Low temperature hardening of titanium

Abstract

The present invention provides a method for producing a Group IV metal workpiece in its final shape; oxidizing the Group IV metal using a gaseous oxidizing species having an upper temperature limit of up to 800°C in an oxidizing atmosphere at a first temperature for an oxidation duration of at least 10 minutes to form a non-layered Group IV metal oxide on the surface of the workpiece, the first temperature being between 500°C and the upper temperature limit of the gaseous oxidizing species; -4 In another aspect, the present invention relates to a method for oxygen hardening a Group IV metal, the method comprising the step of diffusing oxygen from a non-layered Group IV metal oxide into the Group IV metal at a partial pressure of gaseous oxidizing species of 1000 mbar for a diffusion duration of at least 0.1 hours to form an outer diffusion zone containing oxygen in solid solution. 0.025 The present invention relates to a Group IV metal part comprising a material core having a surface hardness of 0.1 to 0.5 wt %, which is obtainable by the method of the present invention.

Description

[Technical Field]

[0001] This invention relates to low temperature hardening of titanium and other Group IV metals. In particular, it relates to a method for oxygen hardening Group IV metals and to hardened Group IV metal parts. The method and parts are useful in applications where hard metals can be used. The parts of the invention are particularly useful where the aesthetic appearance of the metal is important, such as in watches, jewelry, and glass. [Background technology]

[0002] It is well known that titanium and other Group IV metals can be hardened with interstitial oxygen, and that there is a direct correlation between oxygen content and hardness: the higher the oxygen content, the harder the metal. It is also well known that oxygen hardening of titanium should be carried out at the lowest possible temperature to avoid grain growth and general distortion of the material.

[0003] JPH08134625 discloses a method for forming and surface hardening a titanium or titanium alloy sheet. This method uses a forming gas containing CO2 gas and performs gas pressure forming to form a titanium thin sheet into a predetermined shape, while simultaneously forming a hardened layer. This method is carried out at 700°C to 1100°C, but temperatures below the β transformation point are preferred. Excessive oxidation occurs above 1100°C.

[0004] JPH059703 discloses a method for surface hardening of titanium materials. The titanium material is heat-treated in an atmosphere containing CO2 gas, which is reduced and decomposed into oxygen and carbon, so that the surface of the titanium material is hardened by solid solution and strengthened by interstitial elements. Temperatures of 700°C or higher are used, and hardening times of 10 hours or less are preferred.

[0005] JPH08104970 discloses a surface hardening treatment for titanium materials, aiming to easily form a thick surface-hardened layer without roughening the surface. The titanium surface is hardened by heating to a predetermined temperature in an atmosphere consisting of CO2 gas and inert gas or nitrogen gas, followed by heating in an inert gas or vacuum. Specifically, oxygen and carbon atoms were found to penetrate the surface of titanium materials by heating at a specific partial pressure of CO2 gas. Heat treatment in an inert gas or vacuum caused the oxygen and carbon to diffuse inside, thereby forming a thick hardened layer with low surface roughness. In one example, pure titanium was heated at 800°C for 3 hours in a mixture of CO2 and argon gas, followed by a diffusion heat treatment at 850°C for 3 hours in a vacuum.

[0006] JPH1192911 aims to harden a titanium component at a temperature that does not cause surface roughness but does not form colored substances on the surface and results in a Vickers hardness of at least 750 at a depth of 1 μm from the surface. In this method, the component is heated to 700-800°C and treated in a reduced-pressure atmosphere containing nitrogen and oxygen components. Nitric oxide, nitrogen dioxide, nitrous oxide, or ammonia gas can be used as the nitrogen-containing component, and water or oxygen can be used as the oxygen-containing component. For example, a titanium component can be treated with nitrous oxide at 0.3 Torr at 700-800°C for 1-10 hours.

[0007] US 6,221,173 discloses a titanium or titanium alloy having a hard surface layer comprising a first hard layer containing 0.6-8.0 wt. % nitrogen and 1.0-14.0 wt. % oxygen in solid solution, and a second hard layer below the first hard layer containing 0.5-14.0 wt. % oxygen in solid solution. The hard surface layer can be provided by treating the alloy at 650-830°C in a nitrogen atmosphere containing traces of oxygen, water, carbon dioxide, or carbon monoxide at a total pressure of 1.33-1330 Pa. The treatment time is 1-10 hours. This allows nitrogen and oxygen to diffuse into the titanium or titanium alloy without forming titanium nitrides or oxides.

[0008] US2003 / 041922 discloses a method for strengthening titanium alloys to improve their wear resistance. The method involves heating the titanium alloy in a CO2 atmosphere at 600-900°C to diffuse C and O atoms into the titanium alloy without forming titanium oxide. A preferred temperature is 800-850°C, with titanium oxide forming particularly at temperatures above 900°C. The reaction time may be 0.5-50 hours.

[0009] WO 97 / 14820 discloses a method of treating titanium parts in a mixture of nitrogen, methanol, and optionally natural gas or propane at atmospheric pressure at 1450°F to 1850°F, which causes the formation of a coating on the alloy surface consisting primarily of titanium oxide with regions of oxynitride and sometimes carbonitride. The parts can then be treated in a vacuum furnace at 1200°F to 1450°F to reduce the hydrogen level in the parts and improve their yield strength.

[0010] WO2008 / 154593 and WO2007 / 078427 disclose zirconium medical implants having a diffusion hardened zone and optionally also a layered ceramic zone, as well as methods for making hardfaced medical implants. For example, zirconium-niobium alloy samples are oxidized in air at 635°C and subsequently treated in a vacuum furnace.

[0011] WO2017 / 207794 discloses a hardfaced part of a titanium alloy having a diffusion zone containing oxygen and carbon in solid solution and a separate phase of carbon oxides.

[0012] EP0931848 discloses a decorative hardened titanium material, a method for processing the titanium material, and a method for processing the decorative titanium material. The hardened surface layer contains nitrogen and oxygen and has a surface grain size of 0.1 to 60 μm.

[0013] WO 99 / 04055 discloses a method for surface hardening an article formed from titanium, zirconium or an alloy of titanium and / or zirconium, which method comprises heat treating the article in an oxidizing atmosphere to form an oxide layer, and subsequently heat treating the article in a vacuum to diffuse oxygen from the oxide layer into the article.

[0014] In light of the prior art methods, there remains a need for improved methods of hardening titanium or other Group IV metals and their alloys. Summary of the Invention

[0015] In a first aspect, the present invention is a method for oxygen hardening a Group IV metal, said method comprising: - providing a workpiece of Group IV metal in its final shape; - oxidizing a Group IV metal using a gaseous oxidizing species selected from CO2, N2O, and a combination of CO2 and N2O in an oxidizing atmosphere at a first temperature for an oxidation duration of at least 10 minutes to form a non-layered Group IV metal oxide on the surface of the workpiece, wherein the gaseous oxidizing species has an upper temperature limit of up to 800°C, and the first temperature is between 500°C and the upper temperature limit of the gaseous oxidizing species; - Oxygen is added in an inert atmosphere at a second temperature of 500 ° C to 800 ° C and a maximum of 10 -4 Diffusion of oxygen from the non-layered Group IV metal oxide into the Group IV metal at a partial pressure of gaseous oxidizing species of 1000 mbar for a diffusion duration of at least 0.1 hours to form an outer diffusion zone containing oxygen in solid solution. The present invention relates to a method, including:

[0016] In a second aspect, the present invention provides a core hardness and a hardness of at least the core hardness +200HV. 0.025and a diffusion zone 10 μm to 100 μm thick from the surface, in which oxygen in solid solution is at a level that provides a hardness of 120% of the hardness of the material core to a saturation level of the Group IV metal, the diffusion zone further containing carbon and / or nitrogen in solid solution at concentrations that exhibit a maximum in the diffusion zone containing carbon and / or nitrogen that is detectable by glow discharge optical emission spectroscopy (GDOES).

[0017] The Group IV metal component of the second aspect can be obtained by the method of the first aspect, particularly when the oxidizing atmosphere contains carbon-containing molecules or nitrogen-containing molecules, or carbon-containing molecules and nitrogen-containing molecules.

[0018] Any Group IV metal is suitable for both aspects of the present invention. In certain embodiments, the Group IV metal is selected from titanium, titanium alloys, zirconium, and zirconium alloys. In the present invention, the part may consist of a Group IV metal, e.g., a titanium alloy, or the part may include other materials. For example, the part may have a core of another material, polymer, glass, ceramic, or another metal, and an outer layer of titanium alloy or zirconium. Similarly, the workpiece treated with the method of the present invention may have a core of another material. The outer layer need not completely cover the outer surface of the part. The part may be obtained, for example, by additive manufacturing or 3D printing before being treated with the method of the present invention.

