CA2090754A1

CA2090754A1 - Turbine blade repair

Info

Publication number: CA2090754A1
Application number: CA 2090754
Authority: CA
Inventors: Michael James Fraser
Original assignee: Individual
Current assignee: Turbine Blading Ltd
Priority date: 1992-02-27
Filing date: 1993-02-24
Publication date: 1993-08-28
Also published as: EP0558053A1; JPH0610613A; GB9303423D0; GB2264446A; AU3385893A

Abstract

ABSTRACT OF THE DISCLOSURE
"Turbine Blade Repair"

A method of repairing a turbine blade (10) comprises removing damaged material (22) from an area (30) to be repaired, applying by welding a layer of a first material in the form of a soft metal (31) such as Inconel and subsequently applying a layer of second material in the form of a hard metal (36) such as Stellite, the blade may then be worked by machining, grinding or polishing to the desired profile (35,40) and subjected to a heat treatment process for stress relief.
The repair method enables a fast repair that can be carried out without the blade (10) being removed from the rotor (13) or stator and which is economical in use of materials.

Description

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T~tle: Turbine Blade Repair The present invention relates to the repair of turbine blades which may be stator blades or rotor blades primarily for use in steam turbines. Ilowever it is envisaged that the repair method of the present invention may also be used for repair to gas turbine engines.
In a steam turbine it is the outer end of the turbine blade that during use of the turb;ne is most prone to damage either from impact with solid particles or water droplets. Impact with water clroplets or wet steam tends to occur in the later stages of the tLlrbine i.e. the lower pressure encl beyond what is known as the Wilson line where the steam has cooled and hence water condenses more readily.
A combination of the chemicals the water and the stresses to which the blades are subjected can lead to an effect known as stress corrosion cracking which can have a pronounced deleterious effect on a blade particularly if there is any flaw or transitional weakness present in the blade.
In the high pressure stages of the turbine particles carried in the steam from the boiler may be responsible for erosion of the leading edges of the turbine blades.
It is us~lally the outer end of the turbine blades which are most prone to damage in view of their greater linear speed and hence the speed of impact with wet steam or particles as the case may be is considerably greater than at the inner end of the turbine blades. This is emphasized particularly towards the low pressure ends of the turbine since the blades tend to be of considerable length.In order to combat the effects of erosion at the leading edge of the turbine blades it has been known to fit shields of hard material such as Stellite and appl;cants have proposed various repaiI methods including the welding to the blade of a harcl material such as Stellite or a high carbon steel heat treatment to achieve a hardness harder than the normal metal of the turbine blade which is typically 12% cllrome steel. It has also been proposed to have inserts made frommaterial similar to that of the bla(le material bLlt which has been at least in part 2 ~

treated to harden the material. These methocls are disclosed in much greater detail in the following British publications: 2,19,8667; 2,227,190 and 2,229,94~.
The methods proposed by the applicants have been found to be very satisfactory. However applicants have found that in order to virtually eliminatestress in the blade after repair has been carried out the repair methods have tobe carried out under very carefLllly controlled conditions and any member interconnecting blades such as snubbers, lacing wires, shrowds etc should be removed before any substantial heat input s~lch as occurs during welding is carried o-lt otherwise a concentration of stress is likely to arise which is almost impossible to eliminate by normal heat treatment process. Applicants have found by a combination of heating and cooJing and in some cases pre-distorting, excellent repairs can be achieved leaving the blacle in as good if not better condition than the originally manufact~lred blade.
Because of the high cost of material such as Stellite, and in view of the care and time that has to be taken including removal of lacing wires holes, redrilling etc, even though an excellent repair may be carried out, the cost can be considerable and the time taken to carry out such repair to such a high standardmay entail the turbine being o~lt of commission for a longer period than is desired.
It woul(l be desirable therefore in some cases to have a rnethod of repair that enables a blade to be repairecl satisfactorily but more quickly and relatively economically.
It is an object of the present invention to provide a new method of repa;ring turbines.
According to one aspect of the present invention we provide a method of repairing a t~lrbine blade comprising the steps of:-(a) removing any damagecl material from an area of the twrbine bla~leto be rèpaired;
(b) providing a layer of a first metallic material by welding over said repair area;

