High-alloy cast irons are an important group of materials whose production
should be considered separately from that of the ordinary types of cast irons. In
these cast iron alloys, alloy content is well above 4% and, consequently, they cannot
be produced by ladle additions to irons of otherwise standard compositions. The
producing foundries usually have the equipment needed to handle the heat treatment
and other thermal processing unique to the production of these alloys.
The cast iron alloys discussed in this article are alloyed for increased abrasion
resistance, for strength and oxidation resistance at elevated temperatures, and for
improved corrosion resistance. They include the high-alloy graphitic irons and the
high-alloy white irons.
The heat treatment practices for the following high-alloy graphitic irons are described:
- Austenitic gray and ductile irons
- High-silicon irons for heat resisting applications
- High-silicon irons for corrosion resisting applications
The high-alloy graphitic cast irons have found special use primarily in applications
requiring (1) corrosion resistance or (2) strength and oxidation resistance in
high-temperature service. Those alloys used in applications requiring corrosion
resistance comprise the nickel-alloyed (13 to 36% Ni
) gray and ductile
irons, and the high-silicon (14.5% Si
) gray irons.
The alloyed irons produced for high-temperature service comprise the austenitic,
nickel-alloyed gray and nodular irons, the high-silicon (4 to 6% Si)
gray and nodular irons and the aluminum-alloyed gray and nodular irons. Two groups
of aluminum-alloyed irons are recognized: the 1 to 7% Al irons and
the 18 to 25% Al irons.
Austenitic Nickel-Alloyed Graphitic Irons
These nickel-alloyed austenitic irons have found usefulness in applications requiring
corrosion resistance, wear resistance, and high-temperature stability and strength.
Additional properties of benefit are low thermal expansion coefficients, nonmagnetic
properties, and cast iron materials having good toughness at low temperatures.
The procedures and temperatures of the heat treatments for these ductile irons with
nodular graphite are similar to those for gray (flake-graphite), corrosion-resistant
austenitic cast irons.
ASTM Specification A 436 defines eight grades of austenitic gray iron alloys, four
of which are designed to be used in elevated-temperature applications and four
types are used in applications requiring corrosion resistance.
The ASTM Specification A 439 defines the group of austenitic ductile irons. There
are nine alloys listed in the specification. The austenitic ductile iron alloys have
similar compositions to the austenitic gray iron alloys but have been treated with
magnesium to produce nodular graphite. The ductile iron alloys have high strength
and ductility combined with the same desirable properties of the gray iron alloys.
They provide resistance to frictional wear, corrosion resistance, strength and
oxidation resistance at high temperatures, nonmagnetic characteristics and, in some
alloys, low thermal expansivity at ambient temperatures.
Heat Treatment of Austenitic Ductile Irons. Heat treatment of the
nickel-alloyed austenitic irons serves to reduce residual stresses and to stabilize
the microstructure for increased durability. Heat treatments are similar with the
graphite in nodular form (ductile iron) or flake form (gray iron).
Stress Relieving. For most applications, it is recommended that
austenitic cast irons be stress relieved at 620 to 675°C (1150 to 1250°F), for
1 h per 25 mm (1 in.) of section, to remove residual stresses resulting from casting
or machining, or both. Stress relieving should follow rough machining, particularly
for castings that must conform to close dimensional tolerances, that have been
extensively welded, or that are to be exposed to high stresses in service. Stress
relieving does not affect tensile strength, hardness, or ductility. For large,
relatively thin-section castings, mold-cooling to below 315°C (600°F) is
recommended rather than stress relief heat treatment.
Spheroidize Annealing. Castings with hardness above 190 HB may be
softened by heating to 980 to 1040°C (1800 to 1900°F) for 1/2 to 5 h except
those alloys containing 4% or more chromium. Excessive carbides cause this high
hardness and may occur in rapidly cooled castings and thin sections. Annealing
dissolves or spheroidizes carbides. Although it lowers hardness, spheroidize
annealing does not adversely affect strength.
High-Temperature Stabilization. This treatment consists of holding
at 760°C (1400°F) for 4 h minimum or at 870°C (1600°F) for 2 h
minimum, furnace cooling to 540°C (1000°F), and then cooling in air. This
treatment stabilizes the microstructure and minimizes growth and warpage in service.
The treatment is designed to reduce carbon levels in the matrix and some growth and
distortion often accompanies heat treatment. Thus, it is usually advisable to stabilize
castings prior to final machining.
