High-alloy white cast irons are an important group of
materials whose production must be considered separately from
that of ordinary types of cast irons. In these cast iron
alloys, the alloy content is well above 4%, and consequently
they cannot be produced by ladle additions to irons of
otherwise standard compositions. They are usually produced in
foundries specially equipped to produce highly alloyed irons.
The high-alloy white irons are primarily used for abrasion-resistant
applications and are readily cast into the parts needed in
machinery for crushing, grinding, and handling of abrasive
materials. The chromium content of high-alloy white irons
also enhances their corrosion-resistant properties. The large
volume fraction of primary and/or 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 the resistance to abrasion and the toughness needed
to withstand repeated impact.
While low-alloy white iron castings, which have alloy content
below 4%, develop hardnesses in the range of 350 to 550 HB,
the high-alloy irons range in hardness is from 450 to 800 HB.
Specification ASTM A 532 covers the composition and
hardness of the abrasion-resistant white iron grades. Many
castings are ordered according to these specifications.
However, a large number of castings are produced with
composition modifications 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 two major groups:
- Nickel-chromium white irons, which are low-chromium
alloys containing 3 to 5% Ni and 1 to 4% Cr, with one alloy
modification that contains 7 to 11% Cr,
- Chromium-molybdenum irons containing 11 to 23% Cr,
up to 3% Mo and often additionally alloyed with nickel
or copper.
Nickel-Chromium White Irons
The oldest group of high-alloy irons of industrial importance,
the nickel-chromium 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, thus ensuring that a hard martensitic structure
(usually containing significant amounts of retained
austenite) will develop upon cooling in the mold. Chromium
is included in these alloys, at levels from 1.4 to 4%, to
ensure that the irons solidify carbidic, that is, to
counteract the graphitizing effect of nickel.
The optimum composition of a nickel-chromium white iron alloy
depends on the properties required for the service conditions
and the dimensions and weight of the casting. Abrasion
resistance is generally function of the bulk hardness and the
volume of carbide in the microstructure. When abrasion
resistance is the principal requirement and resistance to
impact loading is secondary, alloys having high carbon
contents, ASTM A 532 class I type A (Ni-Hard 1), are
recommended. When conditions of repeated impact are
anticipated, the lower carbon alloys, class I type B (
Ni-Hard 2) are recommended because they have less carbide
and, therefore, greater toughness. A special grade, class
J type C, has been developed for producing grinding balls
and slugs. Here, the nickel-chromium alloy composition has
been adapted for chill casting and specialized sand casting
processes.
The Class I type D (Ni-Hard 4) alloy is a modified
nickel-chromium iron that contains higher levels of chromium,
ranging from 7 to 11%, and increased levels of nickel,
ranging from 5 to 7%. Carbon is varied according to the
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. When impact loading is
expected, carbon content should be held in the range of 2.7
to 3.2%.
Nickel content increases with section size or cooling time of
the casting to inhibit pearlitic transformation. For castings
of 38 to 50 mm thick, 3.4 to 4.2% Ni is sufficient to
suppress pearlite formation upon mold cooling. Heavier
sections may require nickel levels up to 5.5% to avoid the
formation of pearlite. It is important to limit nickel
content to the level needed for control of pearlite; excess
nickel increases the amount of retained austenite and lowers
hardness.
Silicon is needed for two reasons. A minimum amount of
silicon is necessary to improve fluidity of the melt and to
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 (0.2% as 75% Si grade ferrosilicon) have been
reported to increase toughness. It is important to note that
higher silicon contents can promote pearlite and may
increase the nickel requirement.
Chromium is primarily added to offset the graphitizing
effects of nickel and silicon in types A, B,
and C alloys, ranges from 1.4 to 3.5%. Chromium
content must be increased with increasing section size. In
type D alloy, chromium levels range from 7 to 11%
(typically 9%) for the purpose of producing eutectic
carbides of the M7C3 chromium
carbide type, which are harder and less deleterious to
toughness.
Manganese is typically held to a maximum of 0.8% even though
1.3% maximum is allowed according to ASTM A 532
specification. While it provides increased harden-ability to
avoid pearlite formation, it is a more potent austenite
stabilizer than nickel, and promotes increased amounts of
retained austenite and lower as-cast hardness. For this
reason, higher manganese levels are undesirable. When
considering the nickel content required to avoid pearlite in
a given casting, the level of manganese present should be a
factor.
Copper increases both hardenability and the retention of
austenite and therefore must be controlled for the same
reason that manganese must be 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, 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 pearlite.
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, 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.
In the high-chromium irons, as with most abrasion-resistant
materials, there is a trade-off between wear resistance and
toughness. By varying composition and heat treatment, these properties can be adjusted to meet the needs of most abrasive applications.
Specification ASTM A 532 covers the compositions and
hardnesses of two general classes of the high-chromium
irons. The chromium-molybdenum irons (Class II of ASTM A532)
contain 11 to 23% Cr and up to 3.5% Mo and can
be supplied either as-cast with an austenitic or
austenitic-martensitic matrix, or heat-treated with a
martensitic matrix microstructure for maximum abrasion
resistance and toughness. They are usually considered the
hardest of all grades of white cast irons. Compared to the
lower-alloy nickel-chromium white irons, the eutectic
carbides are harder and can be heat-treated to achieve
castings of higher hardness. Molybdenum, as well as nickel
and copper when needed, is added to prevent pearlite and to
ensure maximum hardness.
The high-chromium irons (class III of ASTM A 532) represent
the oldest grade of high-chromium irons, with the earliest
patents dating back to 1917. These general-purpose irons,
also called 25% Cr and 28% Cr irons, contain 23
to 28% Cr with up to 1.5% Mo. To prevent
pearlite and attain maximum hardness, molybdenum is added in
all but the lightest-cast sections. Alloying with nickel and
copper up to 1% is also practiced. Although the maximum
attainable hardness is not as high as in the class II
chromium-molybdenum white irons, these alloys are selected
when resistance to corrosion is also desired.
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