Title:
Method for Producing Wear-Resistant and Fatigue-Resistant Edge Layers in Titanium Alloys, and Components Produced Therewith
Kind Code:
A1


Abstract:
The invention relates to edge layer finishing of functional components, and thereby in particular to a method for producing wear-resistant and fatigue-resistant edge layers in titanium alloys, and components produced therewith. The method according to the invention for producing wear-resistant and fatigue-resistant edge layers in titanium alloys by means of laser gas alloying is essentially characterized in that the laser gas alloying is carried out with a reaction gas that contains or releases interstitially soluble elements in the titanium alloy used, whereby the partial pressure of the reaction gas is selected such that the partial pressure remains below the threshold value above which nitride, carbide, or boride titanium phases are produced. The features according to the invention of the wear-resistant and fatigue-resistant component made of a titanium alloy with a gas-alloyed edge layer essentially are that the wear-resistant edge layer is composed of a fine-grain mixture of α- and β-titanium grains with an interstitially dissolved reaction gas, has a surface hardness Hs, measured on the ground surface, of 360 HV0.5≦Hs≦500 HV0.5, or an edge layer microhardness HR, measured on a polished cross section at 0.1 mm below the surface, of 360 HV0.1≦HR≦560 HV0.1, extends over a depth tR of 0.1 mm≦tR<3.5 mm, and does not contain any nitride, carbide, oxide, or boride phases produced by the reaction gas.



Inventors:
Brenner, Berndt (Dresden, DE)
Bonss, Steffen (Zella-Mehlis, DE)
Tietz, Frank (Dresden, DE)
Kaspar, Joerg (Dresden, DE)
Walter, David (Muelheim, DE)
Application Number:
11/571818
Publication Date:
01/17/2008
Filing Date:
07/08/2005
Assignee:
SIEMENS AG (Munich, DE)
FRAUNHOFER-GESELLSCHAFT ZUR FOERDERRUNG DER ANGEWANDTEN FORSCHUNG E.V. (Munich, DE)
Primary Class:
Other Classes:
148/565
International Classes:
C21D1/09; C22B4/00; C23C8/06; C23C8/08; C23C8/24; C23C24/08
View Patent Images:



Foreign References:
JPS5647560A1981-04-30
Other References:
Wear (http://en.wikipedia.org/wiki/Wear#cite_note-Jones_and_Scott-5)
English translation of JP 56-047560 (1981)
Primary Examiner:
TAKEUCHI, YOSHITOSHI
Attorney, Agent or Firm:
GREENBLUM & BERNSTEIN, P.L.C. (RESTON, VA, US)
Claims:
1. A method for producing wear-resistant and fatigue-resistant edge layers in titanium alloys by high-intensity energy gas alloying, comprising: carrying out high-intensity energy gas alloying in a treatment zone using a reaction gas that contains or releases interstitially soluble elements in the titanium alloy used, and wherein the partial pressure of the reaction gas is selected such that the partial pressure remains below the threshold value above which nitride, carbide, or boride titanium phases are produced.

2. The method according to claim 1, wherein nitrogen is used as the reaction gas which is interstitially soluble in the titanium alloy, and the nitrogen together with an inert gas is fed to the laser treatment zone.

3. The method according to claim 2, wherein the volumetric proportion VN of the nitrogen in the working gas mixture is 1%≦VN≦15%.

4. The method according to claim 3, wherein the volumetric proportion VN of the nitrogen is selected in the range of 1%≦VN≦11% for components subjected to particularly high fatigue stress.

5. The method according to claim 2, wherein the volumetric proportion VN of the nitrogen in the working gas is altered during the processing and is adapted to the localized load conditions and to the ratio of wear to cyclic load.

6. The method according to claim 1, wherein the gas-alloyed edge layer is subjected to an accelerated cooling.

7. The method according to claim 6, wherein the accelerated cooling is achieved by a self-quenching as the result of an external cooling of the untreated portions of the component during the gas alloying.

8. The method according to claim 6, wherein the accelerated cooling is achieved by a localized gas cooling subsequent to the treatment zone.

9. The method according to claim 1, wherein the component is mechanically fixed before the high-intensity energy gas alloying and is maintained in the fixed state during the high-intensity energy gas alloying.

10. The method according to claim 1, wherein the fixing and cooling are implemented by the same device.

11. The method according to claim 1, wherein after being cooled to room temperature the gas-alloyed edge layer is mechanically smoothed by vibratory finishing, grinding, and/or polishing.

