FIELD OF THE INVENTION

The invention relates generally to austenitic stainless steels suited to biomedical and other applications requiring biocompatibility and, more particularly, to processes for improving the corrosion and wear resistance and strength of components formed from essentially nickel-free high- manganese and high-nitrogen austenitic stainless steels intended for surgical implantation into or extended contact with the human body.

BACKGROUND OF THE INVENTION

Austenitic stainless steels are, in principle, ideally suited to biomedical and other applications that require biocompatibility, including surgically implanted components, and components that are in extended contact with the outside of a human body, such as jewelry and watchcases; they are not ferromagnetic and combine desirable mechanical properties, high strength, formability and toughness, with a high degree of corrosion resistance. However, many commonly used austenitic stainless steels use nickel as an austenite stabilizer, but nickel is an alloying element associated with adverse physiological reactions in patients and wearers of items made from such alloys.

Several essentially nickel-free austenitic stainless steels developed for use in surgical implants substitute austenite-stabilizer nickel with austenite-stabilizers manganese and nitrogen. Examples of such essentially nickel-free austenitic stainless steels include BioDur® 108, up to 0.1 wt% Ni, 21 to 24 wt% Mn and 0.85 to 1.1 wt % N, PANACEA P559, up to 0.1 wt% Ni, 10 wt% Mn and up to 0.48 wt% N, and BIOSSN4, up to 0.2 wt% Ni, 15 wt% Mn and 0.46 wt% N. See Indian Published Patent Appl'n No. 2020/11015355 (Indian Institute of Technology, 15 Oct. 2021) Table 5 at pp 10-11. Other essentially nickel-free austenitic stainless steels are disclosed in GB published patent application number 2345491A (Heymark Metals Ltd., 7 Dec. 2000), and contain up to 0.05 wt% Ni, 12 to 25 wt% Mn and 0.3 to 0.55% N.

BioDur® 108 has been used in a range of surgically implantable components, including the remotely magnetically-controlled telescopic intramedullary nail ("bone nail") and locking screws ("bone screws") of the PRECICE STRYDE™ system ("STRYDE system") for limb-lengthening by distraction osteogenesis, introduced in 2018 by NuVasive Specialised Orthopaedics™ ("NSO"). See 22 May 2018 Product Announcement at https://www.nuvasive.com/news/nuvasive-precice-stryde-system -used- in-first-patient-for-stature-lengthening-by-international-li mb-lengthening-expert/. The BioDur® 108 bone nail and bone screw components of the STRYDE system are sufficiently strong to support a typical body weight and therefore allow patients to remain at home during recuperation from the surgical implantation and the many weeks typically required for limb lengthening. See 22 May 2018 Product Announcement.

Despite the notional suitability of essentially-nickel free BioDur® 108 for surgically-implanted load bearing components, a number of adverse patient reports were logged for the STRYDE system, resulting in its withdrawal from the market in early 2019. See Urgent Recall Notification (20 February 2021) at https://www.nuvasive.com/wp-content/uploads/2021/02/NSQ-Prec ice-FSN- United-States-Biodur.pdf. Worldwide marketing approval for the STRYDE system was ultimately withdrawn, pending further assessment of biocompatibility concerns. See, e.g., Letter to Health Care Providers (1 July 2021) at https://www.fda.gov/medical-devices/letters-health-care- providers/potential-biocompatibility-concerns-nuvasive-speci alized-orthopedics-precice-devices- letter-health; H. Hohti et al., Orthopaedic Product News (13 Oct. 2021) at https://www.opnews.com/2021/10/magnetically-controlled-limb- lengthening-nails-the-stryde- story/17205.

STRYDE components made from BioDur® 108 surgically removed from patients show evidence of corrosion, particularly mechanically assisted crevice corrosion and pitting, as revealed by examination of the bone screws and screw entry holes in the bone nail and by the presence of biological material within the notionally hermetically sealed telescopic bone nail. See Jellesen et al., Acta Orthopaedica 92(5), pages 621-27 (2021) available at https://actaorthop.org/actao/article/view/923. As further noted in Jellesen et al., such susceptibility of surgically implanted BioDur® 108 components to mechanically assisted crevice corrosion and pitting presumably derives from a combination of interrelated factors, including the effects of processing, composition and microstructure. Jellesen et al. at page 625, R.H. col., lines 17 to 28.

US Published Pat. Appl'n No. 2012/0101531A1 (Peter Barth, 26 Apr. 2012) describes improvements in corrosion and wear resistance of biocompatible Ni-free ferritic and martensitic stainless steels by a "surface nitrogen case hardening" process, in which such low-Mn steels were "treated at a temperature between 1000° C and 1200° C in a nitrogen-containing gas atmosphere for between 3-6 hours and subsequently cooled", para. [0051], thereby absorbing nitrogen from the atmosphere to form a case hardened region and a transition region extending of the order of a few hundred pm below the surface. See, e.g., para. [0059], The application of such "surface nitrogen case hardening" or similar high temperature processes to surgically implantable and other components formed from BioDur® 108 and other biocompatible high-manganese austenitic stainless steels is hampered by manganese evaporation as a toxic vapor, which poses safety risks to those processing such stainless steels and, as a result of elemental loss, may diminish component properties. See ASDR Toxicological Profile for Manganese at https://www.atsdr.cdc.gov/ToxProfiles/tpl51.pdf; see also US Published Patent Appl'n No. 2008/0141826 (Schlumberger Reservoir Completions, 19 June 2008) para. [0064],

US Published Patent Appl'n No. 2008/0141826 reports that, for certain corrosion resistant austenitic stainless steels containing N, Ni and up to 30 wt% Mn used in oilfield applications, Mn evaporation and N loss during melting and casting can be greatly reduced by using a N2 atmosphere at a pressure of at least 1 bar, para. [0025], and how, during solution annealing, use of a "nitrogen-rich atmosphere" enables "further nitrogen (N) solid solution" and "allow[s] for sufficient nitrogen (N) absorption and homogenization in the austenite (y)". See paras. [0027] and [0029],

It would be advantageous to develop processes that increase the resistance to mechanically assisted crevice corrosion and pitting of surgically implanted components formed from BioDur® 108 and other high-manganese, high-nitrogen and essentially nickel-free biocompatible austenitic stainless steels.

