Articles manufactured from cold worked and case hardened essentially Co-free stainless steel alloys and methods for their manufacture

Abstract

A method of making an article, comprising forming a billet consisting essentially of a stainless steel composition: 2.00%-24.00% by weight manganese, 19.00%-30% by weight chromium, 0.50%-4.0% by weight molybdenum, 0.25%-1.10% by weight nitrogen, <1% by weight carbon, <0.03% by weight phosphorus, <1% by weight sulfur, <22% by weight nickel, <0.10% by weight cobalt, <1% by weight silicon, <0.80% by weight niobium, <1% by weight oxygen, <0.25% by weight copper, balance iron. The billet is annealed and cold worked to form the article. The article is then case hardened at a single case hardening temperature without annealing to form a surface layer on the upper surface. Articles formed of such stainless steel composition with a case hardened surface layer are also provided.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 63 / 291,187, filed December 17, 2021, the entirety of which is incorporated by reference herein for all purposes.

[0002] Embodiments of the present invention relate to methods for treating essentially cobalt-free wrought stainless steel and articles of manufacture made therefrom. More particularly, certain embodiments of the present invention relate to treating stainless steel to increase its strength and create a hardened surface that provides improved resistance to wear, corrosion, and fatigue. Certain articles processed according to various embodiments of the present invention are suitable for medical load-bearing applications such as orthopedic joint implants. [Background technology]

[0003] Stainless steels and cobalt chromium molybdenum alloys (CoCrMo or CCM) are metallic materials commonly used in orthopedic applications due to their strength, wear resistance, fatigue resistance, corrosion resistance, and biocompatibility. CoCrMo alloys, containing 27-30% chromium and 5-7% molybdenum by weight, are used in either the cast condition (ASTM F75) or wrought condition (ASTM F1237). Other CoCrMo alloys with different cobalt / chromium / molybdenum ratios are used in the wrought condition (see, for example, ASTM F90, F562, F799, F1537 specifications). Stainless steels are iron-based with chromium, nickel, and molybdenum additions (ASTM F138) or chromium, nickel, manganese, molybdenum, and nitrogen additions (ASTM F1586) and are used in the wrought condition. Each of these ASTM standards is incorporated herein by reference in its entirety.

[0004] Stainless steel compositions essentially free of Ni and Co are specified in ASTM Standard F2229-21 Standard Specification for Wrought Nitrogen-Strengthened 23 Mn-21 Chromium-1 Molybdenum Low Nickel Stainless Steel Alloy Bar and Wire for Surgical Implants (UNS S29108) (referred to herein as "ASTM F2229 Stainless Steel"), which is incorporated herein by reference in its entirety for all purposes. This alloy is also described, for example, in U.S. Patent Publication 20100116377A1, U.S. Patent No. 9,387,022B2, and U.S. Patent No. 10,214,805B2. Examples of materials conforming to this standard are BioDur® 108 stainless steel available from Carpenter Technology, Inc. (USA) and CHRONIFER® 108 nickel-free stainless steel available from Elle Klein, Inc. (Switzerland).

[0005] A similar Co-free stainless steel composition is specified in ASTM standard F1586-21 Standard Specification for Wrought Nitrogen-Strengthened 21 Chromium-10 Nickel-3 Manganese-2.5 Molybdenum Stainless Steel Alloy Bar and Wire for Surgical Implants (UNS S31675), referred to herein as "ASTM F1586 Stainless Steel", which is incorporated herein by reference in its entirety for all purposes. This alloy is described in U.S. Patent No. 9,695,505 B2 and U.S. Patent No. 9,387,022 B2. An example of a material conforming to this standard is BioDur® 734 stainless steel available from Carpenter Technology, Inc. (USA).

[0006] Another similar Co-free stainless steel composition is the high nitrogen stainless steel described, for example, in U.S. Patent No. 6,168,755, which is referred to herein as "high nitrogen, high chromium stainless steel" ("HNHC stainless steel"). An example of a material that includes this composition is a material conforming to ASM SS-1231, for example Micro-Melt® NCORR available from Carpenter Technology, Inc. (USA). TM It is stainless steel.

[0007] To expand the variety of applications in which they can be used, there is a need to treat the ASTM F2229, ASTM F1586, and HNHC stainless steels to increase their strength and resistance to wear, corrosion, and fatigue. Summary of the Invention

[0008] Embodiments of the present invention may be used in a variety of applications, such as the manufacture of orthopedic joint implants. For orthopedic joint implants, the properties required for the articular surface are significantly different from those required for the bulk material, since the articular surface is preferably wear resistant while at the same time having excellent fatigue, corrosion, and tribocorrosion properties. On the other hand, the bulk of the alloy has its own requirements in terms of Young's modulus, fracture toughness, etc. For these reasons, alloys for orthopedic joint applications are preferably specially processed so that the surface of the material has a significantly higher hardness than the bulk of the material.

[0009] The processes described herein may also be used to provide articles for other applications, such as watch parts, electrical parts, or drill parts, i.e., any application requiring a hard, wear-resistant surface in combination with bulk properties such as those defined by ASTM F2229 stainless steels, ASTM F1586 stainless steels, or high nitrogen, high chromium stainless steels ("HNHC stainless steels") and described in U.S. Pat. No. 6,168,755.

[0010] In one aspect, an embodiment of the invention relates to a method for making an article, the method including the steps of: forming a billet comprising or consisting essentially of a stainless steel composition of 2.00%-24.00% by weight manganese, 19.00%-30% by weight chromium, 0.50%-4.0% by weight molybdenum, 0.25%-1.10% by weight nitrogen, < 1% by weight carbon, < 0.03% by weight phosphorus, < 1% by weight sulfur, < 22% by weight nickel, < 0.10% by weight cobalt, < 1% by weight silicon, < 0.80% by weight niobium, < 1% by weight oxygen, < 0.25% by weight copper, balance iron; annealing the billet; cold working the billet to form an article; and subsequently case hardening the article at a single case hardening temperature without annealing the article to form a surface layer on an upper surface thereof.

[0011] One or more of the following features may be included: The stainless steel composition may include: 21.00%-24.00% by weight manganese, 19.00%-23.00% by weight chromium, 0.50%-1.50% by weight molybdenum, 0.85%-1.10% by weight nitrogen, <0.08% by weight carbon, <0.03% by weight phosphorus, <0.01% by weight sulfur, <0.05% by weight nickel, <0.1% by weight cobalt, <0.75% by weight silicon, 0% by weight niobium, <0.25% by weight copper, balance iron.