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(c) provicling a layer of a second metallic material over said layer of first material, sai~l seconcl material being har(ler than said first material;
(d) said second layer of material being applied in a manner so as not to extend over an area greater than or beyond the edge of said first layer of material;
(e) carrying out a stress relieving heat treatment process on said turbine blade;
(f) working where necessary by machining, grinding or polishing said layers of first and/or said second material to achieve a desired blade profile.
Preferably both saicl layer of first material and said layer of second material are applied to the blade in a manner so as to reduce as far as possibleheat input to the blade and the temperature of the blade may be continuously monitored to ensLIre that the temperature of certain parts of the blade do not rise above a predetermined figure.
Preferably said layer of first material comprises Inconel.
Preferably said layer of second material is harder than the material from which the blade is made.
Preferably said layer of second material comprises Stellite.
It is envisaged that to maintain the temperature of the repair area as low as possible it. is envisaged that more than one turbine blade may be repaired atone time so that for example a weld run having a part of said layers of first orsecond material may be applied to one blade and then a weld run to another to allow the first blade to cool before a subsequent weld run is appliecl.
It is further envisaged that in order to ensure that sensitive parts of the blade do not rise above a predetermined temperature for example lacing wire holes, snubbers and tenons to which shrouds are secured, heat conductive means such as a chill may be applied to ensure that the temperatllre around such sensitive areas is kept to a minirnllm.

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Whereas it is envisagecl that the repair of the present invention can most efficiently be carried out without the need to remove shrowds, lacing wires etc., it is env;saged that is some circumstances removal of lacing wires and filling of the holes may be necessary. Furthermore in some cases removal of the shroud and separation of snubbers may also be necessary.
The removal of a shroud and subsequent replacement with a new shroud can be a time cons~lming and complicated operation and can increase the likelihood of damage to the turbine blade in the area of the tenons to wh;ch theshrolld is attachecl. It is envisagecl therefore that according to another aspect of the invention, if it is desired, in the case where blades are tied together by shrouds to free a blade to be repaired by cutting the shroud and, after the repair and heat treatment processes have been carried out subsequently remaking the shro~ld by welcling or by any other suitable means. The cut and rewelding of theshro~ld preferably takes place at a position mid-point between two blades such that the heat hlput generated cluring the wekling operation is as far away as possible from the two adjacent blades thereby minimising heat input to the area of the blade around the tenon.
An ~mexpected advantage has been found in carrying out repairs according to the present invention in that the layers of material applied by welding appears to a certain extent to be uncler compression after cooling of the blade which has been fo~md to decrease the likelihood of crack propagation and hence enhances the life of the repaired area.
The methocl of the present invention may also include the steps of experimenting on a hlrbine blade to determine the best pattern of welding so as to rninimise excessive temperature gradient and distortion and propagation of heat to sensitive areas and, a satisfactory weld pattern has been determined carrying o~lt repair processes on each blacle in the turbine to be repaired in accordance with an optimum pattern.
Preferably the experimental blacle is a blacle of iclentical type b~lt not one that is act-lally attached to the turbine to be repaired. The experimental blade may however be secured in a manner so as to sim~llate attachment to other bladesby for example shrouds, lacing wires etc., so as to establish a proper control.
Preferably the metho(l of the present invention includes analysis and testing of the repaired area ancl it is envisaged that this may include x-raying and hardness testing so as to establish that a proper repair has been achieved and that the blade is left in a stress free condition as possible.
It is f~lrther env;saged that the blade may be del;berately pre-distorted to counteract any distortion that is expected to occur dwring the repair process sothat after the repair and any heat treatment processes the blade is left in not only a stress -~ree condition b~lt also in its proper position relative to the position of a datum blade so that all the turbine blades in the array may have properly aligned leading and trailing edges.
It is further envisaged that clamp means may be used during the repair method to hold the leacling or the trailing edge of the blade in a set position.However in many cases where shro~lding is still in position on the blade such means may not be necessary.
In some cases of particularly thin blades a single pass may be all that is required to achieve said layer of second material over the layer of said first material that has already been applied. In other cases more than one pass may be necessary and where this is the case, second and s~lbseq~lent passes may be applied after a predetermined cooling period and furthermore may not necessarilybe applied in the same direction as the original pass.
Whereas it is believed that the "cast" metal applied by welding may not be as durable as a piece of forged material connected by welding to a blade the considerable decrease in time necessary to carry out the repair and the combination of a layer of soft first material covered by a layer of hard second material has resulted in a swift and relatively economical repair. The blade maybe machined and/or polishe(l to its correct form.
Where one preferred method of the invention is laying of very hard metal, such as Stellite, on the layer of so~t metal which, as mentioned above can be Inconel, the hard metal may he some other alternative perhaps not as hard as Stellite but which may be swbsequently treated, for example a high carbon steel co~lld be heat treated to provide the desired hardness, or alternatively a hard alloy powder may be applied by a plasma transfer process.
In the case of a plasma deposition, which layer of materia] would be very thin, the polishing of the surface of the repaired blade should be carried out before the plasma deposition since otherwise polishing after plasma deposition is likely to remove the cleposite(l harcl alloy.
The present invention will now be clescr;bed in more detail by way of example only with reference to the accompanying drawings wherein:
FIGURE I is a cliagrammatic view of three turbine blades in an array of blades;
FIGURE 2 is a cross-section of one o~ the blades shown in Figure 1;
FIGURE 3 is a cross-section of a blade shown in Figure 2 after removal of damaged material;
FIGURE 4 is the blade shown in Figure 3 after application of a layer of first soft material;
FIGURE 5 is a view of the blade shown in Figure 4;
FIGURE 6 is a cross-section of the blade after machining of the first layer of soft material;
FIGURE 7 is a view of the blade after application of the seconcl layer of material;
FIGURE 8 is a cross-section of the blade shown in Figure 7; and FIGURES 9 and 10 are views of the repaired blade.
Referring first to Figure 1, three t~lrbine blades are shown 10, 11 and 12, each blade having a root portion attached to a rotor 13.
The o~lter ends of each blade have respective tenons 14, 15 and 16 which extend through apertures in a shroucl 17 by Ineans of which the blades 10, 11 and 12 are connected to each other. Further connection means are provkle(l by a lacing wire 18 passing throllgh holes 19, 2() at 21, the shroll(l 17 an(l lacing wire 2 ~

