High-Alloy White Irons
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Abstract:
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.
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 abrasionresistant
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.
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 to 4.
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.