SE1650705A1 - Method of treating a workpiece comprising a titanium metal and object - Google Patents

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 (TM) and b) subjecting said workpiece to one or more nitriding temperatures (Tm, T2) 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.(Fig.i)

Description

1O Method of treating a workpiece comprising a titanium metal and object Technical Field The present invention generally concerns a method of treating a workpiececomprising titanium metal. More specifically it relates to a method whichcomprises nitriding at least a portion of the workpiece such that a surfacelayer of the titanium alloy is converted to titanium nitride. The invention also concerns an object, which has been subjected to such a method.

Background of the invention Titanium has found an increasing use in various technical fields such as inthe manufacturing of machine parts and other components within theautomotive, aerospace, mining, medical and other industries. In practice,chemically pure titanium is not utilized industrially but the titanium isalloyed to form various alloys with differing characteristics. Examples ofsome frequently used titanium alloys are so called commercially pure (cp)titanium or Grade 2 titanium (sometimes referred to as unalloyed), Grade 5titanium (Ti-6Al-4V) and Grade 9 titanium (Ti-3Al-2.5V). In the followingdescription also the so called commercially pure titanium metal is referred to as alloy.

The titanium alloys generally have some characteristics that are favourable inmany applications. Examples of such characteristics are low density, highspecific strength or strength-to-weight ratio, excellent corrosion resistanceand ability to withstand high temperatures. The low density and high specificstrength e.g. contributes to reduce energy consumption and environmentalimpact when producing machine parts and other components. Some titaniumalloys are also non-toxic which is used e.g. for producing orthopaedic anddental prosthesis and implants. However, one technical disadvantage oftitanium alloys is the risk of adhesive seizure in highly loaded sliding or rolling contacts. 1O Several methods have been developed in order to eliminate this disadvantage.Such known methods comprise different PVD (Physical Vapour Deposition) coatings and plasma sprayed coatings.

An emerging method is conversion of the titanium alloy surface by nitridingthe titanium alloy. By this means, ceramic titanium nitrides such as ö-TiN(face-centered cubic) and s-Ti2N (tetragonal) are formed in an outermost firstsurface layer portion of the work piece. The nitrides considerably increase thehardness and thereby the load-carrying capacity of the surface layer. Inaddition to forming nitrides in the outmost first ceramic layer portion, thenitriding process also results in a second metallic layer portion wherenitrogen is diffused into the titanium alloy just beneath the ceramic layer.Typically the nitrogen concentration in this second layer portion is highestadjacent to the first nitride layer portion and is reduced gradually atincreased depth from the surface, thereby forming a nitrogen gradient in thesurface layer. The nitrogen gradient results in increased hardness and support of the first nitride layer portion.

This method is usually carried out by processing the workpiece in vacuumfurnaces. The main limitations of these treatments are long and costlyprocesses, resulting thin nitride layers and shallow penetration of nitrogeninto the bulk metal, thereby forming merely a relatively weak support for the hard and brittle nitride layers.

Prior Art The conference proceedings paper “The HIP-Nitriding of Steels and Ti-basedAlloys” (M. H. Jacobs, M. A. Ashworth and A. J. Marshall; presented at theInternational Conference on Hot Isostatic Pressing, 20-22 May 1996 inAndover, Massachusetts) discloses a method for improving the productivityof the nitriding process by concurrently increasing both the thickness ofceramic nitride layers and the nitrogen penetration into the bulk metal, bysubjecting the titanium parts to nitriding at very high nitrogen pressures for shorter durations by Hot Isostatic Pressing.

Hot Isostatic Pressing (HIP) is a process that is today mainly used to either eliminate the internal porosity of metal castings of titanium and nickel-based 1O super-alloys or to densify various metallic and ceramic powder bodies to solidmaterials. The HIP process subjects a workpiece to concurrently elevatedtemperature and isostatic gas pressure (whereby pressure is applied to thematerial from all directions) in a high pressure containment vessel. An inertgas such as argon is usually used to prevent chemical reactions, and thepressurizing gas is usually raised to a pressure level between 100-200 MPa bya combination of pumping and electrical heating of the gas surrounding thework pieces. When materials are treated with HIP, the simultaneousapplication of heat and pressure eliminates internal porosity through a combination of plastic deformation, creep, and diffusion bonding.

