IE47003B1 - Hot-forged co-cr-mo alloy articles - Google Patents

Abstract

A wrought alloy consisting of 22 to 27 weight percent chromium, 3 to 6 weight percent molybdenum, 0.10 to 0.25 weight percent nitrogen, up to 0.15 weight percent carbon, up to 1 weight per cent manganese, up to weight percent silicon, up to 2 weight percent iron, up to 2 weight percent nickel, and the balance (except for impurities and incidental constituents) cobalt. Also includes alloy articles, e.g. surgical implants, constructed of the above. Further covers hot forged alloy articles wherein the alloy consists of the ingredients stated above.

Description

This invention relates to hot forged articles made from cobalt-chromium-molybdenum based alloys.

Cobalt-base cast alloys containing chromium 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 CobaltChromium-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 cobaltbase 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 20 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 molybdenum and carbon ranges is - 3 said to optimize the combination 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. weight percent chromium 28 molybdenum 6 manganese 0.65 silicon 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 surgical implant and dental alloy and has been widely and successfully used for these 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 ^lsea, A. R. and McBride, C.C., Trans. A.I.M.E., 188, 154-161 (1950); Pletcher, E.E. and 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)7.

More than twenty years ago, wrought bar stock of Stellite (Registered Trade Mark) 21 (Cabot Corp., Kokomo, Ind.), a cobalt-chromium-molybdenum surgical implant alloy 003 - 4 containing about 0.3 percent carbon and about 3 percent nickel, was said to also contain about 0,09 to 0.17 percent nitrogen /Weeton, J.W. and Signorelli, R.A., Trans. Amer.

Soc. Metals, 47, 815-852 (1955)7· 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 1050°C. /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 (Ripe, M.H. et al., ed.), 136-153 (1973); see also U.S. Patent 2,486,5767· Unfortunately commercial production of wrought surgical implants is economically prohibitive if the final shape of the implant 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-chromiuramolybdenum 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 hy such a reduction (Devine, Cohen and Wulff, op. cit.). Chromium is generally known to increase strength hardness and corrosion resistance at the expense of workability, while molybdenum is generally known to increase strength and hardness at the expense of workability (see U.S. - 5 Patent 3,433,631, but cf. U.S. Patent 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 2,997,244 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 stability against stress-rupture at 1700°F.

It is taught that the presence of nitrogen, like carbon, decreases the cold workability of the wrought cobaltbase alloy disclosed in U.S. Patent 3,356,543 (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 molybdenum, 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).

The present invention provides novel hot forged - 6 articles made of an alloy consisting of 25.80 to 26,93 weight percent chromium, 5.13 to 5.49 weight percent molybdenum, 0.126 to 0.235 weight percent nitrogen, up to 0.099 weight percent carbon, up to 1 weight percent manganese, up to 1 weight percent silicon, up to 2 weight percent iron, up to 2 weight percent nickel, and the balance (except for impurities and incidental constituents) cobalt. The above alloy has a high degree of hot workability and may be readily forged directly into irregular shapes. Hot forged articles constructed of this alloy have excellent room temperature tensile properties, fatigue resistance, wear resistance, hardness, ductility, corrosion resistance and compatibility with biological tissue. The alloy is thus an excellent construction material for hot forged surgical implants such as prosthetic hip stems and surgical nails.

The alloy described in the preceding paragraph 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. 1900°F.) 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 for use in this invention comprises casting an ingot having the desired elemental content and subsequently processing the ingot into a desired final alloy condition (i.e. hot forged), shape and size.

The input metal stock is melted and cast into ingots by methods well known to those skilled in the art. Cobalt, chromium 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 commerc35 tally available metal stock. Manganese and silicon act as - 7 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 required operation in the manufacture of an article of the present invention is hot forging, broadly defined as hot working metal at a temperature above its recrystallization temperature into a shape of finite size by hammering or pressing. The alloy utilized in 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., hammer forging, drop forging, press forging).

The articles of this invention in the as forged condition possess 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 articles of the same composition. A particularly advantageous post-forging - 8 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 nitrogen content is about 0.15 to 0.20 weight percent. An alloy containing said level of nitrogen, and in the forged and partially annealed condition, is a preferred material for surgical implants, e.g., hip stems and surgical nails, of the invention. 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 strength (less than 0.2 percent offset) of at least about 90,000 psi, a percent elongation (2 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 number of about 30 to 35, excellent corrosion resistance to physiological fluids, and an excellent bending fatigue strength /no 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 117· Careful maintenance of the levels of chromium, molybdenum, and especially carbon and nitrogen within the critical limits disclosed herein is required in order to obtain the excellent forging characteristics of the alloy utilized in this invention. These characteristics are not realized when either the chromium, molybdenum of carbon level is more than 26.93, 5.49 or 0.099 weight percent, respectively, or the nitrogen level is less than 0.126 weight percent or greater than 0.235 weight percent. It is to be noted that the levels of chromium and 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 utilized in 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 at a level of 0.4 to 0.6 weight percent each. The alloy may also contain incidental impurities, e.g., sulfur or phosphorus. 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.

