METHOD OF TREATING A WORKPIECE COMPRISING A TITANIUM METAL AND OBJECT

Abstract

Method of treating a workpiece comprising a titanium metal, wherein a titanium metal surface layer of the workpiece is converted to titanium nitrides. The method comprises the following steps; a) heating the workpiece to an initial nitriding temperature (T.sub.n1) and b) subjecting said workpiece to one or more nitriding temperatures (T.sub.n1, T.sub.n2) for predetermined time(s) in a nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions for converting the titanium metal surface layer to a first layer portion consisting of titanium nitrides and a second layer portion comprising a nitrogen gradient in the titanium metal. The method further comprises c) quenching the workpiece in the nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions, in order to strengthen the titanium metal below the in step b) formed first nitride layer portion.

Claims

1. A method of treating a workpiece comprising a titanium metal, wherein a titanium metal surface layer of the workpiece is converted to titanium nitrides, which method comprises the following steps; a) heating the workpiece to an initial nitriding temperature (T.sub.n1); b) subjecting said workpiece to one or more nitriding temperatures (T.sub.n1, T.sub.n2) for predetermined time(s) in a nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions for converting the titanium metal surface layer to a first layer portion consisting of titanium nitrides and a second layer portion comprising a nitrogen gradient in the titanium metal; the method being characterized by; c) quenching the workpiece in the nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions at a quenching rate of at least 150 K/min, in order to strengthen the titanium metal below the, in step b) formed, first nitride layer portion.

2. The method according to claim 1, wherein the work piece, before step c), is subjected to at least one temperature (T.sub.n1) at or above the phase transus temperature for the titanium alloy in question for solution treatment of the titanium alloy, to convert prior phase or + phase structure to solely or predominantly phase structure.

3. The method according to claim 1, wherein step c) comprises quenching the workpiece at a cooling rate which is high enough to, at least partially, transform phase by a martensitic transformation directly to phase or phase.

4. The method according to claim 1, wherein said quenching rate is chosen for delaying the remaining transformation of phase to phase at lower temperatures, resulting in substantially finer microstructures.

5. The method according to claim 1, wherein the workpiece, after step c), is subjected to at least one aging temperature (T.sub.q2) at a predetermined time to obtain precipitation hardening of the titanium metal.

6. The method according to claim 1, wherein the workpiece is cooled to room temperature after quenching or after subjecting the workpiece to the at least one aging temperature for the predetermined time.

7. The method according to claim 1, wherein the workpiece is positioned in one and the same hot isostatic press chamber during the entire execution of the method.

8. The method according to claim 1, wherein the workpiece is subjected to the nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions during the entire execution of the method.

9. The method according to claim 1, wherein the titanium metal comprises at least one of the following; titanium alloy of Grade 1, Grade 2, Grade 5 or Grade 9.

10. The method according to claim 1, wherein said workpiece, in step b), is quenched at a quenching rate sufficient to prevent the formation of phase structure.

11. (canceled)

12. The method according to claim 1, wherein step c) is carried out at hot isostatic pressure conditions where the nitrogen gas pressure is at least initially above 10 MPa.

13. The method according to claim 1, wherein the workpiece comprises titanium alloy of Grade 2, Grade 5 or Grade 9 and wherein step b) comprises quenching the workpiece at a quenching rate of; at least 900 K/min and preferably at least 1200 K/min for Grade 2; at least 210 K/min and preferably at least 420 K/min for Grade 5; or at least 300 K/min for Grade 9.

14. An object comprising a titanium metal alloy of Grade 2, Grade 5 or Grade 9, characterized in that the object exhibits a surface layer comprising a first nitride layer portion and second titanium metal portion which exhibits solely or predominantly phase or phase structures and a nitrogen gradient, which surface layer extends to a depth of at least 50 m when the titanium metal alloy is Grade 2 or Grade 9 and at least 75 m when the titanium metal alloy is Grade 5.

15. The object according to claim 14, wherein the object exhibits a hardness of at least 265 HV0,1 at 25 m; at least 325 HV0,1 at 25 m and at least 420 HV0,1 at 50 m, when the titanium metal alloy is Grade 2, Grade 9 and Grade 5 respectively.

16. (canceled)

17. The object according to claim 14, which object constitutes or forms part of a component chosen from a group of components comprising; automotive, aerospace, mining and medical components.

18. The object according to claim 14, which object has been subjected to a method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The invention is now described, by way of example, with reference to the accompanying drawings, in which:

[0045] FIG. 1 is a diagram representing temperature (T) and time (t) and schematically illustrating a method according to an embodiment of the invention.

[0046] FIG. 2 schematically illustrates a cross section in perspective view of a Hot Isostatic Press containing a work piece.