12. The method according to claim 1, wherein after the component is cooled to room temperature or after a mechanical smoothing by vibratory finishing, grinding, and/or polishing, wherein an aging heat treatment of the entire component is performed at a temperature TA of 350°≦TA≦700° C. for an aging period tA of 2 h≦tA≦24 h.

13. The method according to claim 11, wherein after the mechanical smoothing a stress-free annealing is subsequently carried out at a temperature TSR of 300°≦TSR≦620° C. and a period tSR of 1 h≦tSR≦10 h.

14. The method according to claim 6, wherein the gas-alloyed layer is shot-blasted after the cooling from the last heat treatment.

15. The method according to claim 1, wherein a non-vac electron beam unit is used as a high-intensity energy source.

16. The method according to claim 1, wherein a plasma torch is used as a high-intensity energy source.

17. A wear-resistant and fatigue-resistant component made of a titanium alloy, having a gas-alloyed edge layer formed by carrying out high-intensity energy gas alloying in a treatment zone using a reaction gas that contains or releases interstitially soluble elements in the titanium alloy used, and wherein the partial pressure of the reaction gas being selected such that the partial pressure remains below the threshold value above which nitride, carbide, or boride titanium phases are produced, such that the wear-resistant edge layer is composed of a fine-grain mixture of α-titanium and β-titanium grains with an interstitially dissolved reaction gas, has a surface hardness HS, measured on the ground surface, of 360 HV0.5≦HS≦500 HV0.5, or an edge layer microhardness HR, measured at the polished cross section 0.1 mm below the surface, of 360 HV0.1≦HR≦560 HV0.1, extends over a depth tR of 0.1 mm≦tR≦3.5 mm, and does not contain any nitride, carbide, oxide, or boride phases produced by the reaction gas.

18. The wear-resistant and fatigue-resistant component made of a titanium alloy according to claim 17, wherein the wear-resistant edge layer is composed of a track pattern of overlapping individual tracks, and the track overlap rate ü, wherein 0=a-ca(aΔ_track width,c=^track spacing) and ü is 0.5≦ü≦0.9.

19. The wear-resistant and fatigue-resistant component made of a titanium alloy according to claim 17, wherein the edge layer is composed of a wide individual track which is produced by oscillating a beam of the high-intensity energy source transverse to a feed direction.

20. The wear-resistant and fatigue-resistant component made of a titanium alloy according to claim 17, wherein the component represents a turbine blade subjected to erosion or droplet impingement stress.

21. The wear-resistant and fatigue-resistant component made of a titanium alloy according to claim 20, wherein the wear-resistant edge layer includes a leading edge of the blade in a direction of a concave side of the blade and as well as in a direction of a back side of the blade.

22. The wear-resistant and fatigue-resistant component made of a titanium alloy according to claim 20, wherein the wear-resistant edge layer is composed of overlapping tracks parallel to the leading edge.

23. The wear-resistant and fatigue-resistant component made of a titanium alloy according to claim 17, wherein the sequence of track production is selected in such a way that the tracks are each situated in alternation with the neutral fiber with respect to a bending of the turbine blade in the pliant direction.

24. The wear-resistant and fatigue-resistant component made of a titanium alloy according to claim 20, wherein the wear-resistant edge layer is composed of tracks situated transverse to the longitudinal axis of the turbine blade or to the leading edges thereof, the tracks extend around the leading edge, and the oscillation of the beam from the high-intensity energy source is produced about the longitudinal axis of the blade as a result of an oscillating vibratory motion of the turbine blade.

25. The wear-resistant and fatigue-resistant component made of a titanium alloy according to claim 17, wherein a track field boundary on the blade foot side runs at an angle of 20-65° with respect to the leading edge.

26. The wear-resistant and fatigue-resistant component made of a titanium alloy according to claim 17, wherein the edge layer hardness is correspondingly adapted according to localized load conditions and a ratio of wear to cyclic load.

