Note: Descriptions are shown in the official language in which they were submitted.
1~0~339
Cobalt-base cast alloys containing chxomium and molybdenum
are well known to the art. They are characterized by a highly
desirable combination of room temperature properties, i.e.,
strength, fatigue resistance, ductility, wear resistance,
corrosion resistance, compatibility with biological tissue and
moderate hardness, and thus have been widely used in the dental
and orthopedic arts as a material for cast prostheses such as
dentures and surgical implants (see, e.g., American Society for
Testing Materials Designation F 75-67, "Standard Specification
for Cast Cobalt-Chromium-Molybdenum Alloy for Surgical Implants";
U.S. Reissue Patent 20,877; U.S. Patent 2,674,571).
U.S. Patent 2,180,549 discloses the use of a cobalt-base
alloy containing about 10 to 40 percent chromium, about 5 to 20
percent molybdenum and up to about 0.6 percent carbon as a
material for cast prosthetic articles and wrought wires. This
alloy possesses greater resilience, toughness and resistance to
acid than one containing less molybdenum and/or more carbon.
A denture casting alloy consisting essentially of 50 to 60
percent cobalt, 20 to 28 percent chromium, 10 to 2~ percent
nickel, 3.7 to 4.1 percent molybdenum and 0.18 to 0.22 percent
carbon is disclosed in U.S. Patent 3,544,315. Selection of the
narrow mol~bdenum and carbon ranges is said to optimize the com-
bination of strength, hardness, ductility and toughness properties.
The cast cobalt-chromium-molybdenum surgical implant alloy
sold under the trademark Vitallium (Howmedica, Inc., New York, N.Y.)
has the following approximate composition.
~"~ .
339
~e ish~ l~er-,ene
chromium 28
molybdenum 6
manganese 0.65
silico~ 0.70
nickel 0.5
iron - 0.5
tungsten 0.2
carbon 0.20-0.26
nitrogen 0.125-0.25
cobalt balance
Nitrogen is added to this cast alloy in order to improve the
tensile, fatigue resistance and ductility properties at room
temperature. Vitallium has outstanding properties as a cast
lS ~urgical implant and dental alloy and has been widely and
successfully used for the~e purposes. However, even superior
alloy properties, in particular superior bending fatigue strength,
are sought in order to further improve performance of this alloy
in its critical roles.
It is known that cobalt-base alloys cast in a nitrogen-
containing atmosphere, rather than vacuum melted, will absorb
low levels of nitrogen into their composition LElsea, A. R. and
McBride, C.C., Trans. A.I M.E., 188, 154-161 (1950); Fletcher, E.E.
~nd Elsea, A.R., Trans. A.I.M.E., 215, 917-925 (1959); Lane, J.R.
. .
and Grant, N.J., Trans. Amer. Soc. Metals, 44, 113-137 (1952)].
More than twenty years ago, wrought bar stock of Stellite~
21 tCabot Corp., Kokomo, Ind.), a cobalt-chromium-molybdenum
surgical implant alloy containing about 0.3 percent carbon and
about 3 percent nickel, was said to also contain about 0.09 to
~ ~Je ~ _3_
~10~339
0.17 percent nitrogen lWeeton, J.W. and Signorelli, R.A., Trans.
Amer. Soc. Metals, 47, 815-852 (1955)].
The fabrication of cobalt-chromium-molybdenum alloy surgical
implants as wrought, rather than cast, articles has been recommended
as a means of increasing strength, ductility, hardness, corrosion
resistance, fatigue resistance and wear resistance. The
ductility, corrosion resistance, fatigue resistance and wear
resistance of alloy in the as forged condition are improved by
subsequent heat treatment at about 1050C. [Devine, T.M., Kummer,
F.J., and Wulff, J., Journal of Materials Science, 7, 126-128
(1972); Devine, T.M., Cohen J., and Wulff, J., Proceedings of
the New England Conference on Bioengineering (Pope, M . H . et al.,
ed.), 136-153 (1973); see also U.S. Patent 2,486,576].
-
Unfortunately commercial production of wrought surgical
implants is economically prohibitive if the final shape of theimplant must be realized by a method such as cold working with
subsequent machining. A far preferable route would be to forge
the alloy directly into the desired irregular shape. However, no
economically feasible process is presently known wherein a
commercially available cobalt-chromium-molybdenum surgical implant
alloy may be forged directly into surgical implant articles without
a high percentage of fractures during forging.
It is generally known to the art that the ductility and
ability to hot or cold work a cobalt-base alloy containing
chromium and molybdenum may be improved by reducing the carbon
content of the alloy. Corrosion resistance is also generally
improved by such a reduction (Devine, Cohen and Wulff, op. cit.).
