GB2397544A

GB2397544A - Method of Turbine Refurbishment

Info

Publication number: GB2397544A
Application number: GB0301565A
Authority: GB
Inventors: Richard James Martin; Kenneth Crawford Mitchell; Geoffrey Martin Spink
Original assignee: Innogy PLC; RWE Innogy PLC
Current assignee: RWE Generation UK PLC
Priority date: 2003-01-23
Filing date: 2003-01-23
Publication date: 2004-07-28
Anticipated expiration: 2023-01-23
Also published as: WO2004065051A1; GB2397544B; GB0301565D0

Abstract

A method of replacing the tip portion of a turbine blade, comprising the steps of removing the tip portion of a turbine blade formed from chrome steel to leave an exposed surface and joining a replacement tip section formed from chrome steel and having a hardened portion to the exposed surface by welding, where the material in the weld is chrome steel and the temperature of the hardened portion does not exceed 260{C during the welding operation. The blade may be a high twist last stage blade for a steam turbine. The blade and replacement tip are preferably formed from 12 Cr steel, and are joined by a butt weld using TIG welding. The method is preferably carried out while the blade is still on the rotor. Also claimed is a turbine blade comprising a replacement tip section butt welded to the blade body.

Description

Turbine Refurbishment The present invention relates to turbines and, in

particular, to blade refurbishment in a steam turbine.

The present invention describes a method of replacing the tip portion of a turbine blade for a steam turbine, for example a high twist last stage blade.

The outer end or tip of a turbine blade is susceptible to damage during use from impact with solid particles or water droplets. With regard to the latter, steam turbine last stage blades (LSBs) receive the steam at its lowest temperature, where the water droplets are starting to condense out, and their impingement on the blades causes erosion, especially at the blade tips. It is difficult and expensive to remove blades from the rotor and, in any case, the cost of new blades is very high.

High twist LSBs with articulated tip struts were developed in the 1980s. The high twist aerofoil provides enhanced blade efficiency and the blades are linked by articulated tip struts to control vibration.

The blades are pre-twisted so that there is little or no torsional stress during operation at full speed.

In view of the erosion problems with steam, LSBs are often provided with an erosion shield, which will typically be located at the leading edge of the tip section of the blade, where most of the erosion damage occurs. Two known types of erosion shield are a Stellitershield and a Tool steel shield, which may be attached to the leading edge of the tip of the LSB by a welding or brazing operation. However, such shields suffer detachment problems in use because of the significant changes in torsional stress at the blade - 2 tips. In particular, when stationary, the blades are in a pre-twisted state and this invokes a high torsional stress at the flexible tip portion, which, in turn, invokes a torsional stress in the braze/weld material bonding the erosion shield to the blade.

Accordingly, the shield braze/weld bond degrades through cyclic stress and the shield may become detached in service. As will be appreciated, a detached shield may impact and damages the tip struts and blades. Inconeiweded erosion shields and hard facings are also known to invoke relatively poor stress corrosion cracking resistance.

It is also known to provide complete replacement turbine blades that have been subjected to surface and/or thermal treatments at preselected areas on the leading edge of the tip. Such treatments are designed to effect a local hardening reaction or transformation in the alloy to result in a surface hardened portion, which, in the case of laser hardened blades, has a compressive stress. Examples of such treatments include induction, laser and flame hardening and shot peening. While such blades may be substituted for damaged blades having detached erosion shields such an operation is expensive, complicated and time consuming.

Accordingly, the present invention provides a method of replacing the tip portion of a turbine blade for a steam turbine, the method comprising the steps of: (a) removing the tip portion of a turbine blade formed from chrome steel to form an exposed surface for joining to a replacement tip section; (b) providing a replacement tip section formed from chrome steel and comprising a corresponding - 3 surface for joining to said exposed surface of the turbine blade and at least a hardened portion; (c) joining said exposed surface of the turbine blade to the corresponding surface of the replacement tip section by a welding operation, wherein the material in the weld comprises chrome steel, and wherein the welding operation is carried out such that the temperature of said surface hardened portion does not exceed approximately 260 C, whereby little or no softening of said surface hardened portion occurs.

The blade will typically be a high twist last stage blade for a steam turbine, for example a high twist blade with an articulated tip strut. In this case, the replacement tip section will also be adapted to be used in conjunction with an articulated tip strut.

The hardened portion will usually be provided on the leading edge of the replacement tip section, although in some cases it may be provided on the trailing edge.

