WO2006094935A1

WO2006094935A1 - Turbine blades and methods for depositing an erosion resistant coating on the same

Info

Publication number: WO2006094935A1
Application number: PCT/EP2006/060422
Authority: WO
Inventors: Ian John Chilton; Bruce Wynn Roberts; Stuart Richard Holdsworth
Original assignee: Alstom Technology Ltd
Priority date: 2005-03-05
Filing date: 2006-03-03
Publication date: 2006-09-14
Also published as: GB0504576D0

Abstract

The present invention relates to a method of providing an erosion resistant coating on the surface of a turbine blade having a desired profile. The method includes the step of providing a recess having a suitable shape and depth in the surface of the turbine blade. A laser deposition process is then used to deposit an erosion resistant coating material in the recess such that the outer surface of the resulting erosion resistant coating extends beyond the desired profile of the turbine blade. In a final stage in the method, the erosion resistant coating is machined to remove all parts of the erosion resistant coating that extend outside the desired profile of the turbine blade. The finished turbine blade is therefore provided with a coating with the desired profile having a sufficient thickness to protect it against water droplet erosion.

Description

TURBINE BLADES AND METHODS FOR DEPOSITING AN EROSION RESISTANT COATING ON THE SAME

Field of the Invention The present invention relates to turbine blades and in particular to turbine blades for use in steam turbines. The present invention provides a method for depositing an erosion resistant coating on turbine blades using a laser deposition process, such as a laser weld deposition or laser cladding.

Background of the Invention

As discussed in British Patent Application 1271758 (General Electric Company), water droplet erosion of the leading edges of steam turbine blades is a well-known problem. The problem is particularly acute for very long turbine blades in the last stage of the steam turbine where there is a higher proportion of water in the form of droplets (about 10%), and where the longer length of the turbine blades results in a higher tip speed. There are various ways in which the problem of erosion can be addressed.

It is generally preferred that the last stage turbine blades are formed from a material such as a quenched and tempered steel or precipitation-hardened steel or even a titanium alloy, and that the leading edge of each turbine blade is then protected against water droplet erosion. Protection from water droplet erosion can be achieved by changing the properties of the turbine blade in the region of the leading edge, such as by induction hardening the base material or by forging the leading edge from an erosion resistant material during the manufacturing process, for example, or by welding or brazing an erosion resistant shield or coating directly onto the outer leading edge surface of the turbine blade. In the case of an erosion resistant shield, this can be fitted in a suitably shaped recess formed in the radially outermost leading edge of the turbine blade and then brazed in position.

There is some doubt as to whether or not the use of a conventional brazed erosion resistant shield is suitable for new turbine blades that are longer than those that are currently being used and which are therefore subject to much higher tip speeds. There is a danger that the increased shear stresses encountered at these higher tip speeds could cause the shield break away from the turbine blade.

Another problem is known to exist with erosion resistant coating materials that are welded or deposited directly onto the outer surface of the turbine blade. It will be readily appreciated that the profile of a turbine blade must be carefully controlled to be within very strict tolerances. The deposition of the coating material directly onto the outer surface of the turbine blade will change this profile quite significantly and ideally the coating must therefore be machined back until the desired profile is achieved. Unfortunately, once the coating has been machined it could be too thin to provide effective protection against water droplet erosion.

Laser weld deposition (or laser cladding) is a well-established industrial process for depositing a metallic weldable material on the surface of a substrate using a laser beam. It can be used to deposit a coating material onto the outer surface of a turbine blade. Reference is made to International Patent Application WO 01/12381 (Chromalloy Gas Turbine Corporation), which describes how a laser deposition process can be used to cover a gas turbine blade platform with a hardfacing coating.

In this example, a suitable weld consumable such as a nickel- or cobalt-based alloy powder (or in some cases a nickel or cobalt-based alloy wire) is fed into a nozzle positioned just above the substrate surface. The alloy powder is normally supplied to the nozzle as a dispersion in an inert gas such as argon, for example. The deposition apparatus includes a CO₂ or solid-state laser. The laser beam generated by the laser passes through a passage in the nozzle and melts a small volume of the surface of the substrate (usually referred to as a melt pool). The alloy powder is delivered to the melt pool where it melts and mixes with the substrate material. When the laser beam is removed from this area because of relative movement between the nozzle and the substrate, the melt pool cools rapidly and solidifies to form an erosion resistant coating having the composition of the weld consumable and which is fused or metallurgically bonded to the substrate. The nozzle moves across the surface of the substrate in a number of sequential passes to deposit an erosion resistant coating material over a selected region. The thickness of the deposited coating can range from 0.1 to several millimetres. If necessary, multiple layers of the same coating can be deposited one on top of the other. Besides the choice of turbine blade and coating materials, parameters that can affect the laser weld deposition process include the power of the laser beam, the focal length of the laser beam, the feed of the alloy powder (including the powder-gas flow rate) and the speed of relative movement between the nozzle and the substrate.