[0012] The stainless steel composition may include: 2.00 wt%-4.25 wt% manganese, 19.5 wt%-22.0 wt% chromium, 2.0 wt%-3.0 wt% molybdenum, 0.25 wt%-0.50 wt% nitrogen, ≦0.08 wt%, phosphorus ≦0.025 wt%, sulfur ≦0.01 wt%, nickel 9.0 wt%-11.0 wt%, cobalt <0.10 wt%, silicon ≦0.75 wt%, 0.25 wt%-0.80 wt% niobium, copper ≦0.25 wt%, balance iron.

[0013] The stainless steel composition may contain: 5.85%-15% by weight manganese, 27%-30% by weight chromium, 1.5%-4.0% by weight molybdenum, 0.8%-0.97% by weight nitrogen, <0.02% by weight phosphorus, 8%-22% by weight nickel, <0.01% by weight cobalt, silicon, oxygen, carbon, and sulfur in amounts such that (silicon + oxygen + carbon + sulfur) <1% by weight, 0% by weight niobium, <0.01% by weight copper, and the balance iron.

[0014] Forming the billet may include melting and remelting an ingot under air, vacuum or slag, and forging the ingot to create the billet.

[0015] Forming the billet may include forming a powder including the stainless steel composition; and compressing the powder to form the billet.

[0016] Cold working the billet may include at least one of cold forming, cold rolling, cold drawing, shot peening, or pilgering.

[0017] The case hardening may be carried out at a single temperature selected from the range of 400° C. to 1000° C. (752° F. to 1832° F.). The case hardening may be carried out for a time selected from the range of 1 hour to 16 hours.

[0018] Case hardening may include boriding, carburizing, nitriding, carbonitriding, and / or combinations thereof.

[0019] The surface layer may include a surface hardness of at least 350 HV. The surface layer may include a billet stainless steel composition and may further include at least one of carbon, nitrogen, boron, or a combination thereof diffused therein.

[0020] The article may include an orthopedic joint device component, a watch part, an electrical part, or a drill part.

[0021] In another aspect, an embodiment of the present invention relates to an orthopedic joint device, which may include: a first component including a first articular surface; and a second component including a second articular surface configured to articulate with the first articular surface, each of the first and second components consisting essentially of a stainless steel composition including: 2.00%-24.00% by weight manganese, 19.00%-30% by weight chromium, 0.50%-4.00% by weight molybdenum, 0.25%-1.10% by weight nitrogen, <1% by weight carbon, <0.03% by weight phosphorus, <1% by weight sulfur, <22.00% by weight nickel, <0.10% by weight cobalt, <1% by weight silicon, <0.80% by weight niobium, <1% by weight oxygen, <0.25% by weight copper, balance iron. The first articular surface and the second articular surface each include a surface layer comprising or consisting essentially of a stainless steel composition and further comprising at least one of carbon, nitrogen, and boron diffused therein.

[0022] One or more of the following features may be included: The surface layer may have a surface hardness of at least 350HV. The orthopaedic joint device may further include an acetabular cup disposed between the first component and the second component. The acetabular cup may include a metal, a ceramic, and / or a polymer.

[0023] In yet another aspect, an embodiment of the invention relates to a stainless steel article, the stainless steel article comprising a bulk material having at least one of a hardness of at least 300 HV or a yield strength of at least 145 ksi, and consisting essentially of a stainless steel composition comprising: 2.00%-24.00% by weight manganese, 19.00%-30% by weight chromium, 0.50%-4.00% by weight molybdenum, 0.25%-1.10% by weight nitrogen, < 1% by weight carbon, < 0.03% by weight phosphorus, < 1% by weight sulfur, < 22.00% by weight nickel, < 0.10% by weight cobalt, < 1% by weight silicon, < 0.80% by weight niobium, < 1% by weight oxygen, < 1% by weight copper, < 0.25% by weight balance iron; and a surface layer disposed on the bulk material, the surface layer comprising or consisting essentially of the stainless steel composition, and further comprising at least one of carbon, nitrogen, boron, or a combination thereof diffused therein.

[0024] One or more of the following features may be included: The surface layer may have a thickness selected from the range of 10 micrometers to 1000 micrometers, or the range of 30 micrometers to 40 micrometers. The carbon concentration may range from at least 0.10 wt.% at the top surface of the surface layer to <0.08 wt.% in the bulk material.

[0025] The nitrogen concentration may range from at least 1.10 wt.% at the upper surface of the surface layer to 0.85 wt.% to 1.10 wt.% nitrogen in the bulk material, and the nitrogen concentration in the surface layer may be higher than in the bulk material. The boron concentration may range from at least 0.10 wt.% at the upper surface of the surface layer to 0 wt.% in the bulk material.

[0026] The stainless steel article may be an orthopedic joint device component, a watch part, an electrical part, or a drill part. [Brief description of the drawings]

[0027] FIG. 1 is a graph of yield strength (YS) and ultimate tensile strength (UTS) as a function of percentage of cold work for ASTM F2229 stainless steels, such as BioDur® 108 stainless steel.

[0028] FIG. 2 is a graph of YS and UTS as a function of percentage of cold work for ASTM F1586 stainless steels, such as BioDur® 734 stainless steel.

[0029] FIG. 3 is a graph of YS and UTS as a function of percentage of cold work for a high nitrogen, high chromium stainless steel ("HNHC stainless steel"), such as ASM SS-1231 stainless steel.

[0030] FIG. 4 is a schematic depiction of the case hardening process.

[0031] FIG. 5 is a schematic depiction of an artificial hip joint.

[0032] FIG. 6A is a graph illustrating hardness versus distance from the surface for a BioDur® 108 stainless steel sample treated according to an embodiment of the present invention.

[0033] FIG. 6B is a graph showing carbon concentration in weight percent versus distance from the surface for BioDur® 108 stainless steel samples treated according to an embodiment of the invention.

[0034] FIG. 6C is a series of micrographs of the samples of FIGS. 6A and 6B.

[0035] FIG. 7A is a graph illustrating hardness versus distance from the surface for BioDur® 734 stainless steel samples treated according to an embodiment of the present invention.

[0036] FIG. 7B is a graph showing carbon concentration in weight percent versus distance from the surface for BioDur® 734 stainless steel samples treated according to an embodiment of the invention.

[0037] FIG. 7C is a series of photomicrographs of the samples of FIGS. 7A and 7B.

[0038] FIG. 8A shows the NCORR processed according to an embodiment of the present invention. TM 1 is a graph illustrating hardness versus distance from the surface of a stainless steel sample.