27. The method according to claim 1, wherein a laser is used as a high-intensity energy source.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Stage of International Application No. PCT/EP2005/07393, filed Jul. 8, 2005, which claims priority under 35 U.S.C. §119 of German Patent Application No. 10 2004 033 342.4, filed Jul. 9, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the edge layer finishing of functional components, and in particular to a method for producing wear-resistant and fatigue-resistant edge layers in titanium alloys, and components produced therewith. Articles for which the use thereof is possible and practical are components made of titanium alloys which undergo severe stress from erosion, cavitation, droplet impingement, or sliding abrasion, which in addition to their tribological load are also subjected to high cyclic load. The invention may be used in a particularly advantageous manner for protecting turbine blades in the low-pressure region of steam turbines. Other components which may thus be finished in a very advantageous manner are those from the aviation and aerospace sectors, such as, e.g., landing flap guides, drive shafts, hydraulic system components, bolts, or similar connecting elements; parts used in chemical equipment construction (e.g., sonotrode tips, sonochemical facilities), medical technology (e.g., instruments, implants), and high-performance engine construction (e.g., injection systems, valve seats, valve stems, or the like).

2. Discussion of Background Information

Titanium is a superior construction material whose high specific strength, exceptional fatigue resistance, good stress-crack corrosion resistance, chemical resistance, and biocompatibility make it ideal for various special applications. However, the low resistance of titanium to various types of wear is often an obstacle to its more widespread use. The demand for effective wear protection methods has intensified due to the fact that many thermal and chemicothermal edge layer finishing methods used for steel materials, for example, cannot be employed for titanium alloys.

One current method known for producing very wear-resistant edge layers on titanium and its alloys is laser gas alloying (see, for example, H. W. Bergmann: “Thermochemische Behandlung von Titan und Titanlegierungen durch Laserumschmelzen und Gaslegieren,” Zeitschrift für Werkstofftechnik 16 (1985), pages 392-405). An early application of this technology was the protection of joint endoprostheses, as described in DE 39 17 211. To this end, the component is fused by a laser beam up to a depth of 0.1 to 0.7 mm with simultaneous injection of nitrogen into the melt. Due to the high affinity of the titanium for reactive gases such as nitrogen, for example, the gas is immediately dissolved by the melt, and in the case of nitrogen forms titanium nitride, which precipitates from the melt in the form of dendrites. After solidification, the edge layer is composed of a metallic titanium matrix having an altered α/β proportion compared to the initial state, and finely divided titanium nitride dendrites. The hardness of the edge layer is typically 600-1200 HV. Edge layers produced in this manner have very good resistance to stress from sliding abrasion, abrasive wear, or oscillatory sliding abrasion.

Without limiting the universality, the aim is to identify the disadvantages of edge layers produced in this manner for the case of application of increasing the resistance of steam turbine blades to wear from droplet impingement.

During use, rotor blades of low-pressure stages in steam turbines are subjected to extremely high quasistatic (centrifugal force, blade twisting), cyclic (periodic steam pressure impingement, blade vibrations), mechanical-chemical (vibratory corrosion and stress-crack corrosion), and tribological (droplet impingement) loads. Although selected titanium alloys such as, e.g., Ti6AI4V withstand the quasistatic, cyclic, and mechanical-chemical loads very well, their resistance to droplet impingement wear is not adequate to effectively protect highly stressed steam turbine blades from wear erosion as the result of the continuous impact of water droplets in the vicinity of the leading edge.

In analogy to the above-referenced prior art, Gerdes claims the application of laser gas alloying for increasing the wear resistance of low-pressure steam turbine blades (see EP 0491075 B1). A boride-, carbide-, or nitride-forming gas is thereby added to the melt bath in a concentration that results in precipitation of borides, carbides, or nitrides in the melt. In the case of nitrogen as reaction gas, extremely hard titanium nitride is formed when volumetric proportions of nitrogen of typically 20 to 60% with respect to inert gas are used. Under the stated treatment parameters, a hardness of 500 to 900 HV, preferably 500 to 700 HV, is obtained in the edge layer of the Ti6AI4V alloy. Information was not provided concerning the wear resistance and fatigue resistance achieved.

To avoid cracks, the thickness of the protective layer is limited to 0.4 to 1.0 mm. A mechanical or thermal aftertreatment of the layers thus produced is not mentioned, but should not be provided, since particular reference is made to the fact that protection from droplet erosion is obtained with a single method step, namely, the laser gas alloying.

It is known from the applicant's own investigations that, for specimens from the same Ti6AI4V alloy which were laser gas alloyed with a nitrogen content of 25% or 35%, fatigue resistances of only approximately 67% or 37%, respectively, of the base material are achieved.

Thus, the shortcoming of the method is that an edge layer-finished turbine blade of this type has a cyclic load capacity that is far too low. Therefore, the method cannot be used for turbine blades subjected to high cyclic load.

A further shortcoming results from the very limited depth of the protective layer of 0.4 to 1.0 mm. For erosive types of wear, such as, e.g., the droplet impingement wear on turbine blades, this results in limited service life.