Important technical advantages of the laser weld deposition process include the extremely small regions of dilution of the erosion resistant coating and the small heat affected zone (HAZ) due to the relatively low energy input.

Summary of the Invention

The present invention provides a method of providing an erosion resistant coating at the leading edge of a steam turbine blade having a desired profile, the method comprising the steps of: providing a recess in the leading edge surface of the turbine blade; depositing an erosion resistant coating material in the recess by a laser deposition process operative to produce a fusion bond between the coating material and the surface of the turbine blade, the amount of material deposited being such that the as-deposited erosion resistant coating extends beyond the desired profile of the turbine blade; and removing all portions of the erosion resistant coating that extend beyond the desired profile of the turbine blade.

It is very much preferred that the turbine blade is subjected to a stress relieving treatment after deposition of the coating, but before removal of the coating portions that extend beyond the desired profile of the turbine blade. The laser deposition process operative to produce a fusion bond between the coating material and the surface of the turbine blade may be a laser weld deposition or laser cladding process. A laser deposition process is used to apply the coating because it has certain technical advantages (see above), particularly that of creating a one-step fusion bond rather than a mechanical bond with the turbine blade, and is well understood.

By depositing the coating material in a recess, only a relatively small amount of the applied coating (i.e. only the part that extends outside the desired profile of the turbine blade) needs to be machined away. This contrasts with the prior art method described previously, where the coating material is deposited directly onto the outer surface of the turbine blade. The thickness of the finished coating will normally match the depth of the recess (which can be carefully and accurately controlled) and it therefore provides good protection against water droplet erosion.

It is generally preferred that sufficient erosion resistant coating material is deposited in the recess such that the complete outer surface of the as-deposited coating extends beyond the desired profile of the turbine blade. This ensures that the outer surface of the finished erosion resistant coating does not include depressions or recessed areas lying below the desired profile of the turbine blade.

The recess can be pre-formed when the turbine blade is forged or cast, or it can be formed subsequently during a machining process.

The peripheral edges of the recess, where the floor meets the walls of the recess, are preferably radiused or angled to avoid internal sharp corners and so prevent fatigue crack initiation. In other words, the edges of the recess are, at least to some extent, blended into the surrounding blade surface.

The depth of the recess can be determined with reference to the operating environment and design characteristics of the turbine blade. For example, turbine blades operating in environments with a higher proportion of water in the form of droplets or at higher operating speeds may be provided with a deeper recess so that the erosion resistant coating can be made thicker. Except for the above-mentioned blending of the edges of the recess with the surrounding blade surface, the depth of the recess may be constant throughout its length and breadth, or the depth of the recess may vary according to its position on the blade. For example, in some cases it might be useful to make the deposited coating thicker towards the higher speed tip region of the turbine blade so that this part of the turbine blade is provided with additional protection against water droplet impact. In this case, the depth of the recess could be tapered towards the base of the blade so that the recess is deeper at the end that is closest to the tip region of the turbine blade.

It is important that the depth of the finished coating should be neither too thin nor too thick. If the coating is too thin, then the coating will give insufficient protection against erosion of the blade's leading edge. On the other hand, as the thickness of the coating increases, there is an increasing likelihood of excessive distortion of the blade occurring, due to the localised heating effect of the laser deposition process and subsequent cooling and contraction of the coating.

In experiments, we used a laser weld process to deposit a Stellite-6 coating in recesses of the order of lmm deep. "Stellite" is a registered trademark. Stellite-6 is a cobalt- based alloy having a nominal composition in weight percent of carbon 1.6%, cobalt 66.9%, chromium 28% and tugsten 4%. For example, we found that a coating thickness of 0.5mm was too thin because it did not give sufficient protection against erosion, whereas a coating thickness of 1.5mm was too thick, because the coating and the blade on which it was deposited become subject to excessive distortion during cooling and solidification of the coating. We found that a finished coating thickness in the range of roughly 0.8- 1.2mm generally gave adequate erosion protection without causing unacceptable distortion of the blade. Of course, the depth of the recess should be the same as the desired finished coating thickness.