EXAMPLES 1-3 Cast fatigue bars 0.250 inches thick having the composition weight percent chromium 26.80 molybdenum 5.20 nitrogen 0.235 carbon 0.099 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 2225°P. and 100 to 150 microns Hg. pressure, fan - 10 quenched with nitrogen gas, forged at 2100°F. on a forging press, heat treated for grain recrystallization for 20 minutes at 2100°F., forged again at 2100°F. 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 recrystallization (partially annealed) in air for one hour at 2000°F. (Example 1), cartain others heat treated for grain recrystallization (partially annealed) in air for one hour at 2050°F. (Example 2), and certain others left-in the as forged condition (Example 3). The following data on mechanical properties were obtained. 7 0 0 3 Μ •Η ω ε φ •Η Μ ΡΙ •μ ω -Η Φ η υ ε cu β Ρ rd ε Μ •Η Ρ X Ό id β W S Φ (0 3 to θ' φ •Η Μ •μ 4J rd fo W ΟΐΛ CP β β •Η ·Η ι-Ι 4J CO ’ϋ β β φ μ CQ Φ •Ρ Η ο ο ο κ it ο γΗ Ο ο ιη • Φ to Χί to •Ρ φ μ β •Ρ •Η to 10 φ φ r-1 to •Η to (0 φ β Η Φ •μ μ co -Ρ β 0 β •Η Φ 4J υ Φ μ cn φ β fo 0 to ι—Ι W <-» Ή to Ch to φ Μ -μ μ Φ ω to •Ό Ψί ΐμ r-1 0 Φ •Η <#Ρ [χ ΟΙ Φ γΗ •Η Ο 10 β to Φ fo Φ to •Ρ to rd φ ε Μ •Η Ρ μ ω Η □ 1 φ X Η 6 W fo & η Ο οι ω Ο Ο Ο (Μ ο γΗ ο ο ο ο co t—i ο ο ο k. ΟΙ ο rH Ο Ο Ο » Ο co Η CN β ιϋ Φ ε φ λ •μ >ι Λ Ό Φ •Η > •Η Ό Φ ΨΙ rtf Ο β ~ μ S •Η β η ω CU β ο (9 ·Η d η Μ Λ Φ Φ Μ tJ> •Ρ Η ιο rd Φ Φ Ο .β X! η •Ρ 4J Ο Φ <ο r-j to to Η 0 id •Η !χ 0 Ό (0 φ to Ο» β φ Ο •Η μ ι—1 <μ -μ m Φ co μ cn Ό φ • φ •μ Ο to Η •Η •Η rd ιμ to Ο 0 0 β 0 Φ •Η Φ ο μ 0 •μ •Ρ * β ♦rl rd ιο Η +» μ β co ι4 id id «Η rd μ s φ < S s 0 < * s I Φ Ρ rd Λ 0 ΓΟ *-* Φ γΗ - 12 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 silicon 0.49 iron 0.50 nickel 0.43 cobalt balance This ingot was solution annealed for 3 hours at 215O°F. and then forged at 2100°F. to a 2.5 inch square bar. One portion of this wrought square bar was hot rolled at 2100°F, to 0,25 inch thick plate, and the other portion hot rolled at about 2100°F. 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 for 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 diameter 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. 7 0 0 3 Sample Ultimate Tensile Stress (psi) Yield Stress (0.2% offset, psi) Percent Elongation 3/16 inch 262,000 226,000 6.4 thick plate 3/16 inch diameter rod 263,000 218,000 24 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 for 3 hours at 215O°F. and then forged at 2100°P. to a 2.5 inch square bar. One portion of this wrought square bar was hot rolled at 2100°F. to 5/16 inch thick plate, and the other portion hot rolled and hot swaged at 2100°F. to a 1 inch diameter round bar. The 5/16 inch thick plate was then cold reduced to a 0.25 inch thick plate. - 14 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 inch diameter rod were tested for room temperature tensile properties .

Ultimate Sample Tensile Stress (psi) Yield Stress (0.2% offset, psi) Percent Elongation 0.25 inch 196,000 thick plate 166,000 12.4 0.25 inch 213,000 diameter rod 172,000

Claims (8)

1. CLAIMS:1. A hot forged alloy article consisting of 25.80 to 26.93 weight percent chromium, 5.13 to 5.49 weight percent molybdenum, 0.126 to 0.235 weight percent nitrogen, 5 up to 0.099 weight percent carbon, up to 1 weight percent manganese, up to 1 weight percent silicon, up to 2 weight percent iron, up to 2 weight percent nickel, and the balance (except for impurities and incidental constituents) cobalt.

2. An article as claimed in claim 1 containing 0.15 to 0.20 weight percent nitrogen.

3. An article as claimed in claim 1 or 2 containing 0.4 to 0.6 weight percent each of manganese and silicon.

4. An article as claimed in any one of claims 1 to 3 which is in the as forged condition. 15

5. An article as claimed in any one of claims 1 to 3 which is in the forged and partially annealed condition.

6. An article as claimed in any one of claims 1 to 3 and 5 which is a surgical implant.

7. A surgical implant as claimed in claim 6 in the form of a prosthetic hip stem.

8. An article as claimed in any one of claims 1 to 7 wherein there are no impurities or incidental constituents.