The shortcoming results from the fact that microfissures form when the nitrogen content in the gas mixture exceeds 20%. The tendency for microfissure formation increases with the thickness of the protective layer. In addition, the heterogeneous structure, which is formed from a relatively brittle α-titanium matrix with very hard embedded TiN particles, in principle is not suitable for achieving high fatigue resistance.

For improving the fatigue resistance while retaining the very good wear resistance under droplet impingement stress, it is known to subject the components, after the laser gas alloying, to a combined subsequent heat treatment followed by shot blasting (see Gerdes, EP 0697503 A1). The protective layer formed by laser gas alloying in a boron-carbon- or nitrogen-containing gas atmosphere and based on titanium boride, carbide, and/or nitride thereby undergoes heat treatment with formation of a high-vanadium β-titanium phase at a temperature between 600 and 750° C. If nitrogen is selected as the reaction gas, in combination with a mechanical polishing and subsequent shot blasting with an Almen intensity of 0.3 mmA and at least a double coverage, it is then possible, while retaining the wear resistance, to increase the fatigue resistance from approximately 30% to approximately 85% of the initial value for the untreated blade material. The protective layer, having a thickness between 0.4 mm and 1.0 mm, essentially contains titanium nitrides embedded in an α-titanium matrix. The morphology and distribution of the titanium nitrides depends on the process parameters during laser gas alloying, and the nitrogen concentration in the gas atmosphere. Depending on the nitrogen concentration, the titanium nitrides are to be embodied in a plate-like or dendritic manner. When nitrogen is used as reaction gas and argon is used as inert gas, the proportions are to be between 1:4 and 1:2; i.e., the nitrogen content is to be between 20% and 33%. The protective layer formed may then have a Vickers hardness of 600 to 800 HV, depending on the conditions during the laser gas alloying.

The shortcoming of this enhanced method is that the stated fatigue resistance can be achieved only on polished specimens or new blades. However, a characteristic manifestation of droplet impingement wear is the severe roughening of the surface which occurs very early during the service life of a turbine blade. This roughening has three disadvantageous consequences for the cyclic load capacity of the turbine blade:

Firstly, the notch effect of the super-hardened material state increases.

Secondly, the roughening reduces the near-surface internal pressure stress of the shot blast treatment. Furthermore, the negative effect of the microfissures, which are still present but the effects of which have been removed by the shot blasting, may reoccur.

Thus, the deficiency of this enhanced method results from the fact that the causes for the reduced fatigue resistance of the structure—the formation of microfissures and a structure with very hard, brittle phases with insufficient suitable heterogeneity for higher cyclic loads—are not eliminated, but, rather, their effects are only temporarily controlled.

A further shortcoming of the method results from the fact that the improvement in fatigue resistance presupposes a vanadium-containing titanium alloy, as described there. However, there are a number of economically important titanium alloys having a similar range of application which do not contain vanadium.

The shortcoming results from the fact that the purpose of the claimed heat treatment is to form a high-vanadium phase having a β-titanium lattice structure.

SUMMARY OF THE INVENTION

The present invention provides a method for producing wear-resistant and fatigue-resistant edge layers in titanium alloys by high-intensity energy gas alloying, including: carrying out high-intensity energy gas alloying in a treatment zone using a reaction gas that contains or releases interstitially soluble elements in the titanium alloy used, and wherein the partial pressure of the reaction gas is selected such that the partial pressure remains below the threshold value above which nitride, carbide, or boride titanium phases are produced.

In one embodiment, nitrogen is used as the reaction gas which is interstitially soluble in the titanium alloy, and the nitrogen together with an inert gas is fed to the laser treatment zone.

In another embodiment, the volumetric proportion VN of the nitrogen in the working gas mixture is 1%≦VN≦15%.

In yet another embodiment, the volumetric proportion VN of the nitrogen is selected in the range of 1%≦VN≦11% for components subjected to particularly high fatigue stress.

In one embodiment, the volumetric proportion VN of the nitrogen in the working gas is altered during the processing and is adapted to the localized load conditions and to the ratio of wear to cyclic load.

In another embodiment, the gas-alloyed edge layer is subjected to an accelerated cooling.

In yet another embodiment, the accelerated cooling is achieved by a self-quenching as the result of an external cooling of the untreated portions of the component during the gas alloying.

In one embodiment, the accelerated cooling is achieved by a localized gas cooling subsequent to the treatment zone.