The erosion resistant coating material can have any suitable composition, including the nickel-based coatings alluded to previously. However, cobalt-based alloys are generally preferred. A number of suitable alloy compositions are described in EP-A 1403398 (Nuovo Holding S.P.A.). In addition to the above-mentioned Stellite-6 coating, an example of an alloy composition that has been found to be particularly suitable for coating steam turbine components includes chromium from 28 to 32% by weight, tungsten from 5 to 7% by weight, silicon from 0.1 to 2 % by weight, carbon from 1.2 to 1.7% by weight, nickel from 0.5 to 3% by weight, iron from 0.01 to 1% by weight, manganese from 0.01 to 1% by weight, molybdenum from 0.2 to 1% by weight with the remainder being cobalt. The alloy can be applied using the laser weld deposition process (column 5, lines 39 to 46 of EP -A 1403398).

The recess is formed along the leading edge near the tip of the turbine blade because this is where the most protection against water droplet erosion is required. The recess will preferably start at the tip region of the turbine blade and extend down towards the root or platform of the turbine blade. The recess may have any suitable shape in plan view and can be formed in either or both of the convex and concave aerofoil surfaces of the turbine blade. In other words, the recess can be confined to one surface or can wrap around the leading edge of the turbine blade so that it extends into both of the convex and concave surfaces. This latter configuration can help to reduce the level of distortion that can occur when the coating cools and solidifies (see below).

The erosion resistant coating material may be deposited in a single layer or in two or more layers of any suitable thickness. However, when the coating layers cool they contract and can cause distortion of the turbine blade and the generation of undesirable residual stress. The amount of distortion increases as more coating layers are deposited. It is therefore preferred that the number of coating layers is kept as low as possible.

Other steps can be taken to reduce the amount of distortion of the turbine blade. For example, the turbine blade can be pre-stressed or pre-distorted in an opposite sense before the coating is deposited. As already stated, the turbine blade should also be subjected to a stress relief process after the coating has been deposited, however, each different turbine blade material could require a different stress relief heat treatment. Forming the recess on both convex and concave surfaces of the turbine blade may also help to control distortion since contraction of the deposited coating material on opposing surfaces of the turbine blade will be balanced.

We have found in experiments that a very effective method of preventing distortion of the turbine blade is to stiffen the leading edge section of the blade by providing it with an extra, sacrificial, backing thickness of metal behind the recess into which the erosion-resistant coating is to be deposited. The backing portion of metal is preferably of the same composition as the rest of the blade and is formed integrally therewith, but might conceivably be a separate piece that is attached by brazing after the rest of the blade has been formed. Following laser deposition of the coating and a stress-relieving heat treatment, the backing thickness of material is removed. In the preferred case of an integral backing portion of the blade, it would be removed by machining (e.g. contour milling) to give the desired blade profile.

The coating material is preferably deposited in the recess as a series of mutually parallel passes, with each of the passes starting and finishing outside of the recess. In other words, the coating material is deposited on that part of the outer surface of the turbine blade immediately surrounding the recess, as well as in the recess itself. This may seem wasteful because any coating material that is deposited directly on the outer surface as an upstand will be completely removed during the machining process. However, in practice it has certain technical advantages. This is because the start and end of the weld passes (sometimes called the run-on and run-off) can be quite weak structurally and also act as a stress concentrator. If these parts of the coating material are deposited on the outer surface of the turbine blade beyond the recess and removed during the machining process then the coating that remains (i.e. the coating that is deposited in the recess) will have improved structural integrity. It also means that the coating material can be deposited over a simple rectangular area even if the recess has a more complicated shape, leading to a simplification in the deposition process. The passes can be substantially parallel to the leading edge of the turbine blade or at right angles to the leading edge of the turbine blade. Intermediate angles are also possible but are generally not preferred.

The parts of the coating that extend beyond the desired profile can be removed using any suitable machining process such as milling or grinding.