As will be appreciated, the replacement tip section will typically have a shape (and dimensions) that is the same or substantially the same as the removed tip portion. However, in some cases, it may be desirable for the replacement tip section to differ somewhat in shape and/or dimensions so as to improve efficiency and/or erosion resistance.

The replacement tip section is formed from a material that is compatible with the material from which the blade is formed. The replacement tip section will typically be formed from a material that is the same or substantially the same as the material - 4 from which the blade is formed.

The blade may suitably be formed from chrome steel, preferably 12 Cr steel. The replacement tip section may also be formed from chrome steel, preferably 12 Cr steel.

The surface hardened portion of the replacement tip section may be hardened by any of the conventional techniques, such as one or more of induction, laser and flame hardening and shot peening. Laser hardening is preferred.

The method according to the present invention may include the step of surface hardening the desired portion of the replacement tip section prior to it being welded to the exposed surface of the blade.

This may be effected by a thermal treatment, which may comprise, for example, exposing a preselected area of the replacement tip section to thermal energy to effect a local hardening reaction or transformation in the material, typically a martensitic transformation.

The thermal energy is preferably provided by a laser, which can be accurately scanned over the desired region.

The thermal treatment may further comprise a stress corrosion cracking (SCC) resistance treatment, which will typically involve the blade tip being soaked at a temperature of approximately 250 C for up to 2 hours. The thermal treatment may be followed by a mechanical treatment of the surface, for example controlled shot penning.

The welding operation is advantageously carried out under conditions such that the temperature of the - 5 surface hardened portion does not exceed approximately 260 C. The temperature preferably does not exceed approximately 250 C, more preferably the temperature does not exceed approximately 240 C.

The removed tip portion will typically have an erosion shield, for example a Stellite shield or a Tool steel, brazed or welded to the leading edge of the tip portion. The method according to the present application may be relied on when the shield and/or tip require(s) repair.

The method may also be relied on to repair blades that have had shields removed (to avoid 'in service' ejection) and have therefore become eroded.

An important feature of the method according to the present invention is that tip replacement may be carried out in-situ with the blade in the turbine (i.e. while the blade is still attached to the rotor or stator of the turbine).

The joint between the exposed surface of the turbine blade and the corresponding surface of the replacement tip section preferably comprises a butt weld.

Joining of the exposed surface of the turbine blade to the corresponding surface of the replacement tip section is preferably by a tungsten inert gas (TIC) welding operation.

The material in the weld between the exposed surface of the turbine blade and the corresponding surface of the replacement tip section is preferably the same or substantially the same as the material - 6 from which the blade and/or replacement tip section is/are formed. Thus, the material in the weld preferably comprises chrome steel, more preferably 12 Cr steel. The weld material, therefore, is preferably the same as or similar to the parent metal.

During the welding operation cooling means are advantageously provided to ensure that the temperature of the surface hardened portion does not exceed approximately 260 C (and preferably does not exceed approximately 240 C). Such cooling means may simply comprise a heat sink, for example a copper heat sink in thermal contact with the blade and/or tip. If required, the cooling means may comprise forced cooling means relying on a coolant fluid (liquid or gas).

Joining of the exposed surface of the turbine blade to the corresponding surface of the replacement tip section is preferably performed in an inert atmosphere, more preferably an atmosphere comprising or consisting of argon. As stated above, TIC welding is preferred.

The welding operation advantageously comprises a pre-weld heat treatment in which the replacement tip section and/or the turbine blade is/are heated, preferably at a temperature of up to approximately 200 C. This may be achieved using an electrical pre heat arrangement. Thermocouples may be provided to monitor the temperature and the temperature may be controlled by any suitable control means.

The welding operation advantageously comprises a post-weld heat treatment, preferably an induction post-weld heat treatment. The post-weld heat - 7 treatment may provide stress relief in the weld zone and also in the surrounding material, i.e. the heat affected zone.

The use of cooling means will generally not be required during the preweld heat treatment or during the welding, but will often be required during the post-weld heat treatment to ensure that the temperature of the surface hardened portion does not exceed approximately 260 C, and preferably does not exceed approximately 240 C.