OBJECT OF THE INVENTION

It is an object of the present invention to provide processes that increase the resistance to pitting and mechanically assisted crevice corrosion and the strength of surgically implanted and other components that require compatibility formed from BioDur® 108 or other high-manganese, high- nitrogen and at least essentially nickel-free austenitic stainless steels.

It is another object of the present invention to avoid the above-mentioned disadvantages of the above-mentioned components or articles formed from BioDur® 108 or other high-manganese, high- nitrogen and at least essentially nickel-free austenitic stainless steels.

It is a further object of the invention to provide an alternative to the prior art.

SUMMARY OF THE INVENTION

Thus, the above-described objects and several other objects are intended to be obtained in a first aspect of the invention by providing a method for improving corrosion and wear resistance and yield strength of an at least essentially Ni-free and Mn-rich austenitic stainless steel article, the method comprising: providing an article formed from an at least essentially Ni-free austenitic stainless steel alloy comprising less than 1 wt% Ni and at least 4 wt% Mn; maintaining the article at an elevated temperature of between 1050 and 1250 °C in an atmosphere comprising N2; selecting a pressure of the N2 sufficient to provide a desired concentration of N at the article surface while the article is maintained at the elevated temperature; selecting a total pressure of the atmosphere sufficient to effectively prevent Mn evaporation from the article surface while the article is maintained in the selected pressure of N2 at the elevated temperature; and quenching the article from the elevated temperature at a rate of at least 200 K per minute.

An article could be a powder, a blank, a solid product in its final shape or a near-shape product manufactured from powder. A powder could be used as the starting material for manufacturing of metal components, such as metal injection molding, as well as additive manufacturing, such as powder-bed fusion techniques and binder jetting. Products manufactured from powder by metal injection molding or additive manufacturing may need sintering or hot isostatic pressing (HIP) after the step where they get the product's shape. The present invention, where nitrogen is introduced at elevated temperature and pressure while preventing manganese evaporation, could be an integrated part of the sintering or HIP step. Thus, "an article includes a shaped component, a powder or a blank that will be processed or shaped into a component, including by the removal or addition of material.

When reference is made to "a component", the term includes a component intended for surgical implantation, including a component that forms part of a system for osteosynthesis, such as a bone screw, locking plate, intramedullary nail or external fixator, including as used in limb lengthening devices, either intra- or extramedullary. Moreover, what is referred to as "an article" includes components for joint prostheses. A component need not be intended for surgical implantation, and may include a component that will be in extended contact with human skin, such as a component of a piece of jewelry or watch.

Ni-free means comprising less than a trace amount of Ni and essentially Ni-free means comprising less than 1 wt% Ni.

Mn-rich, also referred to as high-Mn or high-manganese, means comprising at least 4 wt% Mn.

An inert gas means a gas or mixture of gases that will not interact chemically with N2gas at the elevated temperature nor with the austenitic stainless steel article. Inert gases include Heand Ar. As will be described in the following, H2 may also be used as gas in a method according to the present invention even though it is not necessarily inert towards the steel. Allthough all the examples given in the detailed description include the use of an inert gas, the scope of protection also covers embodiments without this feature. An advantage of using an atmosphere comprising N2, is that a given N2 pressure, or partial pressure, may be selected independent of the total pressure of the atmosphere. In order to avoid Mn- evaporation at elevated temperatures, the total pressure of the gas mixture needs to be high, such as above 2 bar. By mixing N2 and for example Ar, one can adjust the amount of N2 (partial pressure) independent of the total pressure (sum of partial pressures of Ar and N2). In the end, the partial pressure of N2 determines, for a given temperature, how much nitrogen that can be dissolved during the heat treatment. Normally, HTSN is done in pure N2, i.e. the partial pressure of N2 is equal to the total pressure. However, since a certain minimum pressure is needed to prevent Mn-evaporation, normal HTSN cannot be used if the N2 partial pressure has to be kept below 2 bar. For most austenitic stainless steels, a N2 partial pressure of 2 bar or more will lead to the formation of chromium nitrides and the steel will lose its "stainless" properties, i.e. it is no longer corrosion resistant. Hence, high-Mn materials cannot be nitrided with the common HTSN process, since this would lead either to Mn-evaporation or to the formation of chromium nitrides.

An advantage of selecting a pressure, or partial pressure, of N2 sufficient to provide a desired concentration of N at the article surface while the article is maintained at the elevated temperature is that, for a given at least essentially Ni-free high-Mn austenitic stainless steel, a desired N depth concentration profile, arising from the solid state diffusion of N below the surface, may be produced in a given time. The pressure may e.g. be selected as will be described in relation to figure 3.

An advantage of selecting a total pressure of the atmosphere sufficient to effectively prevent Mn evaporation from the article surface while the article is maintained in the pressure of N2 at the elevated temperatures, is that the gas will not interact with N2 at the elevated temperature and only to a limited extent with the steel.

A desired N depth concentration profile means the depth concentration profile of N which has beforehand been determined as being sufficient to accomplish a desired modification of the properties of the austenitic stainless steel within the depth and concentration profile of N, such as improved strength, hardness and corrosion resistance arising from the presence of dissolved N in the austenitic phase. In the experiments made in relation to the development of the present invention, the cross-sectional hardness was determined.

