CA2106425A1

CA2106425A1 - Method of restoring the bore of a turbo-machine rotor

Info

Publication number: CA2106425A1
Application number: CA002106425A
Authority: CA
Inventors: Dennis Ray Amos; Timothy Lee Driver; Robert Edward Clark
Original assignee: Individual
Current assignee: CBS Corp
Priority date: 1992-09-18
Filing date: 1993-09-17
Publication date: 1994-03-19
Also published as: ITPD930182A1; KR940007350A; ES2083908R; IT1263691B; ITPD930182A0; ES2083908A2

Abstract

ABSTRACT OF THE DISCLOSURE
A method of restoring the bore of a rotor in a turbo-machine rotor having a centrally disposed bore by the steps of (i) depositing a weld material onto the bore surface over at least a portion of the axial length so as to reduce the inside diameter of the bore and roughened the bore surface over the portion of the axial length and (ii) machining the weld material so as to smoothen the bore surface. According to the method, the step of depositing the weld material is performed so as to deposit a weld bead circumferentially around the inner surface in a multi-pass process by inserting a welding nozzle into the bore and simultaneously impart both rotational and linear relative motion between the rotor and the welding nozzle. The bore inner surface is pre-machined to increase the inside diameter prior to the step of depositing the weld material and post-machined to the original bore diameter following the welding.

