Surface hardening of group IV metals

Abstract

The present invention relates to a method for case hardening a group IV metal or group IV metal alloy, and to components hardened by this method, comprising the steps of: providing a workpiece of a group IV metal or group IV metal alloy, the workpiece being in its final shape; nitriding the workpiece in a nitriding atmosphere containing NH as the nitriding species at a first temperature between 450°C and 750°C for a nitriding time of at least 16 hours to form a hydrogen-containing diffusion zone; -4 removing hydrogen from the hydrogen-containing diffusion zone at a partial pressure of H2 of 100 mbar for a hydrogen removal time of at least 4 hours to form a hydrogen-deficient diffusion zone. The method and components are useful for implants, particularly dental implants.

Description

[Technical field]

[0001] The present invention relates to hardening of Group IV metals or alloys. In particular, a method for case hardening a Group IV metal or a Group IV metal alloy and a hardened Group IV metal or Group IV metal alloy component are provided. The method and component are useful for implants, particularly dental implants. [Background technology]

[0002] Titanium is a light metal with tensile strength comparable to stainless steels that naturally reacts with oxygen to form a titanium oxide layer on its surface that provides corrosion resistance. These characteristics make titanium very attractive in many fields, such as aerospace, military, and industrial processes, and also make titanium suitable for medical applications, such as implants, since it is biocompatible. The naturally formed titanium oxide layer is thin, for example on the nanometer scale, but in certain applications it may be desirable to modify the surface-adjacent regions of titanium and its alloys. It is well known that titanium and other group IV metals can be hardened by interstitial oxygen and other elements, and it is also well known that hardening of titanium must be carried out at the lowest possible temperature to avoid grain growth and overall distortion of the material associated with phase transformations or creep.

[0003] Prior art techniques for hardening titanium are known. For example, EP 885980 A1 discloses a method for forming a surface layer with high hardness and tribological properties on titanium or zirconium parts. The method may include raising the temperature above 500° C. to obtain a homogeneous temperature of the part; injecting a treatment gas containing ammonia, a hydrocarbon and / or an oxidizing gas onto the part to be treated, which has been raised to a temperature above 500° C.; and maintaining a pressure in the furnace of at least 100 mbar for at least several minutes, depending on the desired treatment depth. The use of ammonia to treat titanium results in a high degree of hardness and tribological properties of Ti. x N y A yellow surface layer of

[0004] Preisser et al., 1991 (HTM Harterei - Technische Mitteilungen 46 (1991) Nov. / Dez., No.6, Munchen, DE) discloses high pressure nitriding of titanium workpieces, where the workpieces are treated with ammonia at nitriding temperatures of 700°C or 900°C and a pressure of 12 bar. After treatment, the workpiece has an outer layer of TiN, with TiN beneath the outer layer. 2 A layer of N is formed.

[0005] JP 2-25559 A discloses nitriding a titanium workpiece using ammonia at 400°C to 850°C for at least 1 hour, followed by treatment in an inert gas at least 400°C for at least 1 hour to remove hydrogen from the workpiece. The treatment provides a layer of TiN on the workpiece.

[0006] In JP 54-93700, titanium in the nitriding reaction was converted to NH 3 Gas or N 2 -H 2 After treatment in the mixed gas atmosphere, it is disclosed that the titanium is heated to above 600° C. in an inert atmosphere or vacuum to thermally decompose the hydrides formed by the reaction.

[0007] In view of the prior art, there remains a need for improved methods of hardening titanium or other Group IV metals and their alloys, and it is an object of the present invention to provide improved case hardening of Group IV metals and their alloys. Summary of the Invention

[0008] The present invention relates to a method for case hardening a Group IV metal or a Group IV metal alloy, comprising the steps of: providing a workpiece of a Group IV metal or a Group IV metal alloy, the workpiece being in its final shape; 3 In a nitriding atmosphere containing 3nitriding the workpiece at a partial pressure of 100 to 200° C. for a nitriding time of at least 12 hours to form a hydrogen-containing diffusion zone; and -4 mbar H 2 Partial pressure of (pH 2 ) removing hydrogen from the hydrogen-containing diffusion zone for a hydrogen removal time of at least 4 hours to form a hydrogen-depleted diffusion zone.

[0009] The method includes the step of nitriding the workpiece, which may be referred to herein as the "nitriding step."

[0010] The method includes removing hydrogen from the hydrogen-containing diffusion zone. 3 The workpieces treated with H 2 It should be appreciated that exposure to low partial pressures of hydrogen at elevated temperatures causes hydrogen to diffuse from the workpiece into the surrounding atmosphere, thereby being removed from the workpiece. The step of removing hydrogen may be referred to as "diffusing hydrogen" from the hydrogen-containing diffusion zone or as a "diffusion step," and these terms may be used interchangeably herein. Similarly, the hydrogen removal time may be referred to as the diffusion time, and the two terms may be used interchangeably herein.

[0011] The workpiece is prepared in its final shape. The dissolution of nitrogen and hydrogen into the workpiece generally increases the volume of the workpiece, but the workpiece returns to its pre-treatment, i.e. final, shape. However, even if the workpiece is in its final shape after treatment, it can be further treated with steps that do not substantially affect the final shape, such as polishing.

[0012] NH 3Treatment with nitriding, i.e., at a first temperature between 450°C and 750°C, forms a diffusion zone in the group IV metal, which is believed to contain nitrogen in solid solution and hydrogen in solid solution, and usually also a hydride of the group IV metal. Subsequent diffusion treatment, i.e., at a second temperature up to 750°C, forms a diffusion zone in the group IV metal, which contains nitrogen in solid solution but is depleted in hydrogen. Thus, in this specification, the term "diffusion zone" may refer to either the diffusion zone after nitriding or after diffusion treatment. The diffusion zone after nitriding but before diffusion treatment is commonly referred to as the "hydrogen-containing diffusion zone" and the diffusion zone after diffusion treatment is commonly referred to as the "hydrogen-depleted diffusion zone". However, the diffusion zone after hydrogen removal may be referred to as the "nitrogen diffusion zone".

[0013] The diffusion zone extends from the surface of the group IV metal or group IV metal alloy. As used herein, the diffusion zone extends from the surface of the group IV metal to a surface where the microhardness is 50HV greater than the core hardness of the group IV metal or group IV metal alloy. 0.005 The diffusion zone may also be specified by a thickness, where the thickness is the minimum thickness greater than or equal to the core hardness plus 50HV. 0.005 The depth from the surface to the surface of the group IV metal is calculated as the depth from the surface that is equal to the sum of the above. 3Treatment with , i.e., at a first temperature between 450°C and 750°C, forms a nitride layer on the surface of the group IV metal or group IV metal alloy. The group IV metal may be titanium, zirconium, or an alloy containing both titanium and zirconium, and the nitride layer of the group IV metal or group IV metal alloy may be collectively referred to herein as (Ti,Zr)N for alloys containing both titanium and zirconium. Regardless of the presence of the nitride layer, a diffusion zone is considered to extend from the surface of the group IV metal or group IV metal alloy, and the diffusion zone extends deeper than the nitride layer. Thus, the diffusion zone may be considered to be beneath the nitride layer, or the diffusion zone may be considered to be between the nitride layer and the core of the group IV metal or group IV metal alloy. The nitriding process may also be considered to form a nitride layer of the group IV metal or group IV metal alloy on the surface of the workpiece, and to form a hydrogen-containing diffusion zone between the nitride layer and the core of the group IV metal or group IV metal alloy.