US 4,511,411 further discloses a method for increasing the surface hardness ofa titanium alloy component. The method comprises; placing a component oftitanium alloys in an autoclave, pumping nitrogen gas or ammonia into theautoclave and exposing the component in the autoclave for three hours to a pressure of 900 bar (90 MPa) and a temperature of 1 000°C.

Summary of the inventionAn object of the present invention is to provide an enhanced method of treating a workpiece comprising a titanium metal alloy.

Another object is to provide such a method by which a surface layer of the titanium metal alloy workpiece is efficiently hardened by nitriding.

Yet another object is to provide such a method which allows for an enhanced control of the resulting material properties of the workpiece.

Still another object is to provide such a method by which the resulting microstructure of the workpiece may be efficiently and precisely controlled.

A further object is to provide such a method which may be carried out in an efficient, time saving manner at a comparatively low cost.

These and other objects are achieved by a method of the type specified in thepreamble of claim 1, which method comprises the special technical features defined in the characterizing portion of that claim. 1O The method is used for treating a workpiece comprising at least one titaniummetal and involves that a titanium metal surface layer of the workpiece isconverted to titanium nitrides. The method comprises the steps of; a) heatingthe workpiece to an initial nitriding temperature and; b) subjecting saidworkpiece to one or more nitriding temperatures 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 firstlayer portion consisting of ceramic titanium nitrides and a second layerportion comprising a nitrogen gradient in the titanium metal. The methodfurther comprises c) quenching the workpiece in the nitrogen containing gasunder high pressure at Hot Isostatic Pressing (HIP) conditions as a first stepin a hardening heat treatment, in order to further strengthen the titanium metal below the first ceramic nitride layer portion formed in step b).

The method according to the present invention improves the mechanicalproperties of the workpiece. The high nitrogen gas pressure enhancesnitrogen diffusion from the gas into the titanium metal, resulting inconversion to nitrides in the first ceramic layer portion and creating anitrogen gradient in the second titanium metal layer portion. Nitriding atHIP conditions thus results in thick ceramic ö-TiN and s-Ti2N surface layersand in a deep nitrogen gradient layer which additionally exhibits a highnitrogen content immediately below the nitride layer to improve its load-carrying capacity. According to the invention these favourable features iscombined with an improved heat transfer by the highly pressurized gasduring the quenching step. Quenching at HIP conditions not only increasesthe heat transfer but also promotes equal cooling of all surfaces regardless oftheir position and orientation on the workpieces and within the pressure vessel.

The prior art does not disclose a nitriding method of titanium alloys whichsubsequently comprises quenching under high isostatic pressure as a firststep in a hardening heat treatment, using Hot Isostatic Pressing also for priornitriding and concurrent elimination of casting porosity and/ or residual stresses in the titanium alloys. The present invention is based on the 1O realization that recent developments in HIP equipment makes it possible toutilize the improved heat transfer capability achieved by Hot IsostaticPressing for quenching and thereby to carry out hardening heat treatments under HIP conditions.

Quenching the workpiece at isostatic pressures of up to 200 MPa in a gassuch as nitrogen, which at these pressures has a viscosity resembling water,provides that the titanium metal comprising both the nitrogen gradientportion and the bulk titanium metal can be subjected to very high coolingrates (delta T/ s). The excellent heat transfer capabilities at high isostaticpressures also provide that the cooling rate may be precisely controlledwithin wide intervals. By this means, the resulting material properties afterquenching may be precisely controlled by varying and controlling the coolingrate at different stages of the quenching process. By quenching the workpieceunder HIP conditions, the microstructures and resulting properties can beoptimized for different applications. The quenching may e.g. be carried outsuch that the hardness and/ or ductility of both the nitrogen gradient layerand the bulk metal is increased. The nitrogen gradient layer and the bulkmetal may thus be formed to constitute an excellent support for the hard andbrittle nitride layers at the surface. By this means the method may be utilizedfor producing workpieces and components which have excellent properties inregard of e.g. hardness, low friction, ductility, durability, specific strength, temperature resistance and low density.

If desired, the invention also allows for that the cooling rate is furtherincreased by utilizing heat exchangers and fans within the pressure chamberin which the method according to the invention is carried out. This enableseven larger cross-sections of a workpiece to be cooled at sufficient rates. Sincethe workpiece is within a firm isostatic grip, its macroscopic shape is stillpreserved, and such high cooling rates will therefore not result in large residual stresses, cracks or warpage.

