Gray Iron (Part two)

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Gray Iron (Part two)

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Gray Iron (Part two)

Messaggioda Aldebaran » 29/04/2010, 17:15

Resistance to Normal Wear
The scuff resistance described above should not be used as a basis for selecting a material that must resist normal wear.
Resistance to normal wear, like resistance to scuffing, is affected by both graphite form and matrix microstructure (and
possibly by the composition of the iron), but in a different manner.
Some of the same sleeve materials involved in the scuff tests were operated under conditions that produced normal wear
with very little evidence of scuffing. Tests were run for approximately 1000 h using chromium-plated compression rings
on the pistons. All sleeves were hardened and tempered at 205 °C (400 °F). Each of the
three sleeve materials performed best when different types of wear were considered: the material in No. 2 test gave the
lowest ring wear; in No. 4, the lowest sleeve wear; and in No. 5, the greatest resistance to scuffing. Thus, the choice of
optimum sleeve material is a compromise.
Effect of Graphite Structure. It was concluded from the above tests that graphite produces a surface-roughening
effect, which accounts for both the greater scuff resistance and the higher ring wear as the size and quantity of graphite
are increased. In testing type D graphite sleeves with the rougher finish previously mentioned, improved scuff resistance
was also obtained, but at the expense of greater ring wear, apparently from cutting. The larger quantity of graphite in test
5 leaves less load-carrying metal surface, and greater normal wear on the sleeve is the logical result.
The relatively high sleeve wear in test 2 was probably caused by slight scuffing, which gave high wear values for some
sleeves tested, presumably because of the low safety margin of scuff resistance in this type of iron. Surprisingly, this mild
scuffing did not seem to affect the chromium-plated rings, which apparently wore less severely in the presence of fine
graphite. The plain gray iron oil-control rings, however, wore more in test 2 than in test 4. The principal effect of graphite
on wear resistance is the elimination of scuffing, and when graphite is present in greater quantity and size than required
for this purpose, it will reduce resistance to normal wear unnecessarily.
Effect of Matrix Microstructure. Gray iron is used for wear resistance in both the as-cast and hardened conditions.
To show the effects of some of the matrix variations on wear resistance, comparable engine wear tests were made on
seven types of gray iron cylinders, all of which apparently wore in a normal manner.
Another significant advantage of hardening is that it greatly reduces variations in matrix microstructure and thus provides
a superior iron of more dependable performance. In most applications such as valve guides, latheways, and various
sliding members in machines and engines, satisfactory wear resistance is obtained with properly specified and controlled
as-cast gray iron. Hardened iron is used to obtain maximum wear resistance in severe wearing applications such as highspeed
diesel cylinder sleeves, camshafts, gears, and similar heavily loaded wearing surfaces.
Reference cited in this section
12. G. P. Phillips, Hardened Gray Iron--An Ideal Material for Diesel Engine Cylinder Sleeves and Liners,
Foundry, Vol 80, Jan 1952, p 88-95, 222, 224, 226, 228
Dimensional Stability
Effect of Temperature. The dimensional stability of gray iron is degraded as temperature and exposure time increase.
Dimensional stability at elevated temperatures is affected by factors such as growth, scaling, and creep rate.
Growth of the iron is the result of the breakdown of the pearlite to ferrite and graphite. To prevent this growth, alloying
elements such as copper, molybdenum, chromium, tin, vanadium, and manganese must be added because they will
stabilize the carbide structur. Alloying additions are generally
recommended at temperatures above 400 °C (750 °F). Chromium and chromium-nickel-molybdenum additions are most
effective in retarding growth.
Creep. A gray iron part that operates at elevated temperature will deform by creep if the load is great enough .
Residual stresses are present in all castings in the as-cast condition and are caused by:
· Differences in cooling rate between sections of the same casting because of different cross sections or
locations in the mold
· Resistance of the mold to contraction of the casting during cooling
· High-energy cleaning, such as with shot, which can induce compressive stresses
Residual stresses as high as 220 MPa (32 ksi) have been reported in gray iron wheels. Stresses of 170 MPa (25 ksi) have
been observed in localized areas of other gray iron castings. Residual stresses in engine cylinder blocks have been
measured as high as 130 MPa (19 ksi).
