carbide precipitation for additional strength. The most abundant car-
bide in the structural cobalt alloys is chromium-rich M
23
C
6
, although
M
6
C and MC carbides are common, depending on the type and level of
other alloying additions.
32
8.5.3 Welding and heat treatments
In terms of their weldability, high-performance alloys can be classified
according to the means by which the alloying elements develop the
mechanical properties, namely, solid solution alloys and precipitation
hardened alloys. A distinguishing feature of precipitation hardened
alloys is that mechanical properties are developed by heat treatment
to produce a fine distribution of hard particles in a nickel-rich matrix.
Solid solution alloys are readily fusion welded, normally in the
annealed condition. Some noteworthy examples of solid solution alloys
are Ni 200, the Monel 400 series, the Inconel 600 series, the Incoloy
800 series, Hastelloys and some Nimonic alloys such as 75, and PE13.
Because the HAZ does not harden, heat treatment is not usually
required after welding. Precipitation hardened alloys may be suscepti-
ble to postweld heat-treatment (PWHT) cracking. Some of these alloys
are the Monel 500 series, Inconel 700 series, Incoloy 900 series, and
most of the Nimonic alloys.
Weldability. Co-base high-performance alloys are readily welded by
gas metal arc (GMA) or gas tungsten arc (GTA) techniques. Some cast
alloys and wrought alloys, such as Alloy 188, have been extensively
welded. Filler metals generally have been less highly alloyed Co-base

and Fe-Ni-base alloys can be resistance welded when in sheet form.
Brazing, diffusion bonding, and transient liquid phase bonding also
have been employed to join these alloys. Braze joints tend to be more
ductility limited than welds.
Most nickel alloys can be fusion welded using gas-shielded processes
such as TIG or MIG. Of the flux processes, MMA is frequently used,
but the submerged arc welding (SAW) process is restricted to solid
solution alloys (Nickel 200, Inconel alloy 600 series, and Monel alloy
400 series) and is less widely used. Solid solution alloys are normally
welded in the annealed condition, and precipitation hardened alloys,
in the solution treated condition. Preheating is not necessary unless
there is a risk of porosity from moisture condensation. It is recom-
mended that material containing residual stresses be solution treated
before welding to relieve the stresses.
33
Postweld heat treatment is not usually needed to restore corrosion
resistance, but thermal treatment may be required for precipitation
hardening or stress-relieving purposes to avoid stress corrosion
cracking. Filler composition normally matches the parent metal.
However, most fillers contain a small mount of titanium, aluminum,
and/or niobium to help minimize the risk of porosity and cracking.
Nickel and its alloys are readily welded, but it is essential to clean
the surface immediately before welding. The normal method of clean-
ing is to degrease the surface, remove all surface oxide by machining,
grinding, or scratch brushing, and finally degrease. However, these
alloys can suffer from the following weld imperfections and postweld
damage:
33
Porosity. Porosity can be caused by oxygen and nitrogen from air
entrainment and surface oxide or by hydrogen from surface contami-

Microfissuring. Similar to austenitic stainless steel, nickel alloys
are susceptible to formation of liquation cracks in reheated weld
metal regions or parent metal HAZ. This type of cracking is con-
trolled by factors outside the control of the welder such as grain size
or content impurity. Some alloys are more sensitive than others. For
example, the extensively studied Inconel 718 is now less sensitive
than some cast superalloys, which cannot be welded without induc-
ing liquation cracks.
Postweld heat-treatment cracking. This is also known as strain-age
or reheat cracking. It is likely to occur during postweld aging of pre-
cipitation hardening alloys but can be minimized by preweld heat
treatment. Solution annealing is commonly used but overaging gives
the most resistant condition. Inconel 718 alloy was specifically
developed to be resistant to this type of cracking.
Stress corrosion cracking. Welding does not normally make nickel
alloys susceptible to weld metal or HAZ corrosion. However, when
the material will be in contact with caustic soda, fluosilicates, or HF
acid, stress corrosion cracking is possible.
Heat treatment. Solid-solution-strengthened high-temperature alloys
are normally supplied in the solution-heat-treated condition unless
otherwise specified. In this condition, microstructures generally con-
sist of primary carbides dispersed in a single-phase matrix, with
essentially clean grain boundaries. This is usually the optimum condi-
tion for the best elevated temperature properties in service and the
Materials Selection 673
0765162_Ch08_Roberge 9/1/99 6:01 Page 673
best room-temperature fabricability. Typical solution heat-treatment
temperatures for these alloys are between 1100 and 1200°C.
34
Heat treatments performed at temperatures below the solution