[0014] In the method of the present invention, ammonia (NH 3 ) is used in the method of the present invention. 3 When exposed to temperatures above 800°C, especially higher temperatures, such as above 1000°C, N 2 and H 2 Generally, N 2 can also be used for nitriding metals. However, NH 3 When NH is used as a nitriding species, both nitrogen and hydrogen are dissolved in the metal at the same time. 3 It is believed that the molecule splits into N and H atoms at high temperatures, and both the N and H atoms diffuse into the metal. NH 3 is N 2 The "virtual partial pressure" of nitrogen is much higher than that of H alone. 2 It is believed to provide a virtual partial pressure of hydrogen much higher than that of NH alone, and therefore 3 is N 2 It is a much more powerful nitriding species than H 2 It is a much more powerful hydrogenating species than H. 2 The hypothetical partial pressure of NH at different total pressures and 690 °C3 :N 2 pH as a function of ratio 2 However, hydrogen is generally undesirable for hardening Group IV metals because interstitial hydrogen in Group IV metals is believed to embrittle the metals, and furthermore, hydrogen is not generally desirable for hardening Group IV metals because it is believed that interstitial hydrogen in Group IV metals embrittles the metals. 3 When processed with H 2 More hydrogen dissolves in the group IV metal than when it is treated with NH 3 When treated with 2 and H 2 It would be expected that the adverse effects of interstitial hydrogen would be more pronounced than when treated with NH 3 by introducing hydrogen atoms from the group IV metal, i.e. at 450°C to 750°C, after which hydrogen atoms diffuse out of the group IV metal at up to 750°C, e.g. 400°C to 700°C, in particular 600°C to 700°C, thereby forming NH 3 It has been found that the embrittlement problem can be prevented despite the increased amount of hydrogen in the group IV metal obtained by the nitriding process by NH. Furthermore, it is known that the dissolution of hydrogen in the group IV metal also leads to the formation of hydrides, and that removal of hydrogen from the group IV metal having a group IV metal hydride, for example by treatment at high temperatures above 700°C, leads to grain refinement in the group IV metal. 3 When processed with H 2 The hydrogen concentration in the Group IV metal is higher than that in the H 2 The inventors believe that a more efficient grain refinement of the Group IV metal is thus obtained, as compared to the grain refinement obtained using NH 3 Treatment at 450°C-750°C followed by removal of hydrogen from the group IV metal by diffusion at up to 750°C, e.g., 400°C-700°C, results in a hardened and grain refined group IV metal. Specifically, the hardened group IV metal has a grain refined core zone defined by the core hardness of the group IV metal and a hydrogen depleted diffusion zone due to nitrogen in solid solution between the grain refined core zone and the surface of the group IV metal.

[0015] In the nitriding process, NH3 A nitriding atmosphere containing NH 3 The partial pressure of NH can be freely selected, but must be sufficient to dissolve the desired amount of nitrogen and hydrogen in the Group IV metal. 3 If the partial pressure of N is low, not enough nitrogen will dissolve in the Group IV metal or Group IV metal alloy, and NH 3 The partial pressure of NH should generally be at least 1 mbar, for example between 0.5 bar and 2 bar at ambient pressure. 3 If the partial pressure of NH is less than ambient pressure, the pressure may be reduced by any means as desired. For example, the partial pressure may be equal to the total pressure, i.e., the nitriding atmosphere is pure NH with unavoidable impurities. 3 or the partial pressure may be reduced by including an additional gas species in the nitriding atmosphere. The additional gas species may be an inert gas or a gas species providing a specific function. Since the nitriding is carried out at a first temperature of 450°C to 750°C, nitrogen gas, i.e. N 2 is considered an inert gas that does not result in the dissolution of nitrogen. Further inert gases are the noble gases, such as argon and helium. Nitriding atmospheres are free of oxidizing species, such as CO 2 , O 2 and N 2 It is particularly preferred that the nitriding atmosphere does not contain O. It is even more preferred that the nitriding atmosphere does not contain oxygen-containing species. Avoiding oxidizing and oxygen-containing species ensures that the diffusion zone obtained by this method does not contain interstitial oxygen or oxygen in solid solution. However, since nanometer-scale oxide layers naturally form on the surface of group IV metals in contact with air, components produced by this method may still have an unavoidable oxide layer on the surface. Nanometer-scale oxide layers are considered herein to be unadverse, and components having a naturally formed nanometer-scale oxide layer are considered to be substantially free of an oxide layer. Similarly, group IV metals can contain an unavoidable amount of oxygen dissolved in the group IV metal, and when the group IV metal contains an unavoidable amount of oxygen dissolved in the group IV metal, the group IV metal is substantially free of interstitial oxygen.

[0016] The nitriding atmosphere may further include gas species that provide a specific function. The nitriding atmosphere may include, for example, a carbon-containing gas species. Generally, carbon may dissolve in the group IV metal, but the concentration of the carbon-containing gas species must be low to form carbides or carbonitrides in the group IV metal. Generally, carbide formation is avoided if the first temperature does not exceed 700°C. Exemplary carbon-containing gas species are alkanes (e.g., methane), alkenes (e.g., ethylene), and alkynes (e.g., acetylene). The carbon-containing gas species may also contain nitrogen, but should not contain oxygen.

[0017] The nitriding atmosphere is preferably free of oxygen-containing species. However, for certain oxygen-containing species, the oxygen is not available to dissolve significantly more oxygen in the Group IV metal. For example, CO can be included to obtain high carbon activity but not dissolve oxygen in the Group IV metal. Additionally, urea (H 2 NCONH 2 ) as the nitriding species or NH 3 For example, urea may be heated to produce NH 3 A mixture of , CO and other species may be produced and this mixture may be used as the nitriding atmosphere.

[0018] The hydrogen-containing diffusion zone is formed in the nitriding step at a first temperature between 450°C and 750°C. The nitriding time generally depends on the first temperature, and at a first temperature between 450°C and 580°C, the nitriding step is generally undesirably slow, but if the first temperature is at least 580°C, such as at least 581°C or at least 585°C, the nitriding rate of the workpiece is acceptable. If the first temperature is between 700°C and 750°C, undesirable grain growth may be observed for the group IV metal. Therefore, if grain growth is not acceptable, the first temperature should be at most 700°C, such as between 450°C and 700°C or between 580°C and 700°C, such as between 582°C and 700°C. However, the increased amount of hydrogen in the hydrogen-containing diffusion zone allows the grain refinement brought about by the diffusion treatment to be sufficient to avoid the adverse effects of grain growth above 700°C. Thus, the present invention provides a method that allows hardening of group IV metals at temperatures above 700°C while reducing the risk of undesirable grain growth. Table 1 shows a preferred combination of the first temperature and the nitriding time.

[0019] [Table 1]

[0020] Particularly preferred combinations of first temperature and nitriding time are 450° C. to 580° C. and at least 24 hours (e.g., up to 200 hours); 580° C. to 700° C. and at least 12 hours (e.g., at least 16 hours or at least 20 hours) and up to 100 hours; and 700° C. to 750° C. and at least 1 hour and up to 50 hours. The most preferred combination of first temperature and nitriding time is 600° C. to 700° C. and at least 12 hours (e.g., at least 16 hours).

