion-resistant white iron.
Martensitic white irons usually contain some retained austenite, which is not considered objectionable unless it
exceeds about 15%. Retained austenite is metastable and may transform to martensite when plastically
deformed at the wearing surface of the casting.
Silicon has a substantial influence on the microstructure of any grade of white iron. Normally, silicon content
exceeds 0.3%, and it may range as high as 2.2% in some of the high-chromium grades. During the solidification
of unalloyed or low-alloy irons, silicon tends to promote the formation of graphite, an effect that can be
suppressed by rapid solidification or by the addition of carbide-stabilizing elements. After solidification, either
while the casting is cooling to ambient temperature or during subsequent heat treatment, silicon tends to
promote the formation of pearlite in the structure if it is the only alloy present. However, in the presence of
chromium and molybdenum, both of which suppress ferrite, silicon has a minimal effect on ferrite and
substantially suppresses bainite. In certain alloy white irons with high retained-austenite contents, increasing
the silicon content raises the Ms temperature of the austenite, which in turn promotes the transformation of
austenite to martensite. Silicon is also used to enhance the hardening response when the castings are cooled
below ambient temperature.
Mechanical Properties. Hardness is the principal mechanical property of white iron that is routinely
determined and reported. Other (nonstandard) tests to determine strength, impact resistance, and fracture
toughness are sometimes employed by individual users, producers, or laboratories. Because of the difficulty of
preparing test specimens, especially from heavy-section castings, these nonstandard tests are seldom used for
routine quality control. Two exceptions are the tumbling-breakage test and the repeated-drop test, which have
been routinely used by certain producers for testing grinding balls.
Minimum hardness values for pearlitic white irons are 321 HB for the low-carbon grade and 400 HB for the
high-carbon grade. A chill cast high-carbon 2% Cr white iron may reach a hardness of about 550 HB. A typical
hardness range for a sand cast high-carbon grade is about 430 to 500 HB.
The tensile strength (in reality, the fracture strength) of pearlitic white irons normally ranges from about 205
MPa (30 ksi) for high-carbon grades to about 415 MPa (60 ksi) for low-carbon grades. The tensile strength of
martensitic irons with M3C carbides ranges from about 345 to 415 MPa (50 to 60 ksi), while high-chromium
irons, with their M7C3-type carbides, usually have tensile strengths of 415 to 550 MPa (60 to 80 ksi). Limited
data indicate that the yield strengths of white irons are about 90% of their tensile strengths. These data are
extremely sensitive to variations in specimen alignment during testing. Because of the near-zero ductility of
white irons, the usefulness of tensile test data for design or quality assurance is very limited.
Transverse strength, which is an indirect measurement of tensile strength and tensile ductility, can be
determined with a moderate degree of accuracy on unmachined cast test bars. The product of transverse
strength and deflection provides one measure of toughness.
Physical Properties. The density of white irons ranges from 7.50 to 7.75 g/cm3 (0.271 to 0.280 lb/in.3).
Increasing carbon content tends to decrease density; increasing the amount of retained austenite in the structure
tends to increase density. Other physical properties are summarized in Table 6 for low-carbon white iron and
martensitic nickel-chromium white iron.
Abrasion Resistance. The relative abrasion resistance of various types of white iron has been extensively
studied in commercial service and in many types of laboratory abrasion tests. In general, martensitic white irons
have substantially better abrasion resistance than pearlitic or austenitic white irons. There can be substantial
differences in abrasion resistance among the various martensitic irons. The degree of superiority of one type
over another can also vary considerably, depending on the application and also on whether abrasive wear is due
to gouging, high-stress (grinding) abrasion, or low-stress scratching or erosion. In addition, performance in a
wet environment may be quite different from that in a dry environment.
For nickel-chromium martensitic white irons, there are conflicting data as to the relative serviceability of sand
cast and chill cast parts subjected to abrasive wear. This is not particularly surprising, because many of the data
were obtained in test using abrasive ores where the nature of the gangue was incompletely defined or largely
ignored. As discussed in the article "Wear Failures" in Failure Analysis and Prevention, Volume 11 of ASM
Handbook, formerly 9th Edition Metals Handbook, it is very important to completely characterize any abrasive
substance, particularly ores and other earthy mixtures, so that the results of tests and service evaluations can be
properly analyzed.
The hardness of the abrasive material has a marked influence on relative abrasion rates. For example, when the
abrasive is silicon carbide, which is hard enough to scratch M3C and M7C3 carbides as well as martensite and
pearlite, there may be little difference in relative wear rates among any of the white irons. However, with silica
(the abrasive most commonly encountered in service), which is not hard enough to scratch M7C3 carbides but
may scratch M3C carbides and definitely will scratch martensite and pearlite, high-chromium white irons, with
their M7C3 carbides, tend to provide superior performance. If the abrasive mineral is a silicate of intermediate
hardness such as feldspar, which (theoretically) will not scratch fully hard martensite but will scratch pearlite,
any of the martensitic white irons should perform much better than any of the pearlitic white irons.