Chromium is generally known to increase strength hardness and
corrosion resistance at the expense of worka~ility, while
molybdenum is generally known to increase strength and hardness
at the expense of workability (see U.S. Patent 3,433,631, but cf.
U.S. Paten~ 4,012,229)~
--4--
339
The effect of low levels of nitrogen on the hot workability
of cobalt-chromium-molybdenum alloys and on the properties of
the resulting hot-worked material is not well understood.
A cobalt-base cast alloy designed for high temperature use
containing 23 to 36 percent chromium, 2 to 15 percent nickel,
12 to 16 percent tungsten, up to 3 percent molybdenum (the sum
of tungsten plus molybdenum being not greater than 16 percent),
0.2 to 1.0 percent boron, a deoxidizing amount of manganese, up
to 5 percent iron, 0.3 to 0.9 percent carbon, and up to 0.25
percent nitrogen is disclosed in U.S.Patent 2,746,860. Hot
ductility is seriously impaired when the carbon level is greater
than about 0.4 percent. The nitrogen is included for high
temperature stability against embrittlement. However, U.S. Patent
2,977,244 teaches that a nitrogen content above about 0.04 weight
percent in a cast or wrought cobalt-base alloy containing 19 to 22
weight percent chromium, 11.5 to 13.5 weight percent tungsten, up
to 0.15 weight percent boron, up to 3 weight percent molybdenum
and up to 0.25 weight percent carbon is deleterious to stabil.ty
against stress-rupture at 1700F.
It is taught that the presence of nitrogen, like carbon,
decreases the cold workability of the wrought cobalt-base alloy
disclosed in U.S. Patent 3,356,542 (Co-Ni-Cr-Mo).
U.S. Patent 3,366,478 discloses that the forgeability of
an alloy comprising about 15 to 30 percent chromium, about 10 to
30 percent nickel, about 2 to 12 percent tantalum, up to about
3 percent molybden~m, about 0.03 to 0.20 percent carbon and
the balance cobalt is markedly increased by the addition of about
0.01 to 0.5 percent zirconium. The stated role of the zirconium
is to chemically associate with carbon and undesired elements such
as oxygen and nitrogen (which are to be held to the lowest possible
levels).
A novel alloy has been discovered consisting essentially of
5_
3~9
about 22 to 27 weight percent chromium, about 3 to 6 weight per-
cent molybdenum, about 0.10 to 0.25 weight percent nitrogen, up
to about 0.15 weight percent carbon, up to about 1 weight percent
manganese, up to about 1 weight percent silicon, up to about 2
weight percent iron, up to about 2 weight percent nickel, and the
balance cobalt. It has a high degree of hot workability and may
be readily forged directly into irregular shapes. Wrought articles
constructed of this alloy have excellent room temperature tensile
properties, fatigue resistance, wear resistance, hardness, duct-
ility, corrosion resistance and compatibility with biological
tissue. The novel alloy is thus an excellent construction mater-
ial for wrought surgical implants such as prosthetic hip stems
and surgical nails.
The invention disclosed herein consists of said novel
alloy in the wrought condition and a wrought surgical implant
constructed of said alloy.
The invention further consists of a hot forged alloy
article consisting essentially of about 22 to 27 weight percent
chromium, about 3 to 6 weight percent molybdenum, about 0.10 to
0.25 weight percent nitrogen, up to about 0.15 weight percent
carbon, up to about 1 weight percent mar.ganese, up to about 1
weight percent silicon, up to about 2 weight percent iron, up to
about 2 weight percent nickel, and the balance cobalt.
The alloy of this invention is said to be wrought, as
that term is defined herein, if it has been hot worked at a
temperature above its recrystallization temperature (ca. 1900F.)
at least once subsequent to casting so as to reduce a linear
dimension by at least about 5 percent. The main purpose of such
treatment is to impart strength and fatigue resistance to the
alloy.
The basic method of preparing the alloy of this
invention comprises casting an ingot having the desired elemental
- 6 -
``` ~1(3G339
content and, if desired, subsequently processing the ingot into
a desired final alloy condition, shape and size. The alloy may
exist in many different conditions depending on its process
history, e.q., the as cast condition, as forged condition, forged
and annealed condition, forged and recrystallized condition,
cold rolled condition. The alloy of this invention may be used
as a material for cast dentures.
The input metal stock is melted and cast into ingots
by methods well known to those skilled in the art. Cobalt,
chromium
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11(3 ~339
and molybdenum are added to the melt in the proportion desired
in the alloy product. The carbon content of the ingot is
controlled by using low carbon metal stock. Low levels of
manganese, silicon, iron and nickel are almost inevitably
introduced to the melt as contaminants of commercially available
metal stock. Manganese and silicon act as deoxidizers during
the melting operation.