A further advantage of having a desired concentration depth profile of N is that, for articles with closely spaced surfaces, the depth and concentration profile of N from opposing or adjacent surfaces may overlap and affect bulk properties to the article. HTSN is a surface treatment, i.e. nitrogen is introduced into an article from its surface. While the concentration of N near the surface is determined by the partial pressure and temperature, the concentration depth profile of N depends on temperature and process time. The longer the process time, the deeper N will diffuse into the material. For an article with thin cross section, N may diffuse throughout the thickness of the article, i.e. the N-concentration is not only augmented in the near-surface area but also in the core/bulk of the article. Eventually, for thin articles (or powders) a uniform nitrogen distribution can be obtained throughout the article.

Effective prevention of Mn evaporation means that composition and properties of the article are not noticeably affected by Mn evaporation. This can be determined by cross sectional light optical microscopy (LOM), scanning electron microscopy (SEM) or energy dispersive X-ray microscopy (EDS) analysis.

An advantage of quenching at a rate of at least 200 K/minute is suppression of the formation of unwanted Cr nitrides at intermediate temperatures. Fast quenching is needed to prevent the formation of Cr-nitrides. Conventionally, this is achieved by using IXh-gas with pressures of up to 15 bar. Given the high N-contents in the present invention, quenching needs to be faster than conventionally achieved in IXh-gas.

In a further aspect of the invention, the austenitic stainless steel alloy contains:

- at least 50 wt% Fe,

- 15.0 to 35.0 wt% Cr,

- 0.05 to 8 wt% Mo,

- 4.0 to 10.0 wt% Mn,

- 0.01 to 4.0 wt% Cu,

- 0.8 to 1.5 wt% N, and

- has a C content of 0.20 wt% or less, a Si content of 2.0 wt% or less, a P content of 0.03 wt% or less, a S content of 0.05 wt% or less, a Ni content of 0.5 wt% or less, an Al content of 0.03 wt% or less, and an O content of 0.020 wt% or less.

In a further aspect of the invention, the N2 is mixed with a partial pressure of another gas, such as an inert gas.

In a further aspect of the invention, the article is maintained at the elevated temperature for 0.5 to 48 hours, such as for 0.5 to 20 hours, such as for 0.5 to 4 hours.

In a further aspect of the invention, the inert other gas comprises Ar, He or H2. The pressure of the other gas may be at least 10 mbar, such as at least 100 mbar. In a further aspect, the quenching is provided by He. The He pressure may be at least 2 bar.

An advantage of quenching with He is that rapid quenching rates are possible, as result of the high heat capacity of He. Another advantage of quenching with He is that He is inert and will not react with the article as it cools from the elevated temperature.

A method according to the present invention results in a surface treatment of an article as will be described in the following.

In a further aspect, the N2 pressure, or partial pressure, is at least 500 mbar. The upper limit depends on the process and equipment used. It may e.g. be possible to perform the surface treatment in a hot isostatic pressure (HIP) furnace where the pressure may be up to 2000 bar. The pressure may also be at most 1000 bar, such as at most 100 bar, such as at most 5 bar.

An advantage of having the N2 pressure, or partial pressure, be at least 500 mbar is that this is typically high enough to introduce a desired N composition and depth profile.

In a further aspect of the invention, the total pressure of the atmosphere is at least 2 bar.

An advantage of providing the total pressure of the atmosphere to be at least 2 bar is that this may be a generally sufficient total pressure to effectively prevent Mn evaporation.

In a further aspect of the invention, at least a part of the method is performed in a hot isostatic pressure furnace.

In a further aspect of the invention, the at least essentially Ni-free and high Mn austenitic stainless steel comprises BioDur® 108.

The bulk composition of BioDur® 108, see, e.g., https://www.fwmetals.com/materials/stainless- steel/biodur-108/, is specified, in wt%, as follows:

- C <0.08, Cr 19 to 23, Ni <0.05, Mn 21 to 24, Mo 0.50 to 1.50, Cu <0.25, Co <0.10, Si <0.75, S <0.01, P <0.03, N 0.85 to 1.10, and balance Fe.

In a further aspect of the invention, the method further comprises subsequent application of low temperature surface hardening with C or N, such as a surface hardening with C or N at a temperature of 250-580 degrees C.

Application of low temperature surface hardening with C or N means any process that introduces C and/or N in solid solution in austenite into the surface of the essentially Ni-free and high Mn austenitic stainless steel and so hardens said surface. Such low temperature surface hardening may be gas, liquid salt, ion- or plasma-based. Mechanism of such surface hardening include, but are not limited to solid solution hardening, which involves the introduction of N and/or C into solid solution in austenite.

The advantage of including a low temperature surface hardening treatment is that this can further increase wear resistance and improve other related surface properties especially fatigue performance, though corrosion resistance may also be enhanced. By way of low temperature surface hardening, C and N content in excess of 4 wt% and even 5 to 6 wt% may be realized. See, e.g., Encyclopedia of Iron, Steel, and Their Alloys, book chapter: Low Temperature Surface Hardening of Stainless Steel, F.A.P. Fernandes, T.L. Christiansen, M.A.J. Somers, Published Dec. 2015.

A further advantage of including a low temperature surface hardening treatment is that it can complement the high temperature solution treatment. The effects of the low temperature surface hardening treatment generally extend less far below the article surface than the effects of the overlapping high temperature solution treatment. The low temperature processing affects a case depth of around 50 pm, while the preceding high temperature solution treatment case depth may be 1 to 2 mm. "Case depth" is a measure of the depth of the surface-hardened layer of an article. In the present context, it may be defined as the depth below the article surface wherein the hardness is at least 25 HV above the hardness in the core of the unaffected material. As also described herein, low temperature processing offers the possibility of much higher N/C concentrations than high temperature solution treatment.