annealing. In the everyday fabrication of complex components, it may
be impossible to avoid situations where such low levels of cold work or
strain are introduced.
Annealing during hot forming. Components manufactured by hot-forming
techniques should generally be solution heat treated rather than mill
annealed if in-process heat treatment is required. In cases where form-
ing is required to be performed at furnace temperatures below the solu-
tion treatment range, intermediate mill annealing may be employed
subject to the limits of the forming equipment. Hot-formed components,
particularly when formed at high temperatures, will generally undergo
recovery, recrystallization, and perhaps even grain growth during the
forming operation itself. Similarly, if the hot-forming session involves a
small amount of deformation, the piece to be heat treated may exhibit
674 Chapter Eight
0765162_Ch08_Roberge 9/1/99 6:01 Page 674
a nonuniform structure, which will respond nonuniformly to the heat
treatment.
34
Final annealing. Solution heat treating is the most common form of fin-
ishing operation applied to high-temperature alloys and is often
mandated by the applicable specifications for these materials. Where
more than about 10 percent cold work is present in the piece, a final
anneal is usually mandatory. Putting as-cold-worked material into
service can result in recrystallization to a very fine grain size, which
in turn can produce a significant reduction in stress rupture
strength. A good example of this is vacuum brazing. Often performed
as the final step in the fabrication of some components, such a
process precludes the possibility of a subsequent solution treatment
because of the low melting point of the brazing compound.
Consequently, the actual brazing temperatures used are sometimes

34
Materials Selection 675
0765162_Ch08_Roberge 9/1/99 6:01 Page 675
Use of protective atmosphere. Most of the high-performance alloys may be
annealed in oxidizing environments but will form adherent oxide scales
that normally must be removed prior to further processing. Some high-
temperature alloys contain low chromium. Atmosphere annealing of
these materials should be performed in neutral to slightly reducing
environments. Protective atmosphere annealing is commonly per-
formed for all of these materials when a bright finish is desired. The
best choice for annealing of this type is a low dew point hydrogen envi-
ronment. Annealing may also be done in argon and helium. Annealing
in nitrogen or cracked ammonia is not generally preferred but may be
acceptable in some cases. Vacuum annealing is generally acceptable
but also may produce some tinting depending on the equipment and
temperature. The gas used for forced gas cooling can also influence
results. Helium is normally preferred, followed by argon and nitrogen.
34
8.5.4 Corrosion resistance
High-performance alloys generally react with oxygen, and oxidation is
the prime environmental effect on these alloys. At moderate tempera-
tures, about 870°C and below, general uniform oxidation is not a major
problem. At higher temperatures, the commercial nickel- and cobalt-
base high-performance alloys are attacked by oxygen. The level of oxi-
dation resistance at temperatures below 1200°C is a function of
chromium content, Cr
2
O
3
forming as a protective oxide film. Above

molybdenum and copper. Alloy B (N10001), with 28% Mo, is resistant
676 Chapter Eight
0765162_Ch08_Roberge 9/1/99 6:01 Page 676
to hydrochloric acid. Monel 400 (N04400), with 30% Cu, is widely used
in natural waters and in heat-exchanger applications. It also has good
resistance to hydrofluoric acid, although SCC is a potential problem.
Although Monel 400 is used in similar applications as S31600 stain-
less steel, it is its opposite in many aspects of its behavior. For exam-
ple, it has poor resistance to oxidizing media, whereas stainless steels
thrive in these conditions. If chromium is added to nickel, alloys resis-
tant to a wide range of oxidizing and reducing media can be obtained.
One example is Inconel 600. If molybdenum is further added, the
resulting alloys can possess a resistance to an even wider range of
reducing and oxidizing media with very good chloride pitting resis-
tance, for example, Hastelloy C (N10002).
These high-nickel alloys are resistant to transgranular SCC in ele-
vated temperature chlorides, whereas the regular austenitic stainless
steels are very susceptible to this type of attack. It is interesting to note
that S43000 stainless is also resistant to these corrosive environments.
The pitting resistance of high-nickel, chromium-containing alloys is
generally better than that obtained with stainless steels. However, they
can be more susceptible to intergranular corrosion because
1. The solubility of carbon in austenite decreases as nickel increases,
which in turn increases the tendency to form chromium carbide.
2. The higher alloys are generally more prone to precipitate inter-
metallic compounds that can lower corrosion resistance by deplet-
ing the matrix in Ni, Mo, and so forth.
Chromium carbides and intermetallic compounds precipitate out at
temperatures in the range of about 600 to 1000°C. Therefore, there
are restrictions to the use of these alloys as welded materials. Stress -