Only a small percentage of castings are stress relieved before machining, chiefly those requiring exceptional accuracy of
dimensions or those with a combination of high or nonuniform stress associated with either low section stiffness or an
abrupt change in section size. Castings of class 40, 50, and 60 iron are more likely to contain high residual stresses. The range from 480
to 600 °C (900 to 1100 °F) recommended. For best results, castings should be held at temperature 1 h or more and then
cooled in the furnace to below 450 °C (850 °F). High-strength and alloy gray irons require temperatures approaching 600
°C (1100 °F).
Machining Practice. In castings in the as-cast condition, residual tension and compression stresses are balanced, which
makes the castings dimensionally stable at room temperature. However, when part of the surface is removed in
machining, the balance of forces is altered, which can lead to distortion. If the casting is of relatively stiff section or has
properly designed stiffness ribs, there may be no noticeable change in dimensions. Distortion will be most evident in
castings of low stiffness from which a large volume of highly stressed metal has been removed.
Because the surface of a casting is often the principle site of residual stresses, a large proportion of the stress is relieved
by rough machining, with consequent maximum distortion. If, before final machining, the casting is relocated carefully
and properly supported in the machine tool fixtures, acceptable dimensional accuracy will usually be obtained in the
finished piece.
In designing fixtures, it must be recognized that the usual gray iron (class 30) has a modulus of elasticity of about 97 GPa
(14 × 106 psi), which means that deflection under tool bit loads will be approximately twice as great in a part made of this
cast iron as in a steel part having the same section.
It is difficult to make general statements about the dimensional stability that can be achieved in a gray iron casting
without stress relieving. However, it is well known that automotive engine blocks are taken directly from the foundry
without stress relieving and are machined to tolerances of ±0.005 mm (±0.0002 in.) in parts such as crankshaft bearings,
camshaft bearings, and cylinder bores. Therefore, if a casting is properly designed, is cast under controlled conditions,
and is allowed to cool sufficiently in the mold before shakeout, and if proper machining practice is followed, extremely
high dimensional stability can be obtained in many applications without stress relieving. On the other hand, it is
frequently economical to stress relieve complex castings (other than engine blocks) that are produced in small quantities
and that must be machined to precise dimensions.
References cited in this section
7. R.B. Gundlach, The Effects of Alloying Elements on the Elevated-Temperature Properties of Gray Irons,
Trans. AFS, 1983, p 389
14. J.E. Bevan, Effect of Molybdenum on Dimensional Stability and Tensile Properties of Pearlitic Gray Irons
at 600 to 850 °F (315 to 445 °C), Internal Report, Climax Molybdenum Company
17. J.H. Schaum, Stress Relief Heat Treatment of Gray Cast Iron, Trans. AFS, Vol 61, 1953, p 646-650
Effect of Shakeout Practice
Shakeout practice may be influential in establishing the patterns of residual stresses in castings, because most residual
stresses are basically caused by differences in cooling rate, and thus in contraction behavior, between light and heavy
sections.
In addition to its effect on residual stress, shakeout practice may influence both microstructure and hardness of gray iron
castings. If the iron is austenitic at the time the mold is dumped, higher hardness and residual stress may result. Many
different microstructures may be obtained; the austenite in thin sections may transform in the mold, whereas in heavier
sections, which cool more slowly, transformation may be delayed until air cooling after shakeout. In general, the effect of
shakeout practice on hardness is negligible in unalloyed irons. Alloy irons are the most sensitive. However, the
martensitic irons that contain enough alloying elements to reach full hardness with slow cooling in the mold will display
little if any effect of shakeout practice.
Alloying to Modify As-Cast Properties
The term alloying as used here does not include inoculation because by definition the effect of inoculation on the
mechanical properties of an iron is greater than can be explained by the change in chemical composition.
Strength and hardness, resistance to heat and oxidation, resistance to corrosion, electrical and magnetic characteristics,
and section sensitivity can be changed by alloying, which can extend the application of gray iron into fields where costlier
materials have traditionally been used.
There has also been considerable replacement of unalloyed gray iron by alloy iron to meet increased service demands and
to provide increased safety factors.