In an aspect of the invention, the method further comprises application of a surface or bulk deformation treatment.

Surface deformation treatments include, but are not limited to, shot peening, burnishing, ironing etc. Bulk deformation treatments include, but are not limited to, machining operations, such as rolling, stretching, drawing.

Advantages of bulk and surface deformation treatments include further improvement in wear resistance and strength of the article, beyond those obtained by the high temperature and/or low temperature treatment steps.

Another aspect of the invention is an article that is formed from an at least essentially Ni-free austenitic stainless steel comprising between 4 and 25 wt% Mn, further comprising, within a subsurface region extending to a depth of at least 50 pm: N + C content at least 1.1 wt%; hardness of at least 350 HV0.01 and PREN at least 30.

PREN means pitting resistance equivalent number and is a measure for corrosion resistance of stainless steels, defined as equal to the wt% Cr + 3.3 • wt% Mo + 16 • wt% N. The higher PREN the better is the resistance against localized corrosion as pitting and crevice corrosion. Alternatively, the MARC number can be used, which also includes the effect of carbon on the localized corrosion resistance.

An advantage of a subsurface region extending to a depth of at least 50 pm: N + C content at least 1.1 wt%; hardness of at least 350 HV0.01 and PREN at least 30, is improved corrosion and wear resistance, and possibly also yield strength, of the article. These hardness measurements may be made according to ASTM E384-22.

N + C content means the sum of the N and C content expressed in wt%.

In a yet further aspect of the invention, the subsurface region further comprises: N + C content of at least 1.5 wt% and hardness of at least 400 HV0.01.

In another aspect of the invention, the subsurface region further comprises a plastically deformed surface zone with more than 10% cold work and yield strength of at least 800 MPa. The yield strength may be estimated as three times the hardness (HV) according to the well-known Taborrelationship.

In yet another aspect of the invention, the subsurface region further comprises a grain refined surface zone with grain size less than 30 pm and a yield strength of at least 700 MPa.

In another aspect of the invention, the at least essentially Ni-free austenitic stainless steel is BioDur® 108, i.e. is specified, in wt% as: C <0.08, Cr 19 to 23, Ni <0.05, Mn 21 to 24, Mo 0.50 to 1.50, Cu <0.25, Co <0.10, Si <0.75, S <0.01, P <0.03, N 0.85 to 1.10, and balance Fe.

BRIEF DESCRIPTION OF THE FIGURES

To the extent that the figures show different ways of implementing the present invention they are not to be construed as limiting to other possible embodiments falling within the scope of the attached claim set.

FIGS. 1(a) and (b) show the effects of high-temperature solution treatment in a pure N2 atmosphere on the sub-surface microstructure and visual appearance of BioDur® 108 austenitic stainless steel bone screws.

FIGS. 2(a) to (c) show the effects of high-temperature solution treatment in an Nz/Ar atmosphere on the visual appearance, sub-surface microstructure and sub-surface hardness of BioDur® 108 bone screws.

FIG. 3 shows the isopleth of BioDur® 108 with superimposed nitrogen isobars. FIG. 4 shows the result of high temperature solution treatment in a pure N2 atmosphere on a specimen of CARNIT 90 austenitic stainless steel.

FIGS. 5(a) and (b) show and plot the results of three-point bend test conducted on sets of three BioDur® 108 bone screws, as received and high-temperature solution treated in an Nz/Ar atmosphere.

FIG. 6(a) to (e) show the effects of a highly corrosive environment on BioDur® 108 bone screws, as received and high temperature solution treated in an Nz/Ar atmosphere.

FIG. 7 shows the results of a ball and disc wear test conducted on BioDur® 108 bone screws, as received and high temperature solution treated in an Nz/Ar atmosphere.

FIG. 8 is a schematic representation of possible combinations of high temperature solution treatment with low temperature processing steps.

FIG. 9 shows the effect of low temperature treatment in an NH3/H2 atmosphere on the sub-surface microstructure of a BioDur® 108 bone screw.

FIG. 10 are schematic representations of the effects of high temperature solution treatment and low temperature treatment on the depth/concentration profiles of N and C.

FIGS. 11(a) to (c) show the effects of high-temperature solution treatment in an N2/Ar atmosphere, followed by low temperature treatment in an atmosphere of CO and H2, on sub-surface hardness and microstructure of a BioDur® 108 bone screw.

FIGS. 12(a) and (b) show the effects of low/mid temperature treatment in an atmosphere of N2, NH3 and C2H2 followed by high-temperature solution treatment in an N2/Ar atmosphere.

DETAILED DESCRIPTION

As noted, high temperature solution treatment in a pure N2 atmosphere of high-Mn content austenitic stainless steel alloys can result in Mn loss by evaporation.

FIGS. 1(a) and (b) show the effects of high-temperature solution treatment in an N2-only atmosphere on as-received bone screws formed from BioDur® 108 austenitic stainless steel, which contains between 21 and 24 wt% Mn. The high-temperature solution treatment was performed at 1150 °C for 1 hour in 500 mbar of N2 followed by quenching in around 10 bar of He.

FIG. 1(a) is an image obtained in an optical microscope from a polished cross section through a BioDur® 108 bone screw subjected to high temperature solution treatment in 500 mbar of pure N2.

FIG. 1(a) shows the grain structure characteristic of solution-treated austenitic BioDur® 108 and, towards the top of the image, a portion of the bone screw surface S exposed for 1 hour to 500 mbar of N2 at 1150 °C. In Fig. 1(a), the darker features located at grain boundaries and disposed immediately below surface S, including the three darker features identified by arrows and letter V, correspond to voids formed as a result of Mn evaporation during the IXh-only high temperature solution treatment. Surface S also shows a degree of roughness, indicative of Mn evaporation.