[0039] FIG. 8B shows the NCORR processed according to an embodiment of the present invention. TM 1 is a graph showing carbon concentration in weight percent versus distance from the surface of a stainless steel sample.

[0040] FIG. 8C is a series of photomicrographs of the samples of FIGS. 8A and 8B. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] alloy Processes according to embodiments of the invention may be used with (i) an ASTM F2229 stainless steel composition, such as the commercially available BioDur® 108 stainless steel; (ii) an ASTM F1586 stainless steel composition, such as the commercially available BioDur® 734 stainless steel; or (iii) a high nitrogen, high chromium stainless steel ("HNHC stainless steel"), such as the commercially available Micro-Melt® NCORR stainless steel, as described in U.S. Pat. No. 6,168,755 and in "Alloy Digest - World Data for Metals and Alloys" published by ASM International (American Society for Metals) under file code ASM SS-1231. TM Change the stainless steel alloy to match the stainless steel. All three of these commercially available alloys are manufactured by Carpenter Technology Corporation.

[0042] In particular, ASTM F2229 stainless steel is an essentially nickel- and cobalt-free, nitrogen-strengthened stainless steel with excellent biocompatibility and approved by the Federal Food and Drug Administration for medical use. It is produced by an electroslag remelting (ESR) process to ensure microstructural integrity and cleanliness, and is used in applications such as implantable orthopedic devices, high-strength surgical instruments, and orthopedic devices. The absence of intentionally added cobalt makes it compliant with the European Community Medical Devices Regulation 2017 / 745, which requires that devices containing more than 0.10% cobalt by weight be labeled with the presence of cobalt as a carcinogenic, mutagenic, or reproductive toxicant. ASTM F2229 stainless steel is also free of intentionally added nickel, a metal known to cause skin irritation, allergic reactions, teratogenicity, and carcinogenicity in medical applications (see Yang et al., "Nickel-Free Austenitic Stainless Steels for Medical Use," Journal of Science, Technology, and Advanced Materials 11 (2010), 014105). Despite its known advantages, the use of ASTM F2229 stainless steel is limited by its relative softness, typically about 300 HV, when produced by conventional methods. BioDur® 108 stainless steel, manufactured by Carpenter Technology, Inc., and CHRONIFER® 108 nickel-free stainless steel, sold by Elle Klein (Switzerland), are examples of alloys that meet the ASTM F2229 stainless steel standard.

[0043] ASTM F1586 stainless steel is an essentially cobalt-free, nitrogen-strengthened stainless steel approved by the Federal Food and Drug Administration for medical use. It also complies with the European Community Medical Devices Regulation 2017 / 745 and is used in implantable orthopedic parts such as bone plates, bone screws, and hip and knee components. The ASTM F1586 stainless steel standard requires 9.00 to 11.00 wt.% Ni. BioDur® 734, manufactured by Carpenter Technology, is an example of an alloy that meets the ASTM F1586 stainless steel standard.

[0044] HNHC stainless steels are essentially cobalt-free, nitrogen-strengthened stainless steels manufactured using a powder metallurgy process that essentially consists of atomizing and sintering metal to form a billet. It also complies with the European Community Medical Devices Regulation 2017 / 745 and contains up to 8.00% Ni by weight. Its composition allows for the production of high levels of strength through cold working.

[0045] Stainless steels that may be processed according to embodiments of the present invention have compositions selected from the ranges shown in column 5 of Table 1 below, which encompass the ranges of ASTM F2229, ASTM F1586, and HNHC stainless steel compositions also shown in Table 1. Preferred limits for each element are provided below. The balance of all three alloys is iron, with impurities coming from normal manufacturing processes.

[0046] [Table 1]

[0047] [Table 2]

[0048] Cobalt is an element that is not intentionally added to any of the alloys in Tables 1 and 2. Residual cobalt from normal manufacturing processes is kept below 0.10 wt.%, preferably below 0.010 wt.%, to comply with the European Community Medical Devices Regulation 2017 / 745.

[0049] The primary function of manganese in the present alloys is to increase the solubility of nitrogen; the specified levels of manganese (within the ranges listed in Tables 1 and 2) are selected to provide the desired nitrogen level. In ASTM F2229 stainless steels, 21.00 to 24.00 wt.% Ni is required to allow up to 1.10 wt.% N in solution. In ASTM F1586 stainless steels, only 2.00 to 4.25 wt.% Ni is required because the desired N level is 0.25 to 0.50 wt.% and the extra Ni contributes to N solubility. HNHC stainless steels require up to 15 wt.% Mn to allow 0.80 to 0.97 wt.% N in solution.

[0050] Chromium increases both corrosion resistance and nitrogen solubility. However, increasing chromium also decreases austenite stability. The level of chromium (within the ranges listed in Tables 1 and 2) is selected to provide the desired corrosion resistance; and the levels of the other elements are adjusted as necessary to maintain austenite stability. ASTM F2229 and ASTM F1586 stainless steels require similar levels of Cr (19.00% to 23.00% by weight) to reach the desired corrosion resistance. HNHC stainless steels require 27% to 30% by weight Cr for additional corrosion resistance and use in more severe environments.

[0051] Molybdenum significantly improves resistance to corrosion, especially the localized type of corrosion that is of concern in implant applications. However, molybdenum also significantly reduces austenite stability. Like chromium, the level of molybdenum (within the ranges listed in Tables 1 and 2) is selected to provide the necessary corrosion resistance and is balanced by the other elements; and the levels of the other elements are adjusted as necessary to maintain austenite stability.

[0052] Nitrogen plays a major role in maintaining the stability of austenite in the alloys listed in Tables 1 and 2, and also contributes significantly to corrosion resistance and determines strength levels. High levels of nitrogen increase the strain hardening rate of stainless steels during cold working; that is, the strength gained with a given level of cold work during a cold working deformation process increases with the level of nitrogen. Excessive nitrogen levels can result in difficulties in melting, atomizing, sintering, forging, and other processing steps. Nitrogen levels (within the ranges listed in Tables 1 and 2) are controlled by controlling the levels of other elements that affect the solubility of nitrogen.

[0053] The addition of silicon provides deoxidation during the melting and refining process; the specific level used depends on the melting process employed. Since increasing silicon reduces both the stability of austenite and the solubility of nitrogen, silicon levels are limited to 0.75 wt.% in ASTM F2229 and ASTM F1586 stainless steels, and not to exceed 1 wt.% (combined with silicon, oxygen, carbon, and sulfur) in HNHC stainless steels, as described, for example, in U.S. Patent No. 6,168,755.

