Cast high alloy steels are widely used for their corrosion
resistance in aqueous media at or near room temperature and
for service in hot gases and liquids at elevated and high temperatures
(> 650°C). High-alloy cast steels are most often
specified on the basis of composition using the designation
system, which has been replaced by the Alloy Casting
Institute (ACI), which formerly administered these
Mechanical properties of these grades (for example, hardness
and tensile strength) can be altered by suitable heat
treatment. The cast high-alloy grades that contain more than
20 to 30% Cr+Ni, however, do not show the phase changes
observed in plain carbon and low-alloy steels during heating
or cooling between room temperature and the melting point.
These materials are therefore non hardenable, and their
properties depend on composition rather than heat treatment.
Therefore, special consideration must be given to each grade
of high-alloy cast steel with regard to casting design,
foundry practice, and subsequent thermal processing.
Corrosion-resistant high-alloy cast steels, more commonly
referred to as cast stainless steels, have grown steadily in
technological and commercial importance during the past 40
years. The principal applications for these steels are for
chemical-processing and power-generating equipment involving
corrosion service in aqueous or liquid-vapor environments at
temperatures normally below 315°C. These alloys are also
used for special services at temperatures up to 650°C.
Cast stainless steels are defined as ferrous alloys that
contain a minimum of 17% Cr for corrosion resistance.
Most cast stainless steels are of course considerably more
complex compositionally than this simple definition implies.
Stainless steels typically contain one or more alloying
elements in addition to chromium (for example, nickel,
molybdenum, copper, niobium, and nitrogen) to produce a
specific microstructure, corrosion resistance, or mechanical
properties for particular service requirements.
Corrosion-resistant high-alloy cast steels are usually
classified on the basis of composition or microstructure. It
should be recognized that these bases for classification are
not completely independent in most cases; that is,
classification by composition also often involves
Alloys are grouped as chromium steels, chromium-nickel steels
in which chromium is the predominant alloying element, and
nickel-chromium steels in which nickel is the predominant
alloying element. The serviceability of cast
corrosion-resistant steels depends greatly on the absence of
carbon, and especially precipitated carbides, in the alloy
The high-alloy cast steels can also be classified on the basis
of microstructure. Structures may be austenitic, ferritic,
martensitic, or duplex; the structure of a particular grade
is primarily determined by composition. Chromium, nickel, and
carbon contents are particularly important in this regard. In
general, straight chromium grades of high-alloy cast steel are
either martensitic or ferritic, the chromium-nickel grades are
either duplex or austenitic, and the nickel-chromium steels
are fully austenitic.
Martensitic grades include alloys CA-15, CA-40, CA-I5M,
and CA-6NM. The CA-15 alloy contains the minimum amount of
chromium necessary to make it essentially rustproof. It has
good resistance to atmospheric corrosion as well as to many
organic media in relatively mild service. A higher-carbon
modification of CA-15, CA-40 can be heat treated to higher
strength and hardness levels. Alloy CA-15M is a
molybdenum-containing modification of CA-15 that provides
improved elevated-temperature strength. Alloy CA-6NM is an
iron-chromium-nickel-molybdenum alloy of low carbon content.
Austenitic grades include CH-20, CK-20, and CN-7M. The
CH-20 and CK-20 alloys are high-chromium, high-carbon, wholly
austenitic compositions in which the chromium exceeds the
nickel content. The more highly alloyed CN-7M has excellent
corrosion resistance in many environments and is often used
in sulfuric acid service.
Ferritic grades are designated CB-30 and CC-50. Alloy CB-30
is practically nonhardenable by heat treatment. As this alloy
is normally made, the balance among the elements in the
composition results in a wholly ferritic structure similar to
wrought AISI type 442 stainless steel. Alloy CC-50 has
substantially more chromium than CB-30 and has relatively
high resistance to localized corrosion in many environments.
Austenitic-ferritic alloys include CE-30, CF-3, CF-3A,
CF-8, CF-SA, CF-20, CF-3M, CF-3MA, CF-8M, CF-8C, CF-16F, and
CG-8M. The microstructures of these alloys usually contain 5
to 40% ferrite, depending on the particular grade and the
balance among the ferrite-promoting and austenite-promoting
elements in the chemical composition.