[0019] Group IV metals and their alloys can be described in terms of hardness. Group IV metals can be hardened by dissolving oxygen in the metal, but whether or not they are oxygen hardened, Group IV metals have a core hardness. Thus, the core hardness corresponds to the hardness (e.g., surface hardness) of the Group IV metal before hardening. While the core hardness generally depends on the particular Group IV metal, when a Group IV metal is treated with the method of the present invention, the surface hardness is at least 200 HV greater than the core hardness. 0.025 The surface hardness is measured by the maximum load of 50g, i.e., HV. 0.05It is preferable to analyze the surface hardness in HV unless otherwise specified. 0.025 Maximum load of 50g (e.g. HV 0.01 , HV 0.025 or HV 0.005 The surface hardness value obtained by 0.025 Grade 2 titanium typically has a core hardness of about 200HV. 0.025 Therefore, the surface hardness of grade 2 titanium hardened according to the invention is at least 400 HV 0.025 Grade 5 titanium typically has a core hardness of about 300HV 0.025 Therefore, the surface hardness of grade 5 titanium hardened according to the present invention is at least 500 HV 0.025 However, higher surface hardnesses are generally obtained with the methods of the present invention. In certain embodiments, the surface hardness is at least 650 HV. 0.025 At least 650HV 0.025 A surface hardness of at least 700 HV can be achieved with any Group IV metal. 0.025 (e.g. at least 800HV 0.025 ) is preferred.

[0020] The method of the present invention allows the workpiece to regain its metallic luster after treatment, so that the parts of the present invention are indistinguishable from the workpiece before treatment by visual inspection. Therefore, if the workpiece has a mirror-like finish appearance, the mirror-like finish appearance will also be found in the part after treatment by this method. In the present invention, a "mirror-like finish appearance" is defined as a surface having an arithmetic mean deviation (Ra) roughness of less than 0.1 μm according to the ISO 1302:2002 standard. For example, the Ra value is measured over a length of 1.25 mm using a Taylor Hobson Surtronic S25. A mirror-like finish surface is sometimes referred to as an N3 surface, and the two terms are used interchangeably. In a preferred embodiment, the Group IV metal workpiece is polished before oxidizing the Group IV metal to a surface roughness of less than 0.1 μm according to the ISO 1302:2002 standard. A surface roughness of less than 0.1 μm is also observed in the workpiece after the diffusion process. Group IV metals in general, and titanium in particular, can be made to have a mirror-finish appearance, but if the workpiece contains aluminum, e.g., grade 5 titanium, the first temperature of the method to produce a mirror-finish appearance should not exceed 700°C. On the other hand, if a mirror-finish appearance is appropriate, it is preferred that the Group IV metal does not contain aluminum. Furthermore, if a mirror-finish appearance is appropriate, the oxidizing atmosphere should not contain any carbon-containing molecules other than CO2. When the gaseous oxidizing species is CO2 and the oxidizing atmosphere does not contain any further carbon-containing molecules, a mirror-finish appearance and a surface hardness of at least 1100 HV are obtained. 0.025 Commercially pure (CP) titanium, e.g., grade 2 or grade 4 titanium, can be obtained. Thus, the method of the present invention provides a surface hardness of at least 1100 HV. 0.025 The present invention provides titanium components with a mirror-finished appearance. The presence of additional carbon-containing molecules as unavoidable impurities does not affect the appearance of the mirror finish.

[0021] The inventors have surprisingly found that when a Group IV metal contains aluminum as an alloying element, the metallic appearance after treatment is duller than when a workpiece made from a Group IV metal without aluminum is treated. Without being bound by theory, the inventors believe that during the oxidation process, aluminum forms a more thermodynamically stable oxide than the Group IV metal, thereby preventing all oxygen from diffusing into the Group IV metal during the diffusion process. However, unavoidable aluminum impurities do not pose a problem for the formation of a metallic luster in the Group IV metal. The inventors have further found that the presence of aluminum results in a higher surface hardness than when aluminum is not present. In a preferred embodiment, the Group IV metal does not contain aluminum as an alloying element. In a particularly preferred embodiment, the Group IV metal is a CP Group IV metal, such as titanium grades 2 and 4, or Zr702 zirconium. In another embodiment, the Group IV metal includes aluminum. For example, the Group IV metal may be Grade 5 titanium, also known as Ti6Al4V, or Grade 23 titanium, also known as Ti6Al4V ELI. When the Group IV metal is a CP grade, it is possible to obtain a mirror-finish appearance after treatment with the method of the present invention. The treated workpiece or component of the present invention may contain aluminum, for example, in a compartment separate from the Group IV metal, but the Group IV metal does not contain aluminum as an alloying element. In one embodiment of the present invention, the component of the present invention (e.g., Grade 2 or 4 titanium) has a mirror-finish appearance. If the oxidizing atmosphere contains carbon-containing molecules other than CO2, the mirror-finish appearance may not be obtained because such carbon (e.g., CO) tends to darken the non-stratified oxide layer and may be reflected on the surface of the final component. Therefore, in a preferred embodiment, the oxidizing atmosphere does not contain CO2. Similarly, using CO2 as an oxidizing atmosphere can produce a component of the present invention with a mirror-finish appearance.

[0022] The method of the present invention results in a Group IV metal having a layer of Group IV metal (e.g., titanium oxide or zirconium oxide) in an oxidation step, which acts as a reservoir for oxygen atoms that diffuse into the Group IV metal and dissolve into the lattice of the Group IV metal in a diffusion step. The oxidation step may be referred to as the first step. The inventors have surprisingly found that oxygen atoms can diffuse into the Group IV metal to form an intermediate non-stratified oxide layer that is sufficiently robust and stable to form an outer diffusion zone containing oxygen in solid solution. Diffusion occurs after the oxidation step in a diffusion step, which may be referred to as the second step. Thus, the non-stratified oxide layer is effectively removed from the surface and replaced by the diffusion zone, and the Group IV metal returns to its apparent metallic state, but the surface hardness due to interstitial oxygen is significantly increased, i.e., at least 200 HV greater than the core hardness. 0.025 Units (e.g. at least 650HV 0.025 , at least 800HV 0.025 , or at least 1000HV 0.025 ) is higher. Although the oxide layer is removed during the diffusion process, a native oxide layer inevitably forms on the surface of the hardened Group IV metal. The native oxide layer is nanometer-sized and does not change the metallic appearance of the hardened Group IV metal.

[0023] The method of the present invention uses gaseous oxidation species. Oxidation species that can be made gaseous can be used in this method. The oxide layer obtained in the oxidation step is not layered. The inventors have found that by selecting the first temperature and the oxidation duration based on the oxidation ability of the commonly used gaseous oxidation species, a non-layered oxide layer can be obtained. In the present invention, a workpiece having a non-layered oxide layer may be referred to as an "intermediate workpiece", and these two terms may be used interchangeably. The layering of the oxide layer indicates that it is not stable enough to function as a reservoir for oxygen atoms diffusing into the Group IV metal. Generally, the oxidation ability of CO2, O2, and N2O can be ranked as CO2 < O2 < N2O. A temperature below 500°C is too low to form an oxide layer, and the upper temperature limit of the gaseous oxidation species can be determined. Water vapor, i.e., H2O, is also regarded as a gaseous oxidation species. However, when hydrogen-containing molecules are present in the oxidation atmosphere or the inert atmosphere, hydrogen dissolves in the Group IV metal, and the presence of interstitial hydrogen leads to embrittlement of the Group IV metal. Therefore, it is preferable that neither the oxidation atmosphere nor the inert atmosphere contains hydrogen-containing molecules, especially H2O.

[0024] Regardless of the type of gaseous oxidation species, the first temperature is preferably 600°C to 700°C. When the first temperature is 500°C to 600°C, generally the oxidation process becomes undesirably slow. However, when the first temperature is at least 600°C, the workpiece is oxidized at an acceptable rate. When the first temperature is 700°C to 800°C, i.e., when this temperature is possible with the selected gaseous oxidation species, grain growth may be observed for the Group IV metal. Therefore, if grain growth is not allowed, the first temperature must be at most 700°C (e.g., 500°C to 700°C or 600°C to 700°C).