[0054] Phosphorus is not intentionally added, but is present as an impurity in raw materials commonly used for melting alloys such as those in Tables 1 and 2. Because excessive levels of phosphorus can reduce certain properties such as ductility, melting and refining procedures are used to ensure that phosphorus levels are less than a maximum of 0.020% or 0.025% by weight.

[0055] Like phosphorus, sulfur is not intentionally added, but is present as an impurity in the raw materials commonly used for melting alloys such as those in Tables 1 and 2. Because excessive levels of sulfur can also degrade certain properties such as ductility, melting and refining procedures are used to ensure that sulfur levels are less than 0.010 weight percent in ASTM F2229 and ASTM F1586 stainless steels, and not to exceed 1 weight percent (combined with silicon, oxygen, carbon, and sulfur) in the HNHC stainless steels, as described, for example, in U.S. Patent No. 6,168,755.

[0056] Although copper is often added to stainless alloys to improve resistance to certain types of corrosion, copper is not intentionally added to the alloys in Tables 1 and 2 that are typically intended for implant applications. Copper levels (which may be present as an impurity in raw materials) are limited to less than 0.25% by weight in ASTM F2229 and ASTM F1586 stainless steels, and less than 0.01% by weight in HNHC stainless steels, as described, for example, in U.S. Patent No. 6,168,755.

[0057] Carbon in solid solution helps stabilize the austenite phase. Carbon can also combine with various elements to form carbide phases. Austenitic alloys designed for high corrosion resistance often have low carbon levels because the formation of chromium carbides on grain boundaries can result in reduced corrosion resistance. Carbon is limited to levels less than 0.08 wt.% in ASTM F2229 and ASTM F1586 stainless steel alloys, and not to exceed 1 wt.% (combined with silicon, oxygen, carbon, and sulfur) in HNHC stainless steels, as described, for example, in U.S. Patent No. 6,168,755.

[0058] The process described herein uses methods known to those skilled in the art to produce the alloys described above, including multiple additive steps to form the article, including the surface layer. These steps include melting the raw material, atomizing or casting the molten metal, converting the ingot to a billet during forging, and finally annealing heat treatment of the billet. This is followed by a cold working step, a deformation step performed at a low temperature to form the article and increase its bulk yield strength and ultimate tensile strength. Immediately after cold working, a surface hardening step of the article is performed at a low temperature to improve the physical properties of the surface of the article. Unlike conventional methods, the surface hardening step is not preceded by an annealing step. Cold working strains the material to strengthen it; in contrast, annealing relaxes and softens the material. Thus, annealing the article after cold working would defeat the purpose of the cold working step and would result in the alloy losing the strength gained during cold working.

[0059] Billet forming and annealing Ingots made from the above-mentioned ASTM F2229 stainless steel may be produced by arc melting or vacuum induction melting (VIM) followed by electroslag remelting (ESR). After solidification, the ASTM F2229 stainless steel ingots are homogenized in a furnace to ensure homogenous microstructure and converted into billets by hot working on a rolling mill or radial forging machine, such as in accordance with Carpenter Technology Corporation's BioDur® 108 technical data sheet, which is incorporated herein by reference in its entirety for all purposes (Carpenter Technology Corporation, CarTech® BioDur® 108 alloy). After forging, the ASTM F2229 stainless steel billet may be annealed at a temperature ranging from 1900°F (1038°C) to 2100°F (1149°C) for a period of one hour, then rapidly cooled to room temperature between 1500°F (816°C) and 1800°F (982°C) to prevent the formation of chromium nitride. Within the context of this disclosure, this annealing step is performed in air, a protective atmosphere, or a vacuum and is defined as a heat treatment that relieves internal strains and softens the material through recovery or recrystallization.

[0060] Ingots made from the above-mentioned ASTM F1586 stainless steel may be produced by arc melting or vacuum induction melting (VIM), followed by electroslag remelting (ESR). After homogenization and forging, billets of ASTM F1586 stainless steel may be annealed to a temperature ranging from 1922°F (1050°C) to 2102°F (1150°C) and quenched to prevent the formation of chromium nitrides, such as by following Carpenter Technology Corporation's BioDur® 734 technical data sheet, which is incorporated herein by reference in its entirety for all purposes (Carpenter Technology Corporation, CarTech® BioDur® 734 Alloy).

[0061] HNHC stainless steels may be produced using powder metallurgy (PM) techniques, where the resulting powder is sintered into a fully dense ingot by hot isostatic pressing (HIP). After forging, the billet may be annealed at 2000°F (1093°C) for 1 hour and then quenched to prevent the formation of chromium nitrides, following the recommendations in the Alloy Digest - World Metals and Alloys Data, file code ASM SS-1231.

[0062] Cold working Cold working is defined as, but is not limited to, a forming process consisting of a combination of cold rolling, cold drawing, shot peening, and / or pilgering performed at or below 2 / 3 of the solidus temperature. For most stainless steels, the solidus temperature (defined as the highest temperature below which the alloy is completely solid) is at least 2400°F (1316°C), and 2 / 3 of this temperature is about 1600°F (871°C). In the remainder of this disclosure, cold forming processes are understood to be forming processes performed between room temperature and 1600°F (871°C). Cold working performed in the temperature range of 1000°F (538°C) to 1600°F (871°C) may sometimes be referred to as "warm working". Thus, as used herein, "cold working" encompasses cold working and warm working, i.e., forming processes performed between room temperature and 1600°F (871°C).

[0063] Warm working (cold working) performed in the temperature range of 1000°F (538°C) to 1600°F (871°C) may be a preferred processing step due to the ease of deformation performed in that temperature range, i.e., less external force needs to be applied to the article to reach the desired level of deformation. Deformation performed in the temperature range of 1000°F (538°C) to 1600°F (871°C) introduces fewer defects (e.g., dislocations and point defects) into the article and also generates less energy stored in the article. Articles with less stored energy available for recovery or recrystallization are more likely to retain the required bulk strength during further processing, such as case hardening steps.

[0064] Due to their specific chemistry, ASTM F2229 stainless steels, ASTM F1586 stainless steels, or HNHC stainless steels, unlike most other steel alloys, have no hardening mechanisms available to harden them at high temperatures, and the only processing step that can harden them to make them suitable for load-bearing applications is cold working. Thus, these alloys are preferably cold worked (i.e., deformed at temperatures below 1600°F / 871°C) to make them stronger.

