Alloy Cast Iron(2)

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Alloy Cast Iron(2)

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

High-Nickel Irons. High-nickel austenitic cast irons produced in several compositions, depending on desired
properties and end use. Austenitic gray irons containing large percentages of nickel and copper are fairly
resistant to mildly oxidizing acids, including dilute to concentrated sulfuric acid at room temperature. Their
resistance to sulfuric acid is compared with that of other cast irons in Fig. 17(a). They are also fairly resistant to
hydrochloric and some phosphoric acids at slightly elevated temperatures. Their useful range in hydrochloric
acids is indicated in Fig. 17(c).
The corrosion behavior of high-nickel irons is similar to that of unalloyed gray iron in the presence of nitric
acid. Although the nickel-containing iron exhibits better corrosion resistance than an 18-8 stainless steel, highsilicon
irons are much better for both sulfuric and hydrochloric acids under the conditions shown in Fig. 17(a)
and 17(c).
High-nickel irons exhibit fair resistance to some organic acids (such as acetic, oleic, and stearic acids) and to
red oils. Irons with nickel contents of 18% or more are nearly immune to the effects of weak or strong alkalies
and caustics, although subject to stress corrosion in strong hot caustics at stresses over 70 MPa (10 ksi). High-nickel irons are the toughest of all cast irons containing flake graphite. Although their tensile strength is
relatively low, ranging from 140 to 275 MPa (20 to 40 ksi), they have satisfactory toughness and excellent
machinability. High-nickel ductile irons, which are specially treated so that the graphite forms as spheroids
rather than as flakes, have essentially the same corrosion resistance as high-nickel gray irons, but have much
higher strength and ductility. A similar treatment applied to high-silicon iron provides no improvement in
mechanical properties.
High-nickel irons provide satisfactory corrosion resistance at elevated temperatures up to about 705 to 815 °C
(1300 to 1500 °F). Above this range, high-chromium irons are preferred.
Alloy Cast Irons
Revised by Richard B. Gundlach, Climax Research Services; and Douglas V. Doane, Consulting Metallurgist
Heat-Resistant Cast Irons
Heat-resistant cast irons are basically alloys of iron, carbon, and silicon having high-temperature properties
markedly improved by the addition of certain alloying elements, singly or in combination, principally
chromium, nickel, molybdenum, aluminum, and silicon in excess of 3%. Silicon and chromium increase
resistance to heavy scaling by forming a light surface oxide that is impervious to oxidizing atmospheres. Both
elements reduce the toughness and thermal shock resistance of the metal. Although nickel does not appreciably
affect oxidation resistance, it increases strength and toughness at elevated temperatures by promoting an
austenitic structure that is significantly stronger than ferritic structures above 540 °C (1000 °F). Molybdenum
increases high-temperature strength in both ferritic and austenitic iron alloys. Aluminum additions are very
potent in raising the equilibrium temperature (A1) and in reducing both growth and scaling, but they adversely
affect mechanical properties at room temperature.
. The performance of alloy
gray irons at elevated temperatures is determined by a number of related properties, such as resistance to
growth and oxidation, resistance to thermal shock, response to cyclic heating, creep resistance, rupture strength,
and high-temperature fatigue strength.
Growth is the permanent increase volume that occurs in some cast irons after prolonged exposure to elevated
temperature or after repeated cyclic heating and cooling. It is produced by the expansion that accompanies
graphitization, expansion, and contraction at the transformation temperature, combined with internal oxidation
of the iron. Gases can penetrate the surface of hot cast iron at the graphite flakes and oxidize the graphite as
well as the iron and silicon. The occurrence of fine cracks, of crazing, may accompany repeated heating and
cooling through the transformation temperature ranges because of thermal and transformational stresses.
Silicon contents of less than about 3.5% increase the rate of growth by promoting graphitization, but silicon
contents of 4% or more retard growth. Both manganese and phosphorus decrease growth by acting as carbide
stabilizers.
The carbide-stabilizing alloying elements, particularly chromium, effectively reduce growth in gray irons at 455
°C (850 °F) or above. Growth is not a problem below 400 °C (750 °F), except in the presence of superheated
steam, where it can occur in coarse-grain irons at about 315 °C (600 °F). Even small amounts of chromium,
molybdenum, and vanadium produce marked reductions in growth at the higher temperatures. The influence of
chromium in reducing growth in gray iron at 800 °C (1470 °F) is shown in Fig. 18. These data indicate that in
cyclic heating, including a total of 500 h at the upper temperature, a chromium content of about 2% serves to
eliminate growth.
Chromium irons containing 24 to 34% Cr show no appreciable growth at 1095 °C (2000 °F). In general, cast
irons containing 20 to 35% Cr can be used regularly at about 980 °C (1800 °F) and for short periods up to 1095
°C (2000 °F) with satisfactory resistance to growth and scaling.
High-nickel gray and ductile irons are also quite resistant to elevated-temperature growth. For example, the
growth of gray and ductile versions of high-nickel cast iron is compared to that of unalloyed gray iron in Table
11; these data are for continuous exposure to superheated steam at 480 °C (900 °F). In addition to resistance to
growth, the austenitic gray and ductile irons are resistant to warpage and cracking in cyclic elevatedtemperature
service. This resistance is attributed to the absence of phase transformation, to moderate elastic
moduli, and to good mechanical properties at about 595 to 760 °C (1100 to 1400 °F).