The nitrogen content of the ingot may be introduced at a
reproducible level by any one of several methods, for example:
(a) adding the desired amount of nitrogen to the melt in
the form of a material such as CrN and melting under an inert
atmosphere (e.g. argon);
(b) melting under a nitrogen atmosphere (nitrogen content
determined by nitrogen partial pressure); and
(c) melting under air (nitrogen content determined by
melting time and temperature). --
Subsequent processing of the ingot may include one-or more
common metallurgical operations such as hot working, homogenization,
solution annealing for stress relief, heat treatment; for grain
recrystallization and increase of ductility (i.e. partial
annealing), or cold working, depending on the desired final
properties.
A preferred hot working operation is hot forging, broadly
defined as hot working metal at a temperature above its recrystal-
lization temperature into a shape of finite size by hammering orpressing. The alloy of this invention can be readily hot forged,
without a high incidence of fracturing, directly into strong,
fatigue resistant articles of irregular shape, e.g. prosthetic
hip stems, at much lower costs than by forming it into irregular
shapes by other methods such as cold working and subsequent
machining. Any conventional forging method may be used (e.g.,
nammer forging, drop forging, press forging).
--7--
1~0~339
The alloy of ~his invention in the as fGrsed condition
possesses at room temperature ultimate tensile strength, yield
strength, fatigue resistance, hardness and resistance to
corrosion by physiological fluids significantly superior to those
of cast alloy of the same composition. A particularly advantageous
post-forging operation is to partially anneal an article in the
as forged condition to increase its ductility, without reducing
its ultimate tensile strength by more than about 10 percent, by
subjecting said article to a heat treatment at a temperature
above its recrystallization temperature for a time period
sufficient to cause essentially complete grain recrystallization,
but not sufficient to cause substantial grain growth.
The preferred carbon content is up to about 0.1 weight
percent, tne preferred nitrogen content about 0.15 to 0.20 weight
percent. An alloy of this invention containing said levels of
carbon and nitrogen, and in the forged and partially annealed
condition, is a preferred material for surgical implants, e g.
hip stems and surgical nails. Such an alloy in said condition
possesses a unique combination of properties at room temperature,
i.e., an ultimate tensile strength of at least about 150,000 psi,
a yield stren~th (less than 0.2 percent offset) of at least
about 90,000 psi, a percent elongation (~ inch gage length) of
at least about 18 percent, a percent reduction of area (2 inch
gage length) of at least about 18 percent, excellent ductility,
a Rockwell hardness nu~ber of about 30 to 35, excellent corrosion
resistance to physiological fluids, and an excellent bending
fatigue strength lno failure after ten million cycles of at least
about 50,000 psi alternating stress and at least about 100,000
psi maximum stress ("A" ratio of about 1)]. An alloy of this
invention containing the preferred levels of carbon and nitrogen,
and in the cold-rolled condition, is a particularly suitable
material of construction for compression plates.
`339
Careful maintenance of the levels of chromium, molybdenum,
and especially carbon and nitrogen withir, the critical limits
disclosed herein is required in order to obtain the excellent
forging characteristics of the alloy of this invention. These
characteristics are not realized when either the chromium,
molybdenum or carbon level is more than about 27, 6 or 0.15
weight percent, respectively, or the nitrogen level is less than
about 0.10 weight percent or greater than about 0.25 weight
percent. It is to be noted that the levels of chromium and
10 molybdenum are generally lower in this novel alloy than in
commercially available cast Vitallium, but that superior
properties can nevertheless be practicably realized by hot forging.
The mechanism of this surprising interaction of alloying
ingredients is not fully understood at this time. Photomicrographs
of the alloy of this invention in the forged and partially annealed
condition reveal the existence of fine uniform grains with a
uniform distribution of carbides. An austenitic structure is
maintained in the matrix. No significant levels of nitrides are
visible. Iron and nickel are generally deleterious to corrosion
resistance and should be maintained at the lowest practicable
levels. On the other hand, manganese and silicon aid in
deoxidizing the melt and improving castibility and are preferably
present in the alloy of this invention at a level of about
0.4 to 0.6 weight percent each. The alloy may also contain
incidental impurities, e.g. sulfur or phosphorous, at such low
levels that they do not significantly affect alloy properties.
The following examples illustrate the invention but are not
to be construed as limiting the same.