[0025] The oxidation step has an oxidation duration. The oxidation duration is determined by the time the workpiece is treated with the gaseous oxidizing species at the first temperature. The workpiece may be heated, for example, from ambient temperature to the first temperature before being exposed to the gaseous oxidizing species, or the workpiece may be exposed to the gaseous oxidizing species at the first temperature, for example, while the workpiece is at ambient temperature. If the workpiece is heated before being exposed to the gaseous oxidizing species, the workpiece may be heated in an inert atmosphere, in a vacuum, or in the gaseous oxidizing species, and the time the workpiece is exposed from 500°C to the first temperature is preferably minimized.

[0026] Generally, the thickness of the oxide layer is determined by the duration of oxidation, taking into full consideration the oxidizing power of the gaseous oxidizing species used. The longer the duration of oxidation, the thicker the oxide layer will be, and since the dissolution of elements into Group IV metals generally appears to be parabolic, a fourfold increase in reaction duration is required to double the dissolution depth. The oxide layer typically has the composition MeO2 (where Me is the Group IV metal), e.g., TiO2 or ZrO2, although traces of other elements (e.g., carbon and / or nitrogen, or other metals from alloys of the Group IV metal) may also be present. The thickness of the diffusion zone is therefore proportional to the thickness of the oxide layer. Core hardness + at least 200HV 0.025 Regardless of the Group IV metal being treated, an oxide layer thickness of only 1 μm is considered sufficient to obtain and identify a diffusion zone with a surface hardness and a hardness profile across the cross section of at least 650 HV. 0.025 An oxide layer thickness of at least 2 μm is believed to be sufficient to obtain a diffusion zone with a surface hardness of 1 μm. On the other hand, the oxidation duration is preferably sufficient to result in a non-stratified oxide layer of 5 μm to 15 μm. The oxidation duration to result in a non-stratified oxide layer of 5 μm to 15 μm is determined by cross-sectional examination of the intermediate workpiece.

[0027] Once the oxide layer is formed, the intermediate workpiece can be stored as long as desired before undergoing the diffusion step. Because the non-stratified oxide layer is stable, normal handling of the intermediate workpiece does not affect the non-stratified oxide layer. Therefore, the method of the present invention is highly flexible, as the oxidation step can be performed in a first oven (e.g., an oven without vacuum capabilities) and then transferred to another oven with vacuum capabilities. Advantageously, this allows for logistical optimization of the manufacturing of the components of the present invention. Furthermore, due to the stability of the non-stratified oxide layer and the intermediate workpiece, the conditions for the diffusion step can be selected independently of the gaseous oxidizing species used in the oxidation step.

[0028] CO2, the mildest gaseous oxidizing species, allows the formation of a stable, non-stratified oxide layer above 500°C, regardless of the oxidation duration. However, to limit deformation and grain growth in Group IV metals, the upper temperature limit for CO2 is 800°C. Although a stable, non-stratified oxide layer can be formed even with longer oxidation durations, for practical reasons and limitations on the oxide layer thickness, the CO2 oxidation duration can be up to 16 hours. In the case of CO2, the oxidation duration is preferably between 1 and 16 hours.

[0029] Of the commonly used gaseous oxidizing species, NO is the most potent, with an upper temperature limit of 700°C. It also forms a non-stratified oxide layer more quickly than CO, and the oxidation duration is preferably 10 minutes to 2 hours. In certain embodiments, NO is preferred as the gaseous oxidizing species because it allows for a lower first temperature (e.g., 500°C to 650°C), further reducing the risk of deformation and grain growth in the Group IV metal.

[0030] O2 may be used as the gaseous oxidizing species. However, when O2 is used, the diffusion zone does not contain carbon and / or nitrogen in solid solution after the oxidation treatment. O2 has a higher oxidizing power than CO2, and the upper temperature limit is 750°C. The higher oxidizing power of O2 compared to CO2 also limits the oxidation duration. Therefore, when the gaseous oxidizing species is O2, the oxidation duration is limited to 3 hours to prevent the formation of a stratified oxide layer. When O2 is used, an oxidation duration of 30 minutes results in a surface hardness of at least 1000 HV after the diffusion process.0.025 It is believed that a non-stratified oxide layer of sufficient thickness can be obtained. The duration of the O2 oxidation is preferably 30 minutes to 3 hours.

[0031] Figures 1 and 2 show cross sections of oxide layers obtained using CO2 and N2O at various temperatures. CP Grade 4 titanium specimens were treated in CO2 or N2O at ambient pressure, and the cross sections were analyzed microscopically. The results are shown in Figures 1 and 2. CO2 treatment resulted in the formation of a robust and stable layer of titanium oxide at all times and temperatures. Figure 2 shows that N2O treatment of Grade 4 titanium formed a stable non-stratified oxide layer at 680°C, whereas a strongly stratified oxide layer formed at temperatures above 780°C. The boundary between the formation of a non-stratified and a layered titanium oxide layer when N2O was used as the gaseous oxidizing species on CP Grade 2 titanium is shown in Figure 15. Combinations of first temperature and oxidation duration below the dotted line result in a non-stratified titanium oxide layer. Specific combinations of first temperature and oxidation duration at which a non-stratified titanium oxide layer can be obtained by treating titanium with N2O are listed in Table 1. This combination of first temperature and oxidation duration results in a non-stratified titanium oxide layer, and is considered a limiting value, so that increasing the first temperature beyond the values ​​in Table 1 while keeping the oxidation duration unchanged will result in a stratified titanium oxide layer. Preferred combinations of first temperature and oxidation duration when CO2 and N2O are used alone as gaseous oxidizing species are shown in Table 2. When CO2 and N2O are used in combination, the N2O temperature and time are used.

[0032] [Table 1]

[0033] [Table 2]

[0034] When O2 is used as the oxidizing species, the first temperature may be 500°C to 750°C and the oxidation duration may be 30 minutes to 3 hours, or the first temperature may be 600°C to 650°C and the oxidation duration may be 30 minutes to 2 hours.

[0035] Preferably, the pressure of the oxidizing atmosphere is ambient pressure. It is more preferred that the gaseous oxidizing species is at ambient pressure, i.e., the oxidizing atmosphere contains no other molecules. However, it is contemplated that the oxidizing atmosphere may be at ambient pressure and the partial pressure of the gaseous oxidizing species reduced by the addition of an inert gas (e.g., a noble gas such as argon or helium). Operating at ambient pressure simplifies the process compared to operating at altered, especially reduced, pressures. When the oxidizing atmosphere contains CO, it is preferred that the pressure of the oxidizing atmosphere is ambient pressure.

[0036] The oxidizing atmosphere may contain molecules capable of providing atoms that diffuse into the Group IV metal, which are incorporated into the non-stratified oxide layer. The oxidizing atmosphere may be, for example, ambient air, in which O2 is the gaseous oxidizing species, present at a partial pressure of about 20%, or about 0.2 atmospheres, mixed with N2 at a partial pressure of about 80%, or about 0.8 atmospheres. While N2 can increase the hardness of the workpiece, dissolution of nitrogen atoms from N2 can alter the surface appearance, so a mirror-like finish cannot be achieved when N2 is present. Therefore, air is preferably not used, and N2 is preferably not included in the oxidizing atmosphere. However, the interstitial nitrogen in the parts of the present invention or workpieces processed by the present method increases the surface and cross-sectional hardness of the diffusion zone.

[0037] Oxidation of a Group IV metal workpiece increases its volume due to the inclusion of oxygen atoms in the oxide layer. Therefore, the intermediate workpiece has a larger volume than the untreated workpiece. However, the inventors have now surprisingly discovered that if the intermediate workpiece is subjected to a diffusion step, the oxygen atoms diffuse into the Group IV metal, and the part returns to the volume of its untreated state before the first oxidation step. However, to ensure that the inventive part has the same volume as the untreated workpiece, the thickness of the non-layered oxide layer must be limited to 50 μm, particularly 25 μm. In this way, the method of the present invention can harden a Group IV metal part in its final shape without affecting its shape or size. This is particularly significant when grain growth is undesirable and the first temperature is up to 700°C. The treated part is in its final shape. If a mirror-finish appearance is desired, the part may be polished to a mirror-finish appearance, i.e., a surface Ra roughness of less than 0.1 μm, before treatment by the method of the present invention. Advantageously, this method allows for a mirror finish appearance after treatment, i.e., the surface has an Ra roughness of less than 0.1 μm after treatment.

[0038] In the diffusion step, oxygen from the non-layered Group IV metal oxide diffuses into the Group IV metal, allowing the diffusion step to be carried out independently of the parameters used in the oxidation step. However, to prevent further oxidation of the Group IV metal, the partial pressure of the gaseous oxidizing species should be as low as possible. For example, the partial pressure of the gaseous oxidizing species should be at most 10 -4 mbar, but preferably lower, e.g. at most 10 -5 mbar or 10 -6 mbar.