In another embodiment, the component is mechanically fixed before the high-intensity energy gas alloying and is maintained in the fixed state during the high-intensity energy gas alloying.

In yet another embodiment, the fixing and cooling are implemented by the same device.

In one embodiment, after being cooled to room temperature the gas-alloyed edge layer is mechanically smoothed by vibratory finishing, grinding, and/or polishing.

In another embodiment, after the component is cooled to room temperature or after a mechanical smoothing by vibratory finishing, grinding, and/or polishing, wherein an aging heat treatment of the entire component is performed at a temperature TA of 350°≦TA≦700° C. for an aging period tA of 2 h≦tA≦24 h.

In yet another embodiment, after the mechanical smoothing a stress-free annealing is subsequently carried out at a temperature TSR of 300°≦TSR≦620° C. and a period tSR of 1 h≦tSR≦10 h.

In one embodiment, the gas-alloyed layer is shot-blasted after the cooling from the last heat treatment.

In another embodiment, a non-vac electron beam unit is used as a high-intensity energy source.

In yet another embodiment, a plasma torch is used as a high-intensity energy source.

In one embodiment, a laser is used as a high-intensity energy source.

The present invention also provides a wear-resistant and fatigue-resistant component made of a titanium alloy, having a gas-alloyed edge layer formed by carrying out high-intensity energy gas alloying in a treatment zone using a reaction gas that contains or releases interstitially soluble elements in the titanium alloy used, and wherein the partial pressure of the reaction gas being selected such that the partial pressure remains below the threshold value above which nitride, carbide, or boride titanium phases are produced, such that the wear-resistant edge layer is composed of a fine-grain mixture of α-titanium and β-titanium grains with an interstitially dissolved reaction gas, has a surface hardness HS, measured on the ground surface, of 360 HV0.5≦HS≦500 HV0.5, or an edge layer microhardness HR, measured at the polished cross section 0.1 mm below the surface, of 360 HV0.1≦HR≦560 HV0.1, extends over a depth tR of 0.1 mm≦tR≦3.5 mm, and does not contain any nitride, carbide, oxide, or boride phases produced by the reaction gas.

In another embodiment, the wear-resistant edge layer is composed of a track pattern of overlapping individual tracks, and the track overlap rate ü, wherein 0=a-ca(aΔ_track width,c=^track spacing)

and ü is 0.5≦ü≦0.9.

In yet another embodiment, the edge layer is composed of a wide individual track which is produced by oscillating a beam of the high-intensity energy source transverse to a feed direction.

In one embodiment, the component represents a turbine blade subjected to erosion or droplet impingement stress.

In another embodiment, the wear-resistant edge layer includes a leading edge of the blade in a direction of a concave side of the blade and as well as in a direction of a back side of the blade.

In yet another embodiment, the wear-resistant edge layer is composed of overlapping tracks parallel to the leading edge.

In one embodiment, the sequence of track production is selected in such a way that the tracks are each situated in alternation with the neutral fiber with respect to a bending of the turbine blade in the pliant direction.

In another embodiment, the wear-resistant edge layer is composed of tracks situated transverse to the longitudinal axis of the turbine blade or to the leading edges thereof, the tracks extend around the leading edge, and the oscillation of the beam from the high-intensity energy source is produced about the longitudinal axis of the blade as a result of an oscillating vibratory motion of the turbine blade.

In yet another embodiment, a track field boundary on the blade foot side runs at an angle of 20-65° with respect to the leading edge.

In another embodiment, the edge layer hardness is correspondingly adapted according to localized load conditions and a ratio of wear to cyclic load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the cavitational wear rate Δ {dot over (m)} as a function of the nitrogen content VN in the working gas.

FIGS. 2(a)-(b) illustrate the cross section of a laser-gas-alloyed leading edge.

The aim of the present invention is to provide a method for improving the wear resistance of titanium alloys which results in the least possible decrease in cyclic load capacity in the laser gas-alloyed state compared to the initial state, and which also allows an improved retention of the cyclic load capacity during continued wear. A further aim is to provide wear-resistant and fatigue-resistant components which may be advantageously manufactured using this method.

The object of the present invention, therefore, is to provide an edge layer finishing method which allows establishment of a homogeneous and semi-rigid structural state without the presence of brittle titanium nitride, carbide, or boride phases, regardless of the vanadium content of the titanium alloy, achievement of very good resistance to stress from non-abrasive, cavitational, or droplet impingement wear, and realization of layer thicknesses greater than 1 mm without the formation of microfissures.