Scaling. In addition to the internal oxidation that contributes to growth, a surface scale forms on unalloyed gray
iron after exposure at sufficiently high temperature. The scale formed in air consists of a mixture of iron oxides.
The important factor in scale formation is whether the scale, first, is essentially adherent and protective to the
base metal or, second, tends to flake and permit continued oxidation of the metal.
Silicon, chromium, and aluminum increase the scaling resistance of cast iron by forming a light surface oxide
that is impervious to oxidizing atmospheres. Unfortunately, these elements tend to reduce toughness and
thermal shock resistance. The presence of nickel improves the scale resistance of most alloys containing
chromium and, more important, increases their toughness and strength at elevated temperatures.
Carbon has a somewhat damaging effect above 705 °C (1300 °F) as a result of the mechanism of
decarburization and the evolution of carbon monoxide and carbon dioxide. When these gases are evolved at the
metal surface, the formation of protective oxide layers is hindered, and cracks and blisters may develop in the
scale.
High-Temperature Strength. Measurements of short-time tensile strength, creep strength, and rupture
strength provide a basis for evaluating the performance of metals at elevated temperatures. Creep rate increases
with temperature and becomes an important design factor at elevated temperatures.
Creep is ordinary reported in terms of strain for a specified period of time at a given tensile stress and
temperature. Because cast irons can grow at elevated temperatures without the application of external stress, the
measured increase in length is the sum of growth resulting from metallurgical causes and the mechanical
elongation of creep.
Creep in gray iron is appreciably influenced by microstructure and composition. An unalloyed gray iron with a
carbon equivalent of about 4% can usually be subjected to a tensile stress of 70 MPa (10 ksi) at 400 °C (750 °F)
without exceeding a creep rate of 1% in 10,000 h. Low-alloy irons exhibit even less creep under similar
conditions. Ductile irons may sustain stresses up to 185 MPa (27 ksi) at 425 °C (800 °F) without exceeding a
creep rate of 1% in 10,000 h. Some austenitic ductile irons have about the same creep strength at 540 °C (1000
°F) as the unalloyed ductile irons display at 425 °C (800 °F).
High-Silicon Irons. Although intermediate amounts of silicon increase the rate of growth in cast iron by
increasing the rate of graphitization, additions of 4.5 to 8% Si greatly reduce both scaling and growth. Silicon
also has the advantage of raising the transformation temperature to about 900 °C (1650 °F), thus increasing the
operating temperature range that may be employed without encountering a phase change (Ref 6).
Ferritic high-silicon gray iron is rather brittle and has a low resistance to thermal shock at room temperature.
However, it is superior to ordinary gray iron above about 260 °C (500 °F). An austenitic gray iron containing
5% Si, 18% Ni, and 2 to 5% Cr exhibits considerably better toughness and thermal shock resistance. Both the
plain silicon and the Ni-Cr-Si irons are British developments, the former known commercially as Silal and the
latter as Nicrosilal. They exhibit excellent resistance to scaling in air up to 815 °C (1500 °F), and the Ni-Cr-Si
iron can be successfully employed in sulfurous atmospheres. The maximum temperature for use of these irons
is reported to be 900 °C (1650 °F) for the plain silicon iron and 955 °C (1750 °F) for the Ni-Cr-Si iron. Siliconcontaining
compositions are also available in ferritic ductile iron.
High-Chromium Irons. Chromium is widely used in heat-resistant irons because of its stabilizing influence on
carbides, which deters growth, and its tendency to form a tight, protective oxide. Substantial improvement in
oxidation resistance is obtained by the addition of 0.5 to 1% Cr for many applications up to 760 °C (1400 °F).
Further improvement in resistance to scaling and growth at 760 °C (1400 °F) without excessive loss in
toughness and machinability is reported for irons with up to 2% Cr.
The effect of chromium additions on cyclic heating characteristics and resistance to scaling is discussed in the
section "Growth" in this article. Pertinent data are given in Fig. 18 and 19. Machinable castings with
considerable heat resistance can be obtained with rather small additions of both chromium and nickel to cast
iron. Chromium additions of 15 to 35% are employed for excellent oxidation and growth resistance at about
980 °C (1800 °F) and even up to 1095 °C (2000 °F) in oxidizing atmospheres or in the presence of certain
chemicals.
The high-chromium irons exhibit a characteristically white structure and can be produced with fair
machinability and good strength. Low silicon and carbon contents are desirable when toughness and thermal
shock resistance are required. The thermal shock resistance of these irons is good, but their toughness is quite
limited, even when carbon and silicon contents are low.