The invention further allows for that both nitriding the surface layer andquenching the workpiece is carried out subsequently or even at least partly overlapping in a continuous method step. Irrespective of if the nitriding and 1O quenching operations are carried out overlapping or subsequently, bothprocesses may be carried out in one and the same HIP chamber, therebyeliminating any need for intermediate cooling, transportation, storing,reheating and other handling of the workpiece. The combined nitriding andquenching in a single HIP chamber further eliminates the need for separatechambers, ovens, furnaces and other equipment needed when conducting nitriding and quenching as separate operations.

The method according to the present invention therefore provides a cost-effective way of obtaining titanium nitride layers on titanium alloys, which inaddition is heat treated for superior properties. Additionally, improvedstrength and ductility with reduced scatter may be obtained due to theelimination of all internal porosity in the cast work piece. Further, themethod offers the possibility of manufacturing work pieces with closermachining tolerances since residual stresses are eliminated from the work piece, and batch-processing time may be decreased.

As the quenching step c) is carried out under HIP conditions, a rapid cooling,typically greater than or equal to 100 K /min is possible, exceeding the rate of quenching in oils, since the pressurized gas provides efficient heat transfer.

According to one embodiment the workpiece may, before step c), besubjected to at least one temperature at or above the ß phase transustemperature for the titanium alloy in question for solution treatment of thetitanium alloy to convert prior a phase or a+ß phase structure to solely orpredominantly ß phase. Hereby, microstructures that are particularlyfavourable at some applications may be achieved after completing themethod.

Step c) may comprise quenching the workpiece at a cooling rate which is highenough to, at least partly, transform ß phase by a martensitic transformationdirectly to a' phase or a' ' phase. This also allows for achievingmicrostructures that give the workpiece properties which are desirable at various applications. 1O The quenching rate may be chosen to delay the remaining transformation ofß phase to a phase at lower temperatures, resulting in substantially finer microstructures.

The workpiece may, after step c), be subjected to at least one agingtemperature at a predetermined time to obtain precipitation hardening of thetitanium alloy. When the workpiece comprises titanium Grade 5, it may e. g. be suitable to carry out the aging step at 400-600 °C.

The workpiece may be cooled to room temperature after quenching or aftersubjecting the workpiece to the at least one aging temperature for a predetermined time.

The workpiece may be positioned in one and the same HIP chamber duringthe entire execution of the method. Hereby all intermediate transportation,storing other handling as well as cooling and re-heating of the workpiece is eliminated.

The workpiece may be subjected to the nitrogen containing gas under highpressure at Hot Isostatic Pressing (HIP) conditions during the entire execution of the method.

The titanium metal may preferably comprise at least one of the following;titanium alloy of Grade 1, Grade 2, Grade 5 or Grade 9. However, several other titanium alloys may also be treated by means of the method.

Step c) may be carried out at Hot Isostatic Pressure conditions where thenitrogen gas pressure is, at least initially, above 10 MPa. It may at someinstances be suitable to reduce the gas pressure after quenching such that anexcessive pressure does not to counteract precipitation of particles withhigher specific volume (i.e. lower density) during aging. However, the gaspressure should at all instances be maintained at least at or above 5 MPa for achieving sufficient heat transfer and thus temperature control.

The workpiece may comprise Grade 2 titanium and step b) may thencomprise quenching the workpiece at a quenching rate of at least 900 K / min and preferably at least 1200 K / min. 1O The workpiece may alternatively comprise Grade 5 titanium and step b) maythen comprise quenching the workpiece at a quenching rate of at least 210 K / min and preferably at least 420 K/ min.

If the workpiece comprises Grade 9 Titanium, step b) may comprise quenching the workpiece at a quenching rate of at least 300 K / min.

It should be noted that preferably all of the steps for nitriding, quenching,aging and cooling to room temperature may be carried out at least partlyunder HIP conditions in a single HIP chamber. Hereby, the entire treatmentof the workpiece may be accomplished, without interruptions, in a singlework station. By this means all intermediate handling between differentworks stations and operations is eliminated. It may also be noted that whenalso the cooling to room temperature of the workpiece is carried out in theHIP chamber, the high pressure may be maintained during the entireprocess. The temperature however, naturally needs to be decreased, such thatthis treatment step will not be carried out entirely at the high temperaturesnormally prevailing at HIP conditions. However, not all of the steps neednecessarily be carried out under HIP conditions, as most benefits from HIPare gained during steps a) through c), while the aging treatment may takeplace in another furnace and the cooling to room temperature may be carried out in the aging furnace, another furnace or in the ambient surrounding.

