TURBINE BLADE REPAIR

Abstract: A method of repairing a turbine blade in which a radially outer part of the blade is removed and replaced by a new part (18) comprising the steps of removing the outer part of the blade and securing a new part (18) to the original blade (15) using a weld material (19) having a high nickel content or including an intermediate part of a material having a high nickel content. A weld material (19) of high nickel content or a new part of high nickel content is positioned where the original failure of the blade took place and has been found to reduce the risk of subsequent failure.

Description: Title: Turbine Blade Repair Description of Invention The present invention relates to a method of repairing turbine blades. The method of repair of the present invention is primarily concerned with the repair of turbine blades which have suffered damage and from which a relatively large piece of the turbine blade has become detached from the remainder of the blade. The cause of such damage may, for example, be fatigue due possibly to vibration characteristics of the blade. It is not uncommon for several blades in the turbine blade array to fail at the same position along the length of the blade. United Kingdom Patent No. 2229657B discloses the repair of the end part of a turbine blade in which a piece larger than the original end part of the turbine blade is welded to the blade and subsequently machined to achieve the desired shape. United Kingdom Patent Application No.9209109.9 discloses a method of repairing a turbine blade in which it has been found beneficial to select the area of the join between a remaining piece of a turbine blade and a new piece of turbine blade after consideration of the vibration characteristics of the blade such that the length of the new end piece is dictated not necessarily by the piece that has been damaged, but by determining a selected position, for example a node along the blade so that the join area will not be subjected to excessive strain during use of the turbine. It will be appreciated that where welding has to be carried out to secure a new piece of turbine blade to an existing piece, it is likely that the join area will not be as strong as the original forged blade material. Thus, the method described in UK application no. 9209109.9 enables a satisfactory and long-lasting repair to the carried out with good life expectancy. In some cases, in view of the nature of the blade construction, it may be difficult to select a preferred join position, for example particularly when carrying out repairs where the blades are still in an array, access to a preferred position of join may not be possible. It is an object of the present invention to provide a method of repairing turbine blades which overcomes or reduces the problems mentioned above. According to the present invention, we provide a method of repairing a turbine blade in which it is required to replace an outer end part of the blade by a new end part comprising the steps of removing said outer end part and securing the new end part to the remainder of the blade by using a weld material having a relatively high nickel content compared with the original blade material. Preferably, the nickel content is at least 3% and conveniently, may be in the range of 4% to 8%. A weld material which has found to be particularly suitable is one sold under the designation F.V. 520 (B) having a nickel content of 5% to 6% by weight, chrome 13.2% to 14.7%, silicon 0.2% to 0.5%, manganese 0.5% to 1%, carbon 0.04% and molybdenum 1.2 to 2.0%, the balance being ferrous. It has been found unexpectedly, that use of a weld metal such as F.V 520 (B) provides a repaired blade in which the weld area itself is stronger than the original blade material which is typically a 12% chrome steel. Even though the blade material may be slightly weakened in the region of the repair area, since the position of weakness on the blade is where the weld material is now positioned, the point of weakness on the original blade has now been improved by providing a material which is less prone to failure and any weakness caused by the weld is displaced either side of the point of weakness. It is further envisaged that to further remove the weld area or the heat affected zone of the blade, a new piece of material which is the same as or similar to F.V 520 (B) may be secured in the region where the fracture had taken place. It is further envisaged that a new end piece may be preformed and terminate in a piece of material the same as or similar to F.V. 520 (B). In the latter case, the insert will be longer than the piece which had become separated from the remainder of the blade, thus a further piece of the original blade would have to be removed. It is further envisaged that the new end piece may only be partly finished and may be of a size a little larger than that of the original blade part and after securing to the blade, and heat treatment processes, a machining or polishing operation may be carried out to form the new piece of material to the desired profile. The machining operation may be carried out after heat treatment, for example the formation of lacing wire holes. It has been found that by using a weld metal such as that mentioned above, not only is a strong joint between the original blade material and the new piece obtained, but the possible occurrence of a fault occurring during the welding operation also appears to be considerable decreased. It