The use of alloy iron often depends, in practice, on the relative production requirements in a given foundry. When the
applications requiring alloy iron in a given plant are a small fraction of total production, manufacturing policy may dictate
the use of unalloyed iron for all castings in order to achieve maximum uniformity of production practice. Continuous
production of 450 to 1350 kg (1000 to 3000 lb) heats of alloy iron is usually needed for economical utilization.
Manganese, chromium, nickel, vanadium, and copper can also be used to strengthen cast irons. In many irons a
combination of elements will provide the greatest increase in strength.
To develop resistance to the softening effect of heat and protect against oxidation, chromium is the most effective
element. It stabilizes iron carbide and therefore prevents the breakdown of carbide at elevated temperatures; 1% Cr gives
adequate protection against oxidation up to about 760 °C (1400 °F) in many applications. For temperatures above 760 °C
(1400 °F), the chromium content should be greater than 15% for long-term protection against oxidation. This percentage
of chromium suppresses the formation of graphite and makes the alloy solidity as white cast iron.
For corrosion resistance, chromium, copper, and silicon are effective. Additions of 0.2 to 1.0% Cr decrease the corrosion
rate in seawater and weak acids. The corrosion resistance of iron to dilute acetic, sulfuric, and hydrochloric acids and to
acid mine water can be increased by the addition of 0.25 to 1.0% Cu. For sulfur and acid corrosion, 15 to 30% Cr is
effective. Silicon additions in the range of 14 to 15% give excellent corrosion resistance to sulfuric, nitric, and formic
acids; however, both high-chromium and high-silicon irons are white, and the high-silicon irons are extremely brittle.
The electrical and magnetic properties of cast iron can be modified slightly by minor additions of alloying elements, but a
major change in characteristics can be accomplished by the use of approximately 15% Ni or of nickel plus copper, which
results in an austenitic iron that is virtually nonmagnetic. The austenitic gray irons also have good resistance to oxidation
and growth at temperatures up to about 800 °C (1500 °F).
Molybdenum is an effective alloying addition for retaining strength in heavy sections. It is normally added in amounts of
0.5 to 1.0%, but the low end of this range applies chiefly when molybdenum is added in combination with other elements.
In the casting in thin sections, nickel is the most effective in combating the tendency to form chilled iron.
Base Irons. The selection of alloying elements to modify as-cast properties in gray iron depends to a large extent on the
composition and method of manufacture of the base iron. For example, a foundry producing a base iron containing 2.3%
Si and 3.4% total carbon for automotive castings might add 0.5 to 1.0% Cr if required to make heavier castings with the
same hardness and strength as the normal castings. However, a foundry producing a base iron with 1.7% Si and 3.1% C
for a heavy casting would add 0.5 to 0.8% Si to decrease hardness and chill when pouring this iron in light castings.
Depending on the strength desired in the final iron, the carbon equivalent of the base iron may vary from approximately
4.4% for weak irons to 3.0% for high-strength irons. The method of producing the base iron will affect mechanical
properties and the alloy additions to be made, because factors such as type and percentage of raw materials in the metal
charge, amount of superheat, and cooling rate of the iron after pouring all affect the properties. The base iron used for
alloying will vary considerably from foundry to foundry, as will the alloying elements selected to give the desired
mechanical properties. However, parts produced from different base irons and alloy additions can have the same
properties and performance in service.
Heat Treatment
Gray iron, like steel, can be hardened by rapid cooling or quenching from a suitable elevated temperature. The quenched
iron may be tempered by reheating in the range from 150 to 650 °C (300 to 1200 °F) to increase toughness and relieve
stresses. The quenching medium may be water, oil, hot salt, or air, depending on composition and section size. Heating
may be done in a furnace for hardening throughout the cross section, or it may be localized as by induction or flame so
that only the volume heated above the transformation temperature is hardened. In the range of composition of the most
commonly used unalloyed gray iron castings, that is, about 1.8 to 2.5% Si and 3.0 to 3.5% TC, the transformation range is
about 760 to 845 °C (1400 to 1550 °F). The higher temperature must be exceeded in order to harden the iron during
quenching. The proper temperature for hardening depends primarily on silicon content, not carbon content; silicon raises
the critical temperature.
During the heating of unalloyed gray iron for hardening, graphitization of the matrix frequently begins as the temperature
approaches 600 to 650 °C (1100 to 1200 °F) and may be entirely completed at a temperature of 730 to 760 °C (1350 to
1400 °F). This latter range is used for maximum softening.