In a preferred embodiment, the present invention provides a method for the in-situ replacement of the tip portion of a high twist last stage turbine blade for a steam turbine, the method comprising the steps of: (i) removing the tip portion of a high twist last stage turbine blade formed from 12 Cr steel to form an exposed surface for joining to a replacement tip section, the blade being mounted on the steam turbine rotor; (ii) providing a replacement tip section formed from 12 Cr steel and comprising a corresponding surface for joining to said exposed surface of the turbine blade and at least a hardened portion comprising martensite; (iii) joining said exposed surface of the turbine blade to the corresponding surface of the replacement tip section by a welding operation, wherein the material in the weld comprises 12 Cr steel, and wherein said welding operation is carried out such that the temperature of said surface hardened portion does not exceed approximately 240 C so as to avoid or minimise tempering of said surface hardened portion. - 8

As previously stated, the hardened portion will usually be provided on the leading edge of the replacement tip section. The hardened portion is preferably a laser hardened portion.

The present invention also provides a turbine blade whenever manufactured by a method as herein described.

The present invention also provides a turbine blade for a steam turbine comprising a replacement tip section butt welded to the blade body, wherein the tip section has at least a hardened portion, preferably provided on its leading edge. The turbine blade is preferably a high twist last stage turbine blade for a steam turbine, wherein the blade body and the replacement tip section are formed from 12 Cr steel, and wherein the material in the butt weld is the same or similar to the parent metal, and wherein said hardened portion is a laser hardened portion comprising martensite.

The present invention also provides a steam turbine comprising a turbine blade as herein described.

The method according to the present invention will generally involve establishing the optimum position at which to remove the tip portion of the blade. The whole of the tip (not just an insert portion thereof) may be removed by, for example, a conventional cutting or grinding operation. As will be appreciated, deciding where to cut the tip will depend on a number of factors including the extent of the damaged area (including the erosion shield if provided) and the size of the replacement tip section. 9 -

The tip portion is preferably removed while the blade body and tip thereof are held securely in a jig, which term is intended to encompass a vice, a clamp and the like. This enables accurate cutting/grinding of the tip at the desired position. Since the tip portion is preferably removed with the blade root still attached to the turbine rotor (i.e. removal of the tip in- situ), the jig should preferably be portable.

Pre-formed replacement tip sections are manufactured with the desired shape and dimensions.

The tip sections have a hardened portion, which will usually be a laser hardened portion on the leading edge. The tip sections are then welded without incurring any substantial softening of the laser hardened portion. If the laser hardened portion comprises martensite, then welding is carried out so that there is little or no tempering of the martensite, which would decreases its hardness.

Again, it is preferable that the blade body and the replacement tip section are held securely during the welding operation in a jig, which term is intended to encompass a vice, a clamp and the like. This enables accurate welding of the replacement tip section to the blade. Again, the jig should preferably be portable.

While the same jig may be used for both the removal of the tip and the welding operation, it is preferable to use a tip removal jig and a separate welding jig.

During the welding operation the temperature of the replacement tip section (particularly the hardened portion thereof) is preferably monitored using, for example, one or more thermocouples. Cooling may be achieved by the provision of one or more heats sinks in thermal contact with the tip and/or blade. A copper shoe arrangement is preferred. Forced cooling using a coolant fluid (liquid or gas) may also be used either as an alternative or in combination with the heat sink.

TIG welding is the preferred welding technique using a shielding gas comprising argon. The welding filler wire is preferably the same or at least substantially the same as the parent metal.

As an example, TIG welding may be performed on a 12 Cr blade and replacement tip section using a MSRR9500/10, 1.6, 2.4 mm diameter wire, and a 2% thoriated tungsten electrode (2.4 mm diameter). The Ar flow rate is preferably from approximately 10 to 14 l/minute.

Prior to welding, it is often desirable to carry out a pre-weld heat treatment, typically at a temperature of up to approximately 200 C. This may be achieved using an electrical pre-heat arrangement.

Similarly, after welding, it is often desirable to carry out a post-weld heat treatment in order to relieve any residual stresses in the material. The weld is typically first left to cool below approximately 100 C. The welded area is then heated by, for example, electrical induction to approximately 600 to 700 C (preferably 650 C +/- 15 C) for up to 1 to 2 hours.

During the post-weld heat treatment the temperature of the replacement tip section, particularly the hardened portion thereof, may be monitored using, for example, one or more thermocouples. Cooling may be achieved by the provision of one or more heats sinks in thermal contact with the tip and/or blade. A copper shoe arrangement is preferred. Forced cooling using a coolant fluid (liquid or gas) may also be used either as an alternative or in combination with the heat sink.

As will be appreciated, the final weld may be machined by a grinding/polishing operation to the desired profile. The weld may also be inspected by any conventional NDT technique (for example visual, MPI, and/or radiography) to assess the weld integrity.