the electrochemical interaction of the environment with an alloy. A
case in point is the nickel-molybdenum Hastelloy B-2 (N10665). This
alloy performs exceptionally well in pure deaerated H
2
SO
4
and HCl
but deteriorates rapidly when oxidizing impurities, such as oxygen
and ferric ions, are present.
Ni-base alloys in acid media. Sulfuric acid is the most ubiquitous environ-
ment in the chemical industry. The electrochemical nature of the acid
varies wildly, depending on the concentration of the acid and the impu-
rity content. Pure acid is considered to be a nonoxidizing acid up to a
concentration of about 50 to 60%, beyond which it is generally consid-
ered to be oxidizing. The corrosion rates of nickel-base alloys, in general,
increase with acid concentration up to 90%. Higher concentrations of
the acid are generally less corrosive.
31
The presence of oxidizing impu-
rities can be beneficial to nickel-chromium-molybdenum alloys because
these impurities can aid in the formation of passive films that retard
corrosion. Another important consideration is the presence of chlorides
(Cl
Ϫ
). Chlorides generally accelerate the corrosion attack, but the
degree of acceleration differs for various alloys.
Commercially pure nickel (N02200 and N02201) and Monels have
room-temperature corrosion rates below 0.25 mmиy
Ϫ1
in air-free HCl at

3
.
Pitting corrosion in chloride environments. The nickel-chromium-molybde-
num alloys, such as Hastelloys C-22 and C-276 as well as Inconel 625,
exhibit very high resistance to pitting in oxidizing chloride environments.
The critical pitting temperatures of various nickel-chromium-molybde-
num alloys in an oxidizing chloride solution are shown in Table 8.23.
Pitting corrosion is most prevalent in chloride-containing environments,
although other halides and sometimes sulfides have been reported to
cause pitting. There are several techniques that can be used to evaluate
resistance to pitting. Critical pitting potential and pitting protection
potential indicate the electrochemical potentials at which pitting can be
initiated and at which a propagating pit can be stopped, respectively.
These values are functions of the solution concentration, pH, and tem-
perature for a given alloy; the higher the potentials, the better the alloy.
The critical pitting temperature (i.e., the potential below which pitting
does not initiate), is often used as an indicator of resistance to pitting,
especially in the case of highly corrosion-resistant alloys (Table 8.23).
Chromium and molybdenum additions have been shown to be extremely
beneficial to pitting resistance.
31
Materials Selection 679
TABLE 8.23 Critical Pitting Temperatures
for Nickel Alloys in 6% FeCl
3
during 24 h
Critical pitting
Alloy UNS temperature, °C
825 N08825 0.0 0.0
904L N08904 2.5 5.0