Ordinarily, gray iron is furnace hardened from a temperature of 860 to 870 °C (1575 to 1600 °F). This results in a
combined carbon content of about 0.7% and a hardness of about 45 to 52 HRC (415 to 514 HB) in the as-quenched
condition. The actual hardness of the martensitic matrix is 62 to 67 HRC, but the presence of graphite causes a lower
indicated hardness . Temperatures much above this are not advisable because the as-quenched hardness will be
reduced by retained austenite.
Oil is the usual quenching medium for through hardening. Quenching in water may be too drastic and may cause cracking
and distortion unless the castings are massive and uniform in cross section. Hot oil and hot salt are sometimes used as
quenching media to minimize distortion and quench cracking. Water is often used for quenching with flame or induction
hardening if only the outer surface is to be hardened.
Hardenability of unalloyed gray iron is about equal to that of low-alloy steel. Hardenability can be measured using the
standard end-quench hardenability test employed for steels. The hardenability of cast iron is increased by the addition of
chromium, molybdenum, or nickel. Gray iron can be made air hardenable by the addition of the proper amounts of these
elements.
Mechanical Properties. As-quenched gray iron is brittle. Tempering after quenching improves strength and toughness
but decreases hardness. A temperature of about 370 °C (700 °F) is required before the toughness (impact strength)
approaches the as-cast level. The tensile strength after tempering may be from 35 to 45% greater than the strength of the
as-cast iron.
Heat treatment is not ordinarily used commercially to increase the overall strength of gray iron castings, because the
strength of the as-cast metal can be increased at less cost by reducing the silicon and total carbon contents or by adding
alloying elements. When gray iron is quenched and tempered, it usually is done to increase resistance to wear and
abrasion by increasing hardness. A structure consisting of graphite embedded in a hard martensitic matrix is produced by
heat treatment. Localized heat treatment such as flame or induction hardening can be used in some applications in which
alloy iron or chilled iron has traditionally been used, often with a savings in cost. Heat treatment can be used when
chilling is not feasible, as with complicated shapes or large castings, or when close tolerances that can be attained only by
machining are required. Heat treatment extends the field of application of gray iron as an engineering material.
The hardness of quenched and tempered gray iron measured with either a Brinell or Rockwell C tester is a composite
hardness that superimposes the effects of graphite type, distribution, and size on the hardness of the metal matrix. The
true hardness of the matrix, measured with a microhardness test, is generally from 8 to 10 HRC points higher than the
hardness indicated by the conventional Rockwell C test. The composite hardness was obtained by conventional Rockwell testing; the matrix hardness was
measured with a Vickers indentor and a 200 g load, and the values were converted to equivalent Rockwell hardness. The
hardness of the matrix and the change in matrix hardness with tempering temperature are about the same as in an alloy
steel containing approximately 0.7% C.
After tempering at 370 °C (700 °F) for maximum toughness, the hardness of the metal matrix is still about 50 HRC.
Where toughness is not required and a tempering temperature of 150 to 260 °C (300 to 500 °F) is acceptable, the matrix
hardness is equivalent to 55 to 60 HRC. High matrix hardness and the presence of graphite result in a surface with good
wear resistance for some applications, for instance, farm implement gears, sprockets, diesel cylinder liners, and
automotive camshafts.
Dimensional changes resulting from the hardening of gray iron are quite uniform and predictable if the prior structure and
composition are uniform. Such dimensional changes can be allowed for in machining before heat treatment.
Complex shapes or nonuniform sections may be distorted as a result of the relief of residual casting stresses or a
differential rate of cooling and hardening during quenching, or both. The former condition can be minimized by stress
relieving at about 565 to 590 °C (1050 to 1100 °F) before machining. The latter can be minimized by either
marquenching or austempering; both of these processes are used for cylinder liners where out-of-roundness must be held
to a minimum. Through hardening is employed for gears, sprockets, hub bearings, and clutches.
Localized Hardening. In parts requiring only localized areas of hardness, conventional induction hardening or flame
hardening may be used.