[0021] Preferably, the nitriding atmosphere does not contain oxidizing species. However, in one example, the workpiece is treated to dissolve oxygen in the group IV metal before treatment in the nitriding step or after treatment in the diffusion step. For example, a group IV, e.g., a group IV metal or group IV metal alloy workpiece in its final shape, or a group IV metal or group IV metal alloy workpiece after treatment in the nitriding and diffusion steps, may be treated to have an oxygen diffusion zone containing interstitial oxygen, the oxygen diffusion zone having a thickness of 10 μm to 100 μm from the surface. The oxygen diffusion zone may, for example, have a microhardness from the surface of the group IV metal that is 50 HV greater than the core hardness of the group IV metal. 0.005 Near the surface, for example at a depth of 5 μm, the microhardness of the oxygen diffusion zone is 600 HV 0.005 ~800HV 0.005 The oxygen diffusion zone may be formed, for example, by first oxidizing the group IV metal to form an oxide layer on the group IV metal, and then treating the group IV metal with the oxide layer in a vacuum treatment to dissolve oxygen from the oxide layer in the group IV metal. Alternatively, the group IV metal or group IV metal alloy workpiece treated in the nitriding and diffusion steps may be treated in an oxidizing atmosphere to dissolve oxygen in the group IV metal or group IV metal alloy, or to form an oxide layer on the surface of the group IV metal or group IV metal alloy, and then treated in an oxidizing atmosphere, for example in a vacuum, with a low partial pressure of the oxidizing species. The conditions of the oxidation and dissolution steps, such as temperature, time, and partial pressure of the oxidizing species that are the active species, may be similar to the nitriding step of the present invention and the diffusion step of the present invention with respect to the oxidizing species. When forming an oxygen diffusion zone in a group IV metal, the nitriding step is followed by a diffusion step, and the hydrogen-deficient diffusion zone contains both oxygen and nitrogen.

[0022] Group IV metals and their alloys may be described in terms of their hardness. Group IV metals can be hardened by dissolution of nitrogen, oxygen, and other elements in the metal, but despite any case hardening, Group IV metals have a core hardness. Thus, the core hardness generally corresponds to the hardness of the Group IV metal before case hardening, e.g., the surface hardness. Hardness is generally measured according to the DIN EN ISO 6507 standard. Although the core hardness generally depends on the particular Group IV metal, when Group IV metals are treated with the method of the present invention, the surface hardness is at least 200 HV higher than the core hardness. 0.025 The surface hardness is measured by the load up to 50g, i.e. HV 0.05 Unless otherwise specified, surface hardness values ​​are HV 0.025 A load of up to 50g, e.g. HV 0.01 , HV 0.025 or HV 0.005 The surface hardness value is HV 0.025 These values ​​are also considered to be representative. Grade 2 titanium typically has a core hardness of about 200HV 0.025 The core hardness of grade 5 titanium is typically around 300HV. 0.025 In the nitriding process, the microhardness of the surface of the Group IV metal is reduced by 50HV compared to the core hardness. 0.005 Thus, the thickness of the diffusion zone is such that the microhardness is 50HV more than the core hardness. 0.005 + , to the surface of the Group IV metal.

[0023] Generally, the depth depends on the first temperature and nitridation time as well as the NH 3 The depth also depends on the partial pressure of nitrogen. The higher the first temperature and the longer the nitriding time, the greater this depth will be. Generally, case hardening is already obtained when this depth is about 1 μm, but preferably this depth is at most about 50 μm. The depth is usually between 10 μm and 30 μm. The depth of the diffusion zone containing nitrogen in solid solution in case hardened group IV metals or group IV metal alloys is therefore at most 50 μm, for example between 10 μm and 30 μm. The microhardness of the diffusion zone is 50 HV more than the core hardness. 0.005from the hardness observed on the surface, e.g., about 300HV 0.005 up to 1000HV 0.005 In particular, when the surface has a (Ti,Zr)N layer, the hardness increases to 1000HV at a depth of 2.5μm from the surface. 0.005 ~1500HV 0.005 Generally, the workpieces treated by the method of the present invention, such as the components of the present invention, have a very high surface hardness, but the depth from the surface at which the very high hardness is obtained does not need to be particularly deep, so the components have a microhardness of HV 2.5 μm deep from the surface. 0.005 is explained in.

[0024] In the diffusion step, hydrogen is diffused out of the hydrogen-containing diffusion zone, i.e., removed from the hydrogen-containing diffusion zone. Hydrogen generally diffuses out of the hydrogen-containing diffusion zone when the partial pressure of the hydrogen-containing species is low, and in the method of the present invention, the diffusion step is, inter alia, performed by the diffusion of H 2 Partial pressure of (pH 2 The diffusion step may be carried out at a second temperature of up to 750° C., but for practical reasons the second temperature is usually at least 200° C. For example, the second temperature may be between 300° C. and 750° C., in particular between 600° C. and 700° C. A hydrogen-containing gas species, such as NH 3 is H 2 and other elements of the gas species can dissociate into other gases, thereby H 2 Even if the diffusion process is not included, pH 2 Therefore, the diffusion process can be determined by pH 2 For example, Figure 1 shows the NH 3 :N 2 pH as a function of ratio 2 In particular, pH 2 should be as low as possible, e.g. pH 2 is up to 10 -5 mbar or pH 2 is up to 10 -6 The diffusion step may be carried out in a vacuum, or the diffusion step may be carried out in a vacuum. As used herein, a "vacuum" refers to a total pressure of up to 10 -4mbar, pH 2 Up to 10 -4 The composition is not limited as long as the pressure is within 100 mbar. The conditions for the nitriding step and the diffusion step can be selected independently. 2 The partial pressure of the nitrile can be controlled as desired. For example, pH 2 The total pressure is up to 10 -4 The pH may be equal to the total pressure, such as mbar, or the pH may be adjusted by supplementing the nitriding atmosphere with an inert gas and optionally reducing the total pressure as well. 2 may be reduced.

[0025] The nitriding and diffusion steps may be carried out in the same furnace. For example, the nitriding atmosphere may be directly evacuated for up to 10 -4 pH in mbar 2 or before evacuating the furnace, the nitriding atmosphere may be replaced with an inert atmosphere, e.g., N 2 Alternatively, argon may be substituted. If the nitriding atmosphere is replaced with an inert atmosphere, NH 3 is removed from the furnace, so that the pH is low enough 2 It is also possible to include further steps between the nitriding and diffusion steps. For example, the workpiece may be removed from the furnace in which the nitriding step is performed and cooled to ambient temperature before the diffusion step is performed in the same or another furnace. In addition to ensuring the removal of hydrogen from the hydrogen-containing diffusion zone, the diffusion step also redistributes the interstitial nitrogen and the nitrogen in the nitride layer. In particular, the nitrogen migrates deeper, thereby pushing the interface between the core and the diffusion zone deeper. For example, when a Ti15Zr alloy is treated with a nitriding step, the hardness of the Ti15Zr alloy is 400HV at a depth of 30 μm. 0.005 However, when the diffusion process was performed on Ti15Zr alloy, the hardness was 400HV at a depth of 50μm. 0.005 The results were less than those shown in Figures 2 and 3.