18 collectively damping vibration of the blacles and relative movement between adjacent blades.
As has been explained before, clamage occurs primarily to the outer leading edge of the turbine blades, the damaged areas being ;ndicated in Figure 1 at areas 22, 23 and 24. The damage may take the form of small cracks and erosion which if left can result in large cracks which event~lally may necessitate the complete replacement of the blade.
Referring now to Fig~lre 2 a profile of a blade 10 is shown and the area of damage 22 is indicated and as it can be seen it is primarily on the leading edge~ The remainder of the blade normally being relatively unaffected by wear.
In order to remove clamaged material the blade 10 may be ground such that the damaged material is removed as shown in Figure 3. By grinding away the damaged material any evidence of substantial cracks may be discovered and appropriate action taken to repair such cracks.
In the examp]e shown in the drawing the area of damage extends up to the shro~ld 17 and thus the heat input in the region of the tenon 14 may cause considerable strain to be applied to the tenon 14 in view of the presence of theshroud 17 which woulcl restrain any tendency for movement causecl by the considerable input of heat during a welding operation.
The shroud is therefore cut at positions 28 and 29 thus allowing the end of the blade 10 to move thus alleviating any stress that may be applied to the tenon 14.
A first layer of material comprising a soft metal is then applied to the ground area 30 of the blade, the soft metal for example Inconel may be applied to the blade in runs along the length of the blade and in the example shown fourruns are shown 31, 32, 33 and 34. In some cases depending on the amount of damage, a sing]e run may suffice, in other cases it may be necessary to apply more runs.

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Depending on how many runs of soft metal have to be applied to the blade it may then be necessary to worl~ the soft metal to leave a substantially even layer or in some cases it may be preferred to leave the sol~t metal unworked.
Fig~lre 6 shows the b]ade 10 in whicl1 the soft metal runs 31, 32, 33 and 34 have been worked to leave a substantially constant thickness of soft metal as shown at 35 ready to receive a layer of second material comprising a hard metal.Referring now to Fi~ures 7 and X, hard metal in the form of three weld passes 36, 37 and 38 is shown having been applied to the soft metal 35. The hardmetal for example Stellite, as is the case with the soft metal may not have all three runs app]iecl at the same time but there may be some time delay between the application of each run so that the temperature of the blade 10 is not unduly raised. As has afore been explained there may also be heat sinks or chills provided to limit the temperat~lre of the blacle in the vicinity of a lacing wire hole such as that shown at 20 for example.
The hard metal "runs" 36, 37 and 38 are then machined so that the repaired blade as shown in Figure 9 and Figure 10 is left with a predetermined desired profile having a soft metal layer 35 and a hard metal layer 40 to provide a leading edge portion of the blade that is durable and hence provides a long service life compared with a blade without swch a hardened area.
After the application of the hard metal 36, 37 and 38 and before or after machining the blade may be subjected to stress relieving heat treatment process which may comprise for example raising the blade to a predetermined temperat~lre over a certain length of the time, maintaining the blade at that temperatwre and then allowing the blade to cool in a controlled manner. If required more than one stress relieving heat treatment process may be applied.
After heat treatment and machining the shroud which had been Cllt at 28 and 29 may be remacle by welding for example. However if the adjacent blades such as those shown at ll and 12 also reqwire repair the reconnection of the shroud willonly take place after such repair and heat treatment processes have bcen carriedout.