[0065] ASTM F2229 stainless steels, ASTM F1586 stainless steels, and HNHC stainless steels have different compositions and also have different work hardenability, i.e., they harden differently when subjected to the same cold forming process and require different levels of cold work to reach the same levels of mechanical properties.

[0066] For example, Figure 1 is taken from MJ Walter, "Stainless Steels for Medical Implants: High Levels of Nitrogen in BioDur® 108 Stainless Steel Provide Improved Mechanical and Physical Properties for Medical Implants," Advanced Materials Processing 164 (2006) 84-86, and also from Carpenter Technology's BIODUR® 108 Stainless Data Sheet, which are incorporated herein by reference in their entirety for all purposes. Referring to Figure 1, ASTM F2229 stainless steel in the annealed condition (cold work=0%) provides a yield strength (YS) of 88 ksi / ultimate tensile strength (UTS) of 135 ksi at room temperature and a YS of 270 ksi / UTS of 320 ksi after 80% cold work. A 15% appropriate cold work process is preferred for components made from this alloy to reach the standard mechanical properties of ASTM 799 (120 ksi YS / 170 ksi UTS at room temperature), which is incorporated herein in its entirety by reference for all purposes. For other applications, different levels of cold work may be desired to target specific combinations of YS and UTS.

[0067] In another example, Figure 2 is sourced from Carpenter Technology's BIODUR® 734 stainless data sheet, which is incorporated herein by reference in its entirety for all purposes. Referring to Figure 2, ASTM F1586 stainless steel in annealed condition yields 65 ksi YS / 122 ksi at room temperature and 128 ksi YS / 170 ksi UTS after 35% cold work. For components made from this alloy to reach the ASTM 799 standard mechanical properties (120 ksi YS / 170 ksi UTS at room temperature), a proper cold work process of at least 40% is preferred. Similarly, for other applications, different levels of cold work may be desired to target specific combinations of YS and UTS.

[0068] In a final example, FIG. 3 shows a CarTech® Micro-Melt® NCORR TM The stainless steel data sheet is the source, which is incorporated herein by reference in its entirety for all purposes. HNHC stainless steel in the annealed condition provides a YS of 100 ksi / UTS of 153 ksi at room temperature, and a YS of 264 ksi / UTS of 328 ksi after 70% cold work. For components made from this alloy to reach the standard mechanical properties of ASTM 799 (YS of 120 ksi / UTS of 170 ksi at room temperature), a proper cold work process of at least 15% is preferred. Similarly, for other applications, different levels of cold work may be desired to target specific combinations of YS and UTS.

[0069] surface hardening Case hardening is a surface modification process used to harden the surface layer of stainless steel. Case hardening may thus be used to increase the hardness of the surface of HNHC stainless steels, such as ASTM F2229 stainless steel, ASTM F1586 stainless steel, or ASM SS-1231 stainless steel, through the diffusion of interstitial elements, such as carbon, nitrogen, boron, or combinations thereof, in a gas, ion, or plasma medium or in a vacuum. In the case of pack carburizing, pack nitriding, or pack boriding, the case hardening process may be carried out in a carbon, nitrogen, or boron rich material. This process may include carburizing, nitriding, boriding, carbonitriding, or combinations thereof, at case hardening temperatures below 1000° C. to prevent the formation of deleterious second phases. This process results in the formation of a surface layer that is much harder than the bulk of the material and has improved resistance to wear, corrosion, and fatigue damage.

[0070] Surface hardening can be used to improve the surface properties of ASTM F2229 stainless steel, ASTM F1586 stainless steel, or HNHC stainless steel, and improve the corrosion, fatigue, and wear resistance of these alloys, resulting in better performance in a variety of applications, such as articulated orthopedic applications.

[0071] Specifically, case hardening is a heat treatment process in a C-rich, N-rich, B-rich, or combination of these, or other suitable environment used to change the near-surface chemistry of an alloy through a diffusion process to change the properties. This allows interstitial elements to diffuse into the surface layer of the material. The case hardening process can be boriding (diffusion of boron), carburizing (diffusion of carbon), nitriding (diffusion of nitrogen), carbonitriding (simultaneous diffusion of carbon and nitrogen), or combinations of these. Case hardening processes can be performed using gas, ion, or plasma media, or in a vacuum. During the case hardening process, the interstitial elements diffuse into the surface layer and form a supersaturated solid solution at the surface of the material. The process is performed at a temperature low enough to prevent the formation of precipitates or deleterious second phases such as borides, carbides, nitrides, or carbonitrides. See XY Lee, N Habibi, T Bell, H Dong, "Microstructural features of plasma carburized low carbon cobalt chromium alloys", Surface Technology 23 (2007) 45-51. Surface hardening temperatures can range from 350°C to 1000°C, e.g. 400°C to 1000°C, and times can be up to 60 hours and as short as 1 hour. See for example SR Collins, PC Williams, SV Marx, A Heuer, F Ernst, H Kahn, "Low temperature carburization of austenitic stainless steels", in Steel Heat Treatment, ASM International 2014: pp. 451-460.

[0072] In some embodiments described herein, the case hardening temperature ranges between 400°C and 1000°C, e.g., 500°C, 550°C, 750°C, or 960°C. In some embodiments, the case hardening time ranges from 1 hour to 16 hours, e.g., 1 hour, 4 hours, 5 hours, or 16 hours. At the temperature at which the alloy is case hardened, the diffusivity of carbon, nitrogen, and boron is increased, i.e., the higher the case hardening temperature, the easier it is to introduce a larger amount of interstitial elements into the surface layer, and the thicker the surface layer will be. Case hardening processes performed at higher temperatures are more efficient, i.e., faster, than the same case hardening processes performed at lower temperatures, but lead to the possibility that Cr-rich and N-rich stainless steels will form carbides and nitrides that are detrimental to the corrosion resistance of the alloy. The case hardening temperature and time must be balanced and adapted to the alloy composition to produce the desired thickness of the case hardened layer while avoiding the formation of detrimental carbides and nitrides.

[0073] For the part to be machinable, a minimum hardened surface layer thickness of 10 μm is desired, with 100 μm being a more preferred thickness, and 1,000 μm being an even more preferred thickness. For applications requiring wear resistance, a surface hardness of at least 400 HV, preferably 800 HV, more preferably 900 HV, and even more preferably 1200 HV is desired. The combination of surface hardness and case thickness is determined by the final application, as some applications require a shallow but hard surface layer, while other applications require a thicker but softer hardened surface layer.

