Note: Descriptions are shown in the official language in which they were submitted.
I
CORROSION RESISTANT Curium
PELLET AND METHOD FOR WAXING THE SAME
Technical Field
This invention relates broadly to uranium metal
alloys and their use in ammunition, e.g. pellets of
the shot or generally spherical type. An aspect of
this invention relates to a method err making
ammunition ego. spherical pellets) wherein a molten
chromium-uranium mixture is cooled rapidly, e.g. by
exposing drops of the molten mixture to normal ambient
temperature conditions. Still another aspect of this
invention relates to a chromium-uranium 238 alloy
which is sufficiently corrosion resistant to be used
in the manufacture of high-density shotgun pellets.
Stilt another aspect of this invention relates to a
solid chromium-uranium alloy, pellets made from this
alloy, and methods for making the pellets, wherein the
uranium metal contains at least as much of the U238
isotope as naturally-occurring uranium and preferably
; contains less than about 0.3~ of the U235 isotope.
Description of the Prior Art
metallic lead has been used in ammunition since
the earliest days of ordnance technology. Lead is
plentiful, inexpensive, easily fused and formed, and
very dense - the typical reported density value
; being 11.34 g/cm3. All of these advantages haze been
important in the manufacture of an~nunition pellets,
particularly spherical pellets of the type used in
shotgun shells. The typical shotgun shell comprises
a cylindrical casing enclosing an explosive charge
and a plurality of spherical pellets. The density
of the pellets is particularly important to sportsmen
such as hunters and trapshooters. the high density of
lead has been a popular feature of lead shot among
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sportsmen, many of whom prefer lead over other metals
and alloys having a density less than about 8.0 g/cm3.
Lead shot has, however, one overwhelming dozed-
vantage. Toxic effects of lead upon the systems of
live waterfowl, whether remnant from nonlethal
injury or perhaps ingested with wild grains during
feeding, have prompted protective action on the part
of Governmental agencies concerned with environmental
- - quality. These agencies have urged or even forced
restrictions on the use of lead shot in shotgun shells
used by outdoor sportsmen.
Those branches of the ordnance or ammunition
industries concerned with supplying sportsmen have
responded to this toxicity problem by substituting
various other metal pellets for lead pellets. The
substitutes for lead are metals or alloys which are
generally considered to be less toxic, foremost among
these metals or alloys being stainless steel. Stainless
steel is a more likely substitute than, for example,
2Q nickel, since it is relatively inexpensive. Further-
more, the high industrial and military priority
attached to many of the denser metals such as nickel
makes their continuous availability for purely outdoor
sports usage questionable. The primary advantage
ox such metals is their specific gravity t8~0 in
the case of nickel, which is far below lead jut
still significantly above the 7-8 range typical of
most ferrous metals (particularly the steels). Iron
and steel have other disadvantages besides low
density. The melting point of iron is 1,535 C. -
more than eighty Celsius degrees above the melting
point of nickel. Lead melts at only 327~ C.
Maintaining iron in a molten condition unquestionably
involves large amounts of energy and makes formation
of spherical shot by the drop technique more difficult.
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At the present time, the alloying of iron or
steel with lead does not appear to be a practical
alternative for the manufacture of spherical pellets.
Iron and steel do not appear to have sufficient
compatibility with lead, even in the molten state,
and the formation of a spherical shot from a petal
containing segregated lead or iron or lead- or iron-rich
phases is not an attractive prospect.
There are, of course, many other metals which
do dissolve in molten iron or steel. Many of these
metals have a specific gravity below 8.0 (i.e. a
density below 8.0 g/cm3) and thus do little to
improve the ballistic properties of stainless steel
shot. Other metals, such as wolfram (tungsten have
a density even higher than lead but also have very
high melting points - 3,33U C. in the case of
wolfram. In addition, wolfram is too expensive to be
competitive with nickel and other lower-melting metals
having a higher density than iron.
