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. 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 high-alloy white irons are primarily used for abrasion-resistant applications
and are readily cast in the shapes needed in machinery used for crushing, grinding,
and general handling of abrasive materials. The large volume of eutectic carbides in
their microstructures provides the high hardness needed for crushing and grinding
other materials. The metallic matrix supporting the carbide phase in these irons
can be adjusted by alloy content and heat treatment to develop the proper balance
between resistance to abrasion and the toughness needed to withstand repeated
All high-alloy white irons contain chromium to prevent formation of graphite on
solidification and to ensure the stability of the carbide phase. Most also contain
nickel, molybdenum, copper, or combinations of these alloying elements to prevent
the formation of pearlite in the microstructure. While low-alloyed pearlitic white
iron castings develop hardness in the range 350 to 550 HB, the high-alloyed white
irons range from 450 to 800 HB.
ASTM Specification A 532 covers the composition and hardness of white iron grades
used for abrasion-resistant applications. Many castings are ordered according to
these specifications: however, a large number of castings are produced with
modifications to composition for specific applications. It is most desirable
that the designer, metallurgist, and foundry-man work together to specify the
composition, heat treatment, and foundry practice to develop the most suitable
alloy and casting design for a specific application.
The high-alloy white cast irons fall into three major groups:
- The Ni-Cr white irons, which are low-chromium alloys containing 3 to 5%
Ni and 1 to 4% Cr with one alloy modification which
contains 7 to 11% Cr.
- The chromium-molybdenum irons containing 11 to 23% Cr, up to 3%
Mo, and often additionally alloyed with nickel or copper.
- The 25% Cr or 28% Cr white irons, which may
contain other alloying additions of molybdenum and/or nickel up to 1.5%
Nickel-Chromium White Irons
One of the oldest groups of high-alloy irons of industrial importance, the Ni-Cr white
irons, or Ni-Hard irons, have been produced for more than 50 years and are very
cost-effective materials for crushing and grinding.
In these martensitic white irons, nickel is the primary alloying element because at
levels of 3 to 5% it is effective in suppressing the transformation of the austenite
matrix to pearlite, and thus ensuring that a hard, martensitic structure will develop on
cooling in the mold. Chromium is included in these alloys, at levels from 1.4 to 4%
to ensure that the irons will solidify with carbides to counteract the graphitizing
effect of nickel. The optimum composition of the Ni-Cr white iron alloy depends on
the properties required for the service conditions and the dimensions and weight of
the casting. Abrasion resistance is generally a function of the bulk hardness and
the volume of carbide in Cr-Mo iron.
Carbon is varied according to properties needed for the intended service. Carbon
contents in the range of 3.2 to 3.6% are prescribed when maximum abrasion resistance
is desired. Where impact loading is present, carbon content should be held in the
range of 2.7 to 3.2%.
Silicon is needed for two reasons. A minimum amount of silicon is necessary to
improve fluidity and produce a fluid slag. But of equal importance is its effect
on as-cast hardness. Increased levels of silicon, in the range of 1 to 1.5%, have
been found to increase the amount of martensite and the resulting hardness. Late
additions of ferrosilicon have been reported to increase toughness. Note that
higher silicon contents can promote pearlite and may increase the nickel
Manganese is usually held to 0.8% max. While it provides increased hardenability to
avoid pearlite formation, it is also a potent austenite stabilizer, more so than
nickel, and will promote increased amounts of retained austenite and lower as-cast
hardness. For this reason higher manganese levels are undesirable. In considering
the nickel content required to avoid pearlite in a given casting, the level of
manganese present should be a factor.
Copper increases hardenability and the retention of austenite and, therefore,
must be controlled for the same reason manganese is limited. Copper should be
treated as a nickel substitute and, when properly included in the calculation of
the amount of nickel required to inhibit pearlite in a given casting, it reduces
the nickel requirement. Molybdenum is a potent hardenability agent in these alloys
and is used in heavy section castings to augment hardenability and inhibit
Heat Treatment or Nickel-Chromium White Irons. Nickel-chromium white
iron castings are given a stress-relief heat treatment because, properly made, they have
a martensitic matrix structure, as-cast. Tempering is performed between 205 to 260°C
(400 to 450°F) for at least 4 h. This tempers the martensite, relieves some of
the transformation stresses, and increases the strength and impact toughness by
50 to 80%. Some additional martensite may form on cooling from the tempering temperature.
This heat treatment does not reduce hardness or abrasion resistance.
In the heat treatment of any white cast iron, care must be taken to avoid cracking by
thermal shock; never place the castings in a hot furnace or otherwise subject them to
rapid heating or cooling. The risk of cracking increases with the complexity of the
casting shape and section thickness.
An austenitizing heat treatment usually comprised heating at temperatures between
750 and 790°C (1380 and 1450°F) with a soak time of 8 h. Air or furnace cooling,
not over 30°C/h, was conducted followed by a tempering/stress-relief heat treatment.