[0039] The diffusion step may be carried out in a vacuum. In the present invention, "vacuum" refers to a pressure of up to 10°C regardless of the composition of the atmosphere. -4mbar. In another embodiment, the diffusion step is carried out in an inert atmosphere. In the present invention, an "inert atmosphere" is an atmosphere that does not contain components that interact with the Group IV metal other than unavoidable impurities. A preferred inert atmosphere is a noble gas (e.g., argon). The diffusion step is preferably carried out in a vacuum, since this more easily ensures that the hardened part retains its metallic luster after treatment with the method of the present invention. Surprisingly, the diffusion step pressure is 10 -4 Above mbar, the inventors have observed that contaminants, including gaseous residues, in the oven prevent the reformation of the metallic luster on the part after treatment with the method of the present invention.

[0040] The diffusion step is carried out at a second temperature of 500°C to 800°C for a diffusion duration of at least 0.1 hours (e.g., at least 1 hour). The diffusion duration must be sufficient to allow the non-stratified oxide layer of the intermediate workpiece to be removed by oxygen atoms diffusing into the Group IV metal. There is no upper limit to the diffusion duration. However, if the diffusion duration is excessively long, oxygen may eventually diffuse into the Group IV metal so uniformly that the Group IV metal lacks a diffusion zone that provides sufficient hardness. Therefore, the diffusion duration generally should not exceed 100 hours. The diffusion duration depends on the thickness of the non-stratified oxide layer; the thicker the non-stratified oxide layer, the longer the diffusion duration for its removal. Therefore, the diffusion duration can be considered to be determined by the gaseous oxidizing species, the first temperature, and the oxidation duration. The diffusion duration is independent of the type of Group IV metal.

[0041] Generally, the higher the temperature, the faster the diffusion, and the absence of gaseous oxidizing species to oxidize the workpiece means that the second temperature is less restrictive than the first temperature. Therefore, the second temperature can be higher than the first temperature. Thus, when the second temperature is higher than the first temperature, hardening can be completed more quickly. In some embodiments, the second temperature is higher than the first temperature. In more specific embodiments, the second temperature is 600°C to 750°C (e.g., 650°C to 700°C). Because a second temperature of up to 700°C can prevent grain growth and deformation of the Group IV metal, a second temperature of up to 700°C can be selected when grain growth is undesirable. When the second temperature is 650°C to 750°C, the diffusion duration is typically 2 hours to 40 hours.

[0042] In a particular embodiment, the gaseous oxidizing species is CO2, the first temperature is 600°C to 750°C, the oxidation duration is 1 hour to 8 hours, the second temperature is 650°C to 750°C, and the diffusion duration is 2 to 8 times the oxidation duration (e.g., 2 hours to 64 hours). In another embodiment, the gaseous oxidizing species is CO2, the first and second temperatures are both 650°C to 700°C, the second temperature is higher than the first temperature, the oxidation duration is 2 hours to 6 hours, and the diffusion duration is 3 to 6 times the oxidation duration (e.g., 6 hours to 36 hours).

[0043] In another embodiment, the gaseous oxidizing species is N2O and / or O2, the first temperature is 600°C to 650°C, the oxidation duration is 30 minutes to 2 hours, the second temperature is 650°C to 700°C, and the diffusion duration is 4 to 20 times the oxidation duration (e.g., 2 hours to 40 hours).

[0044] In certain embodiments, the oxidizing atmosphere further comprises CO. For example, the gaseous oxidizing species can be CO, and the oxidizing atmosphere can comprise CO and CO. The presence of carbon in the oxidizing atmosphere is believed to result in the dissolution of carbon in the Group IV metal, even in trace amounts, leading to a more stable incorporation of the oxide layer and the diffusion zone between the Group IV metal core and the oxide layer, resulting in the formation of a highly stable non-stratified oxide layer. When the gaseous oxidizing species is CO, a more stable non-stratified oxide layer is formed than when O or N O is used, even without the addition of CO to the oxidizing atmosphere. However, when O or N O are the gaseous oxidizing species and CO is added to the oxidizing atmosphere, carbon also dissolves in the Group IV metal, resulting in a more stable non-stratified oxide layer. Other carbon-containing molecules are also expected to dissolve carbon in the Group IV metal, but hydrogen-containing molecules (e.g., alkanes) should be avoided to prevent embrittlement of the Group IV metal.

[0045] When the oxidizing atmosphere contains CO2 and CO, CO2 and CO participate in the following reactions 1 and 2.

[0046] [ka]

[0047] Generally, CO is considered to be a carbon donor molecule with no oxidizing ability, and CO2 has oxidizing ability but limited carbon activity. The partial pressure of O2 (pO2) and the activity of carbon (a c ) is determined by Equation 1 and Equation 2. The partial pressure of O2 is

[0048]

number

[0049] And the activity of carbon is

[0050]

number

[0051] So, during the ceremony, ΔG1=-282.200+86.7T(J) ΔG2=-170.550+174.3T(J) is.

[0052] The use of a mixture of CO and CO allows for control of the amount of carbon dissolved in the Group IV metal, thereby adjusting the oxidizing atmosphere and ultimately the diffusion zone of the inventive components. Carbon generally increases the microhardness of the diffusion zone compared to a diffusion zone containing oxygen but no carbon. Furthermore, CO is believed to result in a thicker diffusion zone and a thinner oxide layer. The combination of a thin oxide layer and a thick diffusion zone is considered advantageous in the subsequent process of diffusing oxygen from the non-layered Group IV metal oxide into the Group IV metal because oxygen atoms in the Group IV metal oxide more efficiently diffuse into the Group IV metal. Furthermore, without being bound by theory, the inventors believe that the carbon in the Group IV metal oxide further stabilizes the non-layered oxide layer, thereby improving the process compared to processes using only CO, O, or NO as the gaseous oxidizing species. Thus, in certain embodiments, the oxidizing atmosphere is a mixture of CO and CO, with 40% to 90% CO relative to the total of CO and CO. In another embodiment, the oxidizing atmosphere is a mixture of CO2 and CO with 40% to 60% CO2 relative to the total of CO2 and CO. Similar ratios apply to other gaseous oxidizing species (e.g., O2 or N2O). For example, the oxidizing atmosphere may be a mixture of O2 and CO with 40% to 60% O2 relative to the total of O2 and CO. The oxidizing atmosphere may be a mixture of N2O and CO with 40% to 60% N2O relative to the total of N2O and CO.

[0053] Treatment of a Group IV metal in an oxidizing atmosphere dissolves oxygen into the Group IV metal, forming a diffusion zone between the Group IV metal core and the non-stratified oxide layer. In the present invention, a "diffusion zone" refers to any zone identified at a certain depth from the surface of the Group IV metal, where oxygen dissolves into the Group IV metal. The infiltrated oxygen hardens the Group IV metal, and the diffusion zone can be identified by measuring the hardness of a cross-section of the Group IV metal. After the diffusion process, oxygen atoms from the non-stratified oxide layer have diffused into the Group IV metal, leaving only an unavoidable nanometer-thick oxide layer on the Group IV metal. The concentration of infiltrated oxygen is highest at the surface of the hardened Group IV metal and decreases with depth. Generally, the diffusion zone is considered to extend from the surface of the Group IV metal to a depth where the cross-sectional hardness is 120% of the hardness of the Group IV metal core.

[0054] The concentration of oxygen introduced near the surface of the Group IV metal is preferably close to saturation. Even if the oxygen content near the surface is below the oxygen saturation level, the Group IV metal is still at 1000 HV. 0.025 A surface hardness of 650HV can be achieved. 0.025 A surface hardness of about 1000 HV is believed to be sufficient to provide scratch resistance to the hardened Group IV metal. 0.025 This allows for a surface hardness of approximately 1000 HV. Therefore, in a preferred embodiment, the diffusion zone is at least 5 μm thick. Although increasing the thickness beyond 50 μm is ineffective, the thickness of the diffusion zone is not limited. Therefore, in one embodiment, the diffusion zone is 5 μm to 50 μm thick, i.e., a hardness of 120% of the core hardness of the Group IV metal is obtained at a depth of 50 μm from the surface. A cross section of hardened Grade 4 titanium and its hardness profile are shown in Figure 3. The core hardness is approximately 230 HV, and the hardness at a depth of 0.06 mm from the surface is approximately 280 HV, so the diffusion zone is 60 μm thick.

