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TitleTechnology of Ni-Hard Cast Iron
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High-Alloy Mineral Processing White Irons

Abstract

High-alloy white cast irons are a specific group of materials whose
production must be considered separately from that of ordinary type s 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 ot herwise standard compositions.
They are usually produced in foundries specially equipped to produce high alloy irons.
intMPE inc. pours on the average of more than 300 tons of white metal per month
with a single cast size ability of 10 tons with a 21 inch thick cross section.

intMPE products that are produced with th ese materials are in conformance to
standards set for th in this paper. Our chemical analysis is “state of the art” with
capabili ties of monitor ing more than 35 elements at the same time; from this, we
provide our customers with physical and test coupons.

The high-alloy white irons are prima rily used for abrasion-resistant applications
and are readily cast into parts needed in machinery for crushing, grind ing, and handling
of abrasive materials.

The chromium content of high-a lloy white irons enhances their corrosion-
resistant properties. The large volume fraction of prima ry and/or eutectic carbides in
their mic ro struct ures provides the hardness needed for crushing and grindi ng other
materials. The metall ic matrix support ing the carbide phase in these irons can be
adjusted by alloy content and heat t reatment to develop the proper balance between the
resistance to abrasion and the toughness needed to withstand repeated impact.

Whi le low-alloy white iron castings, which have an alloy content below 4%,
develop hardness in the range of 350 to 550 HB, the high alloy irons range in hardness
from 450 to 800 HB. Specifi cation ASTM A532 covers the composition and hardness of
the abrasion-resistant white iron grade s. Many castings are ordered according to these
specifi cations, but a large number of castings are produced with composition
modific ations for specific applications. The designer, metallu rgist, and foundry man
must 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 t wo major groups:

1) Nickle -Chromium white irons - Low-Chromium alloys containing
3 - 5% Ni and 1 - 4% Cr, with one alloy modifica tion that contains

7 - 11% Cr.

2) Chromium-Molybdenum irons - 11 - 23% Cr, up to 3% Mo and
often additionally alloyed with nickel or copp er.

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A third group comprises the 25% or 28% Cr white irons, which may contain other
alloying additions of molybdenum and/or nickel up to 1.5%. The nickel-chromium irons
are also commonly identified as Ni-Hard types 1 - 4.

The nicke l-chromium white i rons or Ni-Hard irons , are the oldest group
of high-alloy irons of industrial importance, and have been produced for more than 50
years. The Ni-Hard irons have proven to be very cost effective materials that are used
for crushing and grinding.

In these martensitic white irons, nickel is the primary alloying element. Nickel, at
levels of 3 - 5%, is effective in suppressing the transformation of the austenite matrix to
pearlite. This ensures 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 - 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. 4 types of nickel-chromium white iron alloy are listed below:

a) Cl ass I ty pe A - Abrasion resistance is the general function of the bulk
hardness and the volume of carbide in the microstructure. When this resistance
is the principal requirement, and resistance to impact loading is secondary, alloys
having high carbon contents, ASTM A532 class I type A (Ni-Hard 1) are
recommended.

b) Cl ass I ty pe B - In repeated impact, the lower carbon alloys, class I type B
(Ni-Hard 2) are recommended because of the existence of less carbide, and
therefore, greater toughness.

c) Class J ty pe C - This special grade, that has a nickel- chromium alloy
composition, is used for chill casting, specialized sand casting processes, and
producing grinding balls and slugs.

d) Class I type D - Ni-Hard4 alloy is a modified nickel-chromium iron that
contains higher levels of chromium, ranging from 7 - 11%, and increased levels of
nickel, ranging from 5 - 7%. Carbon is varied according to the properities needed
for the intended service.

For example:

1) Carbon content in the range of 3.2% - 3.6% is used when maximum
abrasion resistance is desired

2) Carbon content in the range of 2.7% - 3.2% is used when impact loading
is expected.

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Nickel content increases with section size or cooling time of the casting to inhibit
pearli tic transformation. It i s important to limi t ni ckel content to the level needed for
control of pearlite; excess nickel increases the amount of retained austenite and lowers
hardness. For castings of 38 - 50 mm thic k, 3.4 - 4.2% Ni is sufficient to suppress
pearli te formation upon mold cooling. Heavier sections may requir e nickel levels up to
5.5% to avoid the formation of pearlite.

Silicon is needed for several reasons:

1) A minimum amount of silicon is necessary to improve flu idi ty of the melt and
to produce a fluid slag, plus it has an effect on as-cast hardness.

2) Increased levels of sil icon, in the range of 1 - 1.5% increases the amount of
martensite and the resulting hardness.

3) Late additions of ferrosilic on - 0.2% as 75% Si grade ferrosilicon- i ncreases
toughness.

4) Hi gher silicon contents promotes pearlite and may increase nickel
requirements.

Chromium is used to offset the graphitizi ng effects of nickel and silicon i n types
A, B, C alloys, and in ranges from 1.4 - 3.5%. As the section size increases, the chromium
content must increase. In type D alloy, chromium levels range from 7 - 11% (usually 9%)
for t he purpose of producing eutectic carbides of the M7C3 chromium carbi de 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 A532 specifications. While it provide s increased harden
abili ty to avoid pearli te formation, it is a more potent austenite stabili zer than nickel,
plus it promot es 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 gi ven casting, the level of manganese present
should be a factor.

Molybdenum is a potent harden ability agent in these alloys and is used in
heavy section castings to augment harden ability and inhibi t pearlite.

High-Chromium White Irons

The high-chromium white irons have excellent abrasion resistance and they must
also be able to wit hstand heavy impact loading. These alloyed white irons are
recognized as providing the best combination of t oughness and abrasion resistance
attainable among the white cast irons.

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