Dimensional Stabilization. This treatment normally is limited to
castings that require true dimensional stability, such as those used in precision
machinery or scientific instruments. The treatment is not applicable to castings of
type I alloys. Other alloys may be dimensionally stabilized by the following
- Heat to 870°C (1600°F), and hold for 2 h minimum plus 1 h per 25 mm
(1 in.) of section
- Furnace cool, at a maximum rate of 50°C/h (100°F/h), to 540°C
- Hold at 540°C (1000°F) for 1 h per 25 mm (1 in.) of section, and then
cool uniformly in air
- After rough machining, reheat to 455 to 480°C (850 to 900°F) and hold
for 1 h per 25 mm (1 in.) of section, and cool uniformly in air
- Finish machine and reheat to 260 to 315°C (500 to 600°F), and cool
uniformly in air.
Although this treatment is seldom used, quenching
from high temperatures is capable of producing higher-than-normal strength levels and
slightly higher hardnesses by dissolving some carbon in austenite at elevated
temperatures and by preventing precipitation of the carbon by rapid cooling. This
treatment consists of heating to 925 to 1010°C (1700 to 1850°F) and quenching
in oil or water. Because no metallurgical phase change occurs, the possibility of
cracking is lessened.
High-Silicon Irons for High-Temperature Service
Graphitic irons alloyed with from 4 to 6% Si
have provided good
service, and low cost, in many elevated-temperature applications. These irons,
whether gray or nodular, provide good oxidation resistance and stable ferritic matrix
structures that will not go through a phase change at temperatures up to 815°C.
The elevated silicon content of these otherwise normal cast iron alloys reduces the
rate of oxidation at elevated temperatures, because it promotes the formation of a
dense, adherent film at the surface, which consists of iron silicate rather than iron
oxide. This layer is much more resistant to oxygen penetration and its effectiveness
improves with increasing silicon content.
High-Silicon Nodular Irons. The advent of ductile iron led to the
development of high-silicon nodular irons, which currently represent the greatest
tonnage of these types of irons being produced. Converting the eutectic flake graphite
network to isolated graphite nodules further improved resistance to oxidation and
growth. The higher strength and ductility of the nodular iron versions of these alloys
qualifies them for more rigorous service.
The high-silicon nodular iron alloys are designed to extend the upper end of the
range of service temperatures viable for ferritic nodular irons. These irons are used
to temperatures of 900°C. At 5 to 6% Si, oxidation resistance is
improved and critical temperature is increased, but the iron can be very brittle at
room temperature. For most applications, alloying with 0 to 1% Mo
provides adequate strength at elevated temperatures and creep resistance.
The high-silicon gray and nodular irons are predominantly, ferritic as-cast, but the
presence of carbide stabilizing elements will result in a certain amount of pearlite
and often intercellular carbides. These alloys are inherently more brittle than
standard grades of iron and usually have higher levels of internal stress due to
lower thermal conductivity and higher elevated-temperature strength. These factors
should be taken into account where deciding on heat treatment requirements.
For the high-silicon nodular irons, high-temperature heat treatment is advised in all
cases to anneal any pearlite and stabilize the casting against growth in service. A
normal graphitizing (full) anneal in the austenitic temperature range is recommended
where undesirable amounts of carbide are present.
For the 4 to 5% Si irons this will require heating to at least
900°C (1650°F) for several hours, followed by slow cooling to below
700°C (1300°F). At higher silicon contents (>5%), in which carbides
readily break down, and in castings relatively carbide-free, subcritical annealing
in the temperature range 720 to 790°C (1325 to 1450°F) for 4 h is effective in
ferritizing the matrix. Compared to full annealing, the subcritically annealed material
will have somewhat higher strength, but ductility and toughness will be reduced.
High-Silicon Irons for Corrosion Resistance. Irons with high silicon
content (14.5% Si) comprise a unique corrosion-resistant ferritic
cast iron group. These alloys are widely used in the chemical industry for processing
and for transporting highly corrosive liquids. The most common of the high-silicon
iron alloys are covered in ASTM Specification A 518M.
Because of the very brittle nature of high-silicon cast iron, castings are usually
shaken out only after mold cooling to ambient temperature. However, some casting
geometries demand hot shakeout so that the castings can be immediately
stress-relieved and furnace cooled to prevent cracking.
Castings are stress relieved by heating in the range of 870 to 900°C (1600 to
1650°F) followed by slow cooling to ambient temperatures to minimize the
likelihood of cracking. Heat treatments have no significant effect on corrosion