[0055] Removal of the non-stratified oxide layer is easily detected visually by cross-sectional analysis or by GDOES analysis. Visual inspection of the part surface will reveal whether the part has a metallic luster or whether an oxide layer is present on the surface. Representative parts of grade 2 and 23 titanium are shown in Figures 4 and 6, respectively. A cross-section corresponding to Figure 4 is shown in Figure 5. In Figures 4-6, (a) shows an untreated workpiece, (b) shows an intermediate workpiece, and (c) shows a part of the present invention.

[0056] Representative GDOES analyses with and after oxide layer removal are shown in Figures 9 and 10 for grade 2 titanium and Figures 12 and 13 for grade 5 titanium, respectively. The intermediate workpiece has a stable oxygen content from the surface to a certain depth, indicating the presence of an oxide layer. Conversely, the final part shows a decrease in oxygen content at the surface, indicating a diffusion zone.

[0057] Carbon is present in the non-stratified oxide layer when carbon is present in the oxidizing atmosphere (e.g., when the gaseous oxidizing species is CO) or when CO is added to the oxidizing atmosphere. Similarly, nitrogen is present in the non-stratified oxide layer when nitrogen is present, for example, as N or NO. The carbon and / or nitrogen content can be detected by GDOES, as is evident from FIGS. 9 and 12. When oxygen diffuses into the Group IV metal during the diffusion process, carbon also diffuses into the Group IV metal. However, because the distributions of carbon and oxygen atoms are different and can be detected by GDOES, the carbon intensity exhibits a peak, i.e., a maximum, in the diffusion zone. Similarly, the nitrogen intensity may exhibit a peak, i.e., a maximum, in the diffusion zone. Without being bound by theory, the inventors believe that the maximum in carbon intensity represents a peak in the interstitial carbon concentration in the diffusion zone, which in turn increases the hardness of the diffusion zone at the corresponding location. Without being further bound by theory, the inventors believe that this peak in carbon concentration is also reflected in the surface hardness of the components of the present invention. When nitrogen is present, the same considerations apply to the nitrogen peak in the diffusion zone, which results in a surface hardness higher than that obtained with carbon-free gaseous oxidizing species when hardening using CO2 as the gaseous oxidizing species, or when CO2 is added to the oxidizing atmosphere. For example, when CO2 is used as the gaseous oxidizing species, the surface hardness of grade 5 titanium is about 1300 HV. 0.025 The surface hardness of the untreated metal is approximately 460HV 0.025 For grade 2 titanium, the surface hardness was approximately 1100 HV when CO2 was used as the gaseous oxidizing species. 0.025 The surface hardness of the untreated metal is approximately 360HV 0.025 In a particular embodiment, the part is made of grade 5 titanium with a surface hardness of at least 1300 HV. 0.025 In another embodiment, the part is made of grade 2 titanium and has a surface hardness of at least 1100 HV. 0.025 is.

[0058] The carbon intensity maximum also applies when the oxidizing atmosphere contains CO, in which case high carbon activity leads to a harder diffusion zone and a harder surface. Thus, a minimum of 800 HV is achieved for grade 5 titanium when the oxidizing atmosphere is a mixture of CO2 and CO with 40% to 60% CO2 relative to the total of CO2 and CO, when the oxidizing atmosphere is a mixture of O2 and CO with 40% to 60% O2 relative to the total of O2 and CO, or when the oxidizing atmosphere is a mixture of N2O and CO with 40% to 60% N2O relative to the total of N2O and CO. 0.025 A surface hardness of at least 800HV is achieved for grade 2 titanium 0.025 A surface hardness of 1000 MPa is obtained.

[0059] In general, all variations and features relating to the aspects and embodiments of the invention may be freely combined, and therefore the features described above relating to the method apply equally to the parts of the invention.

[0060] The invention will now be explained in more detail with the aid of examples and by reference to schematic drawings. [Brief explanation of the drawings]

[0061] [Figure 1] Figure 1 shows the oxide layer formed by CO2 treatment. [Figure 2] FIG. 2 shows the oxide layers from the N2O and CO2 treatments. [Figure 3] FIG. 3 shows the hardness profile and cross section of a titanium part of the present invention. [Figure 4] FIG. 4 shows the untreated workpiece, the intermediate part and the hardened part. [Figure 5] FIG. 5 shows cross sections of the untreated workpiece, the intermediate part and the hardened part. [Figure 6] FIG. 6 shows the untreated workpiece, the intermediate part and the hardened part. [Figure 7] FIG. 7 shows the hardness profile of the titanium part of the present invention. [Figure 8]FIG. 8 shows the Glow Discharge Optical Emission Spectroscopy (GDOES) curve of the untreated titanium workpiece. [Figure 9] FIG. 9 shows the GDOES curve of the middle titanium part. [Figure 10] FIG. 10 shows the GDOES curve of the titanium part of the present invention. [Figure 11] FIG. 11 shows the GDOES curve of an untreated titanium workpiece. [Figure 12] FIG. 12 shows the GDOES curve of the middle titanium part. [Figure 13] FIG. 13 shows the GDOES curves of the titanium parts of the present invention. [Figure 14] FIG. 14 shows the hardness profile and cross section of a titanium part of the present invention. [Figure 15] FIG. 15 shows a plot of time versus temperature resulting in a non-stratified titanium oxide layer. [Figure 16] FIG. 16 shows the roughness measurements of a mirror-finished part of the present invention. [Figure 17] FIG. 17 shows a photograph of a mirror-finished part of the present invention. Reference to the drawings serves to illustrate the invention and should not be construed as limiting the features of the particular embodiment shown. DETAILED DESCRIPTION OF THE INVENTION

[0062] The present invention relates to a method for oxygen hardening Group IV metals and to a method for oxygen hardening Group IV metals with a hardness of at least 200 HV greater than the core hardness. 0.025 It relates to Group IV metal parts having high surface hardness.

[0063] In the present invention, a "Group IV metal" is a metal selected from the titanium group of the periodic table, or an alloy containing at least 50% titanium metal. Thus, a "titanium alloy" is an alloy containing at least 50% titanium (a / a), and similarly, a "zirconium alloy" is an alloy containing at least 50% zirconium (a / a). In the present invention, alloys containing at least 50% titanium and zirconium (a / a) are suitable for the present methods and components. Similarly, alloys may contain hafnium, an element from Group IV of the periodic table. Thus, alloys containing at least 50% titanium, zirconium, and hafnium (a / a) are suitable for the present invention.

[0064] The alloys of the present invention may contain other elements as appropriate, and as used herein, "alloying element" may refer to the metal element or elements in the alloy, or to the constituents of the alloy. Titanium and zirconium alloys are well known to those skilled in the art. Alloys of Group IV metals may also contain metals from other groups in the periodic table (e.g., aluminum or niobium). A representative niobium-containing alloy is Ti13Nb13Zr. An aluminum-containing alloy is Ti6Al4V (Grade 5), which exists as an "extra-low interstitial" (ELI) version, Ti6Al4V ELI, commonly referred to as Grade 23.

[0065] For purposes of the present invention, grades of titanium containing at least about 99% (w / w) titanium (e.g., grade 1 titanium, grade 2, or grade 4 titanium) are considered "pure titanium," and therefore, pure titanium may contain trace elements (e.g., oxygen, carbon, nitrogen, or other metals such as iron) up to about 1% (w / w). Pure titanium is sometimes referred to as "commercially pure" (CP). In particular, nitrogen and carbon, which are included in the Group IV metals in the present invention, may be unavoidable impurities. Elements present as "unavoidable impurities" are not expected to affect the workpieces treated by the methods of the present invention or the components of the present invention. Similarly, grades of zirconium containing at least about 99% (w / w) zirconium are considered "pure zirconium" for purposes of the present invention.

[0066] When metals or alloys are given in percentages, the percentages are by weight of the material, e.g., % (w / w), unless otherwise stated. When atmospheres are given in percentages, the percentages are by volume, e.g., % (v / v), unless otherwise stated. Similarly, unless otherwise stated, the composition of gas mixtures may be on an atomic basis and may be given as percentages or ppm (parts per million).

[0067] In the present invention, hardness is generally measured according to the DIN EN ISO 6507 standard, HV 0.025 Unless otherwise specified, the unit "HV" refers to this standard. Hardness of a cross section (of the treated Group IV metal) may be reported, and the depth of the measurement may be noted. Cross section hardness measurements may be referred to as "microhardness" and surface hardness measurements may be referred to as "macrohardness."

