VOLUME
23 | NUMBER 2
| SECOND QUARTER 2002
I N N O V A T I
O N S
Innovator
Gravitates to Niobium for Superconducting Cavity Experiments
By:
Kirk Richardson— Wah Chang
Explorer Christopher Columbus’ sense of curiosity and
desire to search for answers began along the foothills of
Genoa, Italy’s Appennines in the 15th Century. From these
beginnings, the explorer/innovator went on to discover new
frontiers, stretching known boundaries... re-defining horizons.
Though Genoa has grown into a modern industrial city since
then, some things remain as they were centuries ago. The Mediterranean
seaport still yields the inquisitive... those who examine
our world (as well as others) from a different angle, attempting
to expand horizons.
Dr. Renzo Parodi fits the profile. The Genoa native works
for INFN (officially Instituto Nazionale Fisica Nucleare),
an Italian organization focused on research and development
of sub-nuclear and nuclear interactions. Unlike Sr. Columbus,
Dr. Parodi and his team are looking at the smaller picture.
Dr. Parodi is currently focused on two of INFN’s programs,
code-named PACO and TRA.SCO (TRAsmutazione SCOrie), energy-related
experiments that utilize niobium superconducting cavities.
According to Dr. Parodi, a good RF superconductor must have:
high critical temperature Tc, low electric resistivity, high
thermodynamic critical magnetic field Bc, and good mechanical
properties either for easy forming and mechanical stability
in operation. He says that no other materials meet these requirements
as well as niobium.
Case in point. The PACO experiment, a feasibility study for
a novel gravitational wave detector, uses a superconducting
radiofrequency resonator as an active detection element. Periodic
gravitational waves are emitted from massive stellar bodies
spinning around each other, like binary neutron stars, pulsars,
and similar astronomic objects. The aim of the PACO experiment
is to build a detector with enough sensitivity to detect periodic
gravitational waves (if any) coming from binary stars in our
galaxy.
High quality niobium (supplied by Wah Chang) is a fundamental
ingredient in this experiment, according to Dr. Parodi. His
team is using niobium to build the microwave resonator for
the following two reasons: (1) The detector sensitivity is
proportional to the stored energy in the resonator (the greater
the stored energy, the great the energy transferred) and (2)
The detector must operate at low temperatures to keep to a
minimum the noise due to the thermal induced vibrations of
the cavity walls.
“This combination of needs calls for the use of a superconducting
radiofrequency resonator, fully exploiting the properties
of niobium at low temperatures’” Dr. Parodi says.
He explains that the use of a superconductor reduces by a
factor of one million the radiofrequency losses per unit of
stored energy in the electromagnetic field. “In our case
(using a niobium superconductor for a stored energy of one
kilo joule) the power losses are only 20 watts compared with
megawatts of RF power losses when using copper,” he points
out.
Parodi mentions that RF “superconductivity people”
working with large accelerating cavities have looked at alternative
materials “since the beginning of accelerator and detector
applications.” He continues, “no materials have
a better compromise among the different requirements than
niobium.” In fact, the reason niobium works well is that
it forces these scientists and engineers into the fewest compromises.
Copper prototype of microwave resonator for detecting gravitational
waves of massive stellar bodies.
Supporting this point, another reason that niobium was specified
for the PACO project is its mechanical strength at liquid
helium temperature. Dr. Parodi illustrates: “You can
consider the resonator as a bell stricken by the hammer blow
of the gravitational wave,” he says. “If the mechanical
losses are low, the bell will ring for a very long time. At
the next hammer blow, the bell is still ringing, and the mechanical
oscillation will build up, improving the detector sensitivity.
This reason alone calls for the use of niobium as the best
material for this kind of detector.”
Dr. Parodi’s team continues to make progress on the project.
Tests for the prototype detector are due in July 2002. “If
successful,” he says, “we hope to have a proposal
for the construction of the full-scale detector by the end
of next year and to start our quest for the gravitational
waves by the end of 2005.”
Niobium is also a key element in TRA.SCO, a joint project
of INFN and ENEA (the Italian Committee for the development
of Energy Production) financed by a three-year grant of the
Italian Ministry for research. The aim of this project is
to prove the feasibility of the main components of a nuclear
waste disposal plant based on nuclear transmutation.
