Case hardened component of titanium

Abstract

The present invention relates to a case hardened component of a titanium alloy, the component having a diffusion zone of a thickness of at least 50 ll, as calculated from the surface of the component, the diffusion zone comprising oxygen and carbon in solid solution and having a distinct phase of a carbo-oxide compound having the composition TiO.sub.xC.sub.1-x, wherein x is a number in the range of 0.01 to 0.99, which diffusion zone has a microhardness of at least 800 HV0.025 and which carbo-oxide compound has a microhardness of at least 1200 HV0.025. In another aspect the invention relates to a method of producing the case hardened component. In a further aspect the invention relates to a method of oxidising a component of a Group IV metal.

Claims

1. A method of producing a case hardened component of a titanium alloy, the method comprising the steps of: providing a component of a titanium alloy, placing the component in a reactive atmosphere comprising a carbon providing gaseous species at a partial pressure of at least 10.sup.5 bar, the carbon providing gaseous species containing carbon and oxygen, and which reactive atmosphere does not comprise a hydrogen containing species, heating the component in an inert atmosphere or the reactive atmosphere to a dissolution temperature T.sub.D of at least 800 C., maintaining the component in the reactive atmosphere at T.sub.D for a reactive duration of at least 30 min to provide the component with a diffusion zone comprising carbon and oxygen in solid solution and having a distinct phase of a carbo-oxide compound having the composition TiO.sub.xC.sub.1-x, wherein x is a number in the range of 0.01 to 0.99, which diffusion zone has a microhardness of at least 800 HV.sub.0.025 and which carbo-oxide compound has a microhardness of at least 1200 HV.sub.0.025, the diffusion zone having a thickness of at least 10 m, cooling the component from T.sub.D to ambient temperature.

2. The method according to claim 1, wherein the carbon providing gaseous species is CO or CO and CO.sub.2 at a ratio of CO to CO.sub.2 of at least 5.

3. The method according to claim 1, wherein T.sub.D is at least 900 C. and wherein the partial pressure of the carbon providing gaseous species is at least 0.1 bar.

4. The method of producing a case hardened component according to claim 1, wherein the reactive atmosphere further comprises a nitrogen containing species.

5. The method of producing a case hardened component according to claim 1, wherein the method further comprises the steps of: placing the component in a nitriding atmosphere comprising a nitriding gaseous species at a partial pressure of at least 10.sup.5 bar, maintaining the component in the nitriding atmosphere at a nitriding temperature T.sub.N of at least 800 C. for a nitriding duration of at least 5 min to diffuse nitrogen into the component.

6. A method of oxidising a component of a Group IV metal, the method comprising the steps of: providing a component of a Group IV metal, placing the component in an oxidising atmosphere comprising an oxidising gaseous species selected from the list consisting of CO.sub.2, mixtures of CO and CO.sub.2, H.sub.2O and mixtures of H.sub.2O and H.sub.2, or mixtures thereof, wherein the oxidising gaseous species is selected to provide a partial pressure of O.sub.2 of less than 0.1 bar, heating the component in an inert atmosphere or the oxidising atmosphere to an oxidising temperature T.sub.Ox of at least 600 C., maintaining the component in the oxidising atmosphere at T.sub.Ox for a reactive duration of at least 5 min to dissolve oxygen in the component, cooling the component from T.sub.Ox to ambient temperature.

7. The method of oxidising a component of a Group IV metal according to claim 6, wherein the oxidising atmosphere does not comprise a reactive amount of a nitrogen containing species and/or wherein the oxidising atmosphere is not supplemented with O.sub.2.

8. The method of oxidising a component of a Group IV metal according to claim 6, wherein the oxidising atmosphere consists of the oxidising gaseous species, or wherein the oxidising atmosphere consists of an inert gas and the oxidising gaseous species and the total pressure of the oxidising atmosphere is in the range of 0.5 bar to 5 bar.

9. The method of oxidising a component of a Group IV metal according to claim 6, wherein the Group IV metal is selected from the list consisting of titanium, a titanium alloy, zirconium and a zirconium alloy.