[0068] Microhardness measurements are generally independent of test conditions because the measurements are made on a microscale. Microhardness measurements are typically performed under a 25g load, i.e., HV 0.025 , or 50g load, i.e. HV 0.05 In contrast, macrohardness is measured at much higher loads (e.g. HV 0.5 The hardness may be measured from the surface with a load of 25g or 50g (0.50kg, which corresponds to a surface hardness of 1000kJ / 2000kcal), so that the measurement gives an overall value of the hardness of the respective material, regardless of the material contained in the surface layer. Microhardness measurements with a load of 25g or 50g usually result in the same value, "HV", but the 25g measurement is preferred due to the smaller cross-section required for the measurement.

[0069] When hardness is recorded on a cross section, the measurement is considered to represent a homogeneous sample with respect to the direction of the applied pressure. Conversely, when hardness is obtained by measuring on the surface, the measurement may represent the average of several different hardness values, i.e., hardness values ​​at different depths. If the surface hardness is measured with a high load (e.g., 0.50 kg), it can be considered an "average" value for the surface and subsurface depths. Therefore, the surface hardness is preferably measured with a load of 25 g or 50 g. If the surface hardness is measured with a load of 25 g, it is 650 HV. 0.025 A value of 0.05 is considered to indicate that the material is scratch resistant. As a result of oxygen dissolving from the surface, the dissolved oxygen content decreases from the surface to the core of Group IV metals, and similarly, hardness is greatest at the surface. [Example]

[0070] Example 1 CP Grade 4 titanium specimens were prepared and treated in a Netzsch STA449 C furnace using CO as the gaseous oxidizing species or in a Netzsch STA449 F3 furnace using NO at ambient pressure and various first temperatures and oxidation durations. After oxidation, cross sections of the specimens were analyzed microscopically. Figure 1 shows how CO produced a stable, non-stratified oxide layer at the temperatures and durations tested, where the temperatures and durations are indicated in Figure 1. As expected, the thickness increased with increasing oxidation duration. Figure 1 therefore demonstrates that CO as a gaseous oxidizing species provides a robust method for the formation of a non-stratified oxide layer at 730°C, the lowest temperature tested.

[0071] Figure 2 compares the cross-sections of oxide layers formed using CO2 or NO as the gaseous oxidizing species. The oxidation duration was 16 hours. Figure 2 shows that at 880 °C, CO2 produced a thick, stable, non-stratified oxide layer, demonstrating the robustness of using CO2 as the gaseous oxidizing species. Treatment with NO resulted in the formation of a stratified oxide layer above 780 °C. This layered oxide layer was easily removed, even with a fingernail, and was not suitable for diffusing oxygen into titanium. However, both NO and CO2 produced non-stratified oxide layers, e.g., approximately 5 μm thick, suitable for processing in the diffusion step, at an oxidation temperature of 680 °C.

[0072] Example 2 Grade 4 titanium samples were treated with N2O at 600°C and ambient pressure for 64 minutes. Following this oxidation step, a diffusion step was performed at 750°C for approximately 10 minutes. -6 The treatment was carried out in a vacuum of 1000 mbar for 4 hours. This treatment gave the specimens a natural surface finish, and they regained their metallic luster. Cross sections of the hardened specimens were analyzed under a microscope and the hardness profile was measured. The results are shown in Figure 3. Grade 4 titanium has a hardness of approximately 230 HV 0.025 The core hardness is approximately 280HV 0.025 was observed at a depth of 0.06 mm from the surface, so the thickness of the diffusion zone is 60 μm.

[0073] Example 3 CP titanium (grade 2) and Ti6Al4V ELI workpieces with a diameter of 15 mm and a thickness of 2 mm were treated in two separate steps.

[0074] In the first step (oxidation step), the sample was placed in an MTI OFT-1200 glass tube furnace. The furnace was evacuated and refilled with CO. A continuous gas flow of 200 ml / min was used. The workpiece was heated to 650°C at 12 K / min. The furnace was maintained at 650°C for 4 hours, and then the furnace was allowed to cool naturally to room temperature.

[0075] In the second step (diffusion step), the cooled workpieces from the first step were placed in a tube furnace equipped with an Edwards 85 T-station turbo vacuum pump. Before starting the furnace, the vacuum pump was used to evacuate the chamber for 10 minutes. -3 The furnace was evacuated to less than 100 mbar. The furnace was heated at about 50 K / min (above 250°C). The furnace was maintained at 680°C for 16 hours and then cooled to room temperature at about 25 K / min. The final pressure in the furnace chamber was 10 -4 The pressure was less than mbar.

[0076] Photographs of the parts are shown in Figure 4 (Grade 2) and Figure 6 (Ti6Al4V ELI), where (a) is the untreated workpiece, (b) is the intermediate workpiece, and (c) is the part of the invention. The oxidized surface is clearly visible in the intermediate workpiece (b), and part (c) has regained its metallic luster.

[0077] Cross sections of (a), (b), and (c) in Figure 4 (Grade 2) are shown in Figure 5, with the black bar corresponding to 5 μm. The thickness of the non-stratified oxide layer was about 2 μm, resulting in a diffusion zone with a thickness of more than 5 μm. The cross-sectional hardness of the Ti6Al4V ELI part is shown in Figure 7, showing that the thickness of the diffusion zone was about 15 μm. The untreated Grade 2 had a surface hardness of about 361 HV. 0.025 The surface hardness of untreated Ti6Al4V ELI is about 450HV. 0.025 After curing, the surface hardness was 1152HV 0.025 and 1382HV 0.025 It was.

[0078] Example 4 The sample of Example 3 was further analyzed by glow discharge optical emission spectroscopy (GDOES). The results are shown in Figures 8-13. Figure 8 shows a GDOES analysis of an untreated Grade 2 titanium workpiece, Figure 9 shows a GDOES analysis of an intermediate Grade 2 titanium workpiece, and Figure 10 shows a GDOES analysis of a Grade 2 titanium part of the present invention. Similarly, Figure 11 shows a GDOES analysis of an untreated Ti6Al4V ELI workpiece, Figure 12 shows a GDOES analysis of an intermediate Ti6Al4V ELI workpiece, and Figure 13 shows a GDOES analysis of a Ti6Al4V ELI part of the present invention.

[0079] GDOES analyzes the content of a particular element, expressed as intensity (units V) over time (seconds). Thus, intensity reflects the relative amount of the element, and time reflects depth from the surface. By analyzing for a time period long enough to reflect the composition of the layers of the workpiece or part, GDOES analysis provides a good comparison of an untreated workpiece of Group IV metal, an intermediate workpiece with a diffusion zone between the non-layered oxide layer and the material core, and the non-layered oxide layer, and the core.

[0080] Figures 8 and 11 show the through-thickness stability of the metal composition. Figures 9 and 12 show that the oxygen content remains fairly stable, representing the non-layered oxide layer. Over time, the titanium signal gradually increases, corresponding to a corresponding decrease in the oxygen signal, which together represent the diffusion zone. Over time, the oxygen signal remains stable, representing the Group IV metal core. Because CO2 was used as the gaseous oxidizing species, carbon is also present in the non-layered Group IV metal oxide and in the diffusion zone below the non-layered metal oxide layer. Figures 10 and 13 show the final parts for grade 2 titanium and Ti6Al4V ELI, respectively. The carbon signal shows that the carbon intensity increases from the surface, which can be seen as a maximum in the carbon intensity curve. This maximum is believed to represent the peak interstitial carbon concentration in the diffusion zone after removal of the non-layered metal oxide layer. Furthermore, this maximum is believed to increase hardness beyond that achieved in the absence of carbon.

[0081] Example 5 Parts of the present invention were prepared by processing Ti6Al4V (Grade 5) workpieces. Specifically, the workpieces were 3D printed as cylindrical workpieces with a diameter of 12 mm and a height of 15 mm, and then processed in two separate steps.

[0082] In the first step, the sample was placed in an MTI OFT-1200 glass tube furnace, the furnace was evacuated and refilled with CO. A continuous gas flow of 200 ml / min was used. The workpiece was heated to 650 °C at 12 K / min. The furnace was maintained at 650 °C for 4 hours, after which the furnace was allowed to cool naturally to room temperature.

[0083] For the second step, the diffusion step, the cooled workpiece from the first step was placed in a tube furnace equipped with an Edwards 85 T-station turbo vacuum pump. Before starting the furnace, the vacuum pump was used to evacuate the chamber to 100°C. -3 The furnace was evacuated to less than 100 mbar. The furnace was heated at about 50 K / min (above 250°C). The furnace was maintained at 680°C for 16 hours and then cooled to room temperature at about 25 K / min. The final pressure in the furnace chamber was 10 -4 The hardness profile was less than mbar. A photograph of the part and its hardness profile are shown in Figure 14. The part showed an increase in hardness up to a depth of 20 μm. The hardness profile is similar to that seen for non-3D printed Ti6Al4V parts.