Description

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57 , 4 07 METHOD OF RESTORING THE BORE OF A IURBO-MACHINE ROTOR
BACKGROUND OF THE INVENTION : -The present invention relates to a method of restoring the bore of a rotor of a turbo-machine, such as a steam or gas turbine or the like. More specifically, the present invention relates to a method of restoring the surface .. . . .
of a rotor bore by depositing weld material onto its inner surface.
The rotor of a turbo-machine, such as a gas or steam turbine, is typically manufactured with a centrally disposed hollow portion, commonly referred to as a bore. The purpose of the bore is to remove non-metallics, which could become crack initiation sites, that form at the center of the rotor using traditional forging techniques. ~ue to the rotational speed of the rotor, i.e., 1800 or 360~ RPM in a turbine used ! 15 in electrical power generation, the centri~ugal force created thereby induces very large stresses at the bore. In addition, ` thermal stresses are imposed on the rotor material surrounding ; the bore due to di~ferential thermal expansion created by ~ thermal gradients within the rotor during start-up and ; 20 shutdown. Moreover, as a ~esult of exposure to the hot ; ., , working fluid, the temperature of the rotor material ¦ surrounding the bore can become quite high.
Consequently/ afterextendedoperation, the exposure to stress and temperature may result in the ~ormation of 25 ~cracks on the bore sur~ace due to a creep mechanism. ~-ypically, these cracks are discovered during maintenance -inspections~. Since such cracks serve to concentrate the ; -.
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2 57,407 stress at the rotor bore, the cracks must be removed prior to returning the rotor to service.
Traditionally, cracks have been removed by over-sizing the rotor bore along a portion of its length --sometimes referred to as "bottle boring" -- so as to machine away the rotor material surrounding the bore that contains the crack. While this approach is successful initially, the cracks may often reappear. Unfortunately, having been oversized once, there is often insufficient thickness remaining in the rotor to allow further bottle boring. As a result, the rotor must be scrapped or subjècted to undesirable ; operational limitations, such as pre-warming prior to start- ;
up. Although weld repairing has been tried with great success on the exposed surfaces of turbo-machine rotor~ -- see, for 15example U.S. patent application Serial No. 07/801,137, filed December 2, 1991 -- it had heretofore been thought that welds of acceptable quality, given the high stresses at the rotor bore, could not be produced within the relatively inaccessible rotor bore inner surface.
20Consequently, it would be desirable to provide a method ~or restoring, rather than merely removing, the cracked surface o~ a turbo-machine bore by a welding process.
In addition to cracking, the surface of the rotor bore is also subject to corrosive pitting. Like cracking, ~itting serves to con~entrate stresses and, hence, weakens the ;; rotor. Accordingly, it would be desirable to provide a method ~or cladding the surface of a rotor bore with a corrosion resistant material.
SUMMARY OF THE ~NVENTI~N
30Accordinqly, it is the general object of the current ~, invention tc provide a method ~or restoring the cracked surface o~ a turbo-machine bore by a welding process. -~; -~riefly, this object, as well as other objects of .! the current invention, is accomplished in a turbG-machine ~5 rotor, having a centrally disposed bore having an axial length and an inside diameter, by the method of restoring the surface ~; of the bore by the steps of (i) depositing a weld material ., .
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3 57,407 onto the bore surface over at least a portion of th~ axial length so as to reduce the inside diameter of the bore and roughen the bore surface over the portion of the axial length and (ii) machining the weld material so as to smoothen the bore surface. According to the method, the step of depositing the weld material comprises depositing a weld bead circumferentially around the inner surface in a multi-pass process by inserting a welding nozzle into the bore and simultaneously imparting both rotational and linear relative motion between the rotor and the welding nozzle.
According to the preferred embodiment of the method, the bore inner surface is pre-machined to increase the inside diameter prior to the step of depositing the weld material and post-machined to the original bore diameter.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a longitudinal cross-section cf a steam turbine rotor having a crack at the bore surface.
Figure 2 is a view similar to Figure 1, showing the rotor a~ter a portion of the bore has been over~sized to remove the crack by bottle boring.
Figure 3 is a schematic diagram showing the welding equipment in use according to the current invention.
Figure 4 is a view of a detailed portion of the over-sized portion of the bore after weld material has been ;~
deposited according to the current invention.
Figure 5 is a view similar to Figure 2 taken after ~
the weld material has been deposited in the over-sized portion ` of the bore according to the current invention.
Figure 6 i5 a view similar to Figure 5 taken after the weld material has been machined smooth according to the current invention.
` Figure 7 is a transverse cross-section taken through J the rotor during stress relievin~.
~ DESCRIPTION OF THE PRE~FRRED EMBODIMENT
Referring to the drawings, there is shown in Figure 1 the rotor 1 of a steam turbine having a bore 2 that has, typically, an inside diameter of approximately 12-15 cm (5-~ ' , .
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4 57,407 6 inches). By way of example, the rotor bore 2 has a crack 3 emanating approximately radially inward ~rom its inner surface 6. As previously discussed, such a crack 3 may be the result of creep stresses in the rotor and sexves to further ;5 concentrate the stresses in the rotor 1. According to the current invention, the crack 3 is removed by first bottle boring the rotor -- that is, by removing material concentrically around the bore over a portion of its axial length. The material may be removed by a variety of techniques known in the art, such as machining or grinding.
As a result of the bottle boring, an over-sized portion 14 is formed in the rotor bore 2, as show~ in Figure 2. The inner diameter of the over-sized portion should be sufficient to encompass the entirety of the crack 3, typically approximately 5 cm (2 inches) larger than the original bore 2 diameter. In addition, the over-sized portion 14 typically should have a length of approximately 25-30 cm (10-12 inches). ~ -As shown in Figure 2, the over-sized portion 14 of the rotor bore 2 has a substantially cylindrical central portion 4 and conically shaped ends 5~ The conical shape serves to ensure that the machining does not produce any sharp steps in the rotor that could create areas of stress concentration. After ;the bottle boring is completed, the over-sized portion 14 is cleaned, ~or example by wiping with denatured alcohol or acetone.
After cleaning, the portion of the rotor surrounding the over-sized portion 14 is pre-heated. In the preferred embodiment, the temperature of the bore sur~ace is raised to at least about 80 C (175 F) during pre-heating by the use ;'30 of an electric resistance heater 15 placed around the circumference of the rotor, a~ shown in Figure 7. As shown in ~igure 3, after pre-heating, a welding nozzle 9 of an automatic gas tun~sten arc welding machine (not shown) is inserted into the rotor bore 2. The welding nozzle 9 is supported by a conduit 12 through which the weld wire 16 is fed from a feed reel 11. Since the rotor bore may be as long ~s 3 ~ (10 feet), it is imp~rtant that the conduit and its :; :, -:
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57,407 support assembly (not shown) be sufficiently rigid to provide stable support for the welding nozzle 9.
Steam turbine rotors are typically manufactured from ; low alloy steel, i.e., steel having less than 6% alloying elements. Such alloying elements typically include various combinations of nickel, chromium, molybdenum, and vanadium.
The ferrous alloys used to restore the rotor bore according to the current inven~ion include similar alloying elements, with the speci~ic composition being selected to match the yield, creep and fatigue strength of the rotor alloy.
- Once the welding nozzle 9 is in place, a gas tungsten arc welding process is used to deposit weld material 13 onto the surface of the over-sized portion 14 o~ the rotor bore 2. A gas tungsten arc process is used because of the extremely fine microstructural grain size it produces and because the equipment can be sufficiently miniaturized to allow insertion of the welding nozzle into the relatively small diameter of the rotor bore 2. In order to produce welds of acceptable quality it is important that extremely pure -` 20 inert gas be used.
The weld material 13 is applied by depositing a 360~
circumferential weld bead 7 around the surface of the bore in an approximately spiral configuration so that the entire ~j length of the over-sized portion 14 is covered by a layer of weld material 13. This is accomplished by imparting both rotational and linear relative motion between the rotor and the weldin~ nozzle 9. In the preferred embodiment, the welding nozzle 9 is held stationary with respect to rotation and the rotcr is rotated around the welding nozzle.
Simultaneously, the welding nozzle 9 is retracted from the , bore ~ using a rac~ and pinion device 17 to effect linear motion. Alternatively, the invention could also be practiced by holding the rotor stationary and rotating the welding nozzle 9 around the inner surface of the rotor bore. In either case, the result is the formation of a thin layer, i.e., typically 0.125 cm (0.050 inch), of weld material 13 ', ' ' ` '' ' :~ , ., : ~ - . .,: .