older, established gas turbine engines, and for a variety of industrial applications.
Alloy 188 (R30188)
Description and corrosion resistance. Alloy 188 is a cobalt-nickel-chromium-tungsten
alloy developed as an upgrade to Alloy 25. It combines excellent high-temperature
strength with very good oxidation resistance up to about 1095°C. Its thermal stability
is better than that for Alloy 25, and it is easier to fabricate. Alloy 188 also has low-cycle
fatigue resistance superior to that for most solid-solution-strengthened alloys and has
very good resistance to hot corrosion.
Applications. It is widely used in both military and civil gas turbine engines and in a
variety of industrial applications.
Alloy 230 (N06230)
Description and corrosion resistance. This is a nickel-chromium-tungsten-molybdenum
alloy that combines excellent high-temperature strength, outstanding oxidation
resistance up to 1150°C, premier nitriding resistance, and excellent long-term thermal
stability. Alloy 230 also has lower expansion characteristics than most high-temperature
alloys, very good low-cycle fatigue resistance, and a pronounced resistance to grain
coarsening with prolonged exposure at elevated temperatures. Components of Alloy 230
are readily fabricated by conventional techniques, and the alloy can be cast.
Applications. Principal applications for Alloy 230 include
Wrought and cast gas turbine stationary components
Aerospace structurals
Chemical process and power plant internals
Heat treating facility components and fixtures
Steam process internals
0765162_Ch08_Roberge 9/1/99 6:01 Page 680
Materials Selection 681
TABLE 8.24 Brief Description, Corrosion Resistance, and Applications of High-
Performance Alloys and Some Highly Alloyed Stainless Steels (Continued)
Cobalt Alloy 6B (R30016)
Description and corrosion resistance. Cobalt 6B is a cobalt-based chromium-tungsten

Description and corrosion resistance. Cobalt 6BH has the same composition as Cobalt
6B, except the material is hot rolled and then age hardened. The direct age hardening
after hot rolling provides the maximum hardness and wear resistance. The advantages
this creates are increased wear life, retained edge characteristics, and increased
hardness. These properties are in addition to the galling and seizing resistance of the
regular Cobalt 6B. Cobalt 6BH is known in the industry as a metal that retains its
cutting edge. The economic advantages are in its long wear time, less downtime, and
fewer replacements.
Applications. Cobalt 6BH is used for steam turbine erosion shields, chain saw guide
bars, high-temperature bearings, furnace fan blades, valve stems, food processing
equipment, needle valves, centrifuge liners, hot extrusion dies, forming dies, nozzles,
extruder screws, and many other miscellaneous wear surfaces. Applications also
include tile-making machines, rock-crushing rollers, and cement and steel mill
equipment. Alloy 6BH is well suited for valve parts and pump plungers.
0765162_Ch08_Roberge 9/1/99 6:01 Page 681
682 Chapter Eight
TABLE 8.24 Brief Description, Corrosion Resistance, and Applications of High-
Performance Alloys and Some Highly Alloyed Stainless Steels (Continued)
Ferralium 255 (S32550)
Description and corrosion resistance. This alloy’s high critical pitting crevice
temperatures provide more resistance to pitting and crevice corrosion than lesser-
alloyed materials. The very high yield strength of this alloy combined with good
ductility allows lower wall thickness in process equipment.
Applications. Alloy 255 is finding many cost-effective applications in the chemical,
marine, metallurgical, municipal sanitation, plastics, oil and gas, petrochemical,
pollution control, wet phosphoric acid, paper-making, and metal-working industries.
It is called super because it is more alloyed than ordinary stainless steels and has
superior corrosion resistance. Alloy 255 is being used in areas where conventional
stainless steels are inadequate or, at best, marginal. One good example is in the paper
industry, which was hit with an epidemic of corrosion problems when environmental

equipment.
Hastelloy (N10665)
Description and corrosion resistance. Alloy B-2 is a nickel-molybdenum alloy with
significant resistance to reducing environments, such as hydrogen chloride gas and
sulfuric, acetic, and phosphoric acids. Alloy B-2 provides resistance to pure sulfuric acid
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Materials Selection 683
TABLE 8.24 Brief Description, Corrosion Resistance, and Applications of High-
Performance Alloys and Some Highly Alloyed Stainless Steels (Continued)
and a number of nonoxidizing acids. The alloy should not be used in oxidizing media or
where oxidizing contaminants are available in reducing media. Premature failure may
occur if B-2 is used where iron or copper is present in a system containing hydrochloric
acid. Industry users like the resistance to a wide range of organic acids and the
resistance to chloride-induced stress-corrosion cracking.
Alloy B-2 resists the formation of grain boundary carbide precipitates in the weld
heat-affected zone, making it suitable for most chemical process applications in the
as-welded condition. The heat-affected weld zones have reduced precipitation of
carbides and other phases to ensure uniform corrosion resistance. Alloy B-2 also has
excellent resistance to pitting and stress corrosion cracking.
Applications. Alloy B-2 has superior resistance to hydrochloric acid, aluminum
chloride catalysts, and other strongly reducing chemicals and has excellent high-
temperature strength in inert and vacuum atmospheres. Applications in the
chemical process industry involve sulfuric, phosphoric, hydrochloric, and acetic acid.
Temperature uses vary from ambient temperature to 820°C depending on the
environments.
Hastelloy C-22 (N06022)
Description and corrosion resistance. Hastelloy C-22 is a nickel-chromium-
molybdenum alloy with enhanced resistance to pitting, crevice corrosion, and stress
corrosion cracking. It resists the formation of grain boundary precipitates in the weld
heat-affected zone, making it suitable for use in the as-welded condition. C-22 has