[0026] In diffusion processes, even small amounts of contaminants can result in undesirable coloring and other undesirable effects on the surface of the workpiece. For example, contaminants can prevent the Group IV metal from retaining its metallic luster, so NH 3 and other undesirable species, e.g. CO2 , O 2 and N 2 The partial pressure of O is up to 10 times the total pressure during the diffusion process. -4 mbar and / or the partial pressure of each species, especially pH 2 The partial pressure is up to 10 -4 Preferably, the concentration is controlled by including only inert gas species at NH 3 , and any oxidizing species, e.g., CO 2 , O 2 and N 2 Lower partial pressure of O, e.g. up to 10 -5 mbar or up to 10 -6 mbar is particularly preferred. In general, inert gases such as argon and N 2 is contaminant, especially O 2 in sufficient quantities so that the Group IV metal retains its metallic luster, it is preferred that only a very pure form of the inert gas be included in the diffusion step.

[0027] Although a metallic luster is not obtained when the surface layer is (Ti,Zr)N, the method of the present invention allows the workpiece to regain its metallic luster after treatment, so that the component of the present invention cannot be distinguished from the workpiece before treatment by visual inspection. Thus, if the workpiece has a mirror-polished appearance, the mirror-polished appearance is also found in the component after treatment by this method. In this specification, a "mirror-polished 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 can be measured using a Taylor-Hubson Surtronic S25 measuring over a length of 1.25 mm. A mirror-polished surface may be referred to as an N3 surface, and the two terms can be used interchangeably. In a preferred embodiment, the workpiece of group IV metal is polished before nitriding 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 step. Furthermore, where a mirror-polished appearance is important, the nitriding atmosphere must be free of carbon-containing molecules. Commercially pure (CP) titanium, e.g., grade 2 or grade 4, can be nitrided to a temperature of at least 1100 HV while retaining a mirror-polished appearance if the nitriding atmosphere has no additional carbon-containing molecules. 0.005 Thus, the method of the present invention can provide a surface hardness of at least 1100 HV. 0.005 To provide a titanium component having a mirror polished appearance.

[0028] In general, the higher the temperature of the diffusion step, i.e., the higher the second temperature, the faster the diffusion. Therefore, it is preferable that the second temperature is as high as possible while preventing undesired grain growth. A second temperature of up to 700°C is preferable, and for example, the second temperature may be 600°C to 700°C. Representative combinations of second temperature and hydrogen removal time that cover both removal and retention of the nitride layer are shown in Table 2. Here, to ensure removal of the nitride layer, the second temperature must be at least 600°C and the hydrogen removal time must be at least 4 hours.

[0029] [Table 2]

[0030] In general, the nitriding time and the hydrogen removal time can be considered in combination, and the inventors have surprisingly found that, when the nitriding time is at least 12 hours, for example at least 16 hours, in particular at a first temperature of 650°C-700°C, a sufficient amount of nitrogen dissolves in the workpiece of a group IV metal or group IV metal alloy, and the nitride layer can be removed by applying a hydrogen removal time of at least 4 hours at a second temperature of at least 600°C, to obtain a hardened component having no nitride layer, in particular no visible nitride layer, at its surface, while retaining the nitrogen diffusion zone. In particular, the method of the present invention, when treating a component of a titanium-based alloy not containing titanium or zirconium, has a surface hardness of 700HV 0.005 ~2000HV 0.005 Similarly, when processing a component made of zirconium or a group IV metal alloy containing at least 2 wt. % zirconium, the hardness at a depth of 2.5 μm from the surface is 1000 HV. 0.005 ~1500HV 0.005 It is possible to obtain a component having a hardness which requires a nitride layer, but which does not have a nitride layer.

[0031] The inventors have further surprisingly found that when treating a titanium-based alloy component that does not contain titanium or zirconium, the treated component does not contain Ti 2 It has been found that the surface nitride layer of TiN is retained but not TiN. Therefore, in this specification, the removal of nitrides from a component refers to the removal of TiN from a titanium-based alloy component that does not contain titanium or zirconium. 2 Does not include removal of N. Ti 2N is visually indistinguishable from titanium or titanium alloys, whereas TiN is gold or yellow. Similarly, the nitrides of zirconium or zirconium alloys are gold or yellow. By removing the gold or yellow nitride layer, the component regains the metallic luster before treatment and is hardened by additionally having a nitrogen diffusion zone. Thus, the present invention provides a hardened component of a group IV metal with metallic luster, e.g. a titanium-based alloy not containing titanium or zirconium, zirconium or zirconium alloy. Moreover, the component does not exhibit the gold or yellow color due to the nitrides of group IV metals.

[0032] In one example, the nitriding and diffusion steps are repeated on the same workpiece. The nitriding and diffusion steps can be repeated as many times as desired. Typically, the first repetition increases the surface hardness compared to the surface hardness of a workpiece that has been treated with only one nitriding and diffusion step.

[0033] Any group IV metal or group IV metal alloy is suitable for this method. In a particular embodiment, the group IV metal is selected from titanium, titanium alloy, zirconium and zirconium alloy. In this specification, the component may be composed of a group IV metal or a group IV metal alloy, such as a titanium alloy or a titanium-zirconium alloy, or may include other materials. For example, the component may have a part that is another material, a polymer, a glass, a ceramic or another metal, and an outer layer of a titanium alloy or zirconium. Similarly, the workpiece treated with the method of the present invention may also have a core of another material. The outer layer does not have to completely cover the outer surface of the component. The component may be manufactured, for example, by additive manufacturing or 3D printing, before being treated according to the method of the present invention.

[0034] The inventors have surprisingly found that NH 3 It has been found that the presence of zirconium affects the available case hardening when using NH 3causes atomic N and atomic H to diffuse into the group IV metal at low temperatures to form a hydrogen- and nitrogen-containing diffusion zone. The inventors have found that zirconium-containing group IV metal alloys, particularly zirconium-containing titanium-based alloys, allow for at least 5 times, e.g., about 10 times, greater nitrogen uptake than non-zirconium-containing group IV metal alloys, e.g., grade 2, grade 4, or grade 5 titanium, particularly when zirconium is present in a content of at least 2 wt. % (e.g., at least 3 wt. % or at least 5 wt. %), and have found that N 2 The inventors have further observed that such high nitrogen uptake is not available when nitriding a zirconium-containing titanium-based alloy at, for example, at least 800° C. Accordingly, it is possible to nitride a titanium-based alloy containing zirconium or at least 2 wt. % (e.g., at least 3 wt. % or at least 5 wt. %) zirconium at a first temperature between 450° C. and 750° C. using NH 3 Nitriding using N as the nitriding species at least at 800 °C 2 This results in nitrogen uptake that is at least five times greater than when the alloy is nitrided using a hydrogen-containing nitriding agent. The present invention provides a solution to the problem of how to increase the nitrogen uptake of zirconium or titanium-based alloys containing at least 2 wt.% zirconium. In addition to increasing the content of dissolved nitrogen in the hydrogen-containing diffusion zone, nitrides of the zirconium-containing titanium-based alloy are present at the surface of the titanium-based alloy after the nitriding step, and certain nitrides may be dissolved in the diffusion step or may be retained in a modified form.