is a further object of the invention to provide a new or improved turbine blade. According to a further aspect fo the present invention we provide a turbine blade comprising a radially inner part made from a normal turbine blade material and an outer part made from the same or similar material and an intermediate part interconnecting said radially inner part and radially outer part made from a material having a high nickel content compared with said other two parts. It is thought that it is possible that the absence of δ ferrite in a weld metal such as F.V. 520 (B) that may be the cause for elimination of weld failures. The presence of δ ferrite in normal weld material, e.g. a 12% chrome steel which is same as the parent material of the blade, can cause a very minute crack or discontinuity in the welding to grow in size during subsequent weld passes. Such an occurrence does not occur with an F.V. 520 (B) material. The invention will now be described by way of example only with reference to the accompanying drawings wherein: FIGURE 1 shows part of a blade array; FIGURE 2 shows the same blade array wherein two blades have failed in the same position along the length; FIGURE 3 is an edge-on view of a blade being repaired in accordance with one method of the invention; FIGURE 4 is an edge-on view of a blade being repaired by a second method of the invention; FIGURE 5 is an edge-on view of a blade being repaired by a third method of the invention. Referring first to Figure 1, a turbine blade array is shown, the blades having inter-connecting snubber-type formations generally located at 10 separating an inner part of a blade 11 from an outer part 12. The outer part of the blade 12 has blade inter-connections through adjacent blades through lacing wires 13 and 14. Referring now to Figure 2, the same blade array is shown in which two blades 15 and 16 have failed at a position between the lacing wire 13 and snubber formations 10. The position of the failure of the blades 15 and 16 being the same distance from the outer ends of the blades indicative that the failure is probably due to fatigue, possibly due to the vibration characteristic of the blade. The occurrence of failure at identical positions on the blade is indicative that failure in the same place may well be expected to re-occur. Referring to Figure 3, a first method of the present invention is shown in which a new blade end piece 18 is secured to the remaining part of a blade 15 using a relatively high nickel content weld material 19 such as F.V. 520 (B), the contents of which have been described above. Prior to welding, the part 15 of the blade may be prepared to give a "V" like formation in cross section, similar to the formation of the end of the new piece 18 to enable a two-sided penetration weld as shown in Figure 3. It is further envisaged that the blade may be subjected to the treatment processes both before the weld takes place, during the weld operation and further heat treatment processes after welding to relieve stress on the blade. After welding, the weld area may be ground and/or polished to achieve the desired profile and, where the insert 18 is of a slightly oversized nature, further machining may take place to achieve the desired profile. It is envisaged the profile may be identical to that of the blade before it was damaged or alternatively, it may be of modified form possibly to reduce the risk of fatigue failure in the same area. Referring now to Figure 4, an alternative method of repair is shown in which the existing part of the blade is first reduced in length, a piece of high nickel material 20 is welded to the blade using a high nickel, for example F.V. 520 (B) material 21 and a new end piece 22 is then welded using a high nickel material 23, for example F.V. 520 (B) to the high nickel piece 20. Similar heat treatments may be carried out as described with reference to the repair shown in Figure 3. The advantage of having a new piece of high nickel material is that it is positioned exactly where it is known that the design of blade is subject to a risk of fatigue failure and the areas of the blade which may be slightly weakened by the welding operation, are spaced from the high risk area. Figure 5 discloses a third embodiment similar to that shown in Figure 4. However, in this case, the existing blade part 16 is likewise reduced in length and the new blade end piece 24 is welded thereto using a high nickel, for example F.V. 520 (B) material 25. The new end part 24 is a composite part having an outer end piece 26 of normal 12% chrome steel blade material to which has been secured, for example by welding a piece of high nickel material 27, for example F.V. 520 (B). The composite new end piece 24 may be preformed in isolation under carefully controlled conditions. Heat treatment processes also are more easily carried out in isolation and the composite end part has the advantage of presenting the high strength, for example F.V. 520 (B) material 27 in a position where it is known that type of blade may have a tendency to fail. The method of repair of the present invention thus not only enables a repair to a blade to be made, but through the use of different material to effect the repair, enhances the strength of the blade at a position of known weakness, and hence further improves the expected service life of the blade. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method 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.