10. The method of oxidising a component of a Group IV metal according to claim 6, wherein a Magnli phase is formed on the surface of the Group IV metal.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) In the following the invention will be explained in greater detail with the aid of an example and with reference to the schematic drawings, in which

(2) FIG. 1 shows a hardness profile of titanium grade 5 hardened with carbon and nitrogen in a prior art method;

(3) FIG. 2 shows a hardness profile of titanium grade 5 hardened with carbon and nitrogen in a prior art method;

(4) FIG. 3 shows cross-sections of titanium grades 2 and 5 hardened in a prior art method;

(5) FIG. 4 shows hardness profiles of titanium grades 2 and 5 hardened in a prior art method;

(6) FIG. 5 shows a cross-section of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(7) FIG. 6 shows a hardness depth profile of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(8) FIG. 7 shows a cross-section of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(9) FIG. 8 shows a cross-section of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(10) FIG. 9 shows a cross-section of titanium grade 5 hardened with carbon and oxygen in the method of the invention;

(11) FIG. 10 shows a cross-section of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(12) FIG. 11 shows cross-sections of a component of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(13) FIG. 12 illustrates tribological tests of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(14) FIG. 13 illustrates corrosion tests of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(15) FIG. 14 shows hardness profiles of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(16) FIG. 15 shows cross-sections of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(17) FIG. 16 shows hardness profiles of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(18) FIG. 17 illustrates corrosion tests of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(19) FIG. 18 shows a cross-section of titanium grade 2 hardened with carbon and oxygen in the method of the invention;

(20) FIG. 19 shows a cross-section of titanium grade 2 oxidised in the method of the invention;

(21) FIG. 20 shows hardness profiles of a titanium grade 2 oxidised in the method of the invention;

(22) FIG. 21 shows a cross-section of titanium grade 2 oxidised in the method of the invention;

(23) FIG. 22 shows a cross-section of titanium grade 2 treated in the duplex hardening method of the invention;

(24) FIG. 23 shows hardness profiles of titanium grade 2 hardened in the duplex method of the invention;

(25) FIG. 24 shows a hardness profile of a titanium grade 2 treated in the duplex hardening method of the invention;

(26) FIG. 25 shows a hardness profile of a titanium grade 2 treated in the duplex hardening method of the invention;

(27) FIG. 26 shows an X-ray diffraction analysis of a sample of titanium grade 2 hardened according to the invention;

(28) FIG. 27 shows X-ray diffraction analyses of samples of titanium grade 2 hardened according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(29) The present invention in a first aspect relates to a method of producing a case hardened component of a Group IV metal. In a second aspect the invention relates to method of oxidising a component of a Group IV metal. In a third aspect the invention relates to case hardened component of a Group IV metal.

(30) In the context of the invention Group IV metal is any metal selected from the titanium group of the periodic table of the elements or an alloy comprising at least 50% of metals from the titanium group. A titanium alloy is any alloy containing at least 50% (a/a) titanium, and likewise a zirconium alloy is any alloy containing at least 50% (a/a) zirconium. It is contemplated that for the method of the invention and for the component of the invention any alloy containing a sum of titanium and zirconium of at least 50% (a/a) is appropriate; this alloy is also considered a titanium alloy in the context of the invention, in particular if the alloy contains more titanium than zirconium. Likewise, the alloy may also comprise hafnium, which is a member of Group IV of the periodic table of the elements so that any alloy having a sum of titanium, zirconium, and hafnium of at least 50% (a/a) is appropriate for the invention.

(31) When a percentage is stated for a metal or an alloy the percentage is by weight of the weight of material, e.g. denoted % (w/w), unless otherwise noted. When a percentage is stated for an atmosphere the percentage is by volume, e.g. denoted % (v/v), unless otherwise noted.

(32) Any grade of titanium containing at least about 99% (w/w) titanium is, in the context of the invention, considered to be pure titanium, e.g. Grade 1 titanium or Grade 2 titanium; thus, the pure titanium may contain up to about 1% (w/w) trace elements, e.g. oxygen, carbon, nitrogen or other metals, such as iron. In another embodiment the titanium alloy is the titanium alloy referred to as Ti-6Al-4V, which contains about 6% (w/w) aluminium, about 4% (w/w) vanadium, trace elements and titanium to balance. The alloy Ti-6Al-4V may also be referred to as Grade 5 titanium.