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6 57,407 covering the entire surface of the over-sized portion 14 of the bore during each pass.
The rate of linear travel of the welding nozzle 9 and the rotational speed of the rotor will depend on the flow rate of weld material 13, which in turn, is a function of the diameter of the weld wire 16 and the amperage. In general, the deposition of sufficient weld material 13 to restore the over-sized portion 14 of the rotor bore 2 to its original diameter i5 a time consuming process ~ e., the thickness of weld material 13 deposited by each weld pass is typically 0.125 cm (0.050 inch) whereas the diameter in the over-sized portion 14 has been increased by approximately 5 cm (2 inches~. Consequently, the relative speed, wire diameter and amperage will be chosen to maximize the deposition rate, being careful to ensure adequate fusion of the weld material 13 to the rotor base metal, however. In the preferred embodiment, a miniature ca~era ~ is installed on the conduit 12 and allows the welding process to be monitored on a real time basis using a video monitor 10. ~lternatively, an optical eye-piece or boroscope could also be utilized to monitor the welding.
;After a layer has been deposited, the current is jappropriately tapered off before breaking the arc~ The welding nozzle 9 is then withdrawn and the weld bead 7 is wire brushed to ensure cleanliness. The welding nozzle 9 is then re-inserted and the process repeated so that the inside diameter of the over-sized portion 14 of the rotor bore 2 is gradually reducied in a multi-layer operation until the diameter is less than the original bore inside diameter, as shown in Figures 4 and 5. The temperature of the rotor metal 3~ surrounding the over-sized portion 14 is monitored and suf~icient cooling time is provided between successive passes to ensure that the temperature does not exceed 315 C (600 F) to minimize the formation of a heat affacted zone in the base metal.
~ 35Since the deposition of weld material 13 results in ,~ a rough sur~ace on the bore, after the welding i5 completed, ~' ~ the surface of the weld material 13 is machined to smoothen .
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, 7 57,407 its surface. In the preferred embodiemnt, the weld material 13 is machined flush with the diameter of the remaining portion of the rotox bore 2 so that the original diameter is restored in the repaired area, as shown in Figure 6. A~ter machining, the repaired portion of the bore is NDT inspected and then heat-treated to minimize residual weld stresses and temper the weld and the heat affected zone in the surrounding base metal. Ideally, the heat treatment should be performed at as high a temperature as possible without degrading the - 10 base material properties. The specific temperature chosen will, therefore, depend on the specific rotor alloy but will typically be approximately 590 ~C (1100 F) and be held for approximately 10 hours.
Heating the rotor for postweld heat treatment may ; 15 be accomplished in a variety of ways known in the art. In the preferred embodiment, a resistance heater 15 is placed around the rotor in the vicinity of the repair and the temperature of the inner surface of the bore weld material 13 is monitored via a thermocouple 28. Following heat treatment, the repaired portion of the rotor l is given a final NDT inspection.
Although the current invention has been illustrated by reference to weld repair of a crack in the hore of turbo-~ machine rotor, the invention is also applicable to cladding ¦ the bore o~ a rotor with a corrosion resistant material to prevent corrosive pitting of the bore surface. In such cases, pre-machining of the bore to create an over-sized diameter may not be necessary and the entire length bore would be clad, ~; rather than only a portion of the axial len~th of the bore.
Although the multi-pass operation previously discussed can be utilized, it is anticipated that at most only two passes would be necessary to deposit sufficient corrosion resistant material. A variety of weldable corrosion resistant alloys may be used, including a 12% chrome alloy. However, the fine ~i micros~ructural grain size producéd by the tungsten inert gas process will itself produce considerable corrosion resistance in a lower chromium bearing alloy chosen primarily for its strength characteristics.
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210~25 8 57,407 As can be seen, the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.