Performance Alloys and Some Highly Alloyed Stainless Steels (Continued)
Hastelloy X (N06002)
Description and corrosion resistance. This is a nickel-chromium-iron-molybdenum
alloy that possesses an exceptional combination of oxidation resistance, fabricability,
and high-temperature strength. Alloy X is one of the most widely used nickel-base
superalloys for gas turbine engine components. This solid-solution-strengthened grade
has good strength and excellent oxidation resistance beyond 2000°F. Alloy X has
excellent resistance to reducing or carburizing atmospheres, making it suitable for
furnace components. Due to its high molybdenum content, alloy X may be subject to
catastrophic oxidation at 1200°C.
It is exceptionally resistant to SCC in petrochemical applications and to carburization
and nitriding. All of the product forms are excellent in terms of forming and welding.
Although this alloy is primarily noted for heat and oxidation resistance, it also has good
resistance to chloride stress corrosion cracking.
Applications. The alloy finds use in petrochemical process equipment and gas
turbines in the hot combustor zone sections. It is also used for structural components
in industrial furnace applications because of its excellent oxidation resistance. It is
recommended especially for use in furnace applications because it has unusual
resistance to oxidizing, reducing, and neutral atmospheres. Furnace rolls made of
this alloy are still in good condition after operating for 8700 h at 1200°C. Furnace
trays, used to support heavy loads, have been exposed to temperatures up to 1250°C in
an oxidizing atmosphere without bending or warping. Alloy X is equally suitable for use
in jet engine tailpipes, afterburner components, turbine blades, nozzle vanes, cabin
heaters, and other aircraft parts. Alloy X has wide use in gas turbine engines for
combustion zone components such as transition duct, combustor cans, spray bars, and
flame holders. Alloy X is also used in the chemical process industry for retorts, muffles,
catalyst support grids, furnace baffles, tubing for pyrolysis operations, and flash drier
components.
Incoloy 800 (N08800)
Description and corrosion resistance. Alloy 800 is a nickel-iron-chromium alloy

Alloy 825. The alloy has outstanding resistance to general corrosion, pitting, crevice
corrosion, and stress corrosion cracking in many aqueous environments including those
containing sulfides and chlorides.
Applications. Uses include surface and downhole hardware in sour gas wells and oil-
production equipment.
Inconel 600 (N06600)
Description and corrosion resistance. Alloy 600 is a nickel-chromium alloy designed for
use from cryogenic to elevated temperatures in the range of 1093°C. The high nickel
content of the alloy enables it to retain considerable resistance under reducing conditions
and makes it resistant to corrosion by a number of organic and inorganic compounds.
The nickel content gives it excellent resistance to chloride-ion stress corrosion cracking
and also provides excellent resistance to alkaline solutions.
Its chromium content gives the alloy resistance to sulfur compounds and various
oxidizing environments. The chromium content of the alloy makes it superior to
commercially pure nickel under oxidizing conditions. In strong oxidizing solutions like
hot, concentrated nitric acid, 600 has poor resistance. Alloy 600 is relatively unattacked
by the majority of neutral and alkaline salt solutions and is used in some caustic
environments. The alloy resists steam and mixtures of steam, air, and carbon dioxide.
Alloy 600 is nonmagnetic, has excellent mechanical properties and a combination of
high strength and good workability, and is readily weldable. Alloy 600 exhibits cold-
forming characteristics normally associated with chromium-nickel stainless steels. It
is resistant to a wide range of corrosive media. The chromium content gives better
resistance than Alloys 200 and 201 under oxidizing conditions, and at the same time
the high nickel gives good resistance to reducing conditions. Other qualities are as
follows:
Virtually immune to chlorine ion stress corrosion cracking.
Demonstrates adequate resistance to organic acids such as acetic, formic, and stearic.
Excellent resistance to high purity water used in primary and secondary circuits of
pressurized nuclear reactors.
Little or no attack occurs at room and elevated temperatures in dry gases, such as