FIG. 1(b) shows an as-received BioDur® 108 bone screw, upper part of FIG. 1(b), and portions of a BioDur® 108 bone screw after high temperature solution treatment in 500 mbar of N2, lower part of FIG. 1(b), including those bone screw portions used to produce the cross-sectional image in FIG. 1(a). Consistent with the sub-surface voids and surface roughness seen in cross-sectional image of FIG. 1(a), the N2-only high temperature solution treated bone screw portions in the lower part of FIG.

1(b) are discolored and less reflective than the as-received bone screw shown in the upper part of FIG. 1(b).

The presence of such subsurface voids and surface roughness, and observable discoloration and reduced reflectivity in IXh-only high temperature solution treated bone screws is caused by decreased corrosion and wear resistance of the BioDur® 108 austenitic stainless steel bone screw, offsetting any benefits arising from the increased concentration of subsurface dissolved N produced by the IXh-only high temperature solution treatment.

FIGS. 2(a), (b) and (c) show the effects of an IXh/Ar high temperature solution treatment on BioDur® 108 bone screws. The treatment was performed at 1150 °C for 1 hour in a partial pressure of 500 bar of N2 mixed with Ar to reach a total pressure of 3 bar, followed by quenching in around 10 bar of He.

FIG. 2(a) is an image obtained in an optical microscope from a polished cross section though a BioDur® 108 bone screw subjected to such IXh/Ar overpressure during high temperature solution treatment. FIG. 2(a) shows grain structure characteristic of solution treated austenitic BioDur® 108 and, towards the top of the image, the location of bone screw surface S actually exposed to 3 bar of an IXh/Ar mixture with a 500 mbar N2 partial pressure at 1150 °C for 1 hour. Strikingly, the voids associated with Mn evaporation evident in FIG. 1(a)— high temperature solution treatment in 500 mbar of N2— are entirely absent from FIG. 2(a)— same high temperature solution treatment in 500 mbar of N2 mixed with Ar to a total pressure of 3 bar.

FIG. 2(b) is a magnified image of a set of three BioDur® 108 bone screws heated to 1150 °C for 1 hour in 500 mbar of N2 in 3 bar of an IXh/Ar mixture, followed by quenching in around 10 bar of He. FIG. 2(b) shows the bone screw used to produce the cross-sectional image in FIG. 2(a). Consistent with the absence of sub-surface voids in the magnified cross sectional image shown in FIG. 2(a), the IXh/Ar high temperature solution treated bone screws shown in FIG. 2(b) have the same overall color and high degree of reflectivity as the as-received bone screw shown in the upper part of FIG. 1(b). FIG. 2(c) shows a hardness profile, measured using a load of 10 grams, against distance perpendicular to the original bone screw surface, the surface marked S in FIG. 2(a), obtained from the bone screw cross section shown in FIG. 2(a). The hardness measurements were made according to ASTM E384-22. As evident from FIG. 2(c), surface hardness decreases with depth below the original bone screw surface, as would be expected for hardness variation determined by the concentration profile of atomic N introduced from the bone screw surface during Na/Ar high temperature solution treatment. During HTSN surface treatment, N is introduced via the surface into the article, where it locates at certain positions of the lattice, distorting the initial lattice. Lattice distortion results in stress, which reflects in an increase in hardness. The higher the N concentration, the higher the distortion and thus the higher the hardness increase. In essence, the N concentration is concluded from the measured hardness profile; it is highest right underneath the surface and gradually dropping to the level of the base material with increasing distance from the surface.

The examples shown in FIGS. 1 and 2 demonstrate how, for a given high-temperature solution treatment of a given high-Mn alloy, application of an Ar overpressure may allow access to an N2 partial pressure that would otherwise produce detrimental results. This may matter because it is the N2 partial pressure, in combination with processing temperature and, to the extent equilibrium conditions are not reached, processing time, that may ultimately determine the sub-surface N profile achieved in a given Ni-free high-Mn austenitic stainless steel.

FIG. 3 illustrates the importance of the N2 partial pressure for the high temperature solution treatment of BioDur® 108. FIG. 3 shows nitrogen isoactivity lines corresponding to N2 pressures of 0.5, 1 and 2 bar, superimposed onto the isopleth for BioDur® 108, calculated using Thermo-Calc software. In general, ThermoCalc software provides highly reliable predictions of N content in the solid stainless steels as function of the partial pressure of N2 imposed under equilibrium conditions. For some combinations of temperature and N wt%, shown as mass %, a given N2 partial pressure may lie close to the edge or even outside the monophase austenite space (stability region), marked as FCC-A1 in FIG. 3, while a different N2 partial pressure may lie well within the austenite stability region.

In general, the equilibrium N content to be introduced into a given Ni-free high-Mn austenitic stainless steel as function of N2 partial pressure during a given processing temperature may be accurately predicted based on an equilibrium phase diagram, such as that shown in FIG. 3, calculated using the thermodynamic database/software package Thermo-Calc. Further, the diffusion depth/N depth profile in a given Ni-free high-Mn austenitic stainless steel as a function of N2 partial pressure for a given processing temperature and time may also be predicted using software packages, for example, those provided by Thermo-Calc, or by "manual" diffusion calculations. The skilled person will further appreciate that certain non-equilibrium deviations from the calculated diffusion depth/N depth profiles may occur.

In general, any of a range of Ni-free high-Mn austenitic stainless steels may benefit from the type of Na/Ar high temperature solution treatment described herein, as a means to avoid problems associated with excess N2 pressure, by allowing independent control of the total pressure and of the N2 partial pressure. As described herein, such independent control over N2 partial pressure allows for control over the N surface concentration, in turn, the depth profile of the N concentration at the processing temperature and, in turn, if quenching is sufficiently rapid, the N concentration/depth profile in the quenched component of article.