(33) The alloys of relevance may contain any other appropriate element, and in the context of the invention an alloying element may refer to a metallic component or element in the alloy, or any constituent in the alloy. Titanium and zirconium alloys are well-known to the skilled person.

(34) The component of the invention may be described by hardness measurements. In the context of the invention the hardness is generally measured according to the DIN EN ISO 6507 standard. If not otherwise mentioned the unit HV thus refers to this standard. The hardness may be measured at the surface of the component or in a cross-section of the component. The hardness measurement in the cross-section may also be referred to as microhardness, and the hardness measurement at the surface may also be referred to as macrohardness. The microhardness measurement is generally independent of the testing conditions, since the measurement is performed at microscale in the cross-section. Microhardness measurements are typically performed at a load of 25 g, i.e. HV.sub.0.025, or 50 g, i.e. HV.sub.0.05. In contrast, the macrohardness is performed from the surface with a much higher load, e.g. 0.50 kg, corresponding to H.sub.v0.5, so that the measurement represents an overall value of the hardness of the respective material and whatever surface layers it contains. Unless noted otherwise the surface hardness is a macrohardness obtained with a load of 0.5 kg. Microhardness measurements at loads of 25 g or 50 g typically provide the same value, HV, but measurement at 25 g is preferred since the measurement requires less space in the cross-section. The diffusion zone obtained according to the invention has a depth of least 50 m, and in a specific embodiment the hardness of the diffusion zone in a cross-section of the component is at least 800 HV.

(35) In a certain aspect the present invention relates to a component hardened in the method of the invention. In the context of the invention a component can be any workpiece, which has been treated in the method of the invention, and the component can be an individual object, or the component can be a distinct part or element of a whole.

(36) The component of the present invention may inter alia be determined in terms of its thickness, and in an embodiment the component has a thickness of up to 50 mm, e.g. in the range of 0.4 mm to 50 mm. In the context of the invention the term thickness is generally understood as the smallest dimension of the three dimensions so that as long as an object has a dimension in the range of from 0.4 mm to 50 mm it can be said to have a thickness in the range of from 0.4 mm to 50 mm. The diffusion zone obtained in the method of the invention is especially advantageous for components with a thickness in the range of 0.4 mm to 50 mm, since the thickness diffusion zone may constitute up to about 1% or more of the thickness of the component.

(37) The invention will now be described in the following non-limiting examples.

EXAMPLES

Comparative Example 1Carbonitriding

(38) A cylindrical (10 mm) grade 5 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with nitrogen gas twice and a continuous gas flow consisting of 10 ml/min N.sub.2+100 ml/min NH.sub.3 and 10 ml/min C.sub.3H.sub.6 was applied. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 1 hour. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbonitriding of the titanium surface yielding a brownish metallic luster. The total case depth, i.e. including the diffusion zone and the compounds formed with the titanium was 8 m. The hardness profile obtained in the experiment is shown in FIG. 1. Thus, when the titanium sample was treated with a carbon providing gaseous species containing hydrogen but without oxygen a sufficient hardness could not be obtained, and moreover the thickness of the diffusion zone was low.

Comparative Example 2Carbonitriding

(39) A cylindrical (10 mm) grade 5 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with nitrogen gas twice and a continuous gas flow consisting of 10 ml/min N.sub.2+100 ml/min NH.sub.3 and 10 ml/min C.sub.3H.sub.6 was applied. The sample was heated to 850 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 16 hours. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbonitriding of the titanium surface yielding a goldish metallic luster. The hardness profile obtained in the experiment is shown in FIG. 2. Despite formation of compounds, e.g. nitrocarbides, in the surface the obtained hardness was low.