[0084] Example 6 Workpieces with a diameter of 15 mm and a thickness of 2 mm were treated in two separate steps with a modification of the oxidizing atmosphere containing CO. The specimens were CP titanium grade 4 and Ti6Al4V (grade 5).

[0085] In the first step, the sample was placed in an MTI OFT-1200 glass tube furnace. The furnace was evacuated and refilled with a 50 / 50 CO2 / CO2 ratio using a continuous gas flow of 200 ml / min. The workpiece was heated to 650°C at 12 K / min. The furnace was maintained at 650°C for 4 hours, and then the furnace was allowed to cool naturally to room temperature.

[0086] In the second step, the cooled workpieces from the first step were placed in a tube furnace equipped with an Edwards 85 T-station turbo vacuum pump. Before starting the furnace, the vacuum pump was used to evacuate the chamber for 10 minutes. -3 The furnace was evacuated to less than 100 mbar. The furnace was heated at about 50 K / min (above 250°C). The furnace was maintained at 680°C for 16 hours, then the furnace was cooled to room temperature at about 25 K / min. The final pressure in the furnace chamber was 10 -4 The pressure was less than mbar.

[0087] The parts obtained exhibited a surface similar to that found on parts oxidized with CO2 only, and the parts thus obtained regained their metallic luster after the diffusion process.

[0088] Example 7 Grade 4 CP titanium and Ti6Al4V (grade 5) with a diameter of 15 mm and a thickness of 2 mm were treated in a variation using ambient air at ambient pressure as the oxidizing atmosphere.

[0089] In a first step, the sample was placed in a Nabertherm LE4 / 11 R6 furnace and then heated at 12 K / min to 650° C. The furnace was kept at 650° C. for 4 hours, after which the furnace was allowed to cool naturally to room temperature.

[0090] In the second step, the cooled sample from the first step was placed in a tube furnace equipped with an Edwards 85 T-station turbo vacuum pump. Before starting the furnace, the vacuum pump was used to evacuate the chamber for 10 minutes. -3 The furnace was evacuated to less than 100 mbar. The furnace was heated at about 50 K / min (above 250°C). The furnace was maintained at 680°C for 16 hours, then the furnace was cooled to room temperature at about 25 K / min. The final pressure in the furnace chamber was 10 -4 The pressure was less than mbar.

[0091] The parts contained interstitial nitrogen, which was reflected in a high surface hardness. The grade 4 titanium parts had a surface similar to that seen on parts oxidized with CO2 only, while the grade 5 titanium parts were less cosmetically appealing.

[0092] Example 8 Zirconium (Zr702) and niobium-containing alloys (Ti13Nb13Zr) were treated using CO2 as the gaseous oxidizing species at 650 °C for 4 hours, followed by a diffusion process at 680 °C for 16 hours in vacuum as described in Example 3. The Ti13Nb13Zr workpieces were 10 mm in diameter and 1 mm thick, while the Zr702 workpieces were square with 15 mm side lengths and 1.5 mm thick.

[0093] As a result of the treatment, the surface hardness of the parts for Ti13Nb13Zr and Zr702 was 264HV before treatment. 0.025 and 185HV 0.025 Compared to about 860HV 0.025 and approximately 1218HV 0.025 It was.

[0094] Example 9 Workpieces of 15 mm diameter and 2 mm thickness were treated with the two steps outlined in Example 3, followed by an additional anodizing step. The workpieces were of CP titanium, grade 4.

[0095] In the third step, the anodizing step, the parts were washed in succession with first distilled water, then ethanol, and finally turpentine. The parts were then submerged in a solution containing 15% phosphoric acid. A voltage of 70 V was applied for 5-10 seconds. After the diffusion step, the parts regained their metallic luster, but anodizing resulted in a visible oxide layer, typical of anodized titanium not treated with the method of the present invention.

[0096] Example 10 The limits of layered and non-layered titanium oxide layer formation when NO is used as the gaseous oxidizing species were determined experimentally. Specifically, CP Grade 2 titanium test pieces were prepared and treated as in Example 1.

[0097] After oxidation, cross sections of the samples were analyzed under a microscope. The results are plotted in Figure 15. Figure 15 plots time versus temperature, with each dot indicating a limit. The region below the dotted line represents the region where a non-stratified titanium oxide layer forms when NO is used as the gaseous oxidizing species. For example, with NO, treatment at 500°C to 700°C for an oxidation duration of 10 minutes to 2 hours, or at 550°C to 600°C for an oxidation duration of 30 minutes to 2 hours, results in a non-stratified oxide layer thick enough to harden the titanium in a subsequent diffusion step.

[0098] Example 11 Discs of CP titanium, 30 mm in diameter and 15 mm thick, were polished to a mirror-like surface finish. Prior to treatment, the CP titanium specimens were polished to a mirror-like surface finish, i.e., an arithmetic mean deviation (Ra) roughness of less than 0.1 μm (according to the ISO 1302:2002 standard). The Ra values ​​were measured with a Taylor-Hubson Surtronic S25 over a length of 1.25 mm. The measurements were repeated 12 times. The average Ra value was less than 0.1 μm, and the specimen surface was deemed to have a mirror-like finish.

[0099] The specimens were placed in a Nabertherm 3-Zone furnace. The furnace was evacuated and refilled with CO2 twice. It was then heated to 650°C for 4 hours with a continuous flow of CO2 at 500 ml / min. The furnace was allowed to cool. The specimens were then returned to the furnace for a second treatment. -4 The mixture was placed in a furnace capable of reaching a pressure of less than 1000 mbar, heated to 680°C and maintained there for 16 hours, and then cooled.

[0100] The Ra value of the specimen surface was measured using the same procedure as before the thermochemical treatment (described above). The specimen still exhibited a surface roughness of less than 0.1 μm Ra. The surface roughness is shown in Figure 16. Figure 16 is a graph of the surface roughness after treatment, measured with a Taylor-Hubson Surtronic S25. Figure 17 shows a photograph of the mirror-finished surface after thermochemical low-temperature curing. Figure 17 shows how the shape of the plot placed next to the specimen is reflected on the surface of the specimen; in particular, there is no distortion in the shape of the reflected plot, and the color of the plot is also reflected on the surface of the specimen. The following claims as originally filed in this application are appended as embodiments. [1] A method for oxygen hardening a Group IV metal, said method comprising: - providing a workpiece of Group IV metal in its final shape; - the Group IV metal is reacted with CO 2 、N 2 O and CO 2 and N 2 O in an oxidizing atmosphere at a first temperature for an oxidation duration of at least 10 minutes to form a non-layered Group IV metal oxide on the surface of the workpiece, wherein the gaseous oxidizing species has an upper temperature limit of up to 800°C, and the first temperature is between 500°C and the upper temperature limit of the gaseous oxidizing species; - Oxygen is added in an inert atmosphere at a second temperature of 500 ° C to 800 ° C and a maximum of 10 -4 diffusing from the non-layered Group IV metal oxide into the Group IV metal at a partial pressure of the gaseous oxidizing species of 100 mbar for a diffusion duration of at least 0.1 hours to form an outer diffusion zone containing oxygen in solid solution. A method comprising: [2] The gaseous oxidizing species is CO 2 The method for oxygen-hardening a Group IV metal according to [1], wherein the upper temperature limit is 800°C and the oxidation duration is 1 hour to 16 hours. [3] The gaseous oxidizing species is N 2 The method for oxygen-hardening a Group IV metal according to [1], wherein the upper temperature limit is 700°C and the duration of the oxidation is 10 minutes to 2 hours. [4] The method for oxygen-hardening a Group IV metal according to any one of [1] to [3], wherein the pressure of the oxidizing atmosphere is ambient pressure. [5] The method for oxygen-hardening a Group IV metal according to any one of [1] to [4], wherein the first temperature is 600°C to 700°C. [6] The total pressure in the inert atmosphere is up to 10 -4 The method for oxygen-hardening a Group IV metal according to any one of [1] to [5], wherein the oxygen pressure is 1000 kJ / cm2 or 1000 kJ / cm2. [7] The method for oxygen-hardening a Group IV metal according to any one of [1] to [6], wherein the inert atmosphere is a rare gas. [8] The method for oxygen-hardening a Group IV metal according to any one of [1] to [7], wherein the second temperature is 650°C to 750°C. [9] The method for oxygen-hardening a Group IV metal according to [8], wherein the diffusion duration is 2 hours to 40 hours.

[10] The method for oxygen-hardening a Group IV metal according to any one of [1] to [4] and [6] to [9], wherein the Group IV metal contains aluminum as an alloying element, and the first temperature is 500°C to 700°C.