As an illustration of the general need to control N2 partial pressure, FIG. 4 shows the detrimental effects of excess N2 pressure during high temperature solution treatment on a N concentration/depth profile obtained in CARNIT 90, a high-Mn, Ni-free austenitic stainless steel containing around 18 wt% Cr, 18 wt% Mn and 0.5 wt% C and N (X35CrMnN18-18). See WO2017133725A1 (Schaeffler Techs. AG & Co. KG, 10 Aug 2017). FIG. 4 is an image obtained in an optical microscope from a polished cross section though a CARNIT 90 specimen maintained at 1,150 °C for 1 hour in 3 bar of N2 and quenched in around 10 bar of He. The surface region, at the upper part of the figure, exhibits a microstructure containing nitride phases N, following introduction during high temperature solution treatment of a concentration of N in the sub-surface region beyond the solubility limit of N in austenite.

All of the examples of N2-only and N2/Ar high temperature solution treatments, followed by He quenching, described in the present application were performed in a laboratory tube furnace equipped with a gas inlet coupled to the regulated output of a pressurized gas bottle. In all examples, the gas bottle contained 16.66 % N2, premixed with balance Ar. Premixed N2/Ar bottles may have a range of different N2 percentages and, whether used in the laboratory tube furnace or other furnace system, provide a straightforward way to deliver a given N2 partial pressure at a given N2/Ar atmosphere overpressure. It would also be possible to configure, in a laboratory tube furnace or other furnace system, the output from separate N2 and Ar bottles to maintain a given N2 partial pressure and a given N2/Ar total pressure.

The laboratory furnace used for the high temperature solution treatment operates by fluctuating the pressure of the N2/Ar mixture around a set point, alternating between releasing excess gas mixture and backfilling with fresh gas mixture. Furnace system configurations with continuously flowing premixed gas would be possible, as would configurations with a static atmosphere. In general, other gases may be used in place of Ar, such as He or H2. Use of H2 could also be beneficial as a means to remove unwanted oxygen present in the furnace by reacting with the oxygen. A mixture containing more than one inert gas may also be used.

For the He quenching step, the laboratory tube furnace used a large inlet, different from that used for the treatment gas, allowing inert gas from a pressurized inert gas bottle to enter directly into the furnace through a reduction valve. The pressure of the large inlet is adjustable, though for He is typically set between around 10 bar and 20 bar. In general, it would also be possible to use different gaseous quench media, provided that the required rapid quench rates are obtainable. In general, it would also be possible to use liquid media. For example, water quenching, though extremely rapid, would tend to contaminate the surface of the austenitic stainless steel with oxides, removal of which would then be required. In general, when using gas as quench medium, it would also be possible to introduce quenching gas from other than a gas bottle, for example, from a pressurized buffer tank, for recirculation through a heat exchanger, as in certain industrial furnaces.

In terms of attainable quench rates, in the laboratory tube furnace used for the examples described in the present application, He quenching typically empties a 50 L He bottle within 1-2 minutes, providing a rapid quench rate, well in excess of 200 K per minute. In general, the slower the cooling rate the higher the risk of forming unwanted nitrides in the surface region. In general, if N content is near the solution limit in austenite at a given temperature, as shown, for example, by the BioDur 108® isopleth shown in FIG. 3, a higher quenching/cooling rate may be required. 200K/min appears to be a reasonable lower limit for the type of high temperature solution treatment described herein for use with Ni-free high-Mn austenitic stainless steels.

FIG. 5(a) shows the results of three-point bend tests on two sets of three BioDur® 108 bone screws, performed at a rate of 1 mm/ min on a 10 kN machine with a 40 mm distance and a roller size of 10 mm. Three-point bend test may, in general, be performed according to ISO 178. In FIG 5(a), the lefthand set is as-received and the right-hand set was maintained for 1 hour at 1150 °C in a partial pressure of 500 mbar of N2 mixed with Ar to a total pressure of 3 bar and quenched in around 10 bar of He. As seen from the figure, the HTSN treated parts have an excellent ductility and will tolerate a full U-bend without fracture - similar to the reference material. FIG. 5(b) is a plot of the results of the three-point bend tests for the sets of three BioDur® 108 bone screws shown in FIG. 5(a), with force (kN) on the vertical axis and displacement (mm) on the horizontal axis.

As demonstrated by the end portion of the results plotted in FIG. 5(b), after high temperature solution treatment in an N2/Ar atmosphere, BioDur® 108 bone screws retained their high ductility. FIGS. 6(a) to (e) show the results of exposing BioDur® 108 bone screws, untreated and high temperature solution treated in an Na/Ar atmosphere, to highly corrosive environments. FIGS. 6(a) to (c) show results from 4 days immersion in 6 % FeCU pH 0.5 at 23 °C, and FIGS. 6(d) from, yet more aggressive, 2 weeks of immersion in FeCU, pH 0.1. In FIGS. 6(a), (d) and (e), the left hand bone screws were exposed as received, while the right hand bone screws had been maintained for 1 hour at 1150 °C in a partial pressure of 500 mbar of N2 mixed with Ar to a total pressure of 3 bar and quenched in around 10 bar of He before exposure. FIGS. 6(b) and (c) are magnified views of portions of as received bone screws.

The as received bone screw portion, seen in the left hand of FIG. 6(a), and in magnified views in FIGS. 6(b) and (c), shows clear evidence of local corrosion, while the equivalent bone screw portion Na/Ar high temperature solution treated, seen in the right of FIG. 6(a), shows no such evidence of local corrosion. The as received bone screw portion, as identified by the arrows in the magnified view of FIG. 6(c), shows clear evidence of pitting corrosion P and stress corrosion cracking C.

The as received bone screw portions subjected to the yet more aggressive corrosion, seen in the left hand of FIGS. 6(d) and (e), exhibit uniform surface corrosion, while the equivalent portions of high temperature solution treated bone screws, seen in the right of FIGS. 6(d) and (e), show no such evidence of corrosion. The black square in figure 6(e) covers some numbers not relevant for the present description.