Comparative Example 3Hardening According to WO 97/14820

(40) Experiments were set up to repeat the procedure of WO 97/14820. Specifically, specimens of grade 2 and grade 5 titanium were treated in a gas composition of 40% H.sub.2+40% N.sub.2+20% CO at a temperature of 899 C. The total pressure was ambient and the treatment time was 2 hours. Cross-sections of the treated material are shown in FIG. 3 and hardness profiles are shown in FIG. 4. In comparison with Comparative Examples 1 and 2, the treatment gas contained both carbon and oxygen, i.e. CO as a carbon providing species, and the partial pressure of the carbon providing species was within the range relevant to the present invention. However, the gas atmosphere also contained hydrogen, which is believed to cause the insufficient hardening.

(41) Thus, treatment of grade 2 titanium provided (FIG. 3a) a diffusion zone and a top layer of relatively soft and brittle (ceramic) rutile (TiO.sub.2). The surface zone was generally brittle and without being bound by theory the present inventors believe that the hydrogen in the treatment gas has resulted in the embrittlement. There was no formation of compounds in the diffusion zone, nor of a compound layer on the diffusion zone. The treatment did result in a hardening of the grade 2 titanium as seen in FIG. 4a, but the hardening was only superficial, e.g. at a depth of 50 m the microhardness was only slightly higher than the core hardness of the alloy.

(42) For grade 5 titanium the treatment resulted in a thin diffusion zone (FIG. 3b) of a relatively low hardness (FIG. 4b). In particular, there was no formation of compounds in the diffusion zone, nor of a compound layer on the diffusion zone and the same observations made for grade 2 titanium are relevant for grade 5 titanium.

Example 1Carbo-Oxidation of Titanium Grade 2

(43) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 50 ml/min Ar and 10 ml/min CO (17 vol. % CO) was applied. The sample was heated to 925 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 68 hours. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbo-oxidation of the titanium. A mixed interstitial compound TiO.sub.xC.sub.1-x has formed in the surface on top of a zone of mixed interstitial solid solution based on carbon and oxygen (diffusion zone).

(44) FIG. 5 shows, in FIG. 5a and FIG. 5b, respectively, reflected light optical microscopy and stereomicroscopy of the cross-section of the treated component. The hardened case consists of a surface zone of mixed interstitial compound TiO.sub.xC.sub.1-x and a mixed interstitial solid solution (diffusion zone) containing both C and O.

(45) The hardness depth profile of the mixed interstitial solid solution/diffusion zone is given in FIG. 6. The maximum hardness in the diffusion zone is 800 HV. The mixed interstitial compound TiO.sub.xC.sub.1-x, has an average hardness of 1530 HV. The hardened case depth is 300 m. The horizontal dotted lines illustrate the core hardness of the titanium metal.

Example 2Carbo-Oxidation of Titanium Grade 2

(46) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 50 ml/min Ar and 10 ml/min CO (17% CO) was applied. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 20 hours. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbo-oxidation of the titanium as seen in FIG. 7, which shows reflected light optical microscopy of cross-sections. A mixed interstitial compound TiO.sub.xC.sub.1-x and mixed interstitial solid solution based on carbon and oxygen (diffusion zone) have formed. The maximum hardness in the diffusion zone is 1148 HV0.025. The mixed interstitial compound TiO.sub.xC.sub.1-x, has an average hardness of 1819 HV0.025. The hardened case depth is approximately 300 m.

Example 3Carbo-Oxidation Titanium Grade 2

(47) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 20 ml/min Ar and 30 ml/min CO (60 vol. % CO) was applied. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 20 hours. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbo-oxidation of the titanium as seen in FIG. 8, which shows reflected light optical microscopy of cross-sections. A mixed interstitial compound TiO.sub.xC.sub.1-x and a mixed interstitial solid solution based on carbon and oxygen (diffusion zone) have formed. The case depth is approximately 400 m. The core has transformed into a Widmansttten structure, which demonstrates that a simultaneous core hardening and surface hardening took place.

Example 4Carbo-Oxidation Titanium Grade 5

(48) A cylindrical (10 mm) grade 5 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 20 ml/min Ar and 30 ml/min CO (60% CO) was applied. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 20 hours. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbo-oxidation of the titanium as seen in FIG. 9, which shows reflected light optical microscopy of cross-sections. A mixed interstitial compound TiO.sub.xC.sub.1-x and a mixed interstitial solid solution based on carbon and oxygen (diffusion zone) have formed. The hardness of the TiO.sub.xC.sub.1-x is 1416 HV0.025. The case depth is approximately 80 m. The core has transformed into an / structure, i.e. simultaneous core and surface hardening took place.