Brief Description of the Drawings

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram showing a cross-section of a known coating that has been deposited as an upstand;

Figure 2a is a schematic perspective view of a cross section of a turbine blade manufactured according to the present invention with a recess of exaggerated thickness formed on both convex and concave surfaces of the turbine blade;

Figure 2b is a schematic perspective view of a cross section of a turbine blade manufactured according to the present invention with a recess of exaggerated thickness formed on the convex surface of the turbine blade;

Figure 3 is a schematic perspective view of a cross section of a turbine blade manufactured according to the present invention with a recess of exaggerated thickness formed on the convex surface of the turbine blade and before the erosion resistant coating has been deposited;

Figure 4 is a schematic cross-sectional end view of the turbine blade of Figure 3 after two strips of the erosion resistant coating have been deposited; Figure 5 is a schematic side view of the turbine blade of Figure 4;

Figure 6 is a schematic top view of the turbine blade of Figure 4; Figure 7 is schematic cross-sectional end view of an experimental test piece representing part of a turbine blade before deposition of an erosion-resistant coating;

Figure 8 is a view like that of Figure 7, but showing the test piece after deposition of the erosion-resistant coating and;

Figure 9 is a flow diagram summarising the process of the invention. Detailed Description of the Preferred Embodiments

Figure 1 illustrates the problem encountered when an erosion resistant coating material is deposited directly onto the outer surface 2 of a turbine blade using a laser weld deposition process. The coating 4 extends above the level of the outer surface 2 as an upstand and there is a relative small amount of dilution between the base material of the turbine blade and the coating as indicated by the region 6. The dashed line 2a represents the profile that the turbine blade took before coating and which it must take in its finished form. Only very small deviations from this profile are allowed otherwise the turbine blade will not operate efficiently. It will therefore be immediately obvious that once the coating 4 has been machined back to substantially match this profile then any remaining coating is likely to be too thin to provide effective protection against water droplet erosion.

This problem is solved by forming a recess in the outer surface of the turbine blade and then depositing an erosion resistant coating in the recess using a laser weld/ laser cladding deposition process.

Figure 2a shows a turbine blade 10 according to the present invention to which an erosion resistant coating has been applied. The turbine blade 10 is formed with a shaped recess 12 (filled with erosion resistant coating material) in its outer surface 14. The recess 12 extends part-way down the leading edge 16 of the turbine blade from its radially outer end towards its radially inner end and wraps around the leading edge from the convex surface 14a to the concave surface 14b. For ease of illustration, the depth of the recess is shown much exaggerated relative to the blade thickness. In fact, in this instance the recess has a constant depth of 1 mm, except that the peripheral edges 12a, 18 of the recess, where the floor of the recess meets its walls, are radiused to eliminate internal sharp corners and thereby prevent fatigue crack initiation, the overall shape of the recess being selected to blend into the surrounding blade surface and maintain the structural integrity of the deposited coating.

Figure 2b shows an alternative turbine blade 10¹ to which an erosion resistant coating 24 has been applied. The turbine blade 10¹ is formed with a shaped recess 22 which extends part-way along the leading edge 16 of the turbine blade but only on the convex surface 14a. Figure 3 shows the same turbine blade 10¹ but before the erosion resistant coating is deposited in the recess. The recess 22 can be clearly seen. The other features of the turbine blade 10¹ are the same as the turbine blade 10 described above.

The process by which the erosion resistant coating 24 is applied to the turbine blade 10' of Figure 3 will now be described with reference to Figures 4 to 6.

A suitable nickel- or cobalt-based alloy wire or powder (such as one containing chromium from 28 to 32% by weight, tungsten from 5 to 7% by weight, silicon from 0.1 to 2 % by weight, carbon from 1.2 to 1.7% by weight, nickel from 0.5 to 3% by weight, iron from 0.01 to 1% by weight, manganese from 0.01 to 1% by weight, molybdenum from 0.2 to 1% by weight with the remainder being cobalt) is fed to a nozzle of the laser deposition apparatus (not shown). A laser beam provided by a CO₂ or solid-state laser passes through a passage in the nozzle and melts a small volume of the floor of the recess 22 of the turbine blade 10¹. The alloy wire or powder is delivered onto the melt pool formed by the laser beam where it melts and mixes with the substrate material of the turbine blade 10¹. When the laser beam is moved from this area because of relative movement between the nozzle and the turbine blade, the melt pool cools rapidly and solidifies to form an erosion resistant coating 24 which derives from the composition of the weld consumable and which is fused or metallurgically bonded to the substrate material of the turbine blade 10¹. Hence, instead of there being a distinct boundary between the coating and the blade material that formed the floor of the recess, there is a region where the metallurgical composition of the material transitions from the composition of the weld consumable to the composition of the blade material. This is shown by a dotted line indicating that the coating has blended into the floor of the recess.

