Method of case hardening a group IV metal

Abstract

A method of producing a case hardened workpiece of a Group IV metal including: placing a workpiece of a Group IV metal in a vessel, creating a low pressure environment in the vessel in which the pressure, pvac, is less than or equal to 10-5 bar, providing oxygen to the vessel to create a reactive atmosphere in the vessel, the reactive atmosphere comprising oxygen at a partial pressure, pO2, in the range of 10 5 bar to 0.01 bar, heating the workpiece to a hardening temperature in the range of 650° C. to 800° C. in the reactive atmosphere or before the reactive atmosphere is created, maintaining the workpiece in the reactive atmosphere at the hardening temperature for a reactive period of at least 5 hours, cooling the workpiece from the hardening temperature to ambient temperature in the reactive atmosphere or in an inert atmosphere.

Claims

1. A method of producing a case hardened workpiece of a Group IV metal, the method comprising the steps of: placing a workpiece of a Group IV metal in a vessel, creating a low pressure environment in the vessel in which the pressure, p.sub.vac, is less than or equal to 10.sup.−5 bar, providing oxygen to the vessel to create a reactive atmosphere in the vessel, the reactive atmosphere comprising oxygen at a partial pressure, p.sub.O2, in a range of 10.sup.−5 bar to 0.01 bar, heating the workpiece to a hardening temperature in a range of 650° C. to 800° C. in the reactive atmosphere or before the reactive atmosphere is created, maintaining the workpiece in the reactive atmosphere at the hardening temperature for a reactive period of at least 5 hours, cooling the workpiece from the hardening temperature to ambient temperature in the reactive atmosphere or in an inert atmosphere.

2. The method of producing a case hardened workpiece of a Group IV metal according to claim 1, a total pressure of the reactive atmosphere being a sum of p.sub.vac and p.sub.O2.

3. The method of producing a case hardened workpiece of a Group IV metal according to claim 1, wherein oxygen is supplied to the vessel for a total duration of the reactive period.

4. The method of producing a case hardened workpiece of a Group IV metal according to claim 1, wherein the reactive period comprises at least one supply period and at least one non-supply period, a supply period being a period of time in which oxygen is added to the vessel and a non-supply period is a period of time in which no further oxygen is added to the vessel.

5. The method of producing a case hardened workpiece of a Group IV metal according to claim 4, wherein the at least one supply period has a duration in the range of 1 minute to 10 minutes and the at least one non-supply period has a duration of from 1 minute to 3 hours.

6. The method of producing a case hardened workpiece of a Group IV metal according to claim 1, wherein the hardening temperature is in the range of 700° C. to 750° C.

7. The method of producing a case hardened workpiece of a Group IV metal according to claim 1, wherein the reactive period has a duration in the range of 5 hours to 75 hours.

8. The method of producing a case hardened workpiece of a Group IV metal according to claim 1, further comprising polishing the workpiece after the step of cooling the workpiece.

9. The method of producing a case hardened workpiece of a Group IV metal according to claim 1, wherein the Group IV metal is selected from the group consisting of titanium and titanium alloys.

10. The method of producing case hardened workpiece of a Group IV metal according to claim 1, wherein the method further comprises the step of core hardening the workpiece after the step of cooling the workpiece.

11. The method of producing a case hardened workpiece of a Group IV metal according to claim 1, wherein the vessel comprises a graphite retort.

12. The method of producing a case hardened workpiece of a Group IV metal according to claim 11, a total pressure of the reactive atmosphere being a sum of pvac, p.sub.O2, p.sub.CO and p.sub.CO2.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) In the following the invention will be explained in greater detail with the aid of examples and with reference to the figures, in which:

(2) FIG. 1 shows a cross-section of titanium Grade 5 case hardened according to a first embodiment of the invention;

(3) FIG. 2 shows a hardness profile in a cross-section of titanium Grade 5 case hardened according to the first embodiment of the invention;

(4) FIG. 3 shows a cross-section of titanium Grade 5 case hardened according to a second embodiment of the invention;

(5) FIG. 4 shows a higher resolution cross-section of titanium Grade 5 case hardened according to the second embodiment of the invention;

(6) FIG. 5 shows a cross-section of titanium Grade 5 case hardened according to a third embodiment of the invention;

(7) FIG. 6 shows a higher resolution cross-section of titanium Grade 5 case hardened according to the third embodiment of the invention;

(8) FIG. 7 shows a hardness profile in a cross-section of titanium Grade 2 case hardened according to the invention;

(9) FIGS. 8A and 8B show cross-sections of zirconium case hardened according to the invention;

(10) FIG. 9 shows a hardness profile in a cross-section of zirconium case hardened according to the invention;

(11) FIGS. 10A, 1013 and 10C show cross-sections of titanium Grade 2 case hardened according to the invention such that there is an oxygen-carbon double layer; and

(12) FIG. 11 shows the hardness profile of carbon and oxygen-containing titanium Grade 2 hardened according to the invention.