The idea driving the project is the transformation (transmutation)
of long-lived radioactive nuclei (produced as a byproduct
in nuclear reactors used for energy generation) into short-lived,
easily disposable radio nuclides. If proven viable, this idea
will solve the problem of the storage of spent reactor fuel
rods (in highly controlled sites) for many centuries, according
to Dr. Parodi.
“The basic layout of the system uses a powerful accelerator
delivering a proton beam of 30 mA at energy of roughly 1 GeV
(one billion volts),” he explains. The neutrons produced
by the proton accelerator break the heavy long-lived nuclei,
producing light radioactive nuclei with much shorter decay
times. “We can think of this system as a nuclear incinerator
burning dangerous byproduct of nuclear power plants (long-lived
nuclei) to easily disposable ashes (short-lived light nuclei),”
says Parodi. He adds, “With minor changes in the nuclear
kettle, we can think to use this system to burn ashes (of)
the nuclear explosives coming out by the reduction of the
nuclear weapons.”
According to Dr. Parodi, the key point of the incinerator
is the powerful proton linear accelerator (a LIN.AC) delivering
the 30MW beam to the system. In his words, “To accelerate
protons to one billion volts, a conventional machine built
using copper will be a power-hungry monster needing roughly
500MW (millions of watts) of electrical power to drive the
radiofrequency power amplifier needed to build the high voltage
giving the proton acceleration. Using niobium, a good superconductor
for radiofrequency applications, the same voltage can be built
using an RF power of a few kilowatts, really peanuts, to be
dissipated at the operating temperature of 4.5K (-269°C),
the boiling temperature of the liquid helium needed to keep
superconducting the niobium.”
The prototype of this system was built in collaboration with
the CERN SL/CT group at Geneva, using high quality niobium
film for optimum radiofrequency performance, deposited on
a copper substrate, shaped for proton acceleration, and designed
with optimum low temperature performance and cost savings
as key considerations. At the time of this printing, test
results have been positive. The main module, developed under
the careful watch of Dr. Parodi, exceeded the LINAC specification
both in stand-alone RF tests and in-life tests in a machine-ready
cryostat, fully equipped for operation in the prototype accelerator.
As for the in-life test, the module worked smoothly for three
days without any failure at an RF input power level of 250KW,
simulating machine operation. This is encouraging news, considering
that current plans call for the final accelerator to be built
using 100 of these modules.
These and the innovator’s other successes shouldn’t
come as a surprise. Afterall, Dr. Parodi has been working
on applied superconductivity for particle accelerators and
detectors since the early 1970s. Among a long list of accomplishments,
he helped define a realistic scenario for the construction
of 128 superconducting cavities for the LEPII accelerator;
developed a test facility for the characterization of superconducting
materials and cables for high energy physics applications;
designed magnets for detectors; and led a Genoa-based group
in a joint CERN-INFN R&D program to develop new materials
and technology for accelerating cavities. The latter program
ended in the mid-1990s, showing the potential benefit of using
niobium-based composites and innovative building techniques
in achieving high fields in superconducting accelerators.
Over the years, he’s learned and taught a lot about niobium.
After informal discussions with friends in the Mechanical
Engineering Department at Genoa University, Dr. Parodi pursued
an idea to form niobium using a technique common in mass production
of stainless steel products. “We therefore decided to
use deep drawing at the facilities of a small local company
using a 300-ton press to produce stainless steel pans,”
he explains. “In the design process, I added a twist
that resulted (in) the winning idea. I proposed to design
the half cups forming the cavities with a fully rounded equatorial
region. This shape will give some advantages during all the
steps of the production process — at forming and the
electron beam welding stage and at the moment of the chemical
polishing.”
He continues, “I also had the feeling, supported by some
poor man computations, that the rounded equator will kill
the resonant emission of electrons (known as multipacting)
that plagued superconducting cavities at the time, putting
in jeopardy any chance of (their) reliable use in a practical
particle accelerator. The guess was a lucky guess... the rounded
equator shape killed at the origin the resonant discharge.”