As demonstrated by the images shown in FIGS. 6(a) to (e), high temperature solution treatment in an Na/Ar atmosphere increases the corrosion resistance of as received BioDur® 108 bone screws.

FIGS. 7(a) and (b) show the results of a ball and disc wear test performed on BioDur® 108 bone screws, as received, FIG. 7(a), and high temperature solution treated in an Na/Ar atmosphere FIG. 7(b). The bone screw presented in FIG. 7(b) had been maintained for 1 hour at 1150 °C in a partial pressure of 500 mbar of N2 mixed with Ar to a total pressure of 3 bar and quenched in around 10 bar of He. The ball and disc wear tests used a fixed reciprocating 6mm diameter ZrCh ball with a normal load of 10 N, sliding at a radius of 2mm and an angle of 25 degrees at 1 Hz frequency, in conformance with ASTM Standard G99. See ASTM G99, Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus, ASTM International, West Conshohocken, PA (2010). The high temperature treated bone screw seen in FIG. 7(b) shows significantly lower wear volume loss than the untreated bone screw presented in in FIG. 7(a).

As demonstrated by the ball test results shown in FIGS. 7(a) and (b), high temperature solution treatment in an IXh/Ar atmosphere increases the wear resistance of as received BioDur® 108 bone screws, a surprising effect may be attributed to additional N in solution. The high temperature solution treatment in an Na/Ar atmosphere, described above, may be advantageously performed in isolation on articles or components formed from Ni-free high-Mn austenitic stainless steel alloys or may be performed on such articles or components as part of a sequence of one or more surface of bulk treatments, including, for example, low temperature surface treatments. FIG. 8 is a schematic representation of a selection of possible combinations of Na/Ar high temperature solution treatment, as described above, with different low temperature processing steps. High temperature solution treatment in an Na/Ar atmosphere may be used to produce "parts", -"pre-processed high temperature solution nitrided (HTSN) 'blanks'", or powder for further processing into parts. HTSN surface treatment adds to the stability of the material under subsequent mechanical surface deformation. In general, mechanical deformation or cold working of Nz/Ar high temperature solution treated articles or components could include surface deformation— shot peening, burnishing etc.— or bulk deformation— machining operations, such as rolling, stretching, drawing. As referenced in FIG. 8, static strain aging may correspond to annealing at 300 to 500 °C after cold working, for example, shot peening. Though the dashed arrows in FIG. 8 show a specific cycle of processing steps— HTSN, shot peening, recrystallization, low temperature surface hardening with N/C— processing sequences not explicitly shown in FIG. 8 may also be used. Further, as described below in connection with FIG. 12, Nz/Ar high temperature solution treatment may also be performed following surface loading with N/C. As referenced in FIG. 8, recrystallization may correspond to heat treatment in a temperature range between around 500 to 1,200 °C.

FIG. 9 is an image obtained in an optical microscope from a polished cross section though a BioDur® 108 bone screw subjected to low temperature gaseous nitriding treatment in NH3/H2 below 420 °C and for under 24 hours but without an N2 or N2/Ar high temperature solution treatment. In general, high-Mn essentially Ni-free austenitic stainless steels may require gaseous nitriding at lower temperatures than conventional, Ni-containing, austenitic stainless steels. FIG. 9 shows the grain structure characteristic of BioDur® 108, the planar bone screw surface S of the bone screw subjected to low temperature gaseous nitriding and the nitrided layer LT so produced. Here, the nitrogen case of expanded austenite is around 4-6 pm thick. In general, the thickness depends on the treatment time and the temperature; it is usually up to around 30pm. Measurement of the hardness of surface S with nitride layer LT yielded readings of 1134, 1134 and 1061 HV0.05. These measurements may be made according to ASTM E384-22.

The effects of low temperature surface treatments, such as the gaseous nitriding (below 470 degrees C) of FIG. 9 or the gaseous carburising (below 540 degrees C) of FIG 11, typically extend to tens of pms below the treated surface. This is significantly less than the effects of N2/Ar high temperature solution treatment, which may extend of the order of hundreds or thousands of pm, as indicated, for example, by the cross-sectional hardness measurements shown in FIG. 2(c) and FIGS. 11(a) and (b). FIG. 10 is a schematic representation of the N concentration depth profile introduced by high temperature solution treatment and the N/C concentration depth profiles introduced by subsequent low temperature surface treatment. The absence of a process step indicated after low temperature treatment does not preclude the existence of such a processing step, as already discussed, for example, in connection with FIG. 8. As indicated by the "optional" arrow in FIG 10, the combination of a low temperature and high temperature treatment may be particularly advantageous. The trends shown in Fig. 10 are clearly shown by actual results in FIGS. 11(a) to (b).

FIGS. 11(a) to (c) show the effects of high-temperature solution treatment in an Na/Ar atmosphere, followed by low temperature treatment in an atmosphere of CO and F , on sub-surface hardness and microstructure of a BioDur® 108 bone screw (note that FIG. 11(b) is a magnification of the first 50 pm of FIG. 11(a)). The high temperature solution treatment involves maintaining the bone screw for 1 hour at 1150 °C in a partial pressure of 500 mbar of N2 mixed with Ar to give a total pressure of 3 bar, followed by quenching in around 10 bar of He. The low temperature treatment maintains the bone screw for 60 hours at 465 °C in a mixture of 40% CO and 60% H2 at around 1 bar.