This object is attained according to the invention by a method for producing wear-resistant and erosion-resistant edge layers in titanium alloys. Components according to the invention and advantageous embodiments are described herein.

As described herein, the method proceeds from introducing into the melt gaseous elements which are soluble in the melt and which after solidification are interstitially dissolved, up to a concentration in which nitride, carbide, or boride titanium phases are not yet formed. In contrast to the known prior art, this results in a comparatively homogeneous and fine-grained structure having structure lengths of several micrometers, of α-titanium grains, individual β-titanium grains, and an arrangement of a mixture of very fine α-titanium and β-titanium grains located between the α-titanium grains. In this manner, the nitride, carbide, or boride phases heretofore known for their negative effects on microfissure formation and development of localized stress concentrations may be avoided.

For the case that nitrogen is used as an element to be introduced interstitially, in one embodiment it can be advantageous to provide volume proportions of nitrogen in the gas mixture. The specific choice thereby depends on the track overlap rate ü and the extent of wear stress relative to the magnitude of the cyclic load. In general, low nitrogen contents are selected for elevated cyclic loads at low wear stress, and vice versa. Likewise, low nitrogen contents are used for higher track overlap rates.

Since the wear stress and the fatigue stress normally vary along the component surface and their ratio consequently changes, it is particularly advantageous to locally adapt the optimal ratio of strength and ductility by varying the nitrogen content in the working gas.

Use may be made of a further influencing variable for setting the ratio between strength and ductility by virtue of the selection of the cooling rate.

During the gas alloying, a noticeable warping of the component occurs due to the intensive localized energy effect in particular with greater depths of the wear-resistant edge layer and/or greater track overlap rates. In one embodiment, in order to reduce or eliminate the warpage in warp-prone components, the component is mechanically fixed in an initial position before starting the gas alloying, and this fixed position is maintained during the treatment. This procedure also has the advantage that the target coordinates for the laser beam do not drift during the treatment, and the CNC program therefore does not require follow-up correction.

After the laser gas alloying, the surface has a roughness which generally is excessive for components under high cyclic load, for which one embodiment provides mechanical aftertreatment. The mechanical aftertreatment may be performed by grinding, vibratory finishing, and/or polishing. The mechanical properties, in particular the fatigue resistance, may be further improved by the heat treatment. In one embodiment, a procedure which the structure formation resulting from phase transitions and hardening, as well as the simultaneous reduction of the internal tension stress remaining after the gas alloying, or in another embodiment, a procedure wherein the internal tension state may be improved without significant alteration of the structure. Since the latter procedure has an effect primarily on the fatigue resistance, and the former procedure (i.e., the procedure which the structure formation resulting from phase transitions and hardening, as well as the simultaneous reduction of the internal tension stress remaining after the gas alloying) affects both the wear resistance and the fatigue resistance, an improved combination of properties may be obtained through the selection of the type of heat treatment.

It is known that the cyclic load capacity of titanium alloys may be improved in particular by near-surface internal pressure stresses, which cannot be adjusted by the above-referenced heat treatment. Therefore, one embodiment of the present invention provides for shot blasting of the wear-resistant edge layer as a further measure for increasing the fatigue resistance.

According to the prior art heretofore, the gas alloying of titanium is carried out using a laser as an energy source with sufficiently high power density. For suitable components, however, a non-vac electron beam or a plasma source may also be used as an energy source. In this case, the working gas mixture composed of reaction gas and inert gas must be fed in a careful and reproducible manner by use of collimator systems.

In one embodiment, advantageous dimensions and hardnesses of the edge layer for components according to the invention, as well as the design of the wear protection zone and structural features which are necessary for this, are provided. For example, the wear-resistant edge layer can be composed of a fine-grain mixture of α-titanium and β-titanium grains with an interstitially dissolved reaction gas has a surface hardness HS, measured on the ground surface, of 360 HV0.5≦HS≦500 HV0.5, or an edge layer microhardness HR, measured at the polished cross section 0.1 mm below the surface, of 360 HV0.1≦HR≦560 HV0.1, extends over a depth tR of 0.1 mm≦tR≦3.5 mm.

If the required width of the wear protection zone is greater than the track width that is normally possible, overlapping track patterns may be produced, or wider tracks may be realized by a rapid oscillation of the beam from the energy source transverse to the feed direction of the component. If necessary, in individual cases the component may be set in oscillatory motion instead of the beam.