[0035] In particular examples, the group IV metal alloy is zirconium or a group IV metal alloy containing at least 2 wt% (e.g., at least 3 wt% or at least 5 wt%) zirconium, e.g., with other metals, e.g., a titanium-based alloy containing 10 wt% to 20 wt% zirconium. For example, the group IV metal may be a titanium-based alloy containing at least 5 wt% zirconium, or a zirconium-based alloy, e.g., pure zirconium. Exemplary zirconium-containing group IV metals and group IV metal alloys are Zr702 zirconium, titanium / niobium alloys such as Ti13Nb13Zr and Ti15Zr (alpha alloy). If the group IV metal alloy, e.g. titanium-based alloy, contains at least 2 wt% zirconium (e.g. at least 3 wt% or at least 5 wt%), the nitriding step forms a much higher content of nitrogen in the diffusion zone and also forms a nitride layer of zirconium nitride (ZrN) and titanium nitride (TiN), e.g. (Ti,Zr)N, if the alloy is a titanium-based alloy or contains titanium on the surface of a zirconium-containing group IV metal alloy. This nitride layer can be retained or removed by the diffusion step. In particular, if the zirconium content is between 10 wt% and 20 wt%, the nitride layer will have a resistance of at least 1000 HV to a depth of 2.5 μm. 0.05 A hardness of 0.01 mm is obtained.

[0036] The nitride layer formed on the surface of the zirconium-containing Group IV metal during the nitriding step can be retained or removed during the diffusion step, but if the component is titanium or a titanium-based alloy that does not contain zirconium, the component will not lose any Ti regardless of the second temperature. 2The surface nitride layer of N is preserved, but TiN is not. In general, the nitride layer can be preserved by carrying out the diffusion step at a lower second temperature. For example, the nitride layer can be preserved by carrying out the diffusion step at a second temperature significantly lower than the first temperature, for example the first temperature being 650°C-700°C and the second temperature being 400°C-600°C. Thus, if the nitride layer is to be removed, the second temperature must be at least 600°C, and if the nitride layer is to be preserved, the second temperature must be at most 600°C. If the nitride layer is to be removed, the hydrogen removal time must be sufficient to ensure the removal of the nitride layer, and if the nitride layer is to be preserved, the hydrogen removal time must be limited to prevent the removal of the nitride layer. In general, it is possible to stop the diffusion step, for example by lowering the temperature, inspect the progress of the removal or retention of the nitride layer, and then resume the diffusion step.

[0037] It is possible to remove the nitride layer at less than 600°C, but this typically involves performing the hydrogen removal step for an extended period of time and monitoring the status of nitride layer removal. Thus, in a particular example, the step of removing hydrogen from the hydrogen-containing diffusion zone includes selecting a second temperature between 400°C and 600°C, and, at the selected second temperature, removing Ti if the Group IV metal is titanium or a titanium alloy that does not contain zirconium. 2 N, for a hydrogen removal time sufficient to ensure removal of the nitride layer, up to 10 -4 mbar H 2 Partial pressure of (pH 2 ) removing hydrogen from the hydrogen-containing diffusion zone. Removal of the nitride layer may be confirmed by any desired method, such as X-ray diffraction (XRD) analysis. Typically, the nitride layer can be removed in a hydrogen removal time of at least 48 hours when the second temperature is between 500° C. and 550° C., or in a hydrogen removal time of at least 24 hours when the second temperature is between 550° C. and 600° C. The step of removing hydrogen from the hydrogen-containing diffusion zone may also include monitoring for the presence of the nitride layer, for example, by XRD analysis.

[0038] Particularly preferred combinations of second temperature and hydrogen removal time are 600°C to 650°C and at least 4 hours (e.g., up to 200 hours); 650°C to 700°C and at least 4 hours (e.g., up to 100 hours); and 700°C to 750°C and at least 2 hours (e.g., up to 50 hours).

[0039] The most preferred combination of conditions for the nitriding step and the hydrogen removal step is: a first temperature and nitriding time of 580°C to 700°C and at least 12 hours (e.g., at least 16 hours or at least 20 hours), up to 100 hours, in the nitriding step; and a second temperature and hydrogen removal time of 650°C to 700°C and at least 4 hours (e.g., up to 100 hours). When the first temperature is 580°C to 700°C and the nitriding time is 16 hours to 100 hours, a sufficient amount of nitrogen is dissolved in the Group IV metal, and when the second temperature is 650°C to 700°C and the hydrogen removal time is 4 hours to 100 hours, a sufficient amount of nitrogen is dissolved in the Group IV metal while retaining the hardening obtained from nitrogen in solid solution in the nitrogen diffusion zone, and when the Group IV metal is titanium or a titanium alloy not containing zirconium, Ti is removed. 2 The nitride layer is removed, except for N.

[0040] In another aspect, the present invention relates to a component obtainable by the method of the present invention. Thus, in another aspect, the present invention relates to a component of zirconium or a group IV metal alloy containing at least 2 wt. % (e.g. at least 3 wt. % or at least 5 wt. %) zirconium, having a core hardness and a microhardness from the surface of the component of at least 50 HV above the core hardness. 0.005 and a nitrogen-containing diffusion zone extending from the surface to a depth equal to the sum of the hardness of the nitrogen-containing diffusion zone and the hardness of the nitrogen-containing diffusion zone extending from the surface to a depth of 2.5 μm, the hardness being 1000 HV. 0.005 ~1500HV 0.005 This component does not include a nitride layer, but it is conceivable that the nitride layer can be retained, for example if the removal of hydrogen is carried out at temperatures up to 600°C.

[0041] It is also possible to obtain a component further comprising a nitride layer at its surface. The nitride layer comprises ZrN and, if titanium is present in the alloy, also TiN. In particular, since ZrN and TiN are intermixable and isomorphous, the nitride layer can be described as (Ti,Zr)N. If the nitride layer is retained on the component, the surface hardness of the component is generally higher than if the nitride layer is removed, but the diffusion zone near the surface has the highest nitrogen content and a hardness of at least 1000 HV. 0.05 Generally, the surface hardness of a component with a (Ti,Zr)N layer is 1000HV 0.005 ~2000HV 0.005 It is.

[0042] In these examples of components, the diffusion zone does not contain interstitial or dissolved oxygen beyond the naturally unavoidable amount of oxygen, and the hardness of the diffusion zone is due to the dissolved nitrogen content. In particular, the component may be substantially free of interstitial or dissolved oxygen. Without being bound by theory, the inventors believe that there is a direct correlation between the content of nitrogen in the diffusion zone and the hardness of the diffusion zone, with the hardness increasing across the diffusion zone from the core of the zirconium or group IV metal alloy containing at least 2 wt. % (e.g. at least 3 wt. % or at least 5 wt. %) zirconium to the surface of the component. More preferably, the component does not contain an oxide layer, except for the nanometer-scale oxide layer that naturally forms on the surface of the group IV metal in contact with air, and is therefore substantially free of an oxide layer. However, in other examples, the diffusion zone of the component also contains interstitial oxygen. The interstitial oxygen may be provided by treating the workpiece to dissolve oxygen in the zirconium or group IV metal alloy containing at least 2 wt. % zirconium prior to treatment in the nitriding step.

[0043] The components are preferably derived from a titanium-based alloy containing 10 wt% to 20 wt% zirconium, although the titanium-based alloy may also have further elements. Representative Group IV metals and alloys include Zr702 zirconium, Ti13Nb13Zr and Ti15Zr.