Claims: CLAIMS 1. A method of repairing a turbine blade in which it is required to replace an outer end part of the blade by a new end part comprising the steps of removing said outer end part and securing the new end part to the remainder of the blade by using a weld material having a relatively high nickel content compared with the original blade material. 2. A method of repairing a turbine blade according to claim 1 wherein the nickel content is at least 3%. 3. A method of repairing a turbine blade according to claim 2 wherein the nickel content is in the range of 4% to 8%. 4. A method of repairing a turbine blade according to any one of claims 1 to 3 wherein the weld material is F.V.520(B), having, by weight, a nickel content of 5% to 6%, a chrome content of 13.2% to 14.7%, a silicon content of 0.2% to 0.5%, a manganese content of 0.5% to 1%, a carbon content of 0.04%, a molybdenum content of 1.2% to 2%, and wherein the remainder is ferrous. 5. A method of repairing a turbine blade according to any one of the preceding claims wherein the new end part comprises a first portion and a second portion, wherein the second portion is secured to the original blade, and wherein the second portion is of a material the same as or similar to F.V.520(B). 6. A method of repairing a turbine blade according to claim 5 wherein the second portion is secured to the first portion by welding. 7. A method of repairing a turbine blade according to claim 5 or claim 6 wherein the second portion is welded to the first portion using a weld material with a relatively high nickel content. 8. A method of repairing a turbine blade according to claim 7 wherein the second portion is welded to the main portion using F.V.520(B) weld material. 9. A method of repairing a turbine blade according to any one of Claims 5 to 8 wherein the position of securement of said first portion to said second portion and the position of securement of said second portion to the original blade are such that said positions are spaced approximately equidistant from the position of initial failure of the blade. 10. A method of repairing a turbine blade according to any one of claims 5 - 9 wherein the new end part is longer than the original end part of the blade, and wherein a piece of the original blade is removed prior to securing the new end part to the original blade. 11. A method of repairing a turbine blade according to any one of the preceding claims wherein after securing the new end part to the original blade, the new end part is subject to a finishing process. 12. A method of repairing a turbine blade according to claim 10 wherein the finishing process comprises one or more of the following processes: heat treatment, machining, polishing. 13. A turbine blade comprising a radially inner part made from a normal turbine blade material and an outer part made from the same or similar material and an intermediate part interconnecting said radially inner part and radially outer part made from a material having a high nickel content compared with said other two parts. 14. A turbine blade according to Claim 13 wherein said intermediate part is secured by welding to said radially outer part and said radially inner part. 15. A turbine blade according to in Claim 13 wherein said intermediate part comprises a solidified weld material. 16. A turbine blade according to any one of Claims 13 to 15, said intermediate part is made from F.V. 520 (B) material. 17. A turbine blade refurbished in accordance with the method of repair according to any one of Claims 1 to 12. 18. A method of repairing a turbine blade substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing. 19. A turbine blade repair in accordance with a method as substantially hereinbefore described with reference to and as illustrated in the accompanying drawings. 20. A method of repairing a turbine blade including any novel step or combination of steps hereinbefore described and/or shown in the accompanying drawings.

Inventors: FRASER MICHAEL JAMES, LEGROS RAYMOND DONALD, FRASER, MICHAEL, JAMES, LEGROS, RAYMOND, DONALD
Applicants: FRASER MICHAEL JAMES, LEGROS RAYMOND DONALD, TURBINE BLADING LTD, FRASER, MICHAEL, JAMES, LEGROS, RAYMOND, DONALD, TURBINE BLADING LIMITED
Application year: 1995
Main classification: B23P6
IPC codes: B23P6, F01D5