FIGS. 11(a) and (b) are plots of hardness as a function of depth below the surface, measured with a load of 10g from the cross section of the high and low temperature treated BioDur® 108 bone screw shown in FIG. 11(c). The plot in FIG. 11(a) shows an initial steep decline from a hardness value approaching 800 HV0.01 adjacent to the treated sample surface, marked S in FIG. 11(c), declining over the order of a few tens pm to around 460 HV0.01 and then declining over a depth of almost 1 mm towards the core value of 374 HV0.01. FIG. 11(b) expands the horizontal scale to show that relatively hardness decline, corresponding to a C enriched layer, extends to a depth of the order of around 35 pm.

FIG. 11(c) is an image obtained in an optical microscope from a polished cross section though a BioDur® 108 bone screw subjected to the above-described high temperature and low temperature N and C treatments. The dashed line indicates the extent of the depth of the C-enriched layer below the treated bone screw surface, the curvature of which is evident.

As already noted, in contrast to the example discussed in connection with FIGS. 11(a) to (c), high temperature solution treatment in an IXh/Ar atmosphere may be followed by a lower temperature treatment. FIG. 12(a) is an image obtained in an optical microscope from a polished cross section through a BioDur® 108 bone screw maintained at 950 °C for 4 hours in mixture N2, NH3 and C2H2, with flow rates of 20, 50 and 10 ml/minute, respectively. The sub-surface microstructure, shown to a depth of around 100 pm, is characteristic of significant carbide and nitride formation. FIG. 12(b) is an image obtained in an optical microscope from a polished cross section through a BioDur® 108 bone screw subjected to the above-described lower temperature N/C treatment and high temperature solution treatment in an Na/Ar atmosphere. As evident from FIG. 12(b), maintaining the carbide and nitride enriched bone screw for 1 hour at 1150 °C in a partial pressure of 500 mbar of N2 mixed with Ar to give a total pressure of 3 bar, then quenching in around 10 bar of He, recovers a microstructure characteristic of austenitic BioDur® 108, but with additional C content, which may be beneficial, by adding solid solution strengthening and delaying nitride formation during cooling.

In general, low or lower temperature surface treatment, in combination with high temperature solution treatment, that together introduce N and C into a subsurface region may allow for greater N solubility than would be possible in Ni-free high-Mn austenitic stainless steel in the absence of C.

Though the above examples involved principally BioDur® 108, the above-described methods may be applied to a range of Ni-free, or essentially Ni-free, high-Mn austenitic stainless steels. Such austenitic stainless steels may be intended not only for biomedical applications, such as the bone screws, bone nails and other components intended for surgical implantation, but also for applications where the steel may be in extended contact with human skin, such as jewelry or watches. Some examples of materials are given below.

As described in the present application, high temperature solution treatment in a controlled Nz/Ar atmosphere of articles formed from BioDur® 108 may, in a sub-surface region extending to a depth of the order of at least few hundred pm (effectively bulk for articles with sufficiently closely spaced surfaces):

Elevate the N concentration to between 1.2 and 2 wt %

Elevate the hardness to above 350, such as above 400, such as above 450 HV0.01

Elevate the yield strength to above 800 MPa

As mentioned above, the yield strength may be determined as three times the hardness (HV) according to the well-known Tabor-relationship.

For stainless steel alloys other than BioDur® 108, the effect of such high temperature solution treatment in a controlled Nz/Ar atmosphere, values may be to:

Elevate the N concentration to between 0.6 and 2 wt %

Elevate the hardness to above 350, such as above 400, such as above 450 HV0.01

Elevate the yield strength to above 500 MPa

The bulk composition of BioDur® 108, see, e.g., https://www.fwrnetals.com/materials/stainless- steel/biodur-108/, is specified, in wt%, as follows: - C <0.08, Cr 19 to 23, Ni <0.05, Mn 21 to 24, Mo 0.50 to 1.50, Cu <0.25, Co <0.10, Si <0.75, S <0.01, P <0.03, N 0.85 to 1.10, and balance Fe.

The above described high temperature solution treatment in a controlled Na/Ar atmosphere may be extended to other essentially Ni-free high-Mn austenitic steels, for example, those set forth in Table 5 or M. Talha et al., Mater. Sci. & Eng. C 33 (2013) 3563-3575, expressed in wt%:

- F2229: C <0.08, Cr 19 to 23, Ni <0.05, Mn 21 to 24, Mo 0.50 to 1.50, Cu <0.25, Si <0.75, N 0.85 to 1.10, and balance Fe;

- F2581: C 0.15 to 0.25, Cr 16.5 to 18.0, Ni <0.05, Mn 9.5 to 12.5, Mo 2.7 to 3.7, Cu <0.25, Si 0.2 to 0.6, N 0.45 to 0.55, and balance Fe;

- P558: C <0.2, Cr 17.0 to 18.0, Ni <0.08, Mn 9.5 to 10, Mo 2.5 to 3.0, Si 0.5, N 0.45 to 0.5, and balance Fe; and

- BIOSSN4: C <005, Cr 17.0 to 18.0, Ni <0.2, Mn 15 to 16, Mo 2.0 to 2.5, Cu <0.65, Si <0.02, N 0.45 to 0.5, and balance Fe;

The disclosed high temperature solution treatment in a controlled Nz/Ar atmosphere may be applied to any at least essentially Ni-free high-Mn austenitic stainless steel alloy falling within the outer composition limits defined by Ni-free high-Mn stainless steels, including those specifically referenced herein, again expressed in wt%:

- C <2, Cr 12 to 35, Ni <1 Mn 4 to 35, Mo <12, Cu <3, Si <3, N <1.10, and balance Fe.

More generally, the disclosed high temperature solution treatment in a controlled Nz/Ar atmosphere may be applied to Ni-free high-Mn austenitic stainless steel alloy falling within the following composition limits, again expressed in wt%:

- C <2, Cr 12 to 35, Ni <1, Mn 4 to 35, Mo <12, N 1.10, and balance Fe.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an", etc., should not be construed as excluding the plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.