The relatively low hardness of the retained austenite in high-chromium irons deserves special
consideration. Because this austenite tends to work harden rapidly and may also transform to martensite, it is
quite abrasion resistant when severely loaded. However, most abrasion tests and field experience indicate that
irons containing considerable retained austenite are not as abrasion resistant as those put into service with fully
martensitic microstructures.
widely used for handling the corrosive media common in chemical plants, even when abrasive conditions are
also encountered. When the silicon content is 14.2% or higher, these irons exhibit a very high resistance to
boiling sulfuric acid . They are especially useful when the concentration of sulfuric acid is above
50%, at which point they are virtually immune to attack, the high-silicon irons are also
very resistant to nitric acid. Increasing the silicon content to 16.5% makes the alloy quite resistant to corrosion
in boiling nitric and sulfuric acids at nearly all concentrations, but because this is accompanied by a reduction
of mechanical strength, it is not ordinarily done in the United States.
The high-silicon irons are very resistant to organic acid solutions at any concentration or temperature. However,
their resistance to strong hot caustics is not satisfactory for most purposes. They are resistant to caustic
solutions at lower temperatures and concentrations, and (although they are no better than unalloyed gray iron in
this regard) they can be used where caustics and other corrosives are mixed or alternately handled. They have
no useful resistance to hydrofluoric or sulfurous acids.
High-silicon irons have poor mechanical properties and particularly low thermal and mechanical shock
resistance. These alloys are typically very hard and brittle, with a tensile strength of about 110 MPa (16 ksi) and
a hardness of 480 to 520 HB. They are difficult to cast and are virtually unmachinable. Their considerable use
stems from their outstanding resistance to acids such as those mentioned above. They are widely used for drain
pipe in chemical plants, laboratories, hospitals, and schools. High-silicon iron towers, tubes, and fittings are
standard equipment for concentrating sulfuric and nitric acids in the explosives and fertilizer industries.
High-silicon iron pumps, valves, mixing nozzles, tank outlets, and steam jets are widely used for handling
severe corrodents such as chromic acid, sulfuric-acid slurries, bleach solutions, and acid-chloride slurries,
which are frequently encountered in plants that manufacture paper, pigments, or dye stuffs or that use
electroplating solutions. High-silicon irons are also widely used for anodes in impressed-current cathodicprotection
systems, especially where aggressive environments such as seawater or chloride soils are
encountered.
The mechanical strength and shock resistance of high-silicon irons can be improved by lowering the silicon
content to 12% or slightly less; this practice is occasionally followed in the United States and Europe. However,
reducing the silicon 12% causes a significant reduction in corrosion resistance and therefore is feasible only in
applications where the loss in corrosion resistance has a minimal effect on service life or is offset by the benefit
derived from the increase in strength.
High-chromium irons containing 20 to 35% Crcontaining 20 to 35% Cr give good service with oxidizing acids, particularly nitric, but
are not resistant to reducing acids. These irons are also reliable for use in weak acids under oxidizing
conditions, in numerous salt solutions, in organic acid solutions, and in marine or industrial atmospheres.
The corrosion resistance of high-chromium cast iron to nitric acid is exceptional; it resists all concentrations of
this acid up to 95% at room temperature. Its corrosion rate is less than 0.13 mm (0.005 in.) per year at all
temperatures up to the boiling point for concentrations up to 70%. In handling nitric acid, the chromium irons
are complementary to high-silicon irons. The former exhibit excellent corrosion resistance to all concentrations
and temperatures, except for boiling concentrated acids, while the latter give better results in stronger acid for both highchromium
and high-silicon irons.
The low-carbon, high-chromium irons are satisfactory for annealing pots; lead, zinc, or aluminum melting pots;
conveyor links; and other parts exposed to corrosion at high temperature. Because the corrosion resistance is
imparted by chromium present in solid solution in the ferritic matrix, this element must be present in sufficient
quantity to combine with carbon as chromium carbide and still remain in the desired amount in the ferrite.
Chromium contents of 30 to 33% are common in irons for use under conditions of severe acid corrosion.
High-chromium irons are resistant to all concentrations of sulfurous acid at temperatures up to 80 °C (175 °F),
to sulfite liquors used in the papermaking industry, to hypochlorite bleaching liquors at room temperature, to
cold aluminum sulfate in concentrations up to 5%, and to some salts that hydrolyze to give acid solutions. They
resist all concentrations of phosphoric acid up to 60% at temperatures up to the boiling point and 85%
concentrations up to 80 °C (175 °F). They also have good resistance to aerated seawater and most mine waters,
including acidic types.
Chromium cast irons have better mechanical properties than high-silicon irons and respond readily to heat
treatment when chromium and carbon contents are suitably balanced. Tensile strength as high as 480 MPa (70
ksi) is obtained with a hardness of 290 to 340 HB. These alloys are generally resistant to shock and can be
machined; both of these properties are improved when carbon content is lowered to about 1.2%. As carbon is
increased, machinability in the annealed condition decreases; consequently, irons with carbon contents of 3% or
more should be used only when no machining is required. The maximum service temperature for highchromium
irons is generally 815 to 1095 °C (1500 to 2000 °F).
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