For flame hardening, it is generally desirable to alloy the iron with small amounts of either chromium or molybdenum to
stabilize the iron carbide and thus prevent graphitization. Also, adequate localized hardening is enhanced when the
structure contains little or no free ferrite prior to hardening. Compared to a pearlitic matrix, it may take from two to four
times as long at temperature to condition a ferritic matrix for hardening. Even though the diffusion of carbon into
austenite from the adjacent graphite is quite rapid, the attainment of a carbon concentration that can produce full hardness
may require 1 to 10 min (or more) for a previously ferritic matrix, depending on ferrite composition, austenitizing
temperature, and graphite distribution and spacing.
Water may be used as the quenching medium when the depth of hardening is shallow and the part is progressively heated
and quenched. When only small areas are being hardened, the parts may be dropped into an oil quench. Localized
hardening has been used for camshafts, gears, sprockets, and cylinder liners.
Reference cited in this section
18. G.A. Timmons, V.A. Crosby, and A.J. Herzig, Trans. AFS, Vol 49, 1941, p 397
Physical Properties
Patternmakers' rules (shrink rules) allow 1% linear contraction upon solidification and cooling of gray iron. (The
comparable allowances for other cast ferrous metals are 0% for annealed ductile iron, 0.7% for ascast ductile iron, 1 1
2
to
2% for white iron, 2% for cast carbon steel, and 2 1
2
% for cast alloy steel.) Often, in practice this allowance is inaccurate
necessitating more exact evaluation of the effects of mass, composition, shape, mold, and core. Generally, contraction is
less with an increase in mass and in gray irons increases with increasing tensile strength.
Three contraction phenomena are exhibited during the cooling of a cast iron from the molten state to room temperature: in
the liquid, contraction of about 2.8% from 1425 °C (2600 °F) to the liquidus at about 1200 °C (2200 °F); contraction
during solidification; and subsequent to solidification, contraction of about 3.0% from 1150 °C (2100 °F) to room
temperature.
Shrinkage in volume during solidification ranges from negative shrinkage in "soft" irons to +1.94% in an iron containing
about 0.90% combined carbon. The white irons undergo 4.0 to 5.5% contraction in volume within the same temperature
range.
Coefficient of thermal expansion of gray irons is about 13 μm/m · °C (7.2 μin./in. · °F) in the range from 0 to 500
°C (32 to 930 °F) and about 10.5 μm/m · °C (5.8 μin./in. · °F) in the range from 0 to 100 °C (32 to 212 °F). The
coefficient of thermal expansion is highly dependent on the matrix structure. The coefficients of thermal expansion of
ferritic and martensitic irons are slightly higher than those of pearlitic irons.
For the temperature range from 1070 °C (1960 °F) to room temperature, the coefficient of thermal expansion varies from
9.2 to 16.9 μm/m · °C (5.1 to 9.4 μin./in. · °F). At about room temperature, the commonly used figure of 10 m/m · °C
(5.5 μin./in. · °F) is accurate enough for moderate changes in temperature.
Density of gray irons at room temperature varies from about 6.95 Mg/m3 for open-grained high-carbon irons to 7.35
Mg/m3 for close-grained low-carbon irons. The density of white iron is about 7.70 Mg/m3.
Electrical and Magnetic Properties. The resistivity of gray iron, compared with that of other ferrous metals, is
relatively high, apparently because of the amounts and distribution of the graphite. Increases in total carbon content and in
silicon content increase resistivity.
The magnetic properties of gray iron may vary within wide limits, ranging from those of irons having low permeability
and high coercive force (suitable for permanent magnets) to those of irons having high permeability, low coercive force,
and low hysteresis loss (suitable for electrical machinery).
The highest magnetic induction and permeability are found in annealed white irons, such as malleable cast iron. Flake
graphite, as in gray iron, does not affect hysteresis loss, but prevents the attainment of high magnetic induction by causing
small demagnetizing forces.
Damping capacity is the capability of a material to quell vibrations and to dissipate the energy as heat, or simply the
relative capability to stop vibrations or ringing. Because gray iron possesses high damping capacity, it is well suited for
bases and supports, as well as for moving parts. It reduces or eliminates parasitic vibration. The noise level of a machine
operating on a gray iron base is materially reduced. High damping capacity is especially desirable in structures and parts
in which vibration can cause stresses in excess of those that result from direct loading.
Reference cited in this section
19. C.F. Walton, Gray and Ductile Iron Castings Handbook, Iron Founders' Society, 1971
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