Uranium is a very dense metal, which is now
becoming readily available as an isotopic mixture
containing more than the naturally-occurring amount
of U233. The most plentiful type of high-percent
38 is a by-product of the uranium enrichment process
for nuclear power. This by-product uranium metal is
called "depleted uranium". Atari uranium normally
contains I of U23~, 0.7~ of U235, and 0.005% of
U234. (these percentages are approximately the save
on either an atomic or weight-percent basis, due to
the very small differences between these isotopes in
their atomic weights and densities The uranium
enrichment process produces two products: uranium
'enriched" in U235 and the so-called depleted
uranium, which typically contains less than 0.3~
U235. Efforts are now underway to utilize depleted
- 4 - ~2Z~
uranium in nonnuclear applications, since its supply
is large and rowing, and since its radioactivity
level is very low (hardly more hazardous than a
radiuI~-dial watch). Radiation exposure is thus not a
great problem to employees working in the presence
of large quantities of depleted uranium or its alloys,
and techniques for reducing toxicity hazards are
known. For these and other reasons, depleted uranium
and its alloys have been suggested for high-density
applications, including munitions. US. Patent No.
3,773,569 (Edelman et at), issued November 20, 1973
descries a uranium-titanium binary alloy containing
more than 9B% uranium and therefore having a nominal
density in excess of 18 g/cm3. It has also been
suggested to use sistered uranium in the core of
ammunition having a subcaliber shell. See US.
Patent No. 3,4~8,222 ~Birkigt~, issued March 3, 1~70.
Louvre, the applicability of these developments to
the manufacture of sports ammunition is my no means
straightforward. Uranium and its salts are both
toxic. In addition, uranium has a high level of
chemical activity toward elements and compounds which
occur commonly in the environment, e.g. water and
oxygen. The coating or encapsulation of uranium within
corrosion-resistant material Gould appear to be
impractical from an industrial standpoint; furthermore,
the grinding action of the digestive system of waterfowl
- would expose the enclosed uranium core to the animal's
system.
Summary of the Invention
; This invention relates broadly to solid, generally
spherical, generally corrosion-resistant, chromium-
uranium metal pellets and the corrosion resistant
chromium-uranium alloys used to make -them. The
35 invention also relates to a method for making the
chromium-uranium alloy wherein about 3-90 atomic-%
molten uranium is blended with about 10-35 atomic-%
molten chromium and the resulting mixture is cooled very
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quickly to about ambient temperature. The preferred
spherical pellets of this invention are generally
uniform in composition and comprise a chromium-uranium
alloy having a specific gravity of at least about 8.4
and containing at least about 10 atomic-% chromium and
up to 90 atomic-% uranium. It is particularly preferred
-to use a ferro-chromium-uraniu~ alloy which is
essentially a solid blend of about 3-70 atomic-% uranium
with 10-35 atomic-% chromium, the major amount of the
balance of the alloy being iron. Typical iron-chromium-
uranium alloys of this invention comprise 3-70 atomic-%
uranium, 1-87 atomic-% iron, and 10-35 atomic-%
chromium. Small amounts ox other elements such as
carbon, manganese, molybdenum, phosphorus, sulfur,
nickel or the like can be present in the alloy. The
spherical pellets are particularly useful as
corrosion-resistant, non-toxic, but extremely dense
replacement for lead pellets in shotgun shells. (The
; density of lead can be equaled or exceeded, if
desired.) The uranium alloys, per so, have utility in
applications where a dense, corrosion-resistant metal is
needed.
'I It has now been found that uranium (e.g. "depleted
uranium" with its low radioactivity and higher-than-
normal content of U238) can be used successfully in
ammunition, particularly spherical pellets, by alloying
the uranium with a suitable
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amount of chromium and/ if desired, a metal of the
first triad of Group VIII of the Periodic Table.
Although uranium alloys have been known at least
since 1914 (e.g. ferro-uranium alloys), certain
unusual characteristics of the preferred uranium alloy
systems must be taken into account when making
ammunition with the desired characteristics. Uranium
undergoes two phase changes as it is heated up from the
solid state. From room temperature up to 660 C.,
the metal is in the so-called a ha state or phase.
Beta-phase uranium is the allotropic form in the
670-780 C. range. Still another phase change
converts the metal to the gamma phase at above 780
C. Only the gamma phase (between 780 C. and the
melting point, approximately 1,132 C.) has a body-
centered cubic crystalline structure similar to
a structure of iron. If ferro-chromo-uranium alloys
were cooled slowly, the uranium would have a tendency to
segregate, leading to the formation ox microscopic
regions of significantly altered composition as
compared to the overall composition of the alloy.