[0074] To simplify the process and minimize the risk of detrimental phases forming during rapid heating and cooling, the case hardening is performed at a single case hardening temperature. During the case hardening process, for example during the heating step of the cycle, the specimen may be exposed to other temperatures ranging from room temperature to the case hardening temperature, for example the carburizing temperature, or during the cooling step of the cycle, the specimen may be exposed to other temperatures ranging from the case hardening temperature to room temperature. The case hardening cycle may be interrupted (i.e. the introduction of boron, carbon and / or nitrogen is temporarily stopped) while the material is held at the case hardening temperature to allow the interstitial elements to diffuse into the surface layer of the material. The case hardening cycle may also include a sequence of short hardening pulses and diffusion times at the case hardening temperature to allow the interstitial elements to diffuse into the surface layer of the material.

[0075] The case hardening process results in the formation of a surface layer that is much harder than the bulk of the material (see schematic example in FIG. 4) and has improved resistance to wear, corrosion, and fatigue damage. According to an embodiment of the invention, a cold worked (including warm worked) ASTM F2229 stainless steel, ASTM F1586 stainless steel, or HNHC stainless steel article is subjected to a case hardening heat treatment.

[0076] A variety of hardfacing heat treatment cycles are offered by various commercial vendors: Kolsterising® (proprietary technology of Bodycote Corporation), ExpaniteHigh-T, ExpaniteLow-T, and SuperExpanite (proprietary technology of Expanite Corporation), or Infracarb® (proprietary technology of ECM-USA, Inc.) Similarly, commercial nitriding, carbonitriding, or boriding hardfacing heat treatment cycles are available from the same vendors.

[0077] An exemplary system for performing the case hardening process described herein on stainless steels (a carburizing process called pack carburizing) is a high temperature furnace, such as Lucifer Furnace's RD7-KHE24 box furnace. A mixture of pelletized carbon and anhydrous sodium carbonate may be placed in a metal container. A suitable composition may be 98% (by weight) pelletized carbon and 2% (by weight) anhydrous sodium carbonate. The metal container may be a stainless steel rectangular box with a stainless steel lid on top and sealed with high temperature refractory cement. This metal container with the mixture may then be placed in the furnace along with the article to be case hardened. The mixture and article are heated to the desired case hardening temperature for a sufficient length of time to obtain a surface layer with the desired concentration of interstitial elements on the top surface of the article, and then rapidly cooled to room temperature by quenching in water, oil, air, or any other fluid.

[0078] After surface hardening, the interstitial elements in solution create compressive stresses that make the surface layer much harder and more resistant to fatigue and wear than the bulk of the material. The high concentration of interstitial elements in the surface layer makes it more resistant to corrosion damage.

[0079] Suitable levels of diffused interstitials according to embodiments of the present invention are as follows:

[0080] ●ASTM F2229 Stainless Steel

[0081] Carburization: ASTM F2229 stainless steel contains a maximum of 0.08% carbon by weight. After carburization, the carbon concentration in the surface layer is at least 0.10% by weight, and preferably a maximum of 5.00% by weight.

[0082] Nitriding: Typical nitrogen levels in ASTM F2229 stainless steel range between 0.85% and 1.10% by weight. After nitriding, the nitrogen concentration in the surface layer ranges from at least 1.10% by weight at the top surface to 0.85% to 1.10% by weight nitrogen in the bulk material, with the nitrogen concentration in the surface layer being higher than in the bulk material.

[0083] Boronizing: The formula of ASTM F2229 stainless steel does not contain boron. The boriding case hardening heat treatment results in a surface layer containing at least 0.05% by weight of boron.

[0084] ○Combinations of these.

[0085] ●ASTM F1586 Stainless Steel

[0086] Carburization: ASTM F1586 stainless steel contains a maximum of 0.08% carbon by weight. After carburization, the carbon concentration in the surface layer is at least 0.10% by weight, and preferably a maximum of 5.00% by weight.

[0087] Nitriding: Typical nitrogen levels in ASTM F1586 stainless steel range between 0.25% and 0.50% by weight. After nitriding, the nitrogen concentration in the surface layer ranges from at least 0.50% by weight at the top surface to 0.25% to 0.50% by weight nitrogen in the bulk material, with the nitrogen concentration in the surface layer being higher than in the bulk material.

[0088] Boronizing: The formula of ASTM F1586 stainless steel does not contain boron. The boriding case hardening heat treatment results in a surface layer containing at least 0.05% by weight of boron.

[0089] ○Combinations of these.

[0090] ●HNHC stainless steel

[0091] Carburization: HNHC stainless steels contain a maximum of 0.03% carbon by weight. After carburization, the carbon concentration in the surface layer is at least 0.10% by weight, and preferably at most 5.00% by weight.

[0092] Nitriding: Typical nitrogen levels in ASTM F1586 stainless steel range between 0.25% and 0.50% by weight. After nitriding, the nitrogen concentration in the surface layer ranges from at least 0.50% by weight at the top surface to 0.25% to 0.50% by weight nitrogen in the bulk material, with the nitrogen concentration in the surface layer being higher than in the bulk material. Nitriding: Typical nitrogen levels in HNHC stainless steels range between 0.80% and 0.90% by weight. After nitriding, the nitrogen concentration in the surface layer ranges from at least 0.90% by weight at the top surface to 0.80% to 0.80% by weight nitrogen in the bulk material, with the nitrogen concentration in the surface layer being higher than in the bulk material.

[0093] Boriding: The formula of HNHC stainless steels does not contain boron. The boriding case-hardening heat treatment results in a surface layer containing at least 0.05% by weight of boron.

[0094] ○Combinations of these.

[0095] Purpose Examples of articles that may be made from the alloys described herein and processed according to embodiments of the invention are as follows:

[0096] An embodiment of the present invention includes an orthopedic joint implant, such as a hip, knee, or shoulder prosthesis. Referring to FIG. 5, the hip prosthesis 500 may include an acetabular socket 510, an insert 520, a femoral head 530, a femoral shaft 540, and a femoral stem 550. The insert 520 (e.g., a polyethylene insert made of ultra-high molecular weight polyethylene, or UHMWPE) may be positioned in contact with the acetabular socket 510 between the acetabular socket and the femoral head 530. The femoral shaft 540, i.e., the tapered portion of the hip prosthesis 500, is positioned between the femoral head 530 and the femoral stem 550. As shown, the femoral stem 550 may be implanted into a patient's femur 560. Thus, an artificial hip joint typically has at least three components: a femoral stem 550 including a metallic stem portion and neck (femoral shaft 540), a femoral head 530 made of metal or ceramic, and an acetabular socket 510 which can be made of metal, ceramic, or polymer (e.g., polyethylene); these three components may be made from stainless steel according to embodiments of the present invention.