Description and corrosion resistance. The most important property of Alloy 601 is
resistance to oxidation at very high temperatures, up to 1250°C, even under severe
conditions such as cyclical heating and cooling. This is possible due to Alloy 601 having
a tightly adherent oxide layer that is resistant against spalling. Its resistance to
carburization is also good, and it is resistant to carbonitriding conditions. Due to its
high chromium and some aluminium content, Inconel 601 has good resistance in
oxidizing sulfur-bearing atmospheres at elevated temperatures.
Applications. This alloy is used for
Trays, baskets, and fixtures used in various heat treatments such as carburizing and
carbonitriding
Refractory anchors, strand annealing and radiant tubes, high-velocity gas burners,
wire mesh belts, etc.
Insulating cans in ammonia reformers and catalyst support grids used in nitric acid
production
Thermal reactors in exhaust system of petrol engines
Fabricated combustion chambers
Tube supports and ash trays in the power generation industry
Inconel 625 (N06625)
Description and corrosion resistance. This is a material with excellent resistance to
pitting, crevice, and corrosion cracking. It is highly resistant in a wide range of organic
and mineral acids and has good high-temperature strength. Other features include
Excellent mechanical properties at both extremely low and extremely high
temperatures
Outstanding resistance to pitting, crevice corrosion, and intercrystalline corrosion
Almost complete freedom from chloride-induced stress corrosion cracking
High resistance to oxidation at elevated temperatures up to 1050°C
Good resistance to acids, such as nitric, phosphoric, sulfuric, and hydrochloric, as
well as to alkalis makes possible the construction of thin structural parts of high
heat transfer
Applications. Inconel 625 is used for

copper under oxidizing conditions. It does show, however, better resistance to reducing
media than oxidizing ones. It also has
Good mechanical properties from subzero temperatures up to about 480°C.
Good resistance to sulfuric and hydrofluoric acids. Aeration, however, will result in
increased corrosion rates. It may be used to handle hydrochloric acid, but the
presence of oxidizing salts will greatly accelerate corrosive attack.
Resistance to neutral, alkaline, and acid salts is shown, but poor resistance is found
with oxidizing acid salts such as ferric chloride.
Excellent resistance to chloride ion stress corrosion cracking.
Applications. Uses for Monel 400 include
Feed water and steam generator tubing
Brine heaters and seawater scrubbers in tanker inert gas systems
Sulfuric acid and hydrofluoric acid alkylation plants
Pickling bat heating coils
Heat exchangers in a variety of industries
Transfer piping from oil refinery crude columns
Plants for the refining of uranium and isotope separation in the production of
nuclear fuel
Pumps and valves used in the manufacture of perchlorethylene, chlorinated plastics
Monoethanolamine (MEA) reboiling tubes
Cladding for the upper areas of oil refinery crude columns
Propeller and pump shafts
Monel 500 (N05500)
Description and corrosion resistance. Alloy K-500 is a nickel-copper alloy,
precipitation hardenable through additions of aluminum and titanium. Alloy K-500
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688 Chapter Eight
TABLE
8.24 Brief Description, Corrosion Resistance, and Applications of High-
Performance Alloys and Some Highly Alloyed Stainless Steels (Continued)