Since Dr. Parodi presented his test results at the Applied
Superconductivity Conference in Pittsburgh, Pennsylvania,
designers of superconducting accelerating cavities have made
very good use of this so-called “lucky guess”. Truth
is that this innovator’s string of successes really has
little to do with luck, but much to do with curiosity... and
a strong will to triumph over challenges.
What keeps Dr. Parodi in this state of high energy? “I
really like this kind of research because we are forced to
deliver, at the end, a real device meeting the specification
needed to reach the goal set by the physical laws that we
want to investigate,” he says, “...the possibility
to overcome, at any accomplished step, one or more unforeseen
trap set by the physical laws is the real force that keeps
the projects (and ourselves) going. I like so much to do research
in applied physics, a field where the solution from any problem
comes out from a cross fertilization of different branches
of physics, electrical, and mechanical engineering.”
The Italian innovator sums up the drive behind his quest:
“The motivation comes from the feeling of working on
the edge of technology in the applied physics domain... trying
to stretch the knowledge of the fabrication processes beyond
the known frontiers,” he says. Sounds like the spirit
of innovation born out of a desire for discovery is still
alive and well in Genoa.
Niobium’s Role Growing in the Fight Against Corrosion
By:
Ronald A. Graham & Richard C. Sutherlin — Wah Chang
Niobium and niobium alloys have been used in a number of industries
and applications for many years, ranging from alloying in
steels and reactive metals to superconductor devices to rocket
nozzles, aircraft rivets, and even jewelry. Niobium’s
high melting point, superconductivity, strength, and other
properties make it well suited for use in such materials,
components, and consumer products. These favorable attributes
and others, such as corrosion resistance, are opening new
applications for this unique metal.
During the past 10-15 years, niobium has been considered and
applied in more and more chemical processing applications
because it exhibits excellent corrosion resistance in many
environments, including hydrochloric acid, nitric acid, chromic
acid, organic acids, salts and liquid metals. This article
contains information about the corrosion resistance of niobium
in a variety of environments. It also highlights those applications
where the metal and its alloys are currently being used and
describes those applications being considered for the future.
However, before launching into how niobium is solving corrosion
challenges, a brief introduction to this interesting metal
is in order.
Niobium’s Attributes
Niobium (discovered by English Chemist C. Hatchett in 1801)
is a soft, ductile metal that can be cold worked over 90%
before annealing becomes necessary. Its density of 8.57 g/cm3
is moderate compared to the majority of high melting point
metals, being only half that of tantalum at 16.65 g/cm3. Pure
niobium can be strengthened with additions of Zr, Ti and Hf.
Like many other reactive metals, niobium owes its corrosion
resistance to the presence of a readily formed, adherent,
passive oxide film. The oxide film can be composed of NbO,
NbO2, Nb2O5 or a mixture of the three. This oxide film will
exist in the lower valence state under reducing conditions.
Niobium’s oxide coating is therefore conditional, depending
on the media it is exposed to. The presence of oxidizing impurities
in the media will improve the metal’s corrosion resistance.
Niobium reacts readily with water to form a niobium oxide.
If the oxide forms a thin compact film, as is the case in
pure water and many dilute aqueous solutions, the corrosion
resistance is excellent. In the presence of complexing agents
like fluorine ion in H2SO4 and HCl, the corrosion behavior
of niobium is dominated by the dissolution of the oxide layer.
Niobium rupture disk.
The oxide layer also serves to preclude hydrogen pickup. The
bare metal easily absorbs monatomic hydrogen. If the oxide
layer fails in aqueous solutions, niobium will suffer from
embrittlement due to hydride precipitation.
The oxides of niobium have a relatively high vapor pressure
(compared with the metal), and, under conditions of low oxygen
pressures and high temperatures, the loss of metal via evaporation
of the oxides can be substantial. Oxygen uptake can be divided
into three regions as a function of time1: 1) a linear region
where oxygen dissolves without oxide formation; 2) a parabolic
region associated with the formation of a protective NbO2
layer and NbO growing into the metal; and, 3) another linear
region where the formation of porous Nb2O5 on top of the NbO2
occurs. At temperatures greater than 1600°C, evaporation
of the NbO dominates.