(13) It should be understood that combinations of the features in the various embodiments are also contemplated, and that the various features, details and embodiments may be combined into other embodiments. In particular, it is contemplated that all definitions, features, details, and embodiments regarding the method of the invention are also relevant for embodiments of the component of the invention which are not explicitly described and vice versa.

(14) Reference to the figures serves to explain the invention and should not be construed as limiting the features to the specific embodiments as depicted.

DETAILED DESCRIPTION OF THE INVENTION

(15) The present invention relates to a method of producing a case hardened workpiece of a Group IV metal, and in other aspects the invention relates to components of a Group IV metal, which are obtainable by the method of the invention. Thus, it will be understood that the methods claimed in the present application give rise to the products claimed in the present application. The products are obtainable by the methods.

(16) 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. Thus, 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. 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.

(17) Unless otherwise noted a percentage in relation to a metal or a component is by weight of the total weight of material, e.g. denoted % (w/w). Likewise, unless otherwise noted a composition of a mixture of gasses is on an atomic basis and may be provided as a percentage or in ppm (parts per million).

(18) In an embodiment of the invention the Group IV metal is titanium or a titanium alloy. Any grade of titanium containing at least about 99% (w/w) titanium is, in the context of the invention, considered to be “pure 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. Likewise, any grade of zirconium containing at least about 99% (w/w) zirconium is, in the context of the invention, considered to be “pure zirconium”. In another embodiment the Group IV metal 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. In particular, nitrogen and carbon contained in a Group IV metal in the context of the invention may represent unavoidable impurities. Elements present as “unavoidable impurities” are considered not to provide an effect for a workpiece treated according to the method of the invention or for the component of the invention.

(19) The alloys of relevance may contain any other appropriate elements, 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.

(20) In the context of the invention the hardness is generally the HV0.05 as measured according to the DIN EN ISO 6507 standard. If not otherwise mentioned, the unit “HV” thus refers to this standard. The hardness is preferably recorded for a cross-section, e.g. of a treated Group IV metal, and it may be noted with respect to the depth of the measurement. In the context of the invention the “depth” is the distance from the surface. When the hardness is recorded at a cross-section the measurement is considered to represent a homogeneous sample with respect to the direction of the pressure applied. In contrast, when the hardness is obtained from measurements at the surface, the measurement may represent an average of several different values of hardness, i.e. at different depths. In the context of the invention a hardness measurement recorded in a cross-section at a depth of about 1 μm is considered to provide the actual hardness of the surface of the material. As an effect of the fact that oxygen is dissolved from the surface, the content of dissolved oxygen will decrease from the surface towards the core of the Group IV metal, and likewise, the hardness will be maximal at the surface, e.g. as represented by measuring the hardness in a cross-section at a depth of about 1 μm. In oxygen hardening methods of the prior art, surface hardnesses of more than 1000 HV are obtained, but the value of the hardness at the surface, e.g. as represented by a hardness measurement in a cross-section at a depth of about 1 μm, provides no information about the hardness at deeper depths.

(21) In an embodiment of the invention the hardness at a depth of 5 μm is at least 800 HV. For example, for components of the invention of titanium or titanium alloys the hardness in a cross-section at a depth of about 5 μm may be at least 900 HV, and for components of the invention of zirconium or zirconium alloys the hardness in a cross-section at a depth of about 5 μm may be at least 700 HV.

(22) In certain aspects, the present invention relates to component treated 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. The component of the present invention may inter alia be determined in terms of its thickness. 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 0.1 mm to 5 mm it can be said to have a thickness in the range of from 0.1 mm to 5 mm. The component of the invention may have a thickness of up to 20 mm, e.g. up to 10 mm, such as in the range of 0.1 mm to 5 mm. The component need not consist of the Group IV metal and in an embodiment of the invention the component has an outer surface of the Group IV metal and a core of another material, e.g. another metal, a polymer, a ceramic material etc.

