CA1100339A

CA1100339A - Cobalt-chromium-molybdenum alloy containing nitrogen

Info

Publication number: CA1100339A
Application number: CA305,930A
Authority: CA
Inventors: Ajit K. Kesh
Original assignee: Howmedica Inc
Current assignee: MTG Divestitures LLC
Priority date: 1977-06-23
Filing date: 1978-06-21
Publication date: 1981-05-05
Also published as: BE868372A; JPS5410224A; FR2395320A1; GB2000188A; AU503731B1; LU79855A1; IE781250L; GB2000188B; CH633046A5; JPS5628980B2; DE2827440C2; IT1096779B; DE2827440A1; IT7824860A0; ES471033A1; IE47003B1; NL7806739A; FR2395320B1

Abstract

COBALT-CHROMIUM-MOLYBDENUM ALLOY CONTAINING NITROGEN
Abstract of the Disclosure A novel wrought alloy 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 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 suitable for surgical implants, is disclosed.

Description

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.

~"~ .

~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)~

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 ``` ~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 - 6a -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).

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.

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- 110~339 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 `339 Ultimate Tensile Yield Stress Percent Sample Stress (psi) (0.2~ offset, psi) Elongation 3/16 inch 263,000 218,000 24 diamter rod 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 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.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A wrought alloy 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 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.

2. An alloy of Claim 1 containing about 0.15 to 0.20 weight percent nitrogen.

3. An alloy of Claim 1 containing up to about 0.1 weight percent carbon.

4. An alloy of Claim 2 containing up to about 0.1 weight percent carbon.

5. An alloy of Claim 1 containing about 0.4 to 0.6 weight percent each of manganese and silicon.

6. An article constructed of the alloy of Claim 1.

7. A surgical implant constructed of the alloy of Claim 1.

8. 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 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.

9. An article of Claim 8 containing about 0.15 to 0.20 weight percent nitrogen.

10. An article of Claim 8 containing up to about 0.1 weight percent carbon.

11. An article of Claim 9 containing up to about 0.1 weight percent carbon.

12. An article of Claim 8 containing about 0.4 to 0.6 weight percent each of manganese and silicon.

13. An article of Claim 8 wherein said article is in the as forged condition.

14. An article of Claim 8 wherein said article is in the forged and partially annealed condition.

15. An article of Claim 8 wherein said article is a surgical implant.

16. An article of Claim 14 wherein said article is a surgical implant.

17. A surgical implant of Claim 16 in the form of a prosthetic hip stem.