Table 1 compares niobium’s corrosion resistance to zirconium,
tantalum, and titanium in various media. Individuals are cautioned
to test coupons in their own media prior to commissioning
fabrication of large-scale vessels or structures. Specific
conditions may be sufficiently different from published data
that trials are warranted.
Two general references that readers should be aware of include
the ASM Metals Handbook volume on Corrosion and Wah Chang’s
niobium literature.2, 3 Both of these sources deal extensively
with the corrosion performance of the metal and its alloys
in various media.
A Growing List of Applications
As mentioned previously, niobium and niobium alloys have excellent
resistance to a wide variety of corrosive environments. These
environments include mineral acids, most organic acids, liquid
metals, most salts and liquid metals. And the list of applications
continues to grow.
Recently niobium has been considered for service in steel
pickling where hydrochloric acid is used. It is also resistant
to some fluoride-contaminated solutions, which is unique among
the reactive metals.
One of the more common uses for niobium has been in overhead
condensers and heat recovery sections of nitric acid facilities.4
It has also been considered for use in the pharmaceutical
industry, where high corrosion resistance is critical.5
It has been used for over 20 years as cathodic protection
devices for oil drilling rigs, ship hulls, bridges, and underground
storage tanks.6, 7 These cathodic protection devices, such
as platinum clad niobium anodes, use an impressed current
to force the corroding structure to become a cathode. Anodes
provide a cost effective method to protect structures from
corrosion attack caused by galvanic action. Niobium has also
been specified for anodes in a water purity system. This electro-chemical
based system effectively combined seven different proven water
treatment processes in a self-contained unit.8
Engineers have turned to niobium for use in evaporators in
the chrome plating industry to resist the hot, concentrated
chromic acid media.9 The reactive metal was shown to resist
the chromic acid environment, even those that could possibly
contain small amounts of free fluorides. Potential applications
in other environments include aqueous bromine and hydrogen
peroxide.
Recently, niobium has been used as rupture disks for chemical
applications with good success.10 The metal’s role in
this application has decreased the cost of the rupture disks
significantly over the other material of choice, tantalum.
Versatile niobium has excellent compatibility with liquid
metals like sodium, potassium, lithium, and uranium. It’s
currently used in sodium vapor lamps because of its corrosion
resistance to metallic sodium, either in vapor or molten form.11
The Nb-1Zr piece holds the metallic sodium, which is heated
to 875-925°C.12 Small amounts of zirconium scavenge oxides,
which tend to form on the grain boundaries. This precludes
grain boundary attack by some liquid metals. The presence
of excessive amounts of gas impurities, however, may reduce
niobium’s resistance to liquid metals.
Niobium generally has good corrosion resistance to sulfuric
acid at the low concentrations of sulfuric acid at room temperature,
but may embrittle in the higher concentrations. At elevated
temperatures, niobium will corrode rapidly if the sulfuric
acid concentration is above 40%. Fe+3 and Cu+2 ions can improve
the corrosion resistance in sulfuric acid.
In hydrochloric acid (HCl) environments in the lower concentrations
(<13%) above boiling and at the higher concentrations and
lower temperatures, niobium and niobium alloys have proven
corrosion resistant. The addition of ferric ion increases
the metal’s corrosion resistance greatly, especially
at higher temperatures. Niobium is not expected to need aeration
for its corrosion resistance. Recent preliminary testing has
shown that the metal may embrittle at the higher concentrations
of hydrochloric acids. Future work is planned to study this
further. Materials specifiers are also considering niobium
for use in HCl steel pickling applications to replace polyethylene
tubing, where an abundance of iron is present.13
Niobium is very resistant to corrosion in nitric acid environments.
It is resistant through the full range of concentrations and
to temperatures above boiling in nitric acid. Unlike other
reactive metals, niobium is not susceptible to stress corrosion
cracking in higher concentrations of nitric acid.
Like many reactive metals, niobium is susceptible to high
corrosion rates in hydrofluoric acid. It is, however, less
sensitive than other reactive metals in acidic environments
with small amounts of fluoride ion.