The present invention may be applied in a particularly advantageous manner to turbine blades stressed by droplet impingement or erosion. In other embodiments of the present invention, technically expedient and beneficial embodiments of the shape, position, and production of the wear-resistant edge layer, can be provided.

Without limiting the universality, the invention is explained below on the basis of one method example and one component example.

EXAMPLE 1

Functional parts of sonochemical equipment are subject to both cavitational load and high cyclic load during operation. The service life of the sonotrodes in particular is drastically reduced by the cavitational load. The sonotrode is composed of the material Ti6AI4V, and in the initial state has a hardness of 340 HV0.5.

For increasing the sonotrode service life, the aim is to provide better protection of the sonotrode tip and the adjacent regions of the cylindrical surface area from cavitational load while ensuring sufficiently high cyclic load capacity. For carrying out the laser gas alloying according to the invention, the sonotrode is placed in a concentric chuck of a CNC rotational axis of a laser processor. The sonotrode has an oversized dimension of 0.2 mm in the region to be treated. A shielding gas bell designed according to EP 0829325 is located above the component, and ensures that the working gas mixture can be reproducibly set with very good exclusion of oxygen, using simple equipment. By use of a gas mixing station a preset nitrogen-argon mixture in a 9%:91% volumetric ratio is blown into the shielding gas bell. The shielding gas bell is attached to the beam-shaping unit of the laser so as to be movable in the z-direction. By means of an air cushion at its lower boundary, the shielding gas bell is also movable in the two other directions x and y with respect to the component.

A CO2 laser with power set at 3.1 kW is used as the laser. The distance of the laser beam focal point to the component surface is selected such that the laser beam has a beam diameter of 3.8 mm on the component surface. This results in a track width of a=3.5 mm for the laser gas-alloyed zone. A distance of 0.75 mm is set as the track spacing c, resulting in a track overlap rate ü=0.75. A feed rate of 540 mm/min is preselected via the CNC program. After purging for 90 s with the working gas mixture, the process is initiated by starting the CNC program, which travels over the sonotrode tip in a grid-like manner, and by switching on the laser. After the slow cooling, the sonotrode is removed from the apparatus, and the excess dimensions in the laser-treated region are ground off.

The layer thus produced is composed of a relatively homogeneous remelt layer having a depth of tR=0.5 mm and a heat influence zone therebelow with a thickness of 1.2 mm. The surface hardness HS reaches approximately 440 HV0.5. This corresponds approximately to an edge layer hardness HR of approximately 510 HV0.1, measured on the polished cross section at 0.1 below the surface. The fluctuation in hardness transverse to the tracks is approximately 50 HV0.1.

The structure is composed of α-titanium grains and finely dispersed deposits of α- and β-titanium at the grain boundaries of the α-titanium grains. The maximum size of the α-titanium grains is several micrometers.

This treatment allows the wear rate at the sonotrode tip to be reduced by approximately a factor of three.

In this case, the cyclic load capacity thus achieved is sufficient to eliminate the need for a further measure for increasing the fatigue resistance.

FIG. 1 illustrates the wear rate Δ {dot over (m)} as a function of the volumetric proportion of nitrogen VN in the working gas. The load from cavitational wear was measured in accordance with ASTM G32-85, using a VC501 high-frequency generator from Sonics & Materials Inc. The test parameters are as follows: indirect vibrational cavitational load on the specimens; frequency: 20 kHz, amplitude: ±20 μm, water temperature (controlled): 22° C.±1K, immersion depth of the specimen surface: 12 . . . 16 mm; distance sonotrode surface to specimen surface: 0.5 mm, load duration: 20 h, measurement of mass erosion every 1.5 h. The wear rate is determined from the increase in slope of the mass loss-time curve after the end of the incubation period.

FIG. 1 clearly shows that, as already known, the resistance to cavitational wear may be significantly improved by laser gas alloying with nitrogen. Contrary to the preconceptions among those skilled in the art, however, this does not require the precipitation of titanium nitride. Even with nitrogen contents of 7 to 13%, wear properties are achieved that are comparable to those with nitrogen contents greater than 20%. In contrast, it was found that the formation of precipitates of dendritic titanium nitride has a very negative effect on the one-way mechanical (crack formation stress) and cyclic (fatigue resistance) load capacity. Thus, for the selected test parameters the crack formation stress, for example, at the edge layer already begins to decrease at a nitrogen content greater than 13%, i.e., clearly before the precipitation of titanium nitride dendrites. This threshold value is valid for the stated test conditions and shifts to higher nitrogen contents as the overlap rate decreases.