chloride may be used in temperatures up to 550°C
Resistance to mineral acids varies according to temperature and concentration and
whether the solution is aerated or not; corrosion resistance is better in deaerated acid
Applications. It is used in the following:
Manufacture and handling of sodium hydroxide, particularly at temperature above
300°C
Production of viscose rayon and manufacture of soap
Analine hydrochloride production and the chlorination of aliphatic hydrocarbons
such as benzene, methane and ethane
Manufacture of vinyl chloride monomer
Storage and distribution systems for phenol; immunity from any form of attack
ensures absolute product purity
Reactors and vessels in which fluorine is generated and reacted with hydrocarbons
Nickel 201 (N02201)
Description and corrosion resistance. Nickel 201 can be hot formed to almost any
shape. The temperature range 650 to 1230°C is recommended and should be carefully
adhered to because the proper temperature is the most important factor in achieving
hot malleability. Full information of the forming process should be sought and
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Materials Selection 689
TABLE
8.24 Brief Description, Corrosion Resistance, and Applications of High-
Performance Alloys and Some Highly Alloyed Stainless Steels (Continued)
understood before proceeding. Nickel 201 can be cold formed by all conventional
methods, but because nickel alloys have greater stiffness than stainless steels, more
power is required to perform the operations. Nickel 201 is the low-carbon version of
Nickel 200. It is preferred to Nickel 200 for applications involving exposure to
temperatures above 320°C. With low base hardness and lower work-hardening rate, it
is particularly suited for cold forming. Other properties are
Good resistance to corrosion in acids and alkalies; most useful under reducing

The chromium and nickel additions give it comparable corrosion to S30400 and
S31600 stainless steels, while having a twice the yield strengths of regular stainless
steels. The high mechanical strength in annealed parts permits use of reduced cross
sections for weight and cost reductions. Although uniform corrosion resistance of
Nitronic 60 is better than S30400 stainless in most environments, its yield strength is
nearly twice that of S30400 and S31600 steels. Chloride pitting resistance is superior
to that of type S31600 stainless; Nitronic 60 provides excellent high-temperature
oxidation resistance and low-temperature impact.
Nitronic 60 is also readily welded using conventional joining processes. It can be
handled similarly to S30400 and S31600 steels. No preheat or postweld heat treatments
are necessary, other than the normal stress relief used in heavy fabrication. Most
applications use Nitronic 60 in the as-welded condition, unless corrosion resistance is a
consideration. Fillerless fusion welds (autogenous) have been made using GTA. These
0765162_Ch08_Roberge 9/1/99 6:01 Page 689
690 Chapter Eight
TABLE 8.24 Brief Description, Corrosion Resistance, and Applications of High-
Performance Alloys and Some Highly Alloyed Stainless Steels (Continued)
welds are free from cracking and have galling and cavitation resistance similar to the
unwelded base metal. Heavy weld deposits using this process are sound and exhibit
higher strength then the unwelded base metal. The metal-to-metal wear resistance of
the GMA welds are slightly lower than the base metal wear resistance.
Applications. Applications using Nitronic 60 are valve stems, seats and trim,
fastening systems, screening, pins, bushings and roller bearings, pump shafts, and
rings. Other uses include wear plates, rails guides, and bridge pins. This alloy provides
a significant lower-cost way to fight wear and galling compared to nickel- or cobalt-
based alloys. It is also used for
Automotive valves; it can withstand gas temperatures of up to 820°C for a minimum
of 80,000 km
Fastener galling; it is capable of frequent assembly and disassembly, allowing more
use of the fastener before the threads are torn up and also helps to eliminate

Marine hardware, mastings and tie downs
Marine and pump shafts
Valves and fittings
Downhole rigging
0765162_Ch08_Roberge 9/1/99 6:01 Page 690
Cobalt-base alloys. The corrosion behavior of pure cobalt has not been
documented as extensively as that of nickel. The behavior of cobalt is
similar to that of nickel, although cobalt possesses lower overall corro-
sion resistance. For example, the passive behavior of cobalt in 0.5 M
sulfuric acid has been shown to be similar to that of nickel, but the crit-
ical current density necessary to achieve passivity is 14 times higher for
the former. Several investigations have been carried out on binary
cobalt-chromium alloys. In cobalt-base alloys, it has been found that as
little as 10% chromium is sufficient to reduce the anodic current den-
sity necessary for passivation from 500 to 1 mAиcm
Ϫ2
. For nickel, about
14% chromium is needed to reduce the passivating anodic current den-
sity to the same level.
It should be noted that all of these alloys, regardless of their
chromium and molybdenum contents, exhibit similar corrosion resis-
tance in dilute H
2
SO
4
. Thus, the high-chromium alloys show approxi-
mately the same corrosion rates as the lower-chromium alloys.
Similar behavior has been observed in the nickel-iron-chromium-
molybdenum alloys. In H
2