Example 5Carbo-Oxidation

(49) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 50 ml/min Ar and 10 ml/min CO (17% CO) was applied. The sample was heated to 1050 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 20 hours. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbo-oxidation of the titanium as seen in FIG. 10, which shows reflected light optical microscopy of cross-sections. A mixed interstitial compound TiO.sub.xC.sub.1-x and a mixed interstitial solid solution based on carbon and oxygen (diffusion zone) have formed. The case depth is approximately 500 m. The core has transformed into a Wittmansttten structure, i.e. simultaneous core and surface hardening. The hardness of the TiO.sub.xC.sub.1-x is 1859 HV0.025 and the C+O rich diffusion zone up to 1145 HV0.025.

Example 6Carbo-Oxidation Titanium Grade 2

(50) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 20 ml/min Ar and 30 ml/min CO (60% CO) was applied. The sample was heated to 1050 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 20 hours. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbo-oxidation of the titanium as seen in FIG. 11a, which shows a stereomicroscopy picture (8 times magnification) of a cross sectioned 10 mm cylindrical specimen with an ISO metric M4 thread and FIG. 11b, which shows reflected light optical micrographs of the cross-section of the sample. A mixed interstitial compound TiO.sub.xC.sub.1-x and a mixed interstitial solid solution based on carbon and oxygen (diffusion zone) have formed.

(51) Wear and corrosion properties of untreated and treated grade 2 titanium were investigated by ball on disc tribology testing in Ringers solution. Results show less wear for the treated sample with a wear track width of 320 m whereas untreated grade 2 titanium shows a wear track width of 1330 m. There were no indications of corrosion for any of the samples tested. The results are depicted in FIG. 12, which shows SEM images of wear tracks after tribocorrosion ball on disc testing where FIG. 12a shows the results for the untreated sample and FIG. 12b shows the results for the sample treated as described above. The wear counterpart was a 6 mm diameter Al.sub.2O.sub.3 ball loaded with a normal force of 5N on the rotating sample disc for total 50 meter with a speed of 0.5 cm/s. Test solution was Ringers solution containing 0.12 g/l CaCl.sub.2, 0.105 g/l KCl, 0.05 g/l NaHCO.sub.3 and 2.25 g/l NaCl.

(52) Another similar sample was immersed in a 200 ml solution 1 to 10 diluted Keller's reagent at 23 C. for 72 hours and inspected with stereomicroscopy and light optical microscopy for signs of corrosion. Even at high magnification there were no signs of corrosion seen as seen in FIG. 13, where FIGS. 13a and c show the sample before exposure to the Keller's reagent, and FIGS. 13b and d show the sample after exposure to Keller's reagent; the samples are shown at 8 magnification in panels a and c, and panels b and d show the samples at 80 magnification, respectively.

Example 7Carbo-Oxidation Titanium Grade 2

(53) Cylindrical (10 mm) grade 2 titanium sample were treated in a Netzsch 449 Thermal analyzer (furnace). For all experiment, the furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar and 40 ml/min CO was applied. The samples were heated to 1080 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 1, 4 and 16 hours. Cooling was carried out at 50 C./min in the flowing process gas. For all treatment this resulted in carbo-oxidation of the titanium. Mixed interstitial compounds TiO.sub.xC.sub.1-x and mixed interstitial solid solutions based on carbon and oxygen (diffusion zone) formed. The hardness depth profiles are given in FIG. 14, where FIG. 14a shows the hardness profile after 1 hour treatment, FIG. 14b after 4 hours treatment and FIG. 14c after 16 hours treatment; in FIG. 14 the blue symbols illustrate the hardness of the mixed interstitial solid solution and the orange symbols illustrate the hardness of the mixed interstitial compounds. It is seen that the hardness of the mixed interstitial compounds is consistently at least 2000 HV, whereas the hardness of the mixed interstitial solid solution is at least 1000 HV for a depth above 150 m (for 1 hour treatment) to a depth of up to 500 m (for 16 hours treatment).