EX~PLES 1-3
Cast fatigue bars 0.250 inches thick having the
composition
11~)0339
weight percent
chromium 26.80
molybdenum 5.20
nitrogen 0.235
carbon o ogg
manganese 0.52
silicon 0.58
iron 0.43
nickel 0.27
cobalt balance
were solution annealed for one hour under an argon atmosphere at
2225F. and 100 to 150 microns Hg. pressure, fan quenched with
nitrogen gas, forged at 2100F. on a forging press, heat treated
for grain recrystallization for 20 minutes at 2100F., forged
again at 2100F. to a thickness of 0.180 inches and then ground
in the as forged condition to a thickness of 0.125 inches. ~
Certain of the bars were then heat treated for grain recrystal-
lization (partially annealed~ in air for one hour at 2000F.
(Example 1), certain others heat treated for grain recrystallization
(partially annealed~ in air for one hour at 2050F. (Example 2),
and certain others left in the as forged condition (Example 3).
The following data on mechanical properties were obtained.
--10 -
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- 110~339
- EXAMPLE 4
Alloy ingredients were melted by air induction melting
technique and poured into a 2.75 inch diameter by 28 inch
long ingot. Nitrogen was added to the melt as chromium
nitride. The ingot was used as an electrode for electroslag
remelting into a 4 inch diameter by 15 inch long ingot having
the following composition.
weight percent
chromium 25.80
molybdenum 5.49
` nitrogen 0.126
carbon 0.06
manganese 0.57
silico~ 0.49
iron 0.50
nickel 0.43
cobalt balance
mis ingot was solution annealed for 3 hours at 2150F. and
then forged at 2100F. to a 2.5 inch square bar. One poEtion
of this wrought square bar was hot rolled at 2100F. to 0.25
inch thick plate, and the other portion hot rolled at about
2100F. to a 1 inch diameter round bar. The 0.25 inch thick
plate was then cold reduced to a 3/16 inch thick plate.
One portion of the 1 inch diameter bar was used as stock
or hot forging of hip stems. The balance was reduced to 0.25
inch diameter rod by hot working, and subsequently cold worked
to 3/16 inch dia~eter rod. A portion of the 3/16 inch diameter
rod was used to forge screw blanks by hot heading.
The 3/16 inch thick plate and a portion of the 3/16 inch
diameter rod were tested for room temperature tensile properties.
Ultimate
Tensile Yield Stress Percent
Sample Stress (psi) (0.2% offset, psi) Elongation
3~16 inch 262,000 226,000 6.4
thick plate
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`339
Ultimate
Tensile Yield Stress Percent
Sample Stress (psi) (0.2~ offset, psi) Elongation
3/16 inch 263,000 218,000 24
diamter rod
EXAMPLE 5
In like manner to that described in Example 4, a 4 inch
diameter by 15 inch long ingot was fabricated having the
following composition.
weight percent
chromium 26.93
molybdenum 5.13
nitrogen 0.205
carbon 0.07
manganese 0.70
silicon 0.52
iron 0.17 .
nickel 0.10
cobalt balance
This ingot was solution annealed or 3 hours at 2150~F. and
then forged at 2100F. to a 2.5 inch square bar. One portion
of this wrought square bar was hot rolled at 2100F. to 5/16
inch thick plate, and the other portion hot rolled and hot
swaged at 2100F. to a 1 inch diameter round bar. The 5/16
inch thick plate was then cold reduced to a 0.25 inch thick
plate.
One portion of the 1 inch diameter bar was used as stock
for hot forging of hip stems. The balance was reduced to 5/16
inch diameter rod by hot working, and subsequently cold worked
to 0.25 inch diameter rod.
The 0.25 inch thick plate and the 0.25 incn diameter
rod were tested for room temperature tensile properties.
11~339
Ultimate
Tensile Yield Stress Percent
Samp]~ Stress(psi) (0.2% offset, psi) Elongation
0.25 inch 196,000 166,000 12.4
thick plate
0.25 inch 213,000 172,000 19
diameter rod
EXAMPLE 6
Manufacture of Prosthetic Surgical Implant
A cast ingot of prismatic shape having the composition
weight percent
chromium 22 to 27
molybdenum 3 to 6
nitrogen 0.10 to 0.25 (preferably 0.15
to 0.20)
carbon up to about 0.15 (preferably up to
about 0.1)
manganese up to about 1
silicon up to about 1
iron up to about 2
nickel up to about 2
cobalt balance
can be formed into a prosthetic implant in the following manner.
The ingot is processed through one or more cycles of alternating
hot work tat about 2000F.) and solution anneal (for about one
hour per inch of thickness at about 2150F. in air or partial
vacuum~. The alloy is hot worked at about 2000F. for a final
time into a cylindrical bar of about 1 inch diameter, and then hot
forged at about 2050 to 2100F. into a hip stem or other desired
shape. The alloy article is then partially annealed by heat
treatment in air or partial vacuum for about one hour at about
1950 to 2000F. (grain recrystallization), and then machined and
polished into the final prosthetic implant.
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