[11] The method for oxygen-hardening a Group IV metal according to any one of [1] to [9], wherein the oxidizing atmosphere further contains CO.

[12] The method for oxygen hardening a Group IV metal according to any one of [1] to

[11] , wherein, before oxidizing the Group IV metal, the Group IV metal workpiece is polished to a surface roughness of less than 0.1 μm according to the ISO 1302:2002 standard.

[12] The gaseous oxidizing species is CO 2 The method for oxygen-hardening a Group IV metal according to [1], wherein the first temperature is 600°C to 750°C, the oxidation duration is 1 hour to 8 hours, the second temperature is 650°C to 750°C, and the diffusion duration is 2 to 8 times the oxidation duration.

[13] The gaseous oxidizing species is CO 2 wherein the first temperature and the second temperature are both 650°C to 700°C, the second temperature is higher than the first temperature, the oxidation duration is 2 hours to 6 hours, and the diffusion duration is 3 to 6 times the oxidation duration.

[14] The gaseous oxidizing species is N 2 The method for oxygen-hardening a Group IV metal according to [1], wherein the first temperature is 600°C to 650°C, the oxidation duration is 30 minutes to 2 hours, the second temperature is 650°C to 700°C, and the diffusion duration is 4 to 20 times the oxidation duration.

[15] The oxidizing atmosphere is CO 2 and CO relative to the total 2 The ratio of CO is 40% to 90% 2 and CO. The method for oxygen-hardening a Group IV metal according to [1].

[16] The oxidizing atmosphere is N 2 N relative to the sum of O and CO 2 N with 40% to 60% O 2 The method for oxygen curing of Group IV metals according to [1], wherein the oxygen curing agent is a mixture of O and CO.

[17] Core hardness and at least the core hardness + 200HV 0.025 and a diffusion zone 10 μm to 100 μm thick from the surface, in which oxygen in solid solution is at a level that provides a hardness of 120% of the hardness of the material core to a saturation level of the Group IV metal, said diffusion zone further containing carbon and / or nitrogen in solid solution at concentrations that exhibit a maximum in said diffusion zone containing carbon and / or nitrogen detectable by glow discharge optical emission spectroscopy (GDOES).

[18] Surface hardness of at least 650HV 0.025

[17] The Group IV metal part according to

[17] ,

[19] A Group IV metal part according to

[17] or

[18] , wherein the surface roughness of the part is less than 0.1 μm according to the ISO 1302:2002 standard.

[20] A Group IV metal part according to any one of

[17] to

[19] , wherein the part is obtainable by a method according to any one of [1] to

[16] .

[21] The Group IV metal part according to any one of

[17] to

[20] , wherein the thickness of the diffusion zone is at least 5 μm.

[22] The method for oxygen-hardening a Group IV metal according to any one of [1] to

[16] or the Group IV metal part according to any one of

[17] to

[20] , wherein the Group IV metal is selected from titanium, a titanium alloy, zirconium, and a zirconium alloy.

[23] The method for oxygen hardening a Group IV metal according to

[22] or the Group IV metal part according to

[22] , wherein the titanium is grade 2, 4 or 5 titanium, or the zirconium is Zr702 zirconium.

Claims

1. 1. A method for oxygen hardening a Group IV metal, said method comprising: - providing a workpiece of Group IV metal in its final shape; - the Group IV metal is reacted with CO ２ , N ２ O and CO ２ and N ２ O in an oxidizing atmosphere at a first temperature for an oxidation duration of at least 10 minutes to form an intermediate non-layered Group IV metal oxide on the surface of the workpiece, wherein the gaseous oxidizing species has an upper temperature limit of up to 800°C, and the first temperature is between 500°C and the upper temperature limit of the gaseous oxidizing species; - a second temperature of 500°C to 800°C in an inert atmosphere and a maximum of 10 －４ diffusing oxygen from said intermediate non-layered Group IV metal oxide into said Group IV metal at a partial pressure of said gaseous oxidizing species of 1000 psi mbar for a diffusion duration of at least 0.1 hours to remove said intermediate non-layered oxide layer and replace it with a diffusion zone containing oxygen in solid solution and extending from the surface of said Group IV metal; A method comprising:

2. The gaseous oxidizing species is CO ２ 2. The method of claim 1, wherein the upper temperature limit is 800° C. and the duration of the oxidation is 1 hour to 16 hours.

3. The gaseous oxidizing species is N ２ 2. The method of claim 1, wherein the upper temperature limit is 700° C. and the duration of oxidation is from 10 minutes to 2 hours.

4. A method for oxygen hardening Group IV metals according to any one of claims 1 to 3, wherein the pressure of the oxidizing atmosphere is ambient pressure.

5. A method for oxygen hardening Group IV metals according to any one of claims 1 to 4, wherein the first temperature is between 600°C and 700°C.

6. The total pressure in the inert atmosphere is up to 10 －４ 6. The method for oxygen curing of Group IV metals according to any one of claims 1 to 5, wherein the temperature is 1000 K or more and 1000 psi or less.

7. A method for oxygen hardening Group IV metals according to any one of claims 1 to 6, wherein the inert atmosphere is a noble gas.

8. A method for oxygen hardening Group IV metals according to any one of claims 1 to 7, wherein the second temperature is between 650°C and 750°C.

9. 9. The method for oxygen hardening Group IV metals of claim 8, wherein the diffusion duration is between 2 hours and 40 hours.

10. 10. A method for oxygen hardening a Group IV metal according to any one of claims 1 to 4 and 6 to 9, wherein the Group IV metal comprises aluminum as an alloying element and the first temperature is between 500°C and 700°C.

11. A method for oxygen hardening Group IV metals according to any one of claims 1 to 9, wherein the oxidizing atmosphere further comprises CO.

12. The gaseous oxidizing species is CO ２ 2. The method of claim 1, wherein the first temperature is between 600°C and 750°C, the oxidation duration is between 1 hour and 8 hours, and the second temperature is between 650°C and 750°C, and the diffusion duration is between 2 and 8 times the oxidation duration.

13. The gaseous oxidizing species is CO ２ wherein the first temperature and the second temperature are both between 650°C and 700°C, the second temperature is higher than the first temperature, the oxidation duration is between 2 hours and 6 hours, and the diffusion duration is between 3 and 6 times the oxidation duration.

14. The gaseous oxidizing species is N ２ 2. The method of oxygen hardening a Group IV metal as recited in claim 1, wherein the first temperature is between 600°C and 650°C, the oxidation duration is between 30 minutes and 2 hours, the second temperature is between 650°C and 700°C, and the diffusion duration is between 4 and 20 times the oxidation duration.

15. The oxidizing atmosphere is CO ２ and CO relative to the total ２ The ratio of CO is 40% to 90% ２ 2. The method of claim 1 wherein the oxygen curing agent is a mixture of SiO2 and CO.

16. The oxidizing atmosphere is N ２ N relative to the sum of O and CO ２ N with an O ratio of 40% to 60% ２ 2. The method of oxygen curing a Group IV metal according to claim 1, wherein the oxygen curing agent is a mixture of O and CO.

17. A Group IV metal part having a material core and a diffusion zone having oxygen in solid solution to provide a surface hardness of 650 HV 0.025 to the oxygen saturation level of the Group IV metal and a hardness of 10 μm to 100 μm deep that is 120% of the hardness of the material core, said diffusion zone extending from the surface of the Group IV metal, said diffusion zone further containing carbon and / or nitrogen in solid solution in concentrations that exhibit a maximum in said diffusion zone containing carbon and / or nitrogen that is detectable by Glow Discharge Optical Emission Spectroscopy (GDOES).

18. Surface hardness of at least 800HV ０．０２５ 18. The Group IV metal component of claim 17, wherein:

19. 19. A Group IV metal part according to claim 17 or 18, wherein the part has an arithmetic mean deviation (Ra) surface roughness of less than 0.1 μm according to the ISO 1302:2002 standard.

20. A Group IV metal part according to any one of claims 17 to 19, wherein the part comprises a layer of native oxide which inevitably forms on the surface of the hardened Group IV metal.

21. A Group IV metal component according to any one of claims 17 to 20, wherein the diffusion zone has a thickness of at least 5 μm.

22. A Group IV metal component according to any one of claims 17 to 20, wherein the Group IV metal is selected from titanium, titanium alloys, zirconium and zirconium alloys.

23. 23. The Group IV metal component of claim 22, wherein the titanium is grade 2, 4, or 5 titanium, or the zirconium is Zr702 zirconium.

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