Niobium displays excellent resistance to corrosion in phosphoric
acid solutions. Robin and Rosa studied the corrosion behavior
of niobium and niobium-tantalum alloys in hot phosphoric acid
solutions.14 Their data show that large additions of tantalum
are required to significantly improve niobium’s corrosion
resistance. Table 2 presents selected corrosion data for the
metal in its pure state in phosphoric acid solutions.
As discussed earlier, niobium shows high resistance to many
different types of organic acids. Table 3 shows examples of
those acids where the metal has been tested and shown to have
good corrosion resistance. These acids include acetic acid,
citric acid, formaldehyde, formic acid, lactic acid, tartaric
acid, and trichloroacetic acid.
Niobium is resistant to a number of other media, including
bromine, chromium plating solutions and hydrogen peroxide.
Aqueous bromine is a highly corrosive media to many materials;
however, niobium has been shown to have excellent corrosion
resistance in liquid and gaseous bromine. It has been considered
previously for use in the processing, storage and transportation
of liquid and gaseous bromine. Table 4 provides corrosion
rates in bromine, chromium plating solutions, seawater, and
hydrogen peroxide.
Niobium is also resistant to most alkalis at room temperature.
It is, however, seriously attacked by hot alkalis and will
be embrittled in concentrated alkalis even at room temperature.
Similar to tantalum, niobium will embrittle in salts, such
as sodium, potassium carbonates and phosphates that hydrolyze
to form alkaline solutions. Table 5 shows that most all combinations
of alkaline solutions lead to embrittlement. Embrittlement
is typically by absorption of hydrogen species.
In salt solutions, except those that hydrolyze to form alkalis,
niobium exhibits excellent corrosion resistance. It is resistant
to chloride solutions, even with oxidizing agents present.
It does not corrode in 10% ferric chloride solution at room
temperature and is resistant to attack in seawater. Niobium
exhibits resistance similar to tantalum in salt solutions.
In gaseous environments, niobium has a large solubility for
oxygen, nitrogen and hydrogen. The gas atoms exist in interstitial
locations. As hydrogen is dissolved, the level will eventually
exceed the terminal solid solubility (TSS) limit, and niobium
hydrides will precipitate. Hydrides are brittle and create
internal stress risers that fail under low applied loads.
Oxygen and nitrogen (in solution) are potent hardeners and
will significantly decrease the ductility of the matrix. Additionally,
oxygen forms surface oxides that are non-protective; for example,
they spall since the oxides have a larger molecular volume
than the base metal.
Alloys Expand the Possibilities
In addition to the alloys mentioned in the preceding section,
there are several others that find use in corrosion resistant
applications. One of the biggest success stories is Nb-55Ti,
a material of choice in high pressure, highly oxygenated,
autoclaves used to leach gold and nickel. The alloy is particularly
resistant to ignition under these conditions.
Nb-55Ti has also been used in a wet oxidation process for
oxidizing organic materials in an aqueous waste stream to
improve biodegradability. The LOPROX® process requires
material that can survive 200°C and 20 bar pressures while
injecting pure oxygen into an acidified waste stream.15 Pure
titanium has sufficient corrosion resistance, but it is self-ignitable
under the stated conditions. The addition of niobium reduces
the propensity for ignition and improves the corrosion resistance.
Nb-55Ti has been used in the oxygen injectors, and recently
in a Nb-55Ti clad pressure vessel.
In other applications, principally nuclear-fuel-related, niobium
is used as an alloying element with zirconium to confer strength
and corrosion resistance in steam and/or heated water environments.
Zr-1Nb is used in the Russian VVER reactors for the fuel cladding.
It has been given the designation, Alloy E-610, by the Russians.
A variant of this alloy, M5, is being proposed for high burnup
fuel in reactors that have high coolant exit temperatures.
The salient feature of M5 is outstanding corrosion resistance
in high temperature, high pressure water with trace amounts
of lithium hydroxide. Another zirconium-niobium alloy used
in nuclear applications is Zr-2.5Nb.
The Canadian CANDU reactors use Zr-2.5Nb as horizontally oriented
pressure tubes that must resist sagging and provide corrosion
resistance to heavy water. The pressure tubes contain the
fuel assemblies and provide a channel for flowing coolant
(heavy water.)