The finished blade geometry should be examined to ensure that it is within the desired tolerances.

The present invention will now be described further with reference to the following drawings, which are provided by way of example, and in which: Figure 1 is a photograph of a series of LP turbine high twist last stage blades (LSBs) mounted on a rotor; Figure 2 is a photograph showing an arrangement for replacing the tip portion of a LSB; Figure 3 is a photograph showing an arrangement for pre-heating the blade and replacement tip section prior to welding; Figure 4 is a photograph showing an arrangement for welding the blade and replacement tip section; Figure 5 is a photograph showing a completed butt weld; and Figure 6 is a photograph showing the finished turbine with replacement laser hardened tip sections welded in place.

With reference to Figure 1, there is shown a series of LP steam turbine high twist last stage blades (LSBs) 1. As can be seen, a number of the blades have had their tip portions removed (see, for example, blade 10). Some of the blades are still intact (see, for example, blade 20), and these can be seen to have a tool steel erosion shield 25 on the leading edge of the blade. The roots of the blades 10, 20 are attached to the turbine rotor 30. Thus, replacement of the tip portions takes place in-situ.

The blades 10, 20 are linked by articulated tip struts at their outer ends to control vibration. The blades 10, 20 are formed from 12 Cr steel and pre-twisted so that there is approximately zero stress during operation.

Figure 2 shows an arrangement for replacing the tip portion of a LSB. A pre-formed replacement tip section 40 formed from 12 Cr steel is provided. The tip section 40 has a laser hardened surface region (typically comprising martensite) on its leading edge.

The replacement tip section 40 and the cut blade 10 are held securely in a welding jig 50. As can be seen, the exposed surface of the cut blade 10 faces the corresponding surface of the replacement tip section 40. In Figure 3, pre-heating means are shown comprising electrically conducting wires 60 and 65 wrapped around the adjacent ends portions of the cut blade 10 and replacement tip section 40. Thermocouples are provided to monitor the temperature, which may be adjusted by any suitable control means (not shown).

Figure 4 is a side view showing the replacement tip section 40 butt welded 80 to the blade 10. The photograph shows a copper backing shoe arrangement 70, which ensures that the temperature of the laser hardened portion does not exceed 240 to 250 C during the TIC welding operation.

Figure 5 shows the completed butt weld 80 in greater detail. The metal in the butt weld 80 is the same or at least similar to the parent metal (i.e. 12 Cr steel).

Following a post-weld heat treatment to relieve any residual stresses in the weld and the surrounding material (i.e. the heat affected zone), the weld is machined by a grinding/polishing operation to the desired profile. The weld is then inspected to assess I the weld integrity. The finished blade geometry is examined to ensure that it is within the desired tolerances. Figure 6 shows the finished turbine with replacement laser hardened tip sections 40 welded in place.