Such segregation, which may be undesirable in the
context of this invention, can be mitigated by rapid-
cooling of the uranium alloy from the molten state.
Fortunately, such rapid cooling can be easily achieved
with a conventional shot vower, where drops of the
molten alloy are formed and permitted to fall through
an atmosphere or medium which is substantially at
normal ambient temperatures. The rapidly cooled
solid and generally spherical pellets have a greater
; tendency to comprise alloys of generally uniform
composition, both microscopically and microscopically,
as compared to melts or liquid-solid mixtures cooled
more slowly. To provide an alloy which is corrosion
resistant and essentially inert toward the environment
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and the interior biological systems of animals and
birds, it is particularly preferred that the uranium be
alloyed with an amount of chromium sufficient to be
corrosion resistant under a variety of conditions.
Although this invention is not bound by any theory,
it is believed that an important principle of stainless
steel technology applies to uranium-chromium alloys;
namely that an alloy (whether or not: it contains iron)
may have the property of forming a microscopic
layer of protective oxide similar to the Allen layer
on aluminum metal exposed to air provided that the
chromium content of the alloy is at least about 10
atomic percent, more preferably at least about 12 atomic
percent. on other words, at least one out of every
10 (more preferably one out of every I atoms in the
alloy should be chromium to obtain "stainless"
properties. Other desiderata which normally play a
predominant role in metallurgy (e.g. cold-working and
stress-induced corrosion properties of the alloy
are of minor significance in the context of this
invention, wherein the major factors to be considered
are density, melting point or melting range, passivity
or chemical/biological inertness, resistance to
segregation in the molten phase and upon rapid cooling,
and compensation for any loss of free shim from
formation of inter metallic compounds and other
combined forms of chromium.
Accordingly, this invention contemplates a
solid, generally spherical, generally corrosion-
resistant metal pellet comprising a relati~elynon-se~regating corrosion-resistant chromium-uranium
alloy which has been cooled from the molten state to
below its solidification temperature (e.g. to below
about 670 I quickly enough to prevent the formation
of a segregated uranium-rich phase wherein the atomic
percent of chromium in the phase is less than about
10~, the resulting alloy having a specific gravity
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of at least about 8.4. It is desirable to avoid
formation of any phase whose altered chromium might
adversely detract from the passivated character of
the alloy. The invention also contemplates a solid
chromium-uranium 23~ alloy comprising at least about
10 weight or about 3 atomic % solid uranium
distributed uniformly (microscopically as well as
microscopical through a corrosion-resistant chromium
or ferro-chromium matrix, which alloy has so called
"stainless" characteristics. The meting point of
such chromium-uranium alloys is below the melting
point of chromium tl~05 Cal, typical alloys of this
invention melting at a lower temperature than iron,
i.e. below 1535 C.
Detailed Description
As will be apparent from the foregoing Summary,
alloys of this invention and pellets or other
ammunition made from these alloys can ye considered
to be chrornium-uranium alloys because at least about
10 or 11 atom-% of chromium is needed for corrosion
resistance and at least about 3 atom-% uranium is
needed to make a significant contribution to the density
and solidification point depression of the alloy.
Theoretically up to 90 autumn of the alloy can be
"depleted uranium" (uranium substantially free of
I; U-235~ without losing some corrosion-resistant
character; however, there is then a risk that the
necessary free chromium may become partially tied up
in uranium-chromium inter metallic compounds. Other
elements can be included in the alloy to reduce cost,
depress the melting point, form inter metallic
compounds with uranium, etc., but the density of
uranium (almost I gjcm3) and the stainless imparting
properties of, say, 12 20 autumn chromium are
difficult to improve upon. aluminum! Minoans and
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molybdenum can enhance corrosion-resistance in some
alloys (aluminum helps the corrosion-resistance of
iron by aiding in the formation of thin,
adherent metal oxides, but aluminum is low in
density - its principal drawback. rletals such as
molybdenum, tantalum, and niobium could ye useful
but for their cost, which is prohibitive in the
context of spherical shot manufacture. Of the
nonmetallic elements, nitrogen is perhaps a consider-
lion, and silicon and carbon can be present. Ills difficult to totally eliminate carbon from a
ferro-chromium~uranium alloy, and such elimination is
ordinarily not necessary. Conventional steel making
techniques can reduce carbon levels well below
1 White, e.g. to less than 0.05 White, if
desired.