As shown in Figures 5 and 6, the nozzle of the laser deposition apparatus can be initially positioned, for example, adjacent to the outer surface of the turbine blade 10' at the location marked A. During operation the nozzle moves towards the outer end of the blade in the direction indicated by the arrow to lay down a strip 24a of coating in a first pass. It will be appreciated that location A is not within the recess 22 but instead is on a part of the outer surface 14 of the turbine blade 10' immediately adjacent the recess. As shown, the strip 24a will therefore be deposited as an upstand on the surface of the turbine blade 10'. Once the first pass is complete, the nozzle moves incrementally away from the leading edge 16 of the turbine blade 10' to the location marked B and lays down a strip 24b adjacent to the strip 24a in a second pass. It will be appreciated that as shown in the Figures, the movement of the nozzle when laying down the strip 24a is from left to right and that the movement of the nozzle when laying down the strip 24b from right to left. However, it also possible, though less efficient, for the nozzle to move back to the location marked C and then lay down the strip 24b moving from left to right. Further strips are laid down in respective passes. The width of each strip is determined by the deposition apparatus. Although only two passes are shown in Figures 4 and 6, it will be readily appreciated that in practice several to many such passes might be needed.

In the case of the turbine blade 10 of Figure 2a, where the recess wraps around the leading edge 16 from the convex surface 14a to the concave surface 14b, the turbine blade may be rotated or turned so that the erosion resistant coating 20 can be deposited in a series of passes that start on the surface 14a and extend around the leading edge 16 onto the other surface 14b in one continuous process.

The region of the surface 14a over which the coating is deposited can be rectangular even though the recess 22 includes a curved edge 22a. This is for practical purposes because it is often convenient to make each of the passes a constant length. However, the length of each pass can be varied to more closely match the shape of the recess 22. This is shown in Figure 6 where the second strip 24b is slightly shorter than the first strip 24a. The passes are substantially parallel to the leading edge 16 of the turbine blade 10' but they can also be at right angles to the leading edge, or at any intermediate angle, though this is not preferred. Figures 4 to 6 assume that more than one coating layer will be required to completely fill the recess. In this case, once a layer of coating 24 has been deposited over the whole of the selection region in a number of parallel passes, the process is repeated so that a second layer is deposited on top of the first and fused with it. Each layer has a thickness such that the accumulated coating, comprising, say, two or three layers, extends slightly beyond the finished profile of the blade.

Although Figures 4 to 6 illustrate a process requiring the application of more than one coating layer, for reasons of manufacturing economy and to minimise distortion of the coating and the blade, we prefer that if possible only one coating layer should be applied in order to fill the leading edge recess.

Once the coating 24 has been deposited, any regions of the coating that extend above the desired profile of the turbine blade 10' are removed by a machining process such as milling or grinding. This will also remove all the parts of the coating that have been deposited as an upstand on the outer surface 14 of the turbine blade 10'.

As already emphasised, it is important to minimise distortion of the blade caused by the localised heating, melting, cooling and solidification of the applied coating and a small volume of the substrate blade material. We have found that a very effective way to minimise such distortion is to stiffen the leading edge of the blade behind the recess by thickening the leading edge with a sacrificial portion that is removed after the coating has been applied to the leading edge recess. In an experiment, a test piece 30, shown in Figures 7 and 8, was used to prove this principle. It also served to illustrate that it is possible to apply sufficient coating material in each pass of the laser deposition apparatus to fill a leading edge recess using only one coating layer.

The test piece 30 was a length of alloy steel plate of the same composition as a material from which last stage steam turbine blades are made. One side edge 32 of the plate was machined to approximate the shape of a leading edge of a turbine blade provided with a recess 34 of depth d that extends around the blade's leading edge, similar to that shown in Figure 2a. The edges of the recess 34 were blended in to the surrounding surface of the test piece by angling the wall 35 of the recess so that it met both the floor of the recess and the surrounding surface obliquely. Moreover, except for a portion 36 of the test piece 30 adjacent the side (leading) edge 32 and behind the recess 34, one face 38 of the test piece was machined to reduce its thickness by an amount D to simulate the thickness of the finished turbine blade. Hence, portion 36 of thickness D simulated the above-mentioned sacrificial portion of the blade, used as a backing piece to stiffen the leading edge of the blade against distortion.

Having appropriately shaped the test piece 30, the recess 34 was filled with coating material 40 by means of a number of closely-spaced parallel passes 42 of the laser deposition apparatus (not shown). In this case, twenty-four passes 42 were used. Sufficient Stellite-6 weld wire material was deposited in each pass 42 to slightly overfill the recess 34, which had a depth of lmm. Hence, the recess was filled using only one layer of coating material. It should also be noted that the coating was deposited so that it overlapped the desired final extent of the recess, for example at areas 44 and 46.