disks, and pressure vessels are but some of the shapes that have been
Materials Selection 691
0765162_Ch08_Roberge 9/1/99 6:01 Page 691
produced. These metals have been used in aircraft, industrial and
marine gas turbines, nuclear reactors, aircraft skins, spacecraft struc-
tures, petrochemical production, and environmental protection appli-
cations. Although developed for high-temperature applications, some
are used at cryogenic temperatures.
The Ni-Cr-Fe alloys are also extensively used in refining and petro-
chemical plant equipment for both liquid and gaseous low-temperature
corrosion resistance and for heat-resistant applications. Table 8.24
describes the practical behavior of the main high-performance alloys and
highly alloyed stainless steels in some of the very demanding operational
situations in which these alloys are expected to perform satisfactorily.
The chemical composition of these alloys can be found in App. E.
8.6 Refractory Metals
8.6.1 Introduction
Refractory metals are characterized by their high melting points,
exceeding an arbitrary value of 2000°C, and low vapor pressures, two
properties exploited by the electronics industry. Only four refractory
metals, molybdenum, niobium, tantalum, and tungsten, are avail-
able in quantities of industrial significance and have been produced
commercially for many years, mainly as additives to steels, nickels,
and cobalt alloys and for certain electrical applications. In addition
to high-temperature strength, the relatively low thermal expansions
and high thermal conductivity of the refractory metals suggest good
resistance to thermal shock. Table 8.25 contains additional data on
physical and mechanical properties of refractory metals.
There are, however, two characteristics, ready oxidation at high tem-
peratures and, in the case of molybdenum and tungsten, brittleness at

Ϫ3
) 10.2 8.57 16.6 19.3
Thermal properties
Melting point (°C) 2610 2468 2996 3410
Boiling point, °C (°C) 5560 4927 6100 5900
Linear coefficient of expansion per °C 4.9ϫ10
Ϫ6
7.1ϫ10
Ϫ6
6.5ϫ10
Ϫ6
4.3ϫ10
Ϫ6
Thermal conductivity, 20°C Wиm
Ϫ1
K
Ϫ1
147 219 54 167
Specific heat, 20°C (Jиkg
Ϫ1
K
Ϫ1
) 255 525 151 134
Electrical properties
Conductivity % IACS (Cu) 30 13.2 13 31
Resistivity, 20°C ␮⍀иcm 5.7 15 13.5 5.5
Coefficient of resistivity per °C (0–100°C) 0.0046 0.0038 0.0046
Mechanical properties
Tensile strength, 20°C (MPa) 700–1400 195 240–500 700–3500
500°C (MPa) 240–450 170–310 500–1400

35
Molybdenum has been used for many years in the lamp industry for
mandrels and supports, usually in wire form. Today, several unique
properties of molybdenum that satisfy more demanding industry
requirements have increased the use of molybdenum as a material in
applications requiring other mill forms.
Molybdenum alloys. Molybdenum has several alloys:
■
TZM (titanium, zirconium, molybdenum). Molybdenum’s prime alloy
is TZM. This alloy contains 99% Mo, 0.5% Ti, and 0.08% Zr with a trace
of carbon for carbide formations. TZM offers twice the strength of pure
molybdenum at temperatures over 1300°C. The recrystallization tem-
perature of TZM is approximately 250°C higher than molybdenum,
and it offers better weldability.
The finer grain structure of TZM and the formation of TiC and ZrC
in the grain boundaries of the molybdenum inhibit grain growth and
the related failure of the base metal as a result of fractures along the
grain boundaries. This also gives it better properties for welding.
TZM costs approximately 25 percent more than pure molybdenum
and costs only about 5 to 10 percent more to machine. For high-
strength applications such as rocket nozzles, furnace structural
694 Chapter Eight
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components, and forging dies, it can be well worth the cost differen-
tial. TZM is available in sheet and rod form in basically the same
size range as molybdenum with the exception of thin foil.
■
Molybdenum/30% tungsten. This is another molybdenum alloy
that offers unique properties. It was developed for the zinc industry.
This alloy resists the corrosive effects of molten zinc. Mo/30W has

■
Low vapor pressure
■
Electrical resistivity
■
Corrosion resistance
■
Purity
■
Ductility and fabricability
■
Machinability
Some combination of these properties and characteristics predicts
increased usage of molybdenum in such applications as rocket nozzles,
Materials Selection 695
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