It is ordinarily impractical to include more than
about 30 or 35 White of chromium in alloys of this
invention. Because of the high atomic weight of
uranium this level of chromium works out to be roughly
85 or 90 atomic percent. Needless to say, much
lower atom-% levels of chromium will ensure a high
level of free (uncombined) chromium in the microscopic
structure of the alloy. It is a fortunate circumstance
that uranium happens to form alloys rather easily
with chromium, thus underscoring further the
usefulness of this metal as a corrosion resistance-
imparting element of the alloy composition Just as
in stainless steel technology.
Because of the high cost and periodic scarcity
of chromium metal, it is desirable to introduce
chromium into alloys of this invention in forms other
than high-purity chromium metal, e.g. as stainless
steel scrap or as chromium ores or compounds (such as
cremate which have been reduced with aluminum,
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carbon, or other reducing agents. The ferrochrome
which is obtained in the carbon reduction process
is typically contaminated with a significant amount
of carbon, but conventional oxidation treatments
can reduce this carbon level if such reduction be
necessary or desirable.
Accordingly, alloys of this invention will
typically contain a metal of Group VIII, first triad,
of the Periodic Table, most preferably iron or
mixtures of iron and nickel. Nikolai is the most
economically practical replacement for iron, but
cobalt is technically operative, as are any mixtures
of these three Group VIII metals.
An alloy for shot of this invention can comprise
as much as about 87 autumn or about 80 White of
iron, nickel, cobalt, or combinations of these
Group VIII metals. Ordinarily the amount of Group
VIII metal or metallic mixture will be at least
one-tenth of the amount of chromium Jon a weight
basis), more typically at least about twice the weight
of the chromium component. For example, it is
ordinarily preferable to "dilute" ferrochromium master
alloy with further iron, so that the Faker weight
ratio ranges from 1:1 to about 6:1 or even as high
as depending upon the amount of uranium to be
added to form the ferro-chromium-uranium alloy.
Silicon is a very abundant element which is
present in many types of steel r though rarely in any
amount greater than 1 weight-%, still rarer at 3 or
I by weight. It is also difficult to avoid the
presence of at least some carbon in iron-containing
alloys. Amounts of manganese and molybdenum are
typically in the range of 0-4 weight-%. Still other
elements such as aluminum, copper, vanadium,
wolfram tungsten), zirconium, boron, tantalum,
niobium and Group VIII elements such as nickel and
lo- I
cobalt, are ordinarily optional in the context of
this invention. They can, in any event, he tolerated
if already present in steel scrap used in making an
alloy of this invention, the only requirement being
that they do not detract from the passive character
of the alloy Mickey, if present, may be used in
significant quantities, e.g. up to a one-for-one
replacement of the iron.
Many nonmetallic or residual-type elements can
be present in alloys of this invention as in
conventional stainless steel. These so-called trace
elements can be introduced either through the
chromium or ferro-chro~ium component or the depleted
uranium. It is ordinarily preferred to keep their
amount below about 200 parts per million, although
this upper limit is not critical in this invention.
Typical of such elements snot previously mentioned
in connection with desirable alloys) are oxygen and
hydrogen, phosphorous, Selfware, and the like. Nitrogen,
selenium, phosphorous, and sulfur may either be present
as trace elements or as deliberately added elements,
Carbon, even if present in amounts greater than
1 weight-%, can be reduced substantially to trace
levels. Alternatively, about 1.0 weight% Or can
be added for each 0.1 weight-% of carbon in excess
of 0.1 weight-%.
Among the ferro-chromium materials used to form
the ferro-chromium component of an alloy ox this
invention are the conventional stainless steels,
preferably in the form of stainless steel scrap.
stainless steels commonly contain elements other than
iron and chromium, e.g. carbon, manganese, and
silicon. The amount of carbon is typically in the
range of about 0~01-1% by weight of the stainless
steel, but stainless steels containing up to 1~2%
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by weight carbon are known the amount of manganese
and silicon is typically less than about 2% my weight,
and the amount of phosphorous and sulfur typically
about 0.4% by weight. The other elements discussed
previously are sometimes used in commercially
available steels. These elements can be present
within the limits described previously. Many of the
typical corrosion-resistant steels which contain about
15-30% by weight of chromium contain about 5-19% by
weight of nickel.