The method thus allows for producing and objects with extraordinary favourable properties and the invention also relates to such objects.

According to an aspect, the invention concerns an object comprising atitanium metal alloy of Grade 2, Grade 5 or Grade 9, which object exhibits asurface layer comprising a first nitride layer portion and second titaniummetal portion with a nitrogen gradient, which surface layer extends to a depthof at least 20 um when the titanium metal alloy is Grade 2 and at least 50 um when the titanium metal alloy is Grade 5 or Grade 9.

The object may exhibit a hardness of at least 250 HVO,1 at 30 um; at least300 HV0,1 at 30 um and at least 500 HV0,1 at 15 um, when the titanium metal alloy is Grade 2, Grade 9 and Grade 5 respectively. 1O The titanium alloy of the object may exhibit solely or predominantly a' phase or a' ' phase structures.

The object may constitute or form part of a component chosen from a groupof components comprising; automotive, aerospace, mining and medical COIIIPOIICIIÉS.

The workpiece thus comprises a nitrided titanium alloy having an improvedcombination of high strength, ductility and hardness. Such a workpiece isintended for use particularly, but not exclusively, in applications where highwear resistance is required or in applications in which strict specifications must be consistently met.

Generally, all terms used in the claims are to be interpreted according to theirordinary meaning in the technical field, unless explicitly defined otherwiseherein. All references to "a/ an /the element, apparatus, component, means,step, etc." are to be interpreted openly as referring to at least one instance ofthe element, apparatus, component, means, step, etc., unless explicitly statedotherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

Brief description of the drawingsThe invention is now described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a diagram representing temperature (T) and time (t) andschematically illustrating a method according to an embodiment of the invention.

Fig. 2 schematically illustrates a cross section in perspective view of a Hot Isostatic Press containing a work piece.

Detailed description of embodiments The invention will now be described more fully hereinafter with reference tothe accompanying drawings, in which certain embodiments of the inventionare shown. This invention may, however, be embodied in many differentforms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that 1O this disclosure will be thorough and complete, and will fully convey the scopeof the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

Fig. 1 illustrates a nitriding heat treatment cycle according to an embodimentof the invention. A workpiece 12 consisting of or comprising at least onetitanium alloy, such as Grade 2, Grade 5 or Grade 9, has been placed in a hotisostatic 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.

The workpiece 12 is surrounded by a nitrogen containing gas 14 inside achamber of the hot isostatic press 10. In the shown example, the gas isnitrogen gas (N2). It is however possible also to use other nitrogen-containinggases such as ammonium gas. During an initial phase of the treatment, thepressure of the gas 14 is increased, typically to a range between 100 and 200MPa. The increase of the gas pressure may take place before, simultaneouslywith or after increasing the temperature in the gas 14. Normally, the increaseof the pressure and the temperature of the gas take place at least partly simultaneously.

As illustrated in Fig. 1, the workpiece 12 is in step a) heated to an initialnitriding temperature (Tm). The workpiece is thereafter subjected to thisnitriding temperature Tm for a first predetermined time. As indicated by thedashed line in Fig. 1, the temperature may during the nitriding step b) beincreased or decreased to any further nitriding temperature Tn2 and kept atthis 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.

During the nitriding step b), the titanium alloy surface layer is converted totitanium nitrides. Typically TiN is formed in the outermost layer and Ti2N isformed further in from the outer surface. Additionally, nitrogen is diffusedfurther into the titanium metal layer beneath the ceramic nitrides. In thismetal layer portion the nitrogen content typically varies such that the nitrogen content is higher adjacent the nitrides and gradually decreases with 1O 11 increased material depth from the surface. I.e. the nitriding step b) results inthe formation of a ceramic nitride layer portion and nitrogen gradient layer portion in the titanium metal.

At the example illustrated by solid lines in Fig. 1 the initial nitridingtemperature Tm lies above the ß transus temperature of the titanium alloy inquestion. However, if nitriding is taking place at a nitriding temperaturebelow the ß transus temperature, it may be advantageous that the workpieceis heated above the ß transus temperature to form the ß structure of the titanium alloy in question, before quenching the workpiece.