After deposition of the coating, the test piece 30 was subjected to a standard stress relieving treatment suitable for the particular blade alloy of which the test piece was made and then the backing piece 36 was machined away along dashed line 48, whereby the unwanted area 46 of the coating was also removed. Finally, the other portions of the coating material 40 that extended beyond the desired final profile of the test piece 30 were removed along the dashed lines 50, 51 and 52, leaving a coating having a depth of about lmm, fused to the underlying blade material. In the case of an actual blade rather than a test piece, the blade and the leading edge coating would then be subjected to a final polishing operation to maximise aerodynamic efficiency.

Figure 9 summarises the preferred embodiment of the invention as explained previously in relation to Figure 2a through Figure 8. The box representing the first step in the process has a dashed border to represent that it may not be required in all circumstances. As indicated in Figure 9, we prefer that the step of providing the sacrificial backing thickness is conveniently accomplished during the forming processes that produce the initial blade shape, before the leading edge recess is formed. Nevertheless, it is conceivable that the sacrificial thickness could alternatively be provided by attaching a separate sacrificial piece of material to the trailing edge by brazing after the initial blade shape has been formed and either before or after formation of the recess. The last step in the process (the polishing step) is routine workshop practice and so is not expressly claimed as part of the present invention, but is required for most efficient operation of the blade in the turbine.

The present invention has been described above purely by way of example, and modifications can be made within the scope of the invention as claimed. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings or any combination of any such features or any generalisation of any such features or combination, which extends to equivalents thereof. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments. Each feature disclosed in the specification, including the claims and drawings, may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise.

Any discussion of the prior art throughout the specification is not an admission that such prior art forms part of the common general knowledge in the field.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like, are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Claims

1. A method of providing an erosion resistant coating at the leading edge of a steam turbine blade having a desired profile, the method comprising the steps of: providing a recess in the leading edge surface of the turbine blade; depositing an erosion resistant coating material in the recess by a laser beam powered deposition process operative to produce a iusion bond between the coating material and the surface of the turbine blade, the amount of material deposited being such that the as-deposited erosion resistant coating extends beyond the desired profile of the turbine blade; and removing all portions of the erosion resistant coating that extend beyond the desired profile of the turbine blade.

2. A method according to claim 1, wherein the turbine blade is subjected to a stress relieving treatment after deposition of the coating, but before removal of the coating portions that extend beyond the desired profile of the turbine blade.

3. A method according to claim 1 or claim 2, wherein the erosion resistant coating material is deposited in the recess such that the complete outer surface of the as-deposited erosion resistant coating extends outside the desired profile of the turbine blade.

4. A method according to any preceding claim, wherein the erosion resistant coating material is deposited in a single layer.

5. A method according to any one of claims 1 to 3, wherein the erosion resistant coating material is deposited in a plurality of layers.

6. A method according to any preceding claim, wherein the erosion resistant coating material is deposited using a laser weld deposition or laser cladding process.

7. A method according to any preceding claim, wherein the erosion resistant coating material is deposited in a plurality of mutually parallel passes of the laser beam, each pass starting and finishing beyond a boundary of the recess.

8. A method according to claim 7, wherein the passes are substantially parallel to the leading edge of the turbine blade.

9. A method according to claim 7, wherein the passes are substantially at right angles to the leading edge of the turbine blade.

10. A method according to any preceding claim, wherein the erosion resistant coating is a cobalt-based alloy.

11. A method according to any of claims 1 to 9, wherein the erosion resistant coating is a nickel-based alloy.

12. A method according to any preceding claim, wherein the turbine blade is provided with a sacrificial backing thickness of blade material near the trailing edge and behind the position of the recess into which the erosion-resistant coating is deposited, thereby to stiffen the trailing edge of the blade against process-induced distortion, the backing thickness of material being removed after application of the coating to give the desired blade profile.

13. A method according to claim 12, wherein the backing thickness of material is removed after the stress relieving treatment.

14. A method according to any preceding claim, wherein the recess in the leading edge surface of the turbine blade has a depth in the range of roughly 0.8 to 1.2 millimeters.

15. A method according to claim 14, wherein the recess has a depth of roughly 1 (one) millimeter.

16. A steam turbine blade produced by the method of any preceding claim.