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It is envisagec~ that cluring the repair process or before the repair process if ;t is known what distortion is likely to occur to the blade dllring the repair proced~lre some compensating force or heat differential may be applied to the blade s-lch that after repair the blacle is in as near as possible its proper aligned position. It is fllrther envisagecl that gllide means or clamp means may be applied to the blade being repairecl to maintain the leading eclge or the trailing edge in its proper position whilst permitting of movement of the trailing edge or lead;ng edge respectively so as not to encoLlrage the build up of stress in the blade during any welding process.
Whereas in the examp]e shown the shroud has been cut to prevent the possibility of und~le stress being applied to the tenon, in some cases it may not be necessary to c~lt the shroucl ancl the application of a chill to the area of the tenon may prevent excessive build up of temperature in that area and hence reduce the risk of stress in the tenon.
Furthermore by careful application of metal coupled with the application of heating means and cooling means carefully applied to the blade it may be thatthe transference of heat to the area o~ the tenon or for example a lacing wire hole may be such that the risk of damage occ~lrring through in built stress is minimal.
However such factors will be determinecl by the size and nat~lre of the blade, the amount of welding to be carried out and the proximity of the weld area to the tenons and/or lacing wire hole.
Whereas the second hard layer of material has been described as a layer of Stellite, alternative materials may be used, e.g. a high carbon steel which may sllbseq~lently be heat hardened.
Alternatively the second layer of material may be a metal which can be s-lbjected to plasma hardening. In this case, it is necessary to machine the blade to the desired profile prior to the application of plasma harclening since the depth of hardness may not cope with any suhstantial machhling thereafter.
The features disclosecl in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a 2 ~ 3 L'l means for performing the disclosecl function, or a metho(3 or process for attaining the disclosed result, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

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Claims

1. A method of repairing a turbine blade comprising the steps of:-(a) removing any damaged material from an area of the turbine blade to be repaired;
(b) providing a layer of a first metallic material by welding over said repair area;
(c) providing a layer of a second metallic material over said layer of first material, said second material being harder than said first material;
(d) said second layer of material being applied in a manner so as not to extend over an area greater than or beyond the edge of said first layer of material;
(e) carrying out a stress relieving heat treatment process on said turbine blade.
(f) working where necessary by machining, grinding or polishing said layers of first and/or said second material to achieve a desired blade profile;

2. The method according to Claim 1 wherein said second material is also harder than the material from which the blade is made.

3. The method according to Claim 2 wherein said second material comprises Stellite.

4. The method according to Claim 1 wherein said second material is hardened subsequent to it being applied to said layer of first material.

5. The method according to Claim 4 wherein a layer of hard metal is applied by plasma deposition.

6. The method according to Claim 4 wherein said second layer of material comprises a high carbon content steel and is hardened by the application of heat.

7. The method according to Claim 1 wherein the turbine blade is subjected to additional controlled heating during the application of said layer of said second material.

8. The method according to Claim 1 wherein the turbine blade is subjected to additional controlled heating during the application of said layer of said first material.

9. The method according to Claim 1 wherein said first material comprises Inconel.

10. The method according to Claim 1 wherein prior to the application of said layer of first material the blade is subjected to a force resulting in a controlled distortion of the blade.

11. The method according to Claim 10 wherein said controlled distortion is achieved by the application of heat to the blade.

12. The method according to Claim 10 wherein said controlled distortion is achieved by the application of a mechanical force to the blade.

13. The method according to Claim 1 wherein the blade is allowed to cool between the application of separate "runs" of first or second material.

14. The method according to Claim 1 wherein chills or other means for conducting heat is applied in areas of discontinuity in the blade.

15. The method according to Claim 1 wherein the turbine blades are provided with means for connecting adjacent blades and wherein at least some of said means are cut or otherwise removed or separated to disconnect said connecting means.

16. The method according to Claim 15 wherein said connecting means comprises a shroud and wherein said separating means is accomplished by cutting said shroud intermediate its connection with adjacent blades and wherein after said method of repair is completed said shroud is rewelded.

17. The method according to Claim 1 wherein temperature sensing devices are applied to the blade prior to any welding and the temperature of the blade is monitored along the repair, the rate at which said first and/or said second layers are applied is controlled to ensure that the blade does not reach an excessive temperature or that the blade does not experience an excessive temperature gradient.