In step c) the workpiece 12 is rapidly cooled during the quenching step of themethod. During the quenching step c) the temperature of the workpiece 12 isdecreased from an initial quenching temperature Tql to a final quenchingtemperature Tq2. Normally, the initial quenching temperature Tq2 is equal tothe last nitriding temperature (Tm 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 outunder an essentially constant cooling rate throughout the quenching step.However, it may be advantageous to vary the cooling rate such that thetemperature decrease per second is different during the passages of differenttemperature intervals during the quenching process. Such a varying quenching is indicated by the dashed line c” in Fig. 1.

By this means it is possible to control the grain size and formation of differentphase structures of the titanium alloy. It should be noted that the quenchingprocess primarily influences the properties of the titanium metal includingthe 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.

An important aspect of the invention is that the quenching step is carried outunder hot isostatic pressing conditions. The high isostatic pressuresprevailing in the chamber greatly contributes to an enhanced heat transferbetween the surrounding gas and the workpiece. By this means, not only is itpossible 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 1O 12 the workpiece is accurately and precisely controlled throughout the quenching process.

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

In the shown example, where the nitriding has taken place above the ßtransus temperature and the titanium alloy of the workpiece has been fullytransformed to the ß structure, it is in step c) quenched quenching rate of 150 K/ min or higher under maintained HIP conditions.

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 apredetermined time. As seen in fig. 1, at this example, the aging temperatureis equal to the final quenching temperature Tq2. However, it is also possiblethat the aging of the material of the workpiece is carried out at any othersuitable temperature. Further, in the shown example, the aging step d) iscarried out immediately subsequent to finalizing the quenching and under high isostatic pressure in the chamber of the hot isostatic press 10.

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

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 isostaticpress 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.

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.

Furthermore, the workpiece may be machined, either before the heating stepa) or after the nitriding, quenching and aging is completed, for example, if some particular surface treatment is required. 1O 13 Carrying out the heating and nitriding step a) under HIP conditionsaccelerates the heating rate, nitriding rate and deep diffusion of nitrogen intothe bulk titanium alloy. Carrying out the quenching step c) under HIPconditions accelerates the cooling rate and concurrently reduces residualstresses due to superplastic conditions during a substantial part of the quenching process.

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

Fig. 2 shows a hot isostatic press 10 in which one workpiece 12 is subjected toa 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 hotisostatic press 10 and that the work piece(s) can be of any shape and size aslong as it/ they can fit inside the hot isostatic press 10. The workpiece 12 isradially and axially outwardly surrounded firstly by a pressurized gas 14acting normally at all surfaces, secondly by furnace walls, thirdly by a heatinsulating mantle and fourthly by the water-cooled pressure vessel walls, being held in compression by pre-stressed wire windings 16.

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

Example Work pieces comprising commercially pure titanium (Grade 2) in the form ofthin-walled tubes (t=1.0 mm) were placed in a hot isostatic press of the typeillustrated in Fig. 2. Nitrogen gas, N2 was supplied to the chamber of the press 10.

During step a) the temperature of the gas was increased until the temperatureof the workpiece reached 960°C. Simultaneously, the pressure of the gas was increased to 170 MPa. 1O 14 In step b) the same temperature and gas pressure was maintained for 2 hours.

Since this temperature was already above the “ß transus” temperature, an increase in temperature in step b) was not needed for this titanium alloy.

In step c), the workpiece was quenched by cooling nitrogen gas according to the following cooling rates of the gas: 3600 K/min between 960-900°C,2460 K/min between 900-800°C,1440 K/min between 800-700°C,1020 K/min between 700-600°C and 600 K/ min between 600-500°C, The temperature of the gas was measured by thermocouples.In this case, no aging treatment was carried out.In step e) the work pieces were cooled to room temperature.

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.

The thin-walled tubes were then analysed by microstructural determinationand it was found that the material comprised a Widmannstätten structurewith non-continuous ß-phase. The microstructural analyse furtherdetermined the material of thin-walled tubes to have been cooled at >1200K/ min (>20 K/ s) through the 888-868°C interval for o1+ß structureformation at this cooling rate, corresponding to a water quench (WQ) rate.Since the cooling rate of the nitrogen gas was more than twice as highthrough at this temperature interval and since the heat transfer coefficient(>1000 W/m2xK) 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.