Refrigeration heat treatment is the more commonly practiced remedy for low hardness
High-Chromium White Irons
The oldest high-alloy white irons produced commercially were the high-chromium
) white irons. The high-chromium white irons have excellent
abrasion resistance and are used effectively in slurry pumps, brick molds,
coal-grinding mills, rolling mill rolls, shot blasting equipment, and components for
quarrying, hard-rock mining and milling. In some applications they must also be able
to withstand heavy impact loading.
These alloyed white irons are recognized as providing the best combination of toughness
and abrasion resistance attainable among the white cast irons. Through variations in
composition and heat treatment these properties can be adjusted to meet the needs of
most abrasive applications.
Special High-Chromium Iron Alloys for Corrosion Resistance. Alloys with
improved resistance to corrosion, for applications such as pumps handling, are produced
with high chromium contents (26 to 28% Cr) and low carbon contents
(1.6 to 2.0% C). These high-chromium, low-carbon irons will provide
the maximum chromium content in the matrix. Addition of 2% Mo is
recommended for improving resistance to chloride-containing environments. Chromium
causes the formation of an adherent, complex, chromium-rich oxide film providing
resistance to scaling at temperatures up to 1040°C (1900°F).
The high-chromium irons designated for use at elevated temperatures fall into
one of three categories, depending upon the matrix structure:
- The martensitic irons alloyed with 12 to 28% Cr
- The ferritic irons alloyed with 30 to 34% Cr
- The austenitic irons which in addition to containing 15 to 30% Cr
also contain 10 to 15% Ni to stabilize the austenite phase
Carbon contents of these alloys range from 1 to 2%.
Optimum performance is usually achieved with heat treated martensitic structures. As
described in the previous section, alloying must be sufficient to ensure that a
pearlite-free microstructure is obtained in heat treatment. Of necessity, the heat
treatment requires an air quench from the austenitizing temperature. Faster cooling
rates should not be used, because the casting can develop cracks due to high thermal
and/or transformation stresses. Thus the alloy must have sufficient hardenability to
allow air hardening. Over-alloying with manganese, nickel, and copper will promote
retained austenite, which detracts from resistance to abrasion and spalling.
Austenitization. There is an optimum austenitizing temperature to
achieve maximum hardness, which varies for each composition. The austenitizing
temperature determines the amount of carbon that remains in solution in the austenite
matrix. Too high a temperature increases the stability of the austenite, and the higher
retained austenite content reduces hardness. Low temperatures result in low-carbon
martensite reducing both hardness and abrasion resistance. Class II irons containing
12 to 20% Cr are austenitized in the temperature range 950 to
1010°C (1750 to 1850°F). Class III irons containing 23 to 28% Cr
are austenitized in the temperature range 1010 to 1090°C (1850 to 2000°F).
Quenching. Air quenching (vigorous fan cooling) the castings from the
austenitizing temperature to below the pearlite temperature range (that is, between 550
and 600°C, or 1020 and 1110°F) is highly recommended. The subsequent cooling
rate should be substantially reduced to minimize stresses; still-air or even furnace
cooling to ambient is common. Complex and heavy section castings are often placed back
into the furnace, which is at 550 to 600°C, and allowed sufficient time to reach
uniform temperature within the casting. After temperature is equalized, the castings are
either furnace or still-air cooled to ambient temperature.
Tempering. Castings can be put into service in the hardened (as cooled)
condition without further tempering or subcritical heat treatments; however, tempering
in the range of 200 to 230°C (400 to 450°F) for 2 to 4 h is recommended to
restore some toughness in the martensitic matrix and to further relieve residual
Sub critical Heat Treatment. Sub critical heat treatment (tempering)
is sometimes performed, particularly in large heat-treated martensitic castings, to
reduce retained austenite contents and increase resistance to spalling. The tempering
parameters necessary to eliminate retained austenite are very sensitive to time and
temperature and vary depending on the castings composition and prior thermal history.
Typical tempering temperatures range from 480 to 540°C (900 to 1000°F) and times
range from 8 to 12 h. Excess time or temperature results in softening and a drastic
reduction in abrasion resistance.
Annealing. Castings can be annealed to make them more machinable,
either by sub-critical annealing or a full anneal. Subcritical annealing is accomplished
by pearlitizing, via soaking in the narrow range between 690 and 705°C for from 4 to
12 h, which will produce hardness in the range 400 to 450 HB. Lower hardness can often
be achieved with full annealing, whereby castings are heated in the range 955 to
1010°C followed by slow cooling to 760°C and holding at this temperature for
10 to 50 h depending on composition.
Stress-Relieving. Very little information is available on the amount of
stress relief that occurs with tempering. The predominant stresses present in
heat-treated castings develop as a result of the volume change accompanying austenite
to martensite transformation. Low-temperature tempering, in the range of 200 to
230°C, is particularly desirable because a substantial improvement (20%) in
fracture toughness occurs when tempering the martens lie phase. Tempering at
temperatures sufficient to significantly relieve stresses, that is, above 540°C, will
substantially reduce abrasion resistance.