EXAMPLE 2

A component according to the invention is to be explained using an end stage rotor blade of a large steam turbine. The leading edges of these rotor blades are subjected to an intensive droplet impingement stress, which in their component stress, wear manifestations, and mechanisms of localized material damage share many commonalities with cavitational wear. Methods and material states which result in improved cavitational wear resistance also have improved resistance to droplet impingement wear. The end stage rotor blade is produced from the titanium alloy Ti6AI4V due to the high stress from centrifugal force. The width of the wear zone is 17 mm on the back side of the blade and 6 mm on the concave side of the blade. Because of the high cyclic load on the blade, other passive protection methods such as soldering of stellite plates, electric spark coating, vacuum plasma injection, or laser gas alloying carried out according to the prior art are excluded.

The component designed according to the invention (see FIG. 2) has a layer with a depth up to tR≈2.5 mm, composed of a fine-particle mixture of α- and β-titanium grains with interstitially dissolved nitrogen. This edge layer has a width of 20 mm on the back side of the blade and a width of 10 mm on the concave side of the blade, and therefore covers a region larger than the width of the wear zone. The track width is a=3.7 mm, the track spacing is c=0.8 mm. A laser power of 4.2 kW with a feed rate of 650 mm/min are selected.

There is a direct correlation of the quantity of the interstitially dissolved nitrogen to the surface hardness HS or the edge layer hardness HR. The layer has a surface hardness of HS≈425 HV0.5. The hardness measured below the surface at the polished cross section is HR≈550 HV0.1. This hardness is obtained with a nitrogen proportion of VN=7% in the working gas.

It is known that the fatigue resistance decreases along with the proportion of dissolved nitrogen. On the other hand, it is known that wear stresses are highest at positions on the turbine blade in the vicinity of the blade tip, but the cyclic loads at these positions are very low. Along the blade leading edge, starting from the blade tip, the wear stress decreases, whereas the cyclic load greatly increases. This circumstance is taken into account in a continuation of the approach according to the invention, in that the volumetric proportion VN of nitrogen in the working gas is set as a function of the blade position, i.e., the nitrogen proportion is reduced, starting at an initial value of VN=11% at the blade tip to a proportion of VN=0% at the end of the laser-treated zone. This simultaneously achieves a smoother transition of hardness and a reduction in the internal tension stresses at the end of the track field. Likewise, by changing the volumetric proportion VN on a track-by-track basis it is possible to achieve a similar hardness and property gradient during the production of the tracks 1, 2, 3, etc. or tracks 4, 6, 8, etc. (see FIG. 2b) at the lateral boundary of the processing region.

The provision of thermal stresses during the blade processing causes warpage, which generally is intolerably large in the direction of the lowest bending moment. This is counteracted by forming the wear-resistant zone from individual overlapping tracks and selecting the sequence of the track production such that, after a certain number of starting tracks (see FIG. 2b, two starting tracks 1, 2), the additional tracks are applied in alternation on the concave side and the back side of the blade. To minimize warpage, more tracks may be applied on the concave side of the blade than would be necessary on the concave side of the blade, based on the dimensions of the wear zone. The last track is applied along the leading edge. Depending on the circumstances, the last track is produced with a lower remelting depth and with higher feed rates. In addition to these measures for warpage reduction, the turbine blade is mechanically fixed before the start of treatment and is maintained in the fixed state during the gas alloying.

LISTING OF REFERENCE NUMBERS AND TERMS USED

  • A—Turbine blade
  • B—Back side of blade
  • C—Concave side of blade
  • D—Wear protection zone, laser gas alloyed edge layer
  • E—Leading edge
  • F—Trailing edge
  • HS—Surface hardness, measured according to Vickers with a 5-N load on the ground surface, average hardness from 10 measurements
  • HR—Edge layer hardness, measured according to Vickers with a 1-N load on the polished cross section along a track 0.1 mm below the surface, averaged over the track spacing and 21 measurements
  • TA—Temperature of the aging heat treatment
  • TSR—Temperature of the stress-free annealing
  • VN—Volumetric proportion of nitrogen in the working mixture
  • a—Track width
  • c—Track spacing
  • tA—Annealing time for the aging heat treatment
  • tR—Track depth
  • tSR—Annealing time for the stress-free annealing
  • ü—Overlap rate
  • Δ {dot over (m)}—Wear rate
  • 1 . . . 17—Track number