The other class of important niobium alloys for corrosion
resistant applications is the metal combined with tantalum.
In this case niobium serves principally as a diluent to the
tantalum to reduce the cost. As mentioned earlier, niobium
has a density of 8.57 g/cm3, compared with tantalum’s
density of 16.65 g/cm3, which means that more volume is available
per unit weight. Since these metals are typically sold on
a weight basis, the cost of pure tantalum can be reduced by
the addition of less expensive niobium. Robin and Rosa have
performed extensive testing of niobium and Nb-Ta alloys in
hot hydrochloric and phosphoric acid solutions.14
Lupton, et al., have reported on the performance of Nb-Ta
alloys in boiling 70% H2SO4. The tantalum-rich alloys showed
a decrease in corrosion rate with increasing exposure time,
as was found for pure tantalum, whereas Nb-50Ta and Nb-40Ta
showed a constant or increasing corrosion rate.16 They further
report that the corrosion rate of tantalum-rich alloys like
Ta-40Nb are very much lower than would be expected for a simple
mixture of niobium and tantalum in which corrosion rate was
proportional to niobium activity. They propose that Ta-40Nb
could be considered for service in 70% H2SO4 at 165°C.
There may be many other applications for niobium and its alloys.
Studies are underway to (1) determine the effect of fluoride
ion in chemical media on the corrosion resistance of the metal;
(2) to determine the alloy’s resistance to acid pickling
applications in contaminated HCl solutions; and (3) to evaluate
niobium alloys for some pharmaceutical applications where
tantalum is currently being applied.
In summary niobium is a highly corrosion resistant metal in
mineral acids under oxidizing conditions. This reactive metal
is a good candidate for use in applications where low concentrations
of fluoride ions are present. It has excellent corrosion resistance
in liquid metals.
More detailed data, including corrosion charts and tables,
as well as information on fabrication of niobium and associated
alloys are available through Wah Chang’s Technical Services
group. Technical Services also provides corrosion testing,
failure analysis, consulting, and other services. For more
information, contact Customer Service at 541.967.6977.
References
1. F. Fairbrother, "The Chemistry of Niobium and Tantalum",
Topics in Inorganic Chemistry and General Chemistry, Ed. P.
Robinson, Monograph 10, Elsevier, Amsterdam, 1967.
2. Metals Handbook, 9th Edition, Corrosion. Metals Park, OH:
ASM International, 1987, 722-724.
3. Niobium, Wah Chang Data Sheets, Wah Chang, Albany, Oregon,
July, 2001.
4. H.C. Starck website, www.hcstarck.com/main26.html, viewed
on June 22, 2001.
5. Coscia, Mike, "Tantalum and Niobium for the Pharmaceutical
Industry", Tantalum Press Monitor, 1996.
6. Outlook, Vol. 6, No. 2, Teledyne Wah Chang Albany, Albany,
Oregon, 1985 (cathodic protection devices).
7. Outlook, Vol. 7, No. 2, Teledyne Wah Chang Albany, Albany,
Oregon, 1986 (cathodic protection of bridge structures).
8. Outlook, Vol. 9, No. 1, Teledyne Wah Chang Albany, Albany,
Oregon, 1988 (anodes for water purifying systems).
9. Outlook Vol. 8, No. 4, Teledyne Wah Chang Albany, Albany,
Oregon, 1987 (niobium evaporator used in chromic acid recovery).
10. Personal discussion with B.J. Sanders, Consultant, 2001.
11. Outlook Vol. 5, No.2, Teledyne Wah Chang Albany, Albany,
Oregon, 1984 (sodium vapor lamps, cathodic protection).
12. Outlook Vol. 9, No. 2, Teledyne Wah Chang Albany, Albany,
Oregon, 1988 (sodium vapor lamps).
13. B. S. Covino, Jr., J. P. Carter and S.D. Cramer, US Bureau
of Mines, "The Corrosion Behavior of Niobium in Hydrochloric
Acid Solutions", NACE, 1980.