DETAILED DESCRIPTION OF EMBODIMENTS

[0047] The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

[0048] FIG. 1 illustrates a nitriding heat treatment cycle according to an embodiment of the invention. A workpiece 12 consisting of or comprising at least one titanium alloy, such as Grade 2, Grade 5 or Grade 9, has been placed in a hot isostatic press 10 as shown by FIG. 2. The workpiece may be, for example, engine parts such as wrist pins, hydraulic suspension parts, transmission parts, valves, pumps, orthopaedic or dental implants or prosthesis or the like.

[0049] The workpiece 12 is surrounded by a nitrogen containing gas 14 inside a chamber of the hot isostatic press 10. In the shown example, the gas is nitrogen gas (N.sub.2). It is however possible also to use other nitrogen-containing gases such as ammonium gas. During an initial phase of the treatment, the pressure of the gas 14 is increased, typically to a range between 100 and 200 MPa. The increase of the gas pressure may take place before, simultaneously with or after increasing the temperature in the gas 14. Normally, the increase of the pressure and the temperature of the gas take place at least partly simultaneously.

[0050] As illustrated in FIG. 1, the workpiece 12 is in step a) heated to an initial nitriding temperature (T.sub.n1). The workpiece is thereafter subjected to this nitriding temperature T.sub.n1 for a first predetermined time. As indicated by the dashed line in FIG. 1, the temperature may during the nitriding step b) be increased or decreased to any further nitriding temperature T.sub.n2 and kept at this temperature for any second predetermined time. The nitriding step b) may thus comprise subjecting the workpieces to any number of different nitriding temperatures for any respective desirable times.

[0051] During the nitriding step b), the titanium alloy surface layer is converted to titanium nitrides. Typically TiN is formed in the outermost layer and Ti.sub.2N is formed further in from the outer surface. Additionally, nitrogen is diffused further into the titanium metal layer beneath the ceramic nitrides. In this metal layer portion the nitrogen content typically varies such that the nitrogen content is higher adjacent the nitrides and gradually decreases with increased material depth from the surface. I.e. the nitriding step b) results in the formation of a ceramic nitride layer portion and nitrogen gradient layer portion in the titanium metal.

[0052] At the example illustrated by solid lines in FIG. 1 the initial nitriding temperature T.sub.n1 lies above the transus temperature of the titanium alloy in question. However, if nitriding is taking place at a nitriding temperature below the transus temperature, it may be advantageous that the workpiece is heated above the (transus temperature to form the structure of the titanium alloy in question, before quenching the workpiece.

[0053] In step c) the workpiece 12 is rapidly cooled during the quenching step of the method. During the quenching step c) the temperature of the workpiece 12 is decreased from an initial quenching temperature T.sub.q1 to a final quenching temperature T.sub.q2. Normally, the initial quenching temperature T.sub.q2 is equal to the last nitriding temperature (T.sub.n1 in the example shown by solid lines in FIG. 1). As indicated by the solid line c in FIG. 1, the quenching may be carried out under an essentially constant cooling rate throughout the quenching step. However, it may be advantageous to vary the cooling rate such that the temperature decrease per second is different during the passages of different temperature intervals during the quenching process. Such a varying quenching is indicated by the dashed line c in FIG. 1.

[0054] By this means it is possible to control the grain size and formation of different phase structures of the titanium alloy. It should be noted that the quenching process primarily influences the properties of the titanium metal including the nitrogen gradient layer portion below the nitride layers in the work piece. By this means the quenching step of the method may be favourably used for controlling the material properties of the entire workpiece.

[0055] An important aspect of the invention is that the quenching step is carried out under hot isostatic pressing conditions. The high isostatic pressures prevailing in the chamber greatly contributes to an enhanced heat transfer between the surrounding gas and the workpiece. By this means, not only is it possible to achieve very high actual cooling rates of the material in the workpiece but it also allows for that the actual cooling rates of the material in the workpiece is accurately and precisely controlled throughout the quenching process.

[0056] It should also be noted that, while not illustrated in the figures, the efficiency of the quenching process may be further enhanced by introducing heat exchangers, fans and other heat transfer enhancing means in the chamber.

[0057] In the shown example, where the nitriding has taken place above the transus temperature and the titanium alloy of the workpiece has been fully transformed to the structure, it is in step c) quenched at a quenching rate of 150 K/min or higher under maintained HIP conditions.

[0058] In the shown example the quenching step c) is followed by an aging step d). At this step d), the workpiece is held at and aging temperature for a predetermined time. As seen in FIG. 1, at this example, the aging temperature is equal to the final quenching temperature T.sub.q2. However, it is also possible that the aging of the material of the workpiece is carried out at any other suitable temperature. Further, in the shown example, the aging step d) is carried out immediately subsequent to finalizing the quenching and under high isostatic pressure in the chamber of the hot isostatic press 10.