Example 8Carbo-Oxidation Titanium Grade 2

(54) Cylindrical (10 mm) grade 2 titanium sample were treated in a Netzsch 449 Thermal analyzer (furnace). For all experiment, the furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar and 40 ml/min CO was applied. The samples were heated to different temperatures (840, 920 and 1000 C.) at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 16 hours. Cooling was carried out at 50 C./min in the flowing process gas. For all treatment this resulted in carbo-oxidation of the titanium, as is evident from the reflected light optical microscopy images shown in FIG. 15a-c. Different morphologies of the hard case was obtained: at 840 C. a diffusion zone without visible a phase of carbo-oxide compounds was observed (FIG. 15a), at 920 C. a compact mixed interstitial compound layer on top of a diffusion zone was formed (FIG. 15b), and at 1000 C. the diffusion zone contained large a phase of mixed interstitial compound (FIG. 15c). Thus, when the treatment temperature was below 900 C. microhardnesses for the diffusion zone and the carbo-oxide layer could not be measured at the same depth from the surface, whereas when the temperature was increased above 900 C. microhardnesses for the diffusion zone and the carbo-oxide layer could be measured at the same depth from the surface.

Example 9Carbo-Oxidation

(55) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 30 ml/min Ar and 20 ml/min CO was applied. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 4 hours. Cooling was carried out at 50 C./min in the flowing process gas. A mixed interstitial compound TiO.sub.xC.sub.1-x and a mixed interstitial solid solution based on carbon and oxygen (diffusion zone) have formed. The case depth is approximately 200 m. The hardness profiles of the TiO.sub.xC.sub.1-x and the C+O rich diffusion zone are illustrated in FIG. 16, which also shows (as a dotted line) the hardness of the untreated material, which corresponds to the core hardness of the treated material.

Example 10Carbo-Oxidation

(56) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar and 40 ml/min CO was applied. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 4 hours. Cooling was carried out at 50 C./min in the flowing process gas. A mixed interstitial compound TiO.sub.xC.sub.1-x and a mixed interstitial solid solution based on carbon and oxygen (diffusion zone) have formed. The sample was immersed in 0.25 wt % HF with pH adjusted to 1 with HCl; the results after 16 days of treatment are shown in FIG. 17, where FIG. 17a shows that the untreated reference suffered from corrosion upon exposure to the solution, whereas no signs of corrosion for the sample hardened according to the invention were observed after 16 days (FIG. 17b). The sample not hardened according to the invention showed signs of corrosion immediately upon exposure to HF as evidenced by discoloration of the solution in which the sample was placed.

Example 11Carbo-Oxidation

(57) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar, 35 ml/min CO and 5 ml/min CO.sub.2 was applied. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 4 hours. Cooling was carried out at 50 C./min in the flowing process gas. The presence of CO.sub.2 increases the partial pressure of O.sub.2 and lowers the carbon activity. The result is illustrated in FIG. 18. A mixed interstitial compound TiO.sub.xC.sub.1-x and a mixed interstitial solid solution based on carbon and oxygen (diffusion zone) have formed. The diffusion zone is now the dominant feature. The case depth is approximately 120 m.

Example 12Oxidation of Titanium Grade 2 in CO/CO.SUB.2

(58) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar, 30 ml/min CO.sub.2 and 20 ml/min CO was applied (pCO=0.33 atm and pCO.sub.2=0.50 atm). The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 20 hours. Cooling was carried out at 50 C./min in the flowing process gas. The applied gas resulted in oxidation of the titanium, as shown in FIG. 19, which shows a layer of titanium oxide of a thickness of about 25 m and a diffusion layer of oxygen in solid solution in titanium (below the oxide layer)the diffusion layer had a thickness of about 100 m thickness.

(59) The hardness profiles of the treated samples were recorded and these are illustrated in FIG. 20. The dotted horizontal lines illustrate the core hardness of the titanium metal.