Microstructural evaluation further showed a formation of a 20 um layer of ö-TiN + s-Ti2N with hardness up to 1068122 HV0.05 at 1 um depth and 519HV0.025 at 10 um depth, followed by a sloping decrease in hardness 30 umfurther into the titanium metal to the bulk level of 230-250 HVO.1.

The invention has mainly been described above with reference to a fewembodiments. However, as is readily appreciated by a person ski11ed in theart, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims (18)

1. Method of treating a workpiece (12) comprising a titanium metal,wherein a titanium metal surface layer of the workpiece is converted totitanium nitrides, which method comprises the following steps; a) heating the workpiece (12) to an initial nitriding temperature (Tn1); b) subjecting said workpiece to one or more nitriding temperatures (Tm,Tn2) for predetermined time(s) in a nitrogen containing gas (14) under highpressure at hot isostatic pressing (HIP) conditions for converting thetitanium metal surface layer to a first layer portion consisting of titaniumnitrides and a second layer portion comprising a nitrogen gradient in thetitanium metal; the method being characterized by; c) quenching the workpiece (12) in the nitrogen containing gas (14) underhigh pressure at hot isostatic pressing (HIP) conditions, in order tostrengthen the titanium metal below the, in step b) formed, first nitride layer portion.

2. Method according to claim 1, wherein the work piece (12), before step c), is subjected to at least one temperature (Tm) at or above the ß phasetransus temperature for the titanium alloy in question for solution treatmentof the titanium alloy, to convert prior a phase or o1+ß phase structure to solely or predominantly ß phase structure.

3. Method according to claim 1 or 2, wherein step c) comprises quenchingthe workpiece (12) at a cooling rate which is high enough to, at least partially,transform ß phase by a martensitic transformation directly to a' phase or a' ' phase.

4. Method according to any of claims 1-3, wherein said quenching rate ischosen for delaying the remaining transformation of ß phase to a phase at lower temperatures, resulting in substantially finer microstructures. 1O 17

5. Method according to any of claims 1-4, wherein the workpiece (12), afterstep c), is subjected to at least one aging temperature (Tq2) at a predetermined time to obtain precipitation hardening of the titanium metal.

6. Method according to any of claims 1-5, wherein the workpiece (12) iscooled to room temperature after quenching or after subjecting the workpiece to the at least one aging temperature for the predetermined time.

7. Method according to any of claims 1-6, wherein the workpiece (12) ispositioned in one and the same hot isostatic press chamber during the entire execution of the method.

8. Method according to any of claims 1-7, wherein the workpiece (12) issubjected to the nitrogen containing gas under high pressure at hot isostatic pressing (HIP) conditions during the entire execution of the method.

9. Method according to any of claims 1 -8, wherein the titanium metalcomprises at least one of the following; titanium alloy of Grade 1, Grade 2, Grade 5 or Grade 9.

10. Method according to any claims 1-9, wherein said workpiece (12), instep b), is quenched at a quenching rate sufficient to prevent the formation of a phase structure.

11. Method according to any of claims 1-10, wherein said workpiece (12), in step b), is quenched at a quenching rate of at least 150 K/ min.

12. Method according to any of claims 1-11, wherein step c) is carried out athot isostatic pressure conditions where the nitrogen gas pressure is at least initially above 10 MPa.

13. Method according to any of claims 1-12, wherein the workpiececomprises 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 1O 18 at least 300 K / min for Grade 9.

14. Object comprising a titanium metal alloy of Grade 2, Grade 5 or Grade9, characterized in that the object exhibits a surface layer comprising afirst nitride layer portion and second titanium metal portion with a nitrogengradient which surface layer extends to a depth of at least 50 pm when thetitanium metal alloy is Grade 2 or Grade 9 and at least 75 pm when the titanium metal alloy is Grade 5.

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

16. Object according to claim 14 or 15 wherein the titanium metal exhibits solely or predominantly a' phase or a' ' phase structures.

17. Object according to any of claims 14-15, which object constitutes orforms part of a component chosen from a group of components comprising; automotive, aerospace, mining and medical components.

18. Object according to any of claims 14-17, which object has been subjected to a method according to any of claims 1-11.