14. A. Robin, J. Rosa, "Corrosion behaviour of niobium,
tantalum and their alloys in hot hydrochloric and phosphoric
acid solutions", International Journal of Refractive
Metals & Hard Materials, Vol. 18 (1), 2000, 13-21.
15. A. Dannmeyer, "Ti-45Nb: A Material of Construction
for Wet Oxidation Processes", Reactive Metals in Corrosive
Applications Conference Proceedings, Sunriver, OR, Sept. 12-16,
1999, Editors, J. Haygarth & J. Tosdale, Wah Chang, 2000,
21-29.
16. D. Lupton, W. Schiffman, F. Schreiber, and E. Heitz, "Corrosion
Behaviour of Tantalum and Possible Substitute Materials under
Extreme Conditions", Metallic Corrosion, Proceedings,
8th International Congress on Metallic Corrosion, DECHEMA,
Frankfurt, 1981, 1441-1446.
Q & A
Taking Advantage of Castings
By:
Mike Wilcox
Mike Wilcox submitted the Q&A column for this issue of
Outlook. Mr. Wilcox served as Operations Manager for Commercial
Titanium Castings, Inc., an investment castings foundry, prior
to joining the Wah Chang team in March of 2000. Since June
of 2000, he has been a key member of the company's rammed
graphite sales team. Mr. Wilcox is asked many things about
casting titanium, zirconium, and other metals, but says that
three questions pop up most often. This Q&A column addresses
these questions and provides some guidance for those considering
castings for their application(s).
Questions:
• When would a casting be a better design choice than
a forging or fabrication?
• When should a rammed graphite casting be used?
• When should an investment casting be used?
Note: The answers below are intended to provide
a starting place for designers. Please use the contact information
at the end of the article for more detailed, application-specific
information.
Answers:
The casting versus forging issue has been a subject of debate
for some time. When the design of the part goes beyond a simple
shape, a casting may quickly become a more economical choice
than a forged piece. In most cases a titanium casting can
be used in place of a forged part without compromising the
mechanical requirements for the part as long as the casting
has gone through the HIP process.
HIP is the commonly used term for Hot Isostatic Pressing.
This process subjects the casting to high heat and high pressure
over time in an inert gas atmosphere. For titanium the temperature
is from 1550ºF to 1750ºF at 14,500 PSI maximum for at least
2 hours. The pressure is uniform in all directions. HIP collapses
and bonds the internal voids that are inherent in the casting
process.
The HIP process greatly improves the mechanical properties
of the casting. The tensile and yield properties are equal
to a forged product. The elongation and reduction of area
may be slightly less in a casting than in a forged part. The
fatigue properties of castings that have gone through HIP
are generally higher than forged parts. The mechanical properties
in a casting are distributed equally in all directions due
to the unidirectional grain structure of a cast part. A forging
will have higher strength in one direction than another due
to its directional grain structure. If the application goes
beyond the requirement of a simple shape or if tooling costs
and lead-time issues are factors, a casting should be a serious
consideration.
The use of castings to replace fabrications is a more obvious
choice. One casting can replace complex fabrications in many
instances and greatly reduce labor cost and lead-time. A casting
will also be very stable dimensionally from one to another
eliminating complex set up, fixturing and inspections required
for weld fabrication. It is not uncommon to replace 10 to
15 fabricated components with one casting.
The choice of rammed graphite (sand-type) casting or investment
casting has proved confusing to design engineers. It is a
misconception to believe that rammed graphite casting is only
good for large, simple shape requirements and that investment
casting is only for small intricate parts. Rammed graphite
casting can be used to cast complex components. The limiting
factor is often wall thickness that is too thin or part shapes
that do not allow access for ramming the mold or mold removal
after casting. Access limitations can also effect the investment
casting shell process as well. When wall thickness approaches
0.25 in. or less with the rammed graphite casting process,
the metal may not remain fluid long enough to completely fill
the mold due to the heat absorption of the graphite. Investment
castings can be used with wall thickness down to the 0.040-in.
range depending on how far the metal has to flow. The investment
casting process can be used to produce castings well over
100 lb. and in very complex shapes (rammed graphite over 1000
lb.).