High-Nickel Irons. The austenitic cast irons containing 18 to 36% Ni, up to 7% Cu, and 1.75 to 4% Cr are
used for both heat-resistant and corrosion-resistant applications. Known as Ni-Resist, this type of iron exhibits
good resistance to high-temperature scaling and growth up to 815 °C (1500 °F) in most oxidizing atmospheres,
good performance in steam service up to 530 °C (990 °F), and can handle sour gases and liquids up to 400 °C
(750 °F). The maximum temperature of use is 540 °C (1000 °F) if appreciable sulfur is present in the
atmosphere. Austenitic cast iron can be employed at temperatures as high as 950 °C (1740 °F). Austenitic irons
have the advantage of considerably greater toughness and thermal shock resistance than the other heat-resistant
alloy irons, although their strength is rather low.
High-nickel ductile irons are considerably stronger and tougher than the comparable gray irons. Tensile
strengths of 400 to 470 MPa (58 to 68 ksi), yield strengths of 205 to 275 MPa (30 to 40 ksi), and elongations of
10 to 40% can be realized in high-nickel ductile irons.
High-Aluminum Irons. Alloy cast irons containing 6 to 7% Al, 18 to 25% Al, or 12 to 25% Cr plus 4 to 16%
Al are reported to have considerably better resistance to scaling than several other alloy irons, including the
high-silicon type. These irons have been little used commercially because of brittleness and poor castability.
Alloy Ductile Irons. Various corrosion-resistant and heat-resistant alloy irons can also be cast as ductile iron,
with the graphite in the form of spheroids rather than in the normal flake-graphite shape characteristic of gray
iron. When the graphite is present as spheroids, the metal exhibits improved elastic behavior, higher modulus of
elasticity, a definite yield point, higher tensile strength, and improved ductility and toughness. Where service applications require these improved properties, alloy ductile irons can be efficiently utilized, although their
metallurgical and production characteristics are more complex than those of comparable gray irons.
Alloy Cast Irons for Automotive Service. The most important automotive application of alloy cast irons is in
brake drums and disks, for which the material is required to have a combination of high heat capacity, good
thermal conductivity, and high emissivity so that it can dissipate a large amount of heat per unit volume. Also,
to maintain strength and dimensional stability during cyclic heating and cooling, the material must have
adequate high-temperature strength and resistance to thermal shock and must resist growth due to changes in
structure.
Low-silicon chromium-molybdenum gray cast iron is generally preferred for automotive disk or drum brakes.
Composition and graphite content are closely controlled to maintain adequate high-temperature strength, along
with the graphite size and distribution that give adequate thermal conductivity and machinability. In general,
decreasing silicon content from 2.5 to 1.5% increases thermal conductivity by about 10%. At the same time, the
carbon equivalent must be kept relatively high to ensure solidification as gray iron rather than as white iron.
Although most alloying elements decrease the thermal conductivity of gray iron, their effect is not as great as
that of silicon. Gray irons alloyed with chromium and molybdenum are reported to have better thermal
conductivity than unalloyed gray irons of comparable silicon and carbon contents. The alloy gray irons also
have more stable structures and better stress rupture properties than the unalloyed irons.
References cited in this section
4. R.J. Greene and F.G. Sefing, "Cast Irons in High-Temperature Service," National Association of Corrosion
Engineers, March 1954
5. Engineering Properties and Applications of the Ni-Resists and Ductile Ni-Resists, No. 1231, Nickel
Development Institute, International Nickel Company, 1975
6. W. Fairhurst and K. Roehrig, High Silicon Nodular Irons, Foundry Trade J., Vol 146, 1979, p 657-681
Alloy Cast Irons
Revised by Richard B. Gundlach, Climax Research Services; and Douglas V. Doane, Consulting Metallurgist
References
1. "Chrome-Moly White Cast Irons," Publication M-630, AMAX Inc., 1986
2. D.E. Diesburg and F. Borik, Optimizing Abrasion Resistance and Toughness of Steels and Irons for the
Mining Industry, in Materials for the Mining Industry, Symposium Proceedings, Climax Molybdenum
Company, 1974, p 26
3. T.E. Norman, A Review of Materials for Grinding Mill Liners, in Materials for the Mining Industry,
Symposium Proceedings. Climax Molybdenum Company, 1974, p 208
4. R.J. Greene and F.G. Sefing, "Cast Irons in High-Temperature Service," National Association of Corrosion
Engineers, March 1954
5. Engineering Properties and Applications of the Ni-Resists and Ductile Ni-Resists, No. 1231, Nickel
Development Institute, International Nickel Company, 1975
6. W. Fairhurst and K. Roehrig, High Silicon Nodular Irons, Foundry Trade J., Vol 146, 1979, p 657-681
Alloy Cast Irons
Revised by Richard B. Gundlach, Climax Research Services; and Douglas V. Doane, Consulting Metallurgist
Selected References
• J. Dodd and J.L. Parks, Factors Affecting the Production and Performance of Thick Section High
Chromium-Molybdenum Alloy Iron Castings, Publication M-383, AMAX Inc.
• Engineering Properties and Applications of Hi-Hard, The International Nickel Company, Inc.
• R.B. Gundlach, High-Alloy Graphitic Irons, in Castings, Vol 15, Metals Handbook, ASM International,
1988, p 698-701
• C.F. Walton and T.J. Opar, Ed., Iron Castings Handbook, Iron Castings Society, Inc., 1981
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