EXAMPLES

Example 1

Steady O.SUB.2 .Supply

(23) Samples of Titanium Grade 5 (also known as Ti-6Al-4V) were treated for 15 hours at 750° C. in a reactive atmosphere of commercially available oxygen.

(24) The samples were first heated from ambient temperature up to 750° C. in vacuum conditions (10.sup.−2 mbar). When reaching 750° C., oxygen was introduced by a controlled steady flow of 1 ml/min and the samples were retained at 750° C. for 15 hours before cooling to ambient temperature in vacuum conditions (10.sup.−2 mbar).

(25) The treated samples were cut to reveal the cross-sections, which are presented as the photomicrograph in FIG. 1. FIG. 1 shows the interface between the diffusion layer and the core of the Grade 5 titanium as a dark horizontal line. The thickness of the layer is recorded at about 38 μm.

(26) The sample treated according to the method of the invention was also subjected to a microhardness analysis, and the results are shown in FIG. 2. The Grade 5 titanium had a core hardness of 360 HV (0.05), and the hardness recorded at 11 μm was above 1000. Likewise, a microhardness much above 150% of the core hardness is observed at 20 μm, and a microhardness of about 125% of the core hardness is observed at about 30 μm.

Example 2

Cycles of O.SUB.2 .Supply/Non-supply

(27) Samples of Titanium Grade 5 were treated in a reactive atmosphere according to the invention at a temperature of 750° C. for reactive periods of at least 5 hours following the procedure otherwise outline in Example 1, except that the oxygen supply was paused after 2 hours for a period of 2 hours and then started again. This cycle of oxygen supply for two hours followed by pausing the supply for two hours was repeated for the duration of the treatment.

(28) Cross-sections of a sample treated for 5 hours are shown in FIG. 3 and FIG. 4, and cross-sections of a sample treated for 30 hours are shown in FIG. 5 and FIG. 6. The cross-section of a sample treated for 30 hours is shown in FIG. 6 at a higher magnification than shown in FIG. 5. Also, in this case, the diffusion layer is clearly discernible and in particular it is evident that no oxide or other compounds have formed with the titanium.

(29) The thicknesses of the diffusion layers obtained after 5 and 30 hours are indicated in FIGS. 4 and 6, respectively.

Example 3

Steady O.SUB.2 .Supply

(30) Samples of Titanium Grade 2 (i.e. pure titanium) were treated in a reactive atmosphere according to the invention at a temperature of 750° C. for a reactive period of 58 hours following the procedure otherwise outlined in Example 1. FIG. 7 shows the hardness profile of a sample treated for 58 hours. It is evident that the microhardness at 5 μm is above 1100 HV, and that the microhardness at a depth of 20 μm is above 900 HV compared to the core hardness of less than 300 HV.

Example 4

Steady O.SUB.2 .Supply

(31) A sample of zirconium was treated according to the invention at a temperature of 750° C. for 28 hours following the procedure otherwise outlined in Example 1. A cross-section of the treated sample is shown at a low magnification in FIG. 8A and at a higher resolution in FIG. 8B. The diffusion layer is clearly visible at both magnifications, and the thickness of the diffusion layer was recorded as 134 μm from the visual inspection. The hardness profile of the hardened zirconium is shown in FIG. 9. It is evident that the core hardness was about 220 HV, and the hardness at 1 μm depth was above 790 HV. At 20 μm depth the microhardness was above 150% of the core hardness, e.g. at about 690 HV. At a depth of 150 μm the microhardness was above 125% of the core hardness.

Example 5

Graphite Retort

(32) Samples of Titanium Grade 2 (i.e. pure titanium) were treated in a reactive atmosphere according to the invention at a temperature of 750° C. for a reactive period of 15 hours following the procedure otherwise outlined in Example 1, except that the vessel comprised a graphite retort. The presence of carbon in solution in the Grade 2 titanium was subsequently identified.

(33) FIGS. 10A-10C show cross sections of the Titanium Grade 2 obtained under these conditions. The oxygen-carbon double layer can clearly be seen in the hardened zone. The corresponding hardness profile achieved can be seen in FIG. 11. The carbon contribution to the profile can be seen from 22 μm onwards and makes a significant contribution the profile.