Although corrosion-resistant or heat-resistant
steels containing as little as 4% by weight of chromium
are known, which also amounts to roughly 4 atom-%
in a typical steel, the stainless steel scrap used
in this invention should contain more thaw 10 atom-%
of chromium (e.g. 12-20 autumn) to ensure that the
resulting Faker alloy will contain at least about
10 autumn of chromium. In those situations wherein
the alloying with uranium reduces the chromium
fraction to less than 10 atomic percent of the total
alloy (including uranium, it is ordinarily preferred
that a steel higher in chromium be added also or that
some chromium or ferro-chromium be added to keep the
chromium fraction above 10 autumn. If desired,
further adjustments to the melt can be made by adding
other elements discussed previously., Some adjustments
in the composition of the melt may have to be made
to take into account trace elements present in the
depleted uranium or other form of uranium used to
pa make ammunition of this invention; however, these
trace amounts are often small enough to be
disregarded.
One objective of this invention is to prepare a
stainless uranium-steel alloy which has been cooled
so rapidly that the iron and uranium atoms hove not
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had a chance to segregate in two or more of the special
crystalline forms called "phases" in metallurgical
terminology. It is known in the art that the internal
precipitation and formation of alpha uranium can be
avoided or mitigated by a technique known as "beta
quenching", wherein the metal is heated above 668
C. to form the beta phase and then quenched through
the alpha-beta transformation range zoo rapidly to
allow growth of the new phase. This treatment
produces a randomly oriented, fine-grain alpha-prime
tmartensitic alpha) phase. Taking advantage of
this or similar phenomena, the preferred chromium
content of an alloy of this invention will not be
altered locally by the formation of a Croupier phase
when cooling from the melt to solidification. the
process of this invention as ordinarily practiced
provides a very rapid cooling from temperatures at
which the uranium-chromium or uranium-chromium-Group
VIII metal mixture will be molten and generally
homogeneous, and the degree of randomness or
microscopic intermingling ox the metallic elements in
a flash-cooled or shot-tower cooled alloy can be at
a very high level. Microscopic grains containing
less than 10 or 12 autumn chromium alloyed with
uranium - which could microscopically corrode and
become highly toxic - thus do not have sufficient
time to form easily, so that the gross analysis of the
alloy closely parallels the microscopic analysis.
Furthermore, typical shot pellets are smaller than
10 mm in diameter and may even have diameters less
than 1 mm. Accordingly, the surface of the
generally spherical droplet of molten metal in the
shot tower can generally be expected to be less than
5 mm from the innermost regions of the drop, thereby
further insuring a rapid dissipation of the heat
which was stored in the molten metal before it was
~29~
- 13 -
formed into drops. Since typical drops falling down
a shot tower form generally spherical shapes ranging
from about 1 to about 5 mm in diameter, it can be
assumed that the innermost regions of each drop will
have cooled to a temperature near ordinary room
temperature in a tatter of seconds, most certainly
less than 6Q seconds. Shot towers designed to make
spherical pellets out of stainless steel can also
handle ferro-chromium-uranium alloys, since the
melting point of these alloys will ye considerably
lower than that of most steels. Accordingly, the
formation of the ferro~chromium-uranium pellets can-
proceed in the Amy manner as the formation of
stainless steel pellets and, if anything, may occur
even easier.
Not very much is known about chromium-uranium-
Group VIII metal (e.g. Fe and/or Nix systems, but the
iron-uranium system has been discussed in the
scientific literature, and at least two inter~etallic
2Q compounds (UFe2 and U6Fe) have been reported. Very
small amounts of uranium drastically depress the
melting point of iron; similarly, small amounts of
iron produce a drastic melting point depression at
the other end ox this binary system. The compound
foe can be formed substantially free of the
uncombined metals, and it reportedly melts at
1230~ C. The compound U6Fe reportedly melts at only
805 C. Mixtures of U6Fe and UFe2 Semites containing
up to about 20~ uranium oxides, generally free ox
uncombined metals, provide the lowest-melting species
- shown in the constitutional or phase diagram
published by Groan, the amazingly low melt temperature
of 725 C. being possible with such a mixture having
an overall iron content or gross analysis near
lo weight % (32 autumn. A mixture of UFe2 and free
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iron can have a melting point as low as 1055 C.,
which is 70 Celsius degrees below that of pure
uranium and about 480 degrees below that of pure iron.