[0044] In another example, the group IV metal alloy is a zirconium-free titanium-based alloy or pure titanium. Representative group IV metals and group IV metal alloys are commercially pure (CP) titanium, such as grade 2 or grade 4, grade 5 titanium also known as Ti6Al4V, or Ti6Al4V ELI also known as grade 23. When the zirconium-free titanium-based alloy or pure titanium is treated with a nitriding process, the nitriding process imparts a gold color to the alloy or titanium, which is representative of TiN, and when treated with a diffusion process, the gold color is removed, and the treated workpiece of the zirconium-free titanium-based alloy or pure titanium returns to the original appearance, including the metallic luster, of the workpiece before the nitriding process. The inventors have surprisingly found that the gold-colored nitride layer is a mixture of TiN and Ti 2 The diffusion process contains both Ti and N. 2 It was found that TiN can be removed without removing N, and as a result, this method can be used to remove high hardness Ti 2 5 and 6, the surface hardness of the component is 700HV 0.005 ~2000HV 0.005 , for example 1000HV 0.005 ~1800HV 0.005 However, it is not possible to visually distinguish the component from a non-zirconium-containing titanium-based alloy or pure titanium before the nitriding process. The microhardness at a depth of 2.5 μm from the surface of the component is typically 700 HV 0.005 ~1200HV 0.005 In particular, the inventors found that Ti was deposited on the surface of the workpiece by XRD analysis of the workpiece after treatment. 2 A typical XRD plot of the workpiece after treatment is shown in Figure 6. The XRD plot can be compared with the XRD plot obtained after nitriding without the diffusion step, for example in Figure 5. A comparison of Figures 6 and 5 shows that after both steps, the Ti 2 It can be seen that N is present, but after the diffusion step, the XRD plot no longer shows the TiN peak. 2 The N-layer is sufficiently hard so that the workpiece is scratch resistant, e.g., its surface hardness is at least 1000 HV.0.005 Thus, the present invention provides a scratch-resistant workpiece of titanium or a titanium alloy having metallic luster.

[0045] In another aspect, the present invention provides a titanium-based alloy component that does not contain titanium or zirconium, the component having a core hardness and a microhardness of 50 HV or less from the surface of the component. 0.005 a nitrogen-containing diffusion zone extending from the surface to a depth equal to the sum of Ti 2 N layer, especially Ti 2 The component has a surface with a surface nitride layer of N. The surface hardness of the component is 700HV 0.005 ~2000HV 0.005 Ti 2 The layer of N is formed by nitridation at a first temperature between 450° C. and 750° C. for a nitridation time of at least 12 hours, for example at least 16 hours. 3 The workpiece is then nitrided at a second temperature of 600°C to 750°C and a maximum of 10 -4 mbar H 2 The surface is preferably free of TiN. In particular, Ti 2 Both N and TiN are identifiable by XRD, and in certain instances the component is TiN identifiable by XRD. 2 In another example, the component does not contain any TiN identifiable by XRD. 2 It is most preferable that it contains N and does not contain TiN.

[0046] It is further preferred that the component does not have a gold color and has a metallic luster, for example the component may have a mirror-polished appearance, defined as a surface with an arithmetic mean deviation (Ra) roughness of less than 0.1 μm according to the ISO 1302:2002 standard.

[0047] In the example of a titanium or titanium-based alloy component that does not contain zirconium, the diffusion zone does not contain interstitial or dissolved oxygen, and in particular the component may be substantially free of interstitial or dissolved oxygen, and the hardness of the diffusion zone is due to the nitrogen content. Without being bound by theory, the inventors believe that there is a direct correlation between the content of nitrogen in the diffusion zone and the hardness of the diffusion zone, such that the hardness increases across the diffusion zone from the core of the titanium or titanium-based alloy that does not contain zirconium to the surface of the component. However, in another example, the diffusion zone of the component also contains interstitial oxygen. The interstitial oxygen may be provided by treating the workpiece to dissolve oxygen in the titanium or titanium-based alloy that does not contain zirconium, either before the nitriding process or after the diffusion process.

[0048] The components are made of Ti which gives the surface a scratch resistance. 2 N surface layer. Generally, the surface hardness is at least 1000HV 0.025 A component is considered scratch resistant if its surface hardness is, for example, 700HV 0.005 ~2000HV 0.005 , for example 1000HV 0.005 ~1800HV 0.005 The surface hardness of the component may be 1000HV 0.005 ~1800HV 0.005 It is particularly preferable that the surface is mirror-polished in accordance with the ISO1302:2002 standard.

[0049] Any embodiment of the invention may be used with any aspect of the invention, and any advantage of a particular embodiment applies equally when an embodiment is used with a particular aspect.

[0050] In the following, the invention will be explained in more detail with the aid of examples and with reference to schematic drawings. [Brief description of the drawings]

[0051] [Figure 1]FIG. 1 shows the pH2 as a function of the NH3:N2 ratio at different total pressures. [Diagram 2] FIG. 2 shows the hardness profile of Ti15Zr hardened using NH3. [Diagram 3] FIG. 3 shows the hardness profile of Ti15Zr hardened according to the method of the present invention. [Figure 4] FIG. 4 shows the hardness profile of Ti15Zr hardened according to the method of the present invention. [Diagram 5] FIG. 5 shows the XRD analysis of Ti6Al4V hardened using NH3. [Figure 6] FIG. 6 shows an XRD analysis of Ti6Al4V hardened according to the method of the present invention. [Figure 7] FIG. 7 shows the hardness profile of titanium hardened according to the method of the present invention. [Figure 8] FIG. 8 shows a microscope image of a cross section of titanium hardened using NH3. [Figure 9] FIG. 9 shows a microscope image of a cross section of titanium hardened according to the method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] Detailed Description The present invention is not limited to the embodiments shown in the drawings. Thus, when features recited in the claims are followed by reference signs, it will be understood that such signs are included solely for the purpose of improving the clarity of the claims and do not limit the scope of the claims in any way.

[0053] The present invention relates to a method for case hardening components of group IV metals or group IV metal alloys, zirconium or group IV metal alloys containing at least 2 wt. % zirconium, and titanium or titanium-based alloys not containing zirconium, which components can be obtained by the method according to the invention.

[0054] As used herein, a "group IV metal" is any metal selected from the titanium group of the periodic table of elements, or an alloy containing at least 50% of a metal from the titanium group. Thus, a "titanium alloy" is an alloy containing at least 50% (a / a) titanium, and similarly, a "zirconium alloy" is an alloy containing at least 50% (a / a) zirconium. For the purposes of the methods and components of the present invention, alloys in which the sum of titanium and zirconium is at least 50% (a / a) are considered suitable. Similarly, alloys may also include hafnium, a member of group IV of the periodic table of elements, as any alloy in which the sum of titanium, zirconium and hafnium is at least 50% (a / a) is suitable for the present invention.

[0055] The alloys of the present invention may contain other suitable elements, and as used herein, "alloying element" may refer to the metal components or elements in the alloy, or any constituent in the alloy. Titanium and zirconium alloys are well known to those skilled in the art. Alloys of Group IV metals may contain metals from other groups in the periodic table of elements, such as 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 of Ti6Al4V ELI, commonly referred to as Grade 23. Further important alloys are titanium 6Al-2Sn-4Zr-6Mo, titanium 6Al-2Sn-4Zr-2Mo, Ti-3Al-8V-6Cr-4Mo-4Zr (TB9), Ti-5Al-2.5Sn (grade 6), Ti-3Al-2.5V (grade 9), Ti-15V-3Al-3Sn-3Cr, Ti-23Nb-0.7Ta-2Zr-1.2O (gum metal), Ti-6Al-7Nb, Ti-15Zr-4Nb-4Ta, Ti-35Nb-7Zr-5Ta, Ti-29Nb-4.6Zr-13Ta, Ti-15Mo-5Zr-3Al, Ti-15Mo.