[0097] The metal processing methods described herein may be used to create orthopedic joint implants that include metal, such as: 1) metal-on-metal (MoM), 2) metal-on-polyethylene (MoP), 3) metal-on-ceramic (MoC), and (4) ceramic-on-metal (CoM). Thus, components of a hip prosthesis that may be created according to embodiments of the present invention include a femoral stem, a femoral head, and an acetabular socket.

[0098] Components formed by the processes described herein have the requisite mechanical properties for use in the articulation of joint replacements, including bulk strengths of 120 ksi YS and 170 ksi UTS at room temperature, and a hardness of at least 350 HV to meet the requirements of ASTM 799.

[0099] In particular, the orthopedic joint device may include a first component consisting essentially of cold worked ASTM F2229 stainless steel, ASTM F1586 stainless steel, or HNHC stainless steel and having a first articular surface; and a second component consisting essentially of cold worked ASTM F2229 stainless steel, ASTM F1586 stainless steel, or HNHC stainless steel and having a second articular surface configured to articulate with the first articular surface. The first articular surface and the second articular surface each consist of hardened ASTM F2229 stainless steel, ASTM F1586 stainless steel, or HNHC stainless steel and include a surface layer having at least one of carbon, nitrogen, or boron diffused therein. In some embodiments, the first component may be an acetabular socket and the second component may be a femoral head. In other embodiments, the first component may be a femoral head and the second component may be a femoral stem.

[0100] Furthermore, flat disks made from ASTM F2229, ASTM F1586, or HNHC stainless steels can be cold formed to produce acetabular shell blanks that can be further processed according to embodiments of the present invention. Spinal rods can also be made from ASTM F2229, ASTM F1586, or HNHC stainless steels using the processes described herein.

[0101] In use, in a hip replacement, damaged bone and cartilage may be removed and replaced with at least some prosthetic components. For example, a damaged femoral head may be removed and replaced with a femoral head 530 attached to a femoral stem 550, which may be placed into the hollow center of the femur 560 by adhesively or press-fitting the femoral stem 550 into the femur 560. In another example, a damaged femoral head may be removed and replaced by placing the femoral head 530 on top of the femoral stem. In many cases, the femoral head 530 includes a structure (e.g., a ball structure) that connects to the femoral stem 550 via the femoral shaft 540. In another example, a damaged cartilage surface of the socket (acetabulum) may be removed and replaced with an acetabular socket 510. In some cases, screws or cement are used to hold the socket in place. In another example, an insert 520 (eg, a plastic, ceramic, or metallic spacer) is fitted between the femoral head 530 and the acetabular socket 510 to provide a smooth gliding surface.

[0102] The materials and methods described in this application may be used to manufacture structural elements of a watch, such as the case, the rings, the gears, the bracelet or parts thereof, and the pins that hold the bracelet.

[0103] Furthermore, the materials and methods described herein may be used to manufacture non-magnetic components, such as retainer rings, for applications in the electrification and electronics markets that require high wear and corrosion resistance.

[0104] Furthermore, the materials and methods described herein may be used in the manufacture of instrumentation / non-magnetic housings for the oil and gas industry, as well as bearings, gears, gear teeth, and pump shafts.

[0105] Working Example The example compositions in Table 1 represent articles made from ASTM F2229 stainless steel, ASTM F1586 stainless steel, or HNHC stainless steel through a cold deformation and case hardening process. Further exemplary compositions expressed in weight percent are provided in Table 3 below.

[0106] [Table 3]

[0107] BioDur® 108 Stainless Steel, BioDur® 734 Stainless Steel, and NCORR TM Stainless steel samples were treated using conditions representative of embodiments of the present disclosure (Condition A through Condition H as shown in Figures 6A, 6B, 7A, 7B, 8A, and 8B) as follows:

[0108] ●Condition A = Cold working (deformation at room temperature) + low pressure carburizing at 500℃ for 1 hour

[0109] ●Condition B = Cold working (deformation at room temperature) + low pressure carburizing at 550℃ for 4 hours

[0110] ●Condition C = Cold work (warm) (deformation at 1200°F) + low pressure carburization at 50°C for 4 hours

[0111] ●Condition D = Cold working (deformation at room temperature) + low pressure carburizing at 550℃ for 16 hours

[0112] ●Condition E = Cold work (warm) (deformation at 1200°F) + low pressure carburization at 550°C for 16 hours

[0113] ●Condition F = Cold working (deformation at room temperature) + low pressure carburizing at 750℃ for 4 hours

[0114] ●Condition G = Cold work (warm) (deformation at 1200°F) + low pressure carburization at 750°C for 4 hours

[0115] ●Condition H = Cold working (deformation at room temperature) + low pressure carburizing at 960℃ for 5 hours

[0116] FIG. 6A is a graph showing the microhardness (Vickers hardness (HV)) of the surface layer of a BioDur® 108 stainless steel sample after five different processing conditions: Conditions D, E, F, G, and H. As shown in FIG. 6A, the bulk hardness is about 400 HV, and the peak hardness at the surface ranges between at least 510 HV and at least 850 HV depending on the processing condition. FIG. 6B shows the amount of carbon in solution in the surface layer of a BioDur® 108 stainless steel sample after four different processing conditions: Conditions A, B, G, and H. The carbon level in the bulk is close to 0.10 wt. %, and in the surface layer ranges from 0.50 wt. % to 3.65 wt. %, depending on the processing condition. FIG. 6C includes optical micrographs showing the visible surface layer of several conditions as shown in FIG. 6A. The optical micrographs in Figure 6C, taken after etching with Carling's anhydrous etchant (a conventional acid mixture used to reveal the alloy microstructure), show the hardened surface layer (light tones) and the bulk (dark tones). As shown in Figure 6C, the thickness of the visible hardened surface layer is approximately 20-25 μm (conditions A and B), 40-50 μm (condition D), 70-80 μm (condition F) or 200-300 μm (condition H).