Claims

CLAIMS: 1. A method of replacing the tip portion of a turbine blade for a

steam turbine, the method comprising the steps of: (a) removing the tip portion of a turbine blade formed from chrome steel to form an exposed surface for joining to a replacement tip section; (b) providing a replacement tip section formed from chrome steel and comprising a corresponding surface for joining to said exposed surface of the turbine blade and at least a hardened portion; (c) joining said exposed surface of the turbine blade to the corresponding surface of the replacement tip section by a welding operation, wherein the material in the weld comprises chrome steel, and wherein the welding operation is carried out such that the temperature of said surface hardened portion does not exceed 260 C, whereby little or no softening of said surface hardened portion occurs.
2. A method as claimed in claim 1, wherein the blade is a high twist last stage blade for a steam turbine.
3. A method as claimed in claim 1 or claim 2, wherein the replacement tip section has a shape that is the same or substantially the same as the removed tip portion.
4. A method as claimed in any one of the preceding claims, wherein the replacement tip section is formed from a material that is the same or substantially the same as the material from which the blade is formed.
5. A method as claimed in any one of the preceding claims, wherein the turbine blade is formed from chrome steel, preferably 12 Cr steel.
6. A method as claimed in claim 5, wherein the replacement tip section is formed from chrome steel, preferably 12 Cr steel.
7. A method as claimed in any one of the preceding claims, wherein said surface hardened portion is or comprises a laser hardened surface portion.
8. A method as claimed in any one of the preceding claims, wherein said welding operation is carried out such that the temperature of said surface hardened portion does not exceed 250 C, and preferably does not exceed 240 C.
9. A method as claimed in any one of the preceding claims, wherein said surface hardened portion is formed by subjecting the replacement tip section to a thermal treatment.
10. A method as claimed in claim 9, wherein said thermal treatment comprises exposing a preselected area of the replacement tip section to thermal energy to effect a local hardening reaction or transformation in the material and to form a surface hardened portion, preferably a surface hardened portion having a compressive stress.
11. A method as claimed in claim 10, wherein 16 said local hardening reaction or transformation is a martensitic transformation.
12. A method as claimed in claim 10 or claim 11, wherein said thermal energy is provided by a laser.
13. A method as claimed in any one of claims 9 to 12, wherein said thermal treatment further comprises a stress corrosion cracking resistance treatment.
14. A method as claimed in any one of claims 9 to 13, wherein said surface hardened portion is formed by subjecting the replacement tip section to a thermal treatment followed by shot peening.
15. A method as claimed in any one of the preceding claims, wherein the removed tip portion has an erosion shield, preferably a Stellite shield or a Tool steel shield.
16. A method as claimed in any one of the preceding claims, wherein the method is carried out while the blade is still mounted on the rotor.
17. A method as claimed in any one of the preceding claims, wherein the joint between said exposed surface of the turbine blade and the corresponding surface of the replacement tip section comprises a butt weld.
18. A method as claimed in any one of the preceding claims, wherein joining of said exposed surface of the turbine blade to the corresponding surface of the replacement tip section is by a tungsten inert gas (TIC) welding operation. - 17
19. A method as claimed in any one of the preceding claims, wherein the material in the weld between said exposed surface of the turbine blade and the corresponding surface of the replacement tip section is the same or substantially the same as the material from which the blade and/or replacement tip section is/are formed.
20. A method as claimed in claim 19, wherein the material in the weld comprises chrome steel, preferably 12 Cr steel.
21. A method as claimed in any one of the preceding claims, wherein during said welding operation cooling means or a heat sink is provided to ensure that the temperature of said surface hardened portion does not exceed 260 C.
22. A method as claimed in any one of the preceding claims, wherein joining of said exposed surface of the turbine blade to the corresponding surface of the replacement tip section is performed in an inert atmosphere, preferably an atmosphere comprising or consisting of argon.
23. A method as claimed in any one of the preceding claims, wherein said welding operation comprises a pre-weld heat treatment in which the replacement tip section and/or the turbine blade is/are heated, preferably at a temperature of up to 200 C.
24. A method as claimed in any one of the preceding claims, wherein said welding operation comprises a post-weld heat treatment to provide stress relief, preferably an induction post-weld heat - 18 treatment.
25. A method for the in-situ replacement of the tip portion of a high twist last stage turbine blade for a steam turbine, the method comprising the steps of: (i) removing the tip portion of a high twist last stage turbine blade formed from 12 Cr steel to form an exposed surface for joining to a replacement tip section, the blade being mounted on the steam turbine rotor; (ii) providing a replacement tip section formed from 12 Cr steel and comprising a corresponding surface for joining to said exposed surface of the turbine blade and at least a hardened portion comprising martensite; (iii) joining said exposed surface of the turbine blade to the corresponding surface of the replacement tip section by a welding operation, wherein the material in the weld comprises 12 Cr steel, and wherein said welding operation is carried out such that the temperature of said surface hardened portion does not exceed 240 C so as to avoid or minimise tempering of said surface hardened portion.
26. A method as claimed in claim 25, wherein said hardened portion is a laser hardened portion.
27. A method as claimed in any one of the preceding claims, wherein said hardened portion is provided on the leading edge of the replacement tip section.
28. A turbine blade whenever manufactured by a method as defined in any one of claims 1 to 27.
29. A turbine blade for a steam turbine comprising a replacement tip section butt welded to the blade body, wherein the tip section has at least a hardened portion, preferably on its leading edge.
30. A turbine blade as claimed in claim 29 which is a high twist last stage turbine blade for a steam turbine, wherein the blade body and the replacement tip section are formed from 12 Cr steel, and wherein the material in the butt weld is the same or similar to the parent metal, and wherein said hardened portion is a laser hardened portion comprising martensite.
31. A steam turbine comprising a turbine blade as defined in any one of claims 28 to 30.
32. A method of replacing the tip portion of a turbine blade for a steam turbine substantially as hereinbefore described with reference to any of the Figures.
33. A turbine blade or blades for a steam turbine substantially as hereinbefore described with reference to any of the Figures.