In the context of this invention, the
formation or presence of Fe-U inter metallic compounds
is preferable to either Faker or Cur compounds, and
any 105s of cold-workahility or the like resulting
from the ferro-uranium compounds is not detrimental to
the objectives of the invention The formation of
chromium compounds within the Faker system can be
compensated for by further addition of chromium petal
or mixtures containing chromium in the uncombined
metallic state. In any event, uranium, both combined
and uncombined, has a fluxing effect on an alloy used
in this invention and can help to keep melt
temperatures below 1500 C., more typically Howe
1450 C.
It is known that uranium alloys show better
corrosion resistance than unalloyed uranium.
Resistance to general corrosion increases with alloy
content and appears to be inversely related to
stress corrosion behavior. Fortunately, stress-
corrosion resistance is not important in the context
of this invention, thereby insuring the practicality
of alloys containing any amount of uranium,
however small, provided that this amount is sufficient
to make an appreciable contribution to the density of
the alloy. An alloy containing only 3 atom
uranium and 10-18 atom-% chromium, the balance Heinz
essentially the less expensive Group VIII petals,
; could show a density improvement of 0.5 ~/c~3 or
more.
s will be apparent from the foregoing discussion,
this invention is not limited to the selection of an
available stainless steel scrap or ferro-chromium
15 - 8
master alloy for alloying with the uranium. Suitable
ferro-chromium-uranium or chromium-uranium alloys
can be formulated from the elements themselves or
other alloys, inter metallic compounds or the like
and specifically tailored to the objectives of this
invention.
Very high density elements can be obtained
without making the uranium fraction any larger than
about 15 autumn or about 40~ by weight. At the
20 autumn level, one can readily approach the density
of lead, depending upon the iron and chromium content.
Await autumn uranium, the density of lead is easily
exceeded with 12-20 atom-% chromium and the valance
essentially iron. It is believed that toxicity
hazards can best be controlled when the uranium content
is a minor amount in terms of atomic percent;
nevertheless, amounts up to 70 autumn uranium leave
enough room for a reasonably inexpensive ferry-
chromium system with stainless imparting
characteristics. Because of the high atomic weight
of uranium, such an alloy would contain an amazing
90~ by weight of uranium. The equally amazingly low
level of about 3.5 weight-% chromium would still
provide the desired 12 autumn and an ability to form
- 25 microscopically thin, adherent metal oxides in air.
; In a typical ferro-chromium-uranium system, other
elements need not exceed, in total, more than 5 or
10 atomic percent and thus iron will typically ye
the major constituent of any component other than
the uranium or chromium component.
Stated another way, the broadly acceptable and
preferred atomic percentage fractions of the
ferro-chromium-uranium alloy are as follows.
:
: . .
.
999~3 .
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Element Atomic Percent (Autumn)
Broad Preferred
U 3.0-70 15-35
Fe 1-87 50-73
Or 10-35 12~20
Other elements 0-10 0-5
As noted previously, partial replacement of
the preferred chromium fraction with another corrosion
- resistance-enhancingelement can help to provide the
desired passivity at the 10 atom-% level of chromium.
It should ye assumed that the uranium in the
foregoing table is totally or predominantly the ~238
isotope, an economically viable form of this isotope
being the so-called depleted uranium. By using the
lo 23~ isotope, radiation is kept to tolerable levels
and the economics of the invention remain within
reach of practicality.
As is known in the art, all of those preferred
metals can form oxides in the presence of air, and
uranium is a particularly rapid oxide-former. o'er
this and other reasons, it may be desirable to
; utilize a shot tower containing a nono~idizing
atmosphere such as a noble gas or nitrogen although
- nitrogen can combine with metals at elevated
25~ temperatures, and this is sometimes intentionally
done in stainless steel technology, nitrogen is an
optional element in this invention
'..
In the claims which follow, amounts ego. in atom-%)
o* uranium and any other elements should be understood
to be gross amounts, in which both free and combined forms
are included. In the case of chromium, the recited amounts
should be understood to refer to corrosion-resistance or
- stainless imparting forms of the metal, e.g. free (uncombined)
chromium and ferrochromium.
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