[0117] FIG. 7A is a graph showing the microhardness (HV) of the surface layer of BioDur® 734 stainless steel samples processed according to four different processing conditions: conditions D, E, F, and G. The bulk hardness is about 350 HV to 400 HV. The hardness of the case ranges between 500 HV and 900 HV depending on the processing condition. FIG. 7B is a graph showing the carbon composition of the surface layer of BioDur® 734 stainless steel samples processed according to two processing conditions: conditions D and G. The peak carbon composition in the surface layer ranges between 0.90 wt. % and 3.20 wt. %. FIG. 7C includes optical micrographs showing the visible hardened surface layer formed after case hardening according to the conditions as shown in FIG. 7A and FIG. 7B. As shown in FIG. 7C, the thickness of the hardened surface layer is 30 to 40 μm (conditions D and E), or 50 μm to 60 μm (conditions F and G).

[0118] FIG. 8A shows the NCORR processed according to two conditions: condition F (cold work (deformation at room temperature) + carburizing at 750°C for 4 hours) and condition G (cold work (warm work) (deformation at 1200°F) + carburizing at 750°C for 4 hours). TM 8B is a graph showing the microhardness (HV) of the surface layer of the stainless steel alloy sample. The bulk hardness was about 350-400 HV, and the peak hardness at the surface reached 850 HV to 900 HV. FIG. 8C shows the microhardness of the NCORR treated according to two treatment conditions: Condition F and Condition G. TM 8A and 8B are graphs showing the carbon composition of the surface layer of stainless steel samples. The peak carbon composition in the surface layer ranges between 4.10 wt. % and 4.60 wt. %. FIG. 8C includes optical micrographs showing the visible surface layer for each of the conditions shown in FIG. 8A and FIG. 8B. As shown in FIG. 8C, the thickness of the hardened surface layer is about 50 μm to 70 μm (for both conditions).

[0119] All references, issued patents and patent applications cited throughout the specification are hereby incorporated by reference in their entirety for all purposes.

[0120] Although the present invention has been described in detail herein with respect to one or more preferred embodiments thereof, it is to be understood that the present disclosure is merely illustrative and exemplary of the present invention, and is made solely for the purpose of providing a complete and enabling disclosure of the present invention. The above disclosure is not intended to be construed as limiting the present invention, nor is it intended to exclude any other embodiments, applications, variations, modifications, or equivalent arrangements. The present invention is limited only by the appended claims and their equivalents.

Claims

1. 1. A method for manufacturing an article, comprising: Manganese 2.00% to 24.00% by weight Chromium 19.00% to 30.00% by weight Molybdenum 0.50% to 4.00% by weight Nitrogen 0.25% to 1.10% by weight Carbon≦1.00 wt% Phosphorus≦0.03 wt% Sulfur ≦1.00% by weight Nickel <22.00 wt% Cobalt <0.10 wt% Silicon≦1.00 wt% Niobium≦0.80 wt% Oxygen≦1.00 wt% Copper≦0.25% by weight forming a billet consisting essentially of a balance iron stainless steel composition; annealing the billet; cold working the billet to form an article; and then case hardening the article at a single case hardening temperature to form a surface layer on an upper surface thereof without annealing the article, wherein the case hardening comprises at least one of boriding, carburizing, nitriding, carbonitriding, or a combination thereof, without forming borides, carbides, nitrides, and carbonitrides; (i) the surface layer comprises a surface hardness of at least 350 HV, and (ii) the article is an orthopedic joint device comprising: a first element comprising a first articular surface; and a second element comprising a second articular surface configured to articulate with the first articular surface, the first element and the second element each consisting essentially of a stainless steel composition, wherein the first articular surface and the second articular surface each comprise a surface layer comprising the stainless steel composition and further comprising at least one of carbon, nitrogen, or boron diffused therein.

2. The billet is Manganese 21.00% to 24.00% by weight Chromium 19.00% to 23.00% by weight Molybdenum 0.50% to 1.50% by weight Nitrogen 0.85% to 1.10% by weight Carbon≦0.08 wt% Phosphorus≦0.03 wt% Sulfur ≦0.01% by weight Nickel≦0.05 wt% Cobalt <0.10 wt% Silicon≦0.75 wt% Niobium 0.00% by weight No intentionally added oxygen Copper≦0.25% by weight 10. The method of claim 1, consisting essentially of a stainless steel composition with the balance being iron.

3. The billet is Manganese 2.00% to 4.25% by weight Chromium 19.5% to 22.0% by weight Molybdenum 2.00% to 3.00% by weight Nitrogen 0.25% to 0.50% by weight Carbon≦0.08 wt% Phosphorus≦0.025 wt% Sulfur ≦0.01% by weight Nickel 9.00% to 11.00% by weight Cobalt <0.10 wt% Silicon≦0.75 wt% Niobium 0.25% to 0.80% by weight No intentionally added oxygen Copper≦0.25% by weight 10. The method of claim 1, consisting essentially of a stainless steel composition with the balance being iron.

4. The billet is Manganese 5.85% to 15.00% by weight Chromium 27.00% to 30.00% by weight Molybdenum 1.50% to 4.00% by weight Nitrogen 0.80% to 0.97% by weight Phosphorus <0.02 wt% Nickel 8.00% to 22.00% by weight Cobalt <0.01 wt% Silicon, oxygen, carbon, and sulfur such that (silicon + oxygen + carbon + sulfur) is ≦1 wt % Niobium 0.00% by weight Copper≦0.01% by weight 10. The method of claim 1, consisting essentially of a stainless steel composition with the balance being iron.

5. The billet is formed as follows: forming a powder comprising a stainless steel composition; and The method of claim 1 , including compacting the powder to form a billet.

6. The method of claim 1 , wherein cold working the billet comprises at least one of cold forming, cold rolling, cold drawing, shot peening, or pilgering.

7. The method of claim 1, wherein the surface hardening temperature is selected from the range of 400°C to 1000°C.

8. The method described in claim 7, wherein the surface hardening temperature is selected from the range of 400°C to 600°C.

9. The method described in claim 8, wherein the surface hardening temperature is selected from the range of 400°C to 550°C.

10. 10. The method of claim 1, wherein the surface hardening is carried out for a length of time selected from the range of 1 to 16 hours.

11. The method of claim 1, wherein the surface hardening comprises boronizing.

12. The method of claim 1, wherein the surface hardening includes a nitriding treatment.

13. The method of claim 1, wherein the surface hardening includes a carbonitriding process.

14. The method of claim 1, wherein the surface hardness is at least 510 HV.

15. The method of claim 1, wherein the surface hardness is at least 850 HV.

16. The method of claim 1 , further comprising an acetabular cup disposed between the first component and the second component.

17. The method of claim 16 , wherein the acetabular cup comprises at least one of a metal, a ceramic, or a polymer.

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