[0056] Any grade of titanium containing at least about 99% (w / w) titanium is considered to be "pure titanium" herein, e.g., grade 1 titanium, grade 2 or grade 4 titanium; thus, pure titanium may contain up to about 1% (w / w) of trace elements such as oxygen, carbon, nitrogen or other metals (e.g., iron). Pure titanium may also be referred to as "commercially pure" (CP). In particular, nitrogen and carbon contained in the group IV metals herein may be unavoidable impurities. Elements present as "unavoidable impurities" are considered not to affect the workpieces treated according to the methods of the invention or the components of the invention. Similarly, a grade of zirconium containing at least about 99% (w / w) zirconium is considered to be "pure zirconium" herein.

[0057] When percentages are used for metals or alloys, unless otherwise specified, the percentages are by weight relative to the weight of the material, e.g., % (w / w). When percentages are used for atmospheres, unless otherwise specified, the percentages are by volume, e.g., % (v / v). Similarly, unless otherwise specified, the composition of a mixture of gases may be on an atomic basis, in which case it may be given as a percentage or in parts per million (ppm).

[0058] In this specification, hardness is generally measured according to the DIN EN ISO 6507 standard, HV 0.005 or HV 0.025 Therefore, unless otherwise specified, the unit "HV" refers to this standard. Hardness may be measured, for example, on a cross-section of the treated Group IV metal and described with respect to the depth of the measurement. Hardness measurements on a cross-section may also be referred to as "microhardness" and hardness measurements on a surface may also be referred to as "macrohardness". When hardness is measured on a surface, the hardness measurements may also be referred to as top-down measurements.

[0059] Generally, microhardness measurements are performed at 5g, or HV 0.005 , 25g, i.e. HV 0.025, or 50 g, i.e. HV 0.05 In contrast, macro-hardness measurements may be performed at much higher loads, e.g. HV 0.5 The measurements may be carried out from the surface with a load of 0.50 kg, which corresponds to a load of 5 g, i.e. HV, and the measurements therefore represent an overall value of the hardness of the respective material and any surface layers it contains. In this specification, for example, microhardness measurements taken on a cross-section of a component manufactured according to the method of the invention are carried out with a load of 5 g, i.e. HV 0.005 The surface hardness was measured at a load of 25g, i.e. HV 0.025 is obtained as a top-down measurement using

[0060] When hardness is recorded on a cross section, the measurement is considered to represent a homogeneous sample with respect to the direction of pressure application. In contrast, when hardness is obtained from measurements on the surface, the measurement may represent the average of several different hardness values, i.e., hardness values ​​at different depths. Thus, when surface hardness is measured at a high load, e.g. 0.50 kg, the value can be considered to represent the "average" value both at the surface and at the depth below the surface. Surface hardness is preferably measured at a load of 25 g or 50 g. When measuring surface hardness at a load of 25 g, a hardness of 650 HV is considered to be the average of 650 HV. 0.025 It is believed that a value of 0.01 indicates that the material is scratch resistant. The effect of nitrogen dissolving from the surface is that the dissolved nitrogen content decreases from the surface towards the core of the Group IV metal, and similarly the hardness is maximum at the surface and decreases with depth. EXAMPLES

[0061] Example 1 Two samples of Ti15Zr alloy were prepared and analyzed by NH 3 The samples were heated to 690 °C at 20 °C / min and nitrided in NH at ambient pressure for 20 h. 3 The two samples were cooled to ambient temperature, one sample was not further treated, the other sample was then placed in vacuum, i.e. at a total pressure of 100 MPa, obtained using an Edwards 85T-station turbo vacuum pump. -4 It was treated at 690° C. for 4 hours at less than mbar.

[0062] For both samples, the nitridation step gave a gold-colored surface, but the gold color disappeared during the subsequent diffusion step: the nitride layer was thus removed and the nitrogen present in the nitride layer dissolved in the diffusion zone.

[0063] Hardness of the treated sample (HV 0.005 ) were analyzed, and the hardness profiles are shown in Figures 2 and 3. In Figures 2 and 3, the error bars represent one standard deviation from the mean. In both cases, the treatment resulted in a hardness of 1100 HV at 2.5 μm from the surface. 0.005 In the sample treated according to the present invention, the hardness exceeded 400 HV for the first time at a depth of about 50 μm (FIGS. 2 and 3). 0.005 The core hardness reached less than 400 HV (Figure 2). 0.005 The smaller diameter was already reached at 30 μm (Figure 3).

[0064] This hardness corresponds to the incorporation of more than 10 times as much nitrogen into the zirconium-containing titanium alloy as compared to conventional titanium alloys that do not contain zirconium.

[0065] Furthermore, the nitriding process resulted in the formation of needle-like hydrides within the grains and at the grain boundaries of the specimens. After the diffusion process, the formation of new α-grains could be seen in the bulk as a result of the removal of hydrogen during the diffusion process and the transformation of the needle-like hydrides. Thus, the method of the present invention refines the grain of the treated metal.

[0066] Example 2 A sample of Ti13Zr13Nb was prepared and analyzed by NH using a Netzsch449 thermal analyzer. 3 The samples were heated to 690 °C at 20 °C / min and nitrided in NH at ambient pressure for 20 h. 3 After exposure to a total pressure of 10 -4 The specimen was treated at 690 °C for 4 h at less than 1000 mbar. After the nitriding treatment, the specimen had a gold color, which disappeared during the subsequent diffusion process. Thus, the nitride layer was removed. The hardness at 2.5 μm from the surface was 1100 HV. 0.005and 500HV at a depth of about 35μm from the surface. 0.005 The hardness profile revealed that a core hardness of less than 0.05 mm was achieved.

[0067] Example 3 A sample of Ti15Zr was prepared and analyzed by NH using a Netzsch449 thermal analyzer. 3 The samples were heated to 690 °C at 20 °C / min and nitrided in NH at ambient pressure for 20 h. 3 After exposure to 1000 s, the sample was placed in vacuum, i.e., at a total pressure of 10 -4 The specimen was treated at 550°C for 24 hours at <1000 mbar. After nitriding, the specimen had a golden color, which was retained during the subsequent diffusion step. The nitride layer was therefore retained. The hardness profile is shown in Figure 4. The surface hardness was 1200 HV 0.025 The hardness of the surface is over 1100HV at 2.5μm. 0.005 In Figure 4, the error bars represent one standard deviation from the mean. The hardness profile shows that the surface hardness of the treated samples is very high.

[0068] Example 4 A sample of Ti13Zr13Nb was prepared and analyzed by NH using a Netzsch449 thermal analyzer. 3 The samples were heated to 690 °C at 20 °C / min and nitrided in NH at ambient pressure for 20 h. 3 After exposure to 1000 s, the sample was placed in vacuum, i.e., at a total pressure of 10 -4 The treated specimens were treated at 550°C for 24 hours under <1000 mbar, thus preserving the nitride layer during the diffusion process. Similarly, the treated specimens were gold-colored during the nitriding process, and this gold color was visible after the diffusion process. The treated specimens had a surface hardness of 1000HV. 0.025 The hardness at 2.5μm from the surface is 900HV 0.005 Exceeded 400HV at approximately 40μm from the surface. 0.005 A core hardness of less than 100 mm was achieved.