Duplex Alloys. Two duplex alloys CD-4MCu and Ferralium
are currently of interest. Alloy CD-4MCu is the most highly
alloyed duplex alloy. Ferralium was developed by Langley
Alloys and is essentially CD-4MCu with about 0.15% N added.
With high levels of ferrite (about 40 to 50%) and low nickel,
the duplex alloys have better resistance to stress-corrosion
cracking (SCC) than CF-3M. Alloy CD-4MCu, which contains no
nitrogen and has relatively low molybdenum content, has only
slightly better resistance to localized corrosion than CF-3M.
Ferralium, which has nitrogen and slightly higher molybdenum
than CD-4MCu, exhibits better-localized corrosion resistance
than either CF-3M or CD-4MCu.
Improvements in stainless steel production practices (for
example, electron beam refining, vacuum and argon-oxygen
decarburization, and vacuum induction melting) have created a
second generation of duplex stainless steels. These steels
offer excellent resistance to pitting and crevice corrosion,
significantly better resistance to chloride SCC than the
austenitic stainless steels, good toughness, and yield
strengths two to three times higher than those of type 304 or
316 stainless steels.
First generation duplex stainless steels, for example, AISI
type 329 and CD-4MCu, have been in use for many years. The
need for improvement in the weldability and corrosion
resistance of these alloys resulted in the second-generation
alloys, which are characterized by the addition of nitrogen
as an alloying element.
Second generation duplex stainless steels are usually about
a 50-50 blend of ferrite and austenite. The new duplex alloys
combine the near immunity to chloride SCC of the ferritic
grades with the toughness and ease of fabrication of the
austenitics. Among the second-generation duplexes, Alloy 2205
seems to have become the general-purpose stainless.
Precipitation-Hardening Grades. The alloys in this
group are CB-7Cu and CD-4MCu. Alloy CB-7Cu is a low-carbon
martensitic alloy that may contain minor amounts of retained
austenite or ferrite. The copper precipitates in the
martensite when the alloy is heat treated to the hardened
Heat-resistant high-alloy steel castings are
extensively used for applications involving service
temperatures in excess of 650°C. Strength at these elevated
temperatures is only one of the criteria by which these
materials are selected, because applications often involve
aggressive environments to which the steel must be resistant.
The atmospheres most commonly encountered are air, flue gases,
or process gases; such atmospheres may be either oxidizing or
reducing and may be sulfidizing or carburizing if sulfur or
carbon are present.
Carbon and low-alloy steels seldom have adequate strength and
corrosion resistance at elevated temperatures in the
environments for which heat-resistant cast steels are
normally selected. Only heat-resistant steels exhibit the
required mechanical properties and corrosion resistance over
long periods of time without excessive or unpredictable
degradation. In addition to long-term strength and corrosion
resistance, some cast heat-resistant steels exhibit special
resistance to the effects of cyclic temperatures and changes
in the nature of the operating environment.
These alloy types resemble high-alloy corrosion-resistant
steels except for their higher carbon contents, which impart
greater strength at elevated temperature. The higher carbon
content and, to a lesser extent, alloy composition ranges
distinguish cast heat-resistant steel grades from their
Iron-chromium alloys contain 8 to 30% Cr and little
or no nickel. They are ferritic in structure and exhibit low
ductility at ambient temperatures. Iron-chromium alloys are
primarily used where resistance to gaseous corrosion is the
predominant consideration because they possess relatively low
strength at elevated temperatures.
Iron-chromium-nickel alloys contain more than 18% Cr
and more than 8% Ni, with the chromium content always
exceeding that of nickel. They exhibit an austenitic matrix,
although several grades also contain some ferrite. These
alloys exhibit greater strength and ductility at elevated
temperatures than those in the iron-chromium group and
withstand moderate thermal cycling. Examples of these alloys
are the HE, HF, HH, HI, HK, and HL grades.
Iron-nickel-chromium alloys contain more than 10% Cr
and more than 23% Ni, with the nickel content always
exceeding that of chromium. These alloys are wholly austenitic
and exhibit high strength at elevated temperatures. They can
withstand considerable temperature cycling and severe thermal
gradients and are well suited to many reducing, as well as
oxidizing, environments. Examples of iron-nickel-chromium
alloys are the HN, HP, HT, HU, HW, and HX grades. Even though
nickel is the major element in the HW and HX grades, these
grades are ordinarily referred to as high-alloy steels rather
than nickel-base alloys.