There is considerable overlap when either process can produce
castings meeting design requirements. In these cases, tooling
and lead-time considerations may take precedent. Production
tooling for rammed graphite casting is less expensive in most
cases and tooling lead-time shorter. Casting lead-time is
about the same. Investment castings are traditionally more
expensive than rammed graphite castings.
In conclusion, we recommend that design engineers contact
casting manufacturers as early in the design process as possible.
By working together, the engineers and the foundry can determine
early on if castings can offer an advantage over wrought product
and, if so, which casting process will be most cost effective
for the application. In addition, the foundry can help design
the part to take full advantage of the casting process.
For more information on castings or to discuss a potential
application, contact Mike Wilcox at 541.967.6932 or by e-mail
at mike.wilcox@wahchang.com.
E V E N T S
ANPSG/Nitric
Groups to Meet Under the Tucson Sun
October
7–10, 2003 — Tucson, Arizona
About this time of year, it’s customary for Outlook to
feature a wrap-up of the annual Nitric Acid Producers Meeting.
That won’t be happening this time around. For the first
time (at least in modern times), the Nitric Acid Producers
have decided to team up with their brethren in the Ammonium
Nitrate Producers Study Group (ANPSG) for a week-long operations,
engineering, and safety jam session in the Arizona desert.
The event takes place October 7-10 at the Loews Ventana Resort
in Tucson.
As always, the “Friends” of the ANPSG and Nitric
Acid Producers groups will be on hand to offer a veritable
plethora of helpful materials, equipment, and engineering
solutions in a combination exhibit hall-hospitality suite.
Hall organizers Messrs. Bob Gill (Ellett Industries) and sidekick
Kirk Richardson (Allegheny Technologies Incorporated) believe
that this year’s meeting should prove well worth a trip
through a forest of cactus (only if you choose to arrive by
foot). “Exhibitors are getting a two-for-one package,
with both events held at the same resort, back-to-back,”
says Richardson. “In addition, Meeting Chairs Ricardo
Rodriguez (ANPSG), Shawn Rana (Nitric Acid Producers), and
their Executive Committees have done an excellent job of organizing
this year’s sessions. Plus the ever clever Mr. Gill (rgill@ellett.ca)
is scheming up another of his nearly famous golf scrambles.”
For up-to-date information on the technical meetings (along
with overall information), visit anpsg.org, which lists presentation
topics and much more. To sign up for the exhibit hall (and
folks… the spaces are going faster than a jack rabbit
with a hungry coyote on its tail!), contact Ms. Sheryl Renzoni
at 541.926.4211. Spaces are $1750 each and will be doled out,
first-come, first served. Don’t miss the chance to defrost
with us under the Tucson Sun this fall.
LYNN DAVIS
President
PARRY WALBORN
Vice President — Commercial
GARY KNEISEL
Director of Sales
ANDY NICHOLS
Director of Marketing
KIRK RICHARDSON
Editor
©2002 Wah Chang. Outlook is published quarterly
by Wah Chang (Albany, Oregon office). The newsletter contains
information on reactive and refractory metals, including hafnium,
niobium, titanium, vanadium, and zirconium, as well as chemicals.
The properties listed herein are average values based on laboratory
and field test data from a number of sources. They are indicative
only of the results obtained in such tests and should not
be considered as guaranteed maximums or minimums.
Information & Order
Contacts
Wah Chang
(headquarters)
P.O Box 460
Albany, Oregon 97321
T 541.926.4211
F 541.967.6990
www.wahchang.com
www.corrosionsolutions.com
Sales/Tech Support
T 541.967.6977
F 541.967.6994
custserv@wahchang.com
CPI Service Center — US
T 541.917.6739
F 541.924.6882
ellen.baumgartner@wahchang.com
CPI Products
T 541.967.6906
Nuclear-Grade Alloys
T 541.967.6914
Ti, V, and Nb Products
T 541.967.6977
Allvac
PO Box 5030
Monroe North Carolina 28111-5030
T 704.289.4511
www.allvac.com
Allegheny Ludlum
500 Six PPG Place
Pittsburgh Pennsylvania 15222
T 800.258.3586
www.alleghenyludlum.com
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