Example 13Oxidation of Titanium Grade 2 in CO/CO.SUB.2

(60) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar, 10 ml/min CO.sub.2 and 40 ml/min CO was applied. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 20 hours. Cooling was carried out at 50 C./min in the flowing process gas. The applied gas resulted in oxidation of the titanium represented as a zone of oxygen in solid solution (diffusion zone) as shown in FIG. 21.

Example 14Oxidation Titanium Grade 2

(61) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar, 10 ml/min CO and 40 ml/min CO.sub.2 was applied. The sample was heated to 750 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 20 hours. Cooling was carried out at 50 C./min in the flowing process gas. The applied gas mixture resulted in oxidation of the titanium providing an oxide layer and a diffusion zone below the oxide layer of a total thickness of about 20 m.

Example 153-interstitial Component Processing

(62) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with nitrogen gas twice and a continuous gas flow consisting of 10 ml/min N.sub.2 and 40 ml/min CO was applied. The applied gas-mixture contains the interstitial elements N, C and O. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 4 hours. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbo-nitro-oxidation of the titanium as shown in FIG. 22. A mixed interstitial compound TiO.sub.xN.sub.yC.sub.1-x-y and a mixed interstitial solid solution based on carbon, oxygen and nitrogen (diffusion zone) have formed. The surface appearance had a slightly more goldish appearance than pure carbo-oxidation. The hardness profiles of the mixed interstitial compound TiO.sub.xN.sub.yC.sub.1-x-y and the diffusion zone are illustrated in FIG. 23, which also shows (as a dotted line) the hardness of the untreated material, which corresponds to the core hardness of the treated material. The case thickness is approximately 220 m.

Example 16Duplex Processing of Titanium Grade 2; Carbo-Oxidation Followed by Nitriding

(63) A cylindrical (10 mm) grade 2 titanium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar and 40 ml/min CO was applied. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 4 hours. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbo-oxidation of the titanium. The carbo-oxidized component was subsequently treated in a tube-furnace equipped with pure N.sub.2 gas. Nitriding was carried out at 1000 C. for 1 hour in flowing N.sub.2 gas (1 l/min). This resulted in partial conversion the CO-rich surface case into a CON containing surface. The diffusion zone is now significantly harder as illustrated in the hardness profile presented in FIG. 24.

Example 17Duplex Processing of Titanium Grade 2; Nitriding Followed by Carbo-Oxidation

(64) A cylindrical (10 mm) grade 2 titanium sample was nitrided in a tube furnace at 1000 C. for 1 hour in flowing N.sub.2 gas (1 l/min). This resulted in a surface layer of TiN. The nitrided component was subsequently treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar and 40 ml/min CO was applied (carbo-oxidation). The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 4 hours. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in (partial) conversion the N-rich surface case into a CON containing surface. The hardness profile is shown in FIG. 25.

Example 18Zirconium Carbo-Oxidation

(65) A zirconium sample was treated in a Netzsch 449 Thermal analyzer (furnace). The furnace was evacuated and backfilled with argon gas twice and a continuous gas flow consisting of 10 ml/min Ar and 40 ml/min CO was applied. The sample was heated to 1000 C. at a rate of 20 C./min in the same gas mixture and upon reaching the temperature held there for 1 hour. Cooling was carried out at 50 C./min in the flowing process gas. This resulted in carbo-oxidation of the zirconium. The surface hardness was 800 HV.

Example 19Formation of Magnli Phases

(66) The grade 2 titanium sample hardened for 16 hours in Example 7 was analysed for the presence of a Magnli phase using X-ray diffraction. The X-ray diffraction pattern is illustrated in FIG. 26, where it is compared to the X-ray diffraction pattern of untreated titanium. FIG. 26 shows the formation of titanium suboxides also known as Magnli phases. The hardening in Example 7 was performed at 80% CO in argon. The hardening was repeated using reactive durations of 4 hours with 10%, 20% and 80% CO in argon, respectively, and the hardened samples were subjected to X-ray diffraction analysis. The results are shown in FIG. 27, which shows that by decreasing the partial pressure of CO the amount of Ti.sub.4O.sub.7 increases in the Magnli phases.