[0069] Example 5 Two samples of Ti6Al4V (α-β alloy) were subjected to NH3 The treatment increased the surface hardness to 2000HV. 0.005 The nitride layer was gold in color and was subjected to X-ray diffraction (XRD) analysis. The XRD plot is shown in Figure 5, which indicates that Ti 2 The presence of N and TiN is confirmed.

[0070] One of the nitrided samples was subjected to a diffusion process. Specifically, the sample was placed in vacuum, i.e., at a total pressure of 100,000 psi, obtained by an Edwards 85T-station turbo vacuum pump. -4 The nitride layer lost its gold color during the diffusion process. XRD analysis was performed again and the results are shown in Figure 6. Comparing Figure 6 with Figure 5, it is clear that TiN has disappeared, but Ti 2 It is demonstrated that N was still detected.

[0071] The samples treated according to the present invention were compared to samples that were not exposed to a diffusion process (i.e., hardness of 2000 HV 0.005 1300HV (compared to over 1300HV) 0.005 The hardness above 0.05 mm and the surface hardness were reduced, but this reduction in hardness was sufficient to provide scratch resistance. Furthermore, the gold color was removed from the surface, resulting in a much more attractive metallic appearance identical to the initial state. The hydrogen was removed during the diffusion process while the nitrogen was redistributed, thereby significantly reducing the risk of hydrogen embrittlement.

[0072] Example 6 Two samples of commercially pure (CP) titanium were prepared and analyzed by a Netzsch 449 thermal analyzer using NH 3 The samples were heated to 690 °C at 20 °C / min and nitrided in NH at ambient pressure for 20 h. 3 After exposure to the ion beam, one sample was placed in vacuum, i.e., at a total pressure of 10 -4 One sample was treated at 690° C. for 4 hours at less than 1000 mbar, the other sample was not further treated.

[0073] The nitriding step gave a golden surface which disappeared during the subsequent diffusion treatment, demonstrating the absence of TiN after treatment with the two-step method of the present invention.

[0074] The hardness profile of the sample treated by the method of the present invention is shown in Figure 7. The surface hardness is 1200HV 0.005 The hardness at 2.5μm from the surface is 800HV 0.005 It is clear that the sample has been given scratch resistance, as the scratch resistance exceeds 300HV at approximately 20μm from the surface. 0.005 In FIG. 7, error bars represent one standard deviation from the mean.

[0075] Two samples were cut to expose cross sections which were analyzed microscopically and are shown in Figures 8 and 9. Figure 8 clearly shows that the needle-like hydrides were visible after treatment in the nitriding process, and Figure 9 clearly shows that the needle-like hydrides disappeared upon diffusion treatment, thereby proving that hydrogen was removed during the diffusion process.

Claims

1. A method for case hardening a Group IV metal or a Group IV metal alloy, comprising: - preparing a workpiece of a Group IV metal or a Group IV metal alloy, wherein the workpiece is in its final shape; - In a nitriding atmosphere containing NH as the nitride species ３ at a first temperature of 450°C to 750°C and a partial pressure of NH of at least 1 mbar ３ nitriding the workpiece over a nitriding time of at least 0.1 hour to form a hydrogen-containing diffusion zone, and - A second temperature of 600 °C to 750 °C and a maximum of 10 －４ mbar of H ２ partial pressure (pH ２ ) to remove hydrogen from the hydrogen-containing diffusion zone over a hydrogen removal time of at least 4 hours to form a hydrogen-deficient diffusion zone. a method comprising the above.

2. The method for case hardening a Group IV metal or a Group IV metal alloy according to Claim 1, wherein the nitriding atmosphere does not contain oxidizing species.

3. The method for case hardening a Group IV metal or a Group IV metal alloy according to Claim 1 or 2, wherein the first temperature is 580 °C to 700 °C.

4. The total pressure in the step of removing hydrogen from the hydrogen-containing diffusion zone is at most 10 －４ mbar, a method for skin hardening a Group IV metal or a Group IV metal alloy according to any one of claims 1 to 3.

5. The method for case hardening a Group IV metal or a Group IV metal alloy according to any one of Claims 1 to 4, wherein the Group IV metal is zirconium or a Group IV metal alloy containing at least 2 wt% zirconium.

6. The method for case hardening a Group IV metal or a Group IV metal alloy according to any one of Claims 1 to 5, wherein the second temperature is 600 °C to 700 °C.

7. The method for case hardening a Group IV metal or a Group IV metal alloy according to any one of Claims 1 to 6, wherein the Group IV metal is titanium or a titanium-based alloy not containing zirconium.

8. A component of a group IV metal alloy containing zirconium or at least 2 wt% zirconium, wherein the component has a nitrogen-containing diffusion zone extending to a depth from the surface of the component where the microhardness is equal to the core hardness of the component plus 50 HV ０．００５ and the hardness at a depth of 2.5 μm from the surface of the component is 1000 HV ０．００５ to 1500 HV ０．００５ wherein the hardness is measured according to the DIN EN ISO 6507 standard and the component does not contain a nitride layer.

9. A component of a titanium-based alloy that does not contain titanium or zirconium, wherein the surface hardness of the component is 700 HV ０．００５ to 2000 HV ０．００５ and the component has a nitrogen-containing diffusion zone extending to a depth from the surface of the component where the microhardness is equal to the core hardness of the component plus 50 HV ０．００５ Here, the hardness is measured according to the DIN EN ISO 6507 standard, and the component has a surface nitride layer of Ti ２ N. Component

10. The component of a titanium-based alloy not containing titanium or zirconium according to Claim 9, wherein the surface does not contain TiN.

11. Said Ti ２ The component of a titanium-based alloy containing no titanium or zirconium according to claim 9 or 10, wherein the layer of N is identified by X-ray diffraction analysis.

12. The microhardness at a depth of 2.5 μm from the surface of the component is 700 HV ０．００５ to 1200 HV ０．００５ The component of a titanium-based alloy containing no titanium or zirconium according to any one of claims 9 to 11, which is such.

13. The component of a titanium-based alloy not containing titanium or zirconium according to Claim 9, wherein the absence of TiN is identified by X-ray diffraction analysis.

14. The component of a titanium-based alloy not containing titanium or zirconium according to Claim 9, wherein the surface of the component does not contain an oxide layer other than the nanoscale oxide layer naturally formed on the surface of the Group IV metal in contact with air.

15. The component of a titanium-based alloy not containing titanium or zirconium according to Claim 9, wherein the component has a mirror-polished appearance defined as a surface with an arithmetic mean deviation (Ra) roughness of less than 0.1 μm in accordance with the ISO 1302:2002 standard.

16. The component of a Group IV metal alloy containing zirconium or at least 2 wt% zirconium according to Claim 8, wherein the absence of TiN is identified by X-ray diffraction analysis. Component according to claim 8, wherein the surface of the component does not contain an oxide layer other than a nanometer-scale oxide layer naturally formed on the surface of a Group-IV metal in contact with air, said component being a zirconium or a Group-IV metal alloy containing at least 2 wt% zirconium. Component according to claim 8, wherein the component has a mirror-polished appearance defined as a surface with an arithmetic mean deviation (Ra) roughness of less than 0.1 µm in accordance with the ISO 1302:2002 standard, said component being a zirconium or a Group-IV metal alloy containing at least 2 wt% zirconium.

Images