[0059] At alternative embodiments of the invention, aging may be carried out at any pressure including atmospheric pressure inside or outside the hot isostatic press, e.g. in a conventional furnace. At some embodiments the aging step may even be fully dispensed with.

[0060] In step e), the workpiece is cooled to room temperature. Just as with the aging step d) cooling may take place under high pressure in the hot isostatic press 10 or under lower, such as atmospheric, pressure in the same press 10. Alternatively, the cooling step may be carried out outside of the hot isostatic press 10.

[0061] The workpiece may then be directly used in any application in which it is likely to be subjected to stress, strain, impact and/or wear under operation.

[0062] Furthermore, the workpiece may be machined, either before the heating step a) or after the nitriding, quenching and aging is completed, for example, if some particular surface treatment is required.

[0063] Carrying out the heating and nitriding step a) under HIP conditions accelerates the heating rate, nitriding rate and deep diffusion of nitrogen into the bulk titanium alloy. Carrying out the quenching step c) under HIP conditions accelerates the cooling rate and concurrently reduces residual stresses due to superplastic conditions during a substantial part of the quenching process.

[0064] Utilizing HIP conditions during any of the steps a) to d) and particularly steps a), b) and c) also results in the following advantages: elimination of casting porosity, elimination of residual stresses, consistent material properties and consistent machining properties.

[0065] FIG. 2 shows a hot isostatic press 10 in which one workpiece 12 is subjected to a method according to the embodiment of the invention illustrated in FIG. 1. It should be noted that one or more workpieces may be placed inside the hot isostatic press 10 and that the work piece(s) can be of any shape and size as long as it/they can fit inside the hot isostatic press 10. The workpiece 12 is radially and axially outwardly surrounded firstly by a pressurized gas 14 acting normally at all surfaces, secondly by furnace walls, thirdly by a heat insulating mantle and fourthly by the water-cooled pressure vessel walls, being held in compression by pre-stressed wire windings 16.

[0066] All of the surfaces of the workpiece 12 as well as all of the surfaces of the furnace and the heat insulating mantle and the internal surfaces of the pressure vessel may be subjected to high-pressure nitrogen gas 14, such as nitrogen at a pressure of up to approx. 200 MPa.

Example

[0067] Work pieces comprising commercially pure titanium (Grade 2) in the form of thin-walled tubes (t=1.0 mm) were placed in a hot isostatic press of the type illustrated in FIG. 2. Nitrogen gas, N.sub.2 was supplied to the chamber of the press 10.

[0068] During step a) the temperature of the gas was increased until the temperature of the workpiece reached 960 C. Simultaneously, the pressure of the gas was increased to 170 MPa.

[0069] In step b) the same temperature and gas pressure was maintained for 2 hours.

[0070] Since this temperature was already above the transus temperature, an increase in temperature in step b) was not needed for this titanium alloy.

[0071] In step c), the workpiece was quenched by cooling nitrogen gas according to the following cooling rates of the gas: [0072] 3600 K/min between 960-900 C., [0073] 2460 K/min between 900-800 C., [0074] 1440 K/min between 800-700 C., [0075] 1020 K/min between 700-600 C. and [0076] 600 K/min between 600-500 C.,

[0077] The temperature of the gas was measured by thermocouples.

[0078] In this case, no aging treatment was carried out.

[0079] In step e) the work pieces were cooled to room temperature.

[0080] All of the steps a), b) and c) were carried out in the hot isostatic press under nitrogen gas at pressures up to 170 MPa.

[0081] The thin-walled tubes were then analysed by microstructural determination and it was found that the material comprised a Widmannsttten structure with non-continuous -phase. The microstructural analyse further determined the material of thin-walled tubes to have been cooled at >1200 K/min (>20 K/s) through the 888-868 C. interval for + structure formation at this cooling rate, corresponding to a water quench (WQ) rate. Since the cooling rate of the nitrogen gas was more than twice as high through at this temperature interval and since the heat transfer coefficient (>1000 W/m.sup.2K) is high between the dense gas and the thin-walled titanium tube, it is reasonable that the metal core could indeed be cooled by 20 K/s.

[0082] Microstructural evaluation further showed a formation of a 20 m layer of -TiN+-Ti.sub.2N with hardness up to 106822 HV0.05 at 1 m depth and 519 HV0.025 at 10 m depth, followed by a sloping decrease in hardness 30 m further into the titanium metal to the bulk level of 230-250 HV0.1.

[0083] The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.