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Claims

1. In a turbo-machine rotor having a centrally disposed bore having an axial length and an inside diameter, a method of restoring the surface of said bore comprising the steps of:
a) depositing a weld material onto said bore surface over at least a portion of said axial length so as to reduce said inside diameter of said bore and roughen said bore surface over said portion of said axial length; and b) machining said weld material so as to smoothen said bore surface.

2. The method according to claim 1, wherein the step of depositing said weld material comprises the step of depositing a weld bead circumferentially around said bore surface.

3. The method according to claim 2, wherein the step of depositing said weld bead comprises the step of inserting a welding nozzle into said bore and simultaneously impart both rotational and linear relative motion between said rotor and said welding nozzle.

4. The method according to claim 3, wherein said welding nozzle has a video camera mounted thereon, and wherein the step of depositing said weld bead comprises the step of observing said weld bead deposition through said camera.

5. The method according to claim 3, wherein the step of imparting said relative rotational motion comprises holding said welding nozzle rotationally fixed and rotating said rotor around said welding nozzle.

6. The method according to claim 3, wherein the step of imparting said relative rotational motion comprises holding said rotor rotationally fixed and rotating said welding nozzle within said bore.

7. The method according to claim 1, wherein the step of depositing said weld material comprises depositing weld beads onto said bore surface in a multi-pass process.

8. The method according to claim 1, further comprising the step of pre-machining said bore surface to increase said inside diameter over at least said portion of said axial length prior to the step of depositing said weld material.

9. The method according to claim 8, wherein said bore surface has a crack formed therein, and wherein the step of pre-machining said bore surface comprises increasing said inside diameter sufficiently to remove said crack.

10. The method according to claim 8, wherein the step of pre-machining said bore surface comprises pre-machining said portion of said axial length of said bore, said portion having a cylindrical portion and first and second ends, said ends having substantially conically shaped cross-sections.

11. The method according to claim 1, wherein the step of depositing said weld material comprises depositing a weld bead using a gas tungsten arc process.

12. The method according to claim 1, further comprising the step of heat treating said bore after said weld material has been deposited.

13. The method according to claim 12, wherein the step of heat treating said bore comprises applying a heating element to external surfaces of said rotor.

14. In a turbo-machine rotor having a centrally disposed bore having an axial length and an inner surface having a first inside diameter, a defect being present in said inner surface, a method of repairing said defect comprising the steps of:
a) machining at least a portion of said axial length of said bore inner surface, thereby increasing said bore inside diameter over said axial length to a second diameter, said second diameter being sufficiently large to encompass said defect;
b) depositing a weld bead circumferentially around said bore inner surface along said portion of said axial length so as to reduce said bore inside diameter to a third diameter; and c) machining said weld bead so as to increase said bore inside diameter to a fourth diameter.

15. The method according to claim 14, wherein said third diameter is less than said first diameter and said fourth diameter is substantially equal to said first diameter.

16. The method according to claim 14, wherein the step of depositing said weld bead comprises the step of inserting a welding nozzle into said bore and simultaneously impart both rotational and linear relative motion between said rotor and said welding nozzle.

17. In a turbo-machine rotor having a centrally disposed bore, a method of cladding the inner surface of said bore comprising the steps of:
a) inserting a welding nozzle into said bore and simultaneously imparting both rotational and linear relative motion between said rotor and said welding nozzle;
b) depositing a bead of a cladding material onto said bore inner surface using said welding nozzle, thereby roughening said bore inner surface;
and c) machining said weld material so as to smoothen said bore surface.

18. The method according to claim 17, wherein the step of depositing said bead comprises the step of depositing said bead using a gas tungsten are process.

19. The method according to claim 17, wherein the step of imparting said relative rotational motion comprises holding said welding nozzle rotationally fixed and rotating said rotor around said welding nozzle.

20. The method according to claim 17, wherein the step of imparting said relative rotational motion comprises holding said rotor rotationally fixed and rotating said welding nozzle within said bore.