WO2013168072A1

WO2013168072A1 - Method of repairing radially cracked hole

Info

Publication number: WO2013168072A1
Application number: PCT/IB2013/053608
Authority: WO
Inventors: Daniël Gerhardus HATTINGH; Louis George VON WIELLIGH; Phillip DOUBELL; Willem Gerhard PENTZ
Original assignee: Nelson Mandela Metropolitan University
Priority date: 2012-05-07
Filing date: 2013-05-06
Publication date: 2013-11-14
Also published as: ZA201401252B

Abstract

This invention relates to a method of repairing a radially cracked hole (12), and more particularly but not exclusively, to a method of repairing a radially cracked blade locating hole in a turbine rotor using a friction welding process. The method includes the steps of drilling the aperture to a diameter exceeding an extremity of a radial crack (13) that extends from a periphery of the aperture into the metal body; plugging the aperture (14) by way of a frictional welding process in which a consumable welding tool (20) is introduced into the aperture (14) via a second open end opposite the first open end so as to form a plugged body; and drilling a new aperture having a predetermined diameter through the plugged body.

Description

METHOD OF REPAIRING A RADIALLY CRACKED HOLE

BACKGROUND TO THE INVENTION

THIS invention relates to a method of repairing a radially cracked hole, and more particularly but not exclusively, to a method of repairing a radially cracked blade locating hole in a turbine rotor using a friction welding process.

Turbine blades of axial flow turbines are secured to rotors via rotor pin holes or blade-locating holes, in use, stress corrosion radial cracks develop adjacent to these rotor pin holes or blade-locating holes due to forces exerted by the turbine blades on the rotor disc during operation, which is exacerbated by corrosive environments in which these rotors typically operate. At present, these cracks are removed by drilling out the pin holes so as to increase the pinhole diameter beyond the radial extent of the radial cracks. There is, however, a limited diameter to which these pin holes can be drilled to, and once an upper limit has been reached the rotor disc is scrapped. Friction Taper Stud Welding (FTSW) and Friction Hydro Pillar Processing (FHPP) are solid state processes where a consumable tool is co-axially rotated in a hoie under a continuously applied axial force. The generated frictional heat forms a localised plasticized layer at the weld interface, which rises along the consumable tool in FHPP. The development of plasticized materia! forms faster than the plunge rate of the consumable tool so as to form dynamically re-crystallised material deposits below the weld interface. The plasticized material at the weld interface is maintained in a sufficiently viscous condition to transmit hydrostatic forces axialiy and radially to the inside of the hole to form a metallurgical bond.

One of the major advantages of friction welding processing is that the consumable tool is manufactured from the same material as the base material to be welded. This limits material composition variances and other problems associated with conventional welding techniques. In addition FHPP is carried out on an automated platform making it a simple and generally a more repeatable process once the processing window and conditions are well defined for the materia! in question. it is accordingly an object of the invention to provide a new method of repairing a radially cracked hole that will, at least partially, alleviate the above disadvantages. it is also an object of the invention to provide a method of repairing a radially cracked hole which will be a useful alternative to existing methods.

It is a stilt further object of the invention to provide method of repairing a radially cracked hole involving the application of a frictional welding process, and more particularly a FHPP process. SUMMARY OF THE INVENTION

A method of repairing a radially cracked aperture extending into a metal body, the method including the steps of:

- drilling the aperture to a diameter exceeding an extremity of a radial crack that extends from a periphery of the aperture into the metal body;

- plugging the aperture by way of a frictional welding process in which a consumable welding tool is introduced into the aperture so as to form a plugged body; and

- drilling a new aperture having a predetermined diameter through the plugged body.

The aperture may extend through the metal body_s and may have a first open end and a second open end.

The method may include the step of locating a backing plate adjacent a first open end of the aperture.

The welding tool may be inserted via the second open end in order for an end of the welding tool in use to abut the backing plate at the onset of the plugging process.

There is provided for the backing plate to be in the form of a consumable made of the same material as the metal body, and part of which is consumed during the friction welding process.

There is also provided for the backing plate to be at least partially recessed in a zone immediately below the first open end of the drilled out aperture, in order for the friction welding process to start below the metal body thereby increasing the processing temperature at the bottom of the aperture and reducing the risk of a lack of fusion at or towards the first open end of the drilled hole.

The new aperture may be substantially aligned with the original aperture, and may alternatively be axially offset relative to the original aperture.

The new aperture may have a smaller, similar or larger diameter than the original aperture.

There is provided for the method to include the step of machining the remainder of the backing piate protruding from the first open end, as well as the plugging protruding from the second end, flush with upper and lower surfaces of the body.

Locating and/or supporting plates may be used to secure the backing plate in position relative to the metal body.

Preferably, the metal body is in the form of a turbine rotor disc.

There may be a clearance between the outer surface of the welding tool and the inner surface of the aperture.

A ratio of the clearance to the diameter of the welding too! is preferably between 0.03 and 0.12, more preferably between 0.05 and 0.1.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described by way of a non- limiting example, and with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a turbine rotor disc; Figure 2 is an enlarged perspective view of a part of the turbine rotor disc of Figure 1 showing a radial crack extending from one of the blade locating holes;

Figure 3 shows a section of the turbine rotor disc in the process of being repaired utilizing the method in accordance with the invention; and

Figures 4(a) to 4{f) are cross-sectional side views showing the method in accordance with the invention in sequential steps.

DETAILED DESCRIPTION OF INVENTION

Referring to the drawings, in which like numerals indicate like features, a non-iimiting example of the application of the method in accordance with the invention is described in more detail. The method is specifically described with reference to a turbine rotor disc, but it will be appreciated that the method can also be applied to repair cracks in various other structures where radial cracks are found. tn this example, the metal body to be repaired is in the form of a turbine rotor disc 10 having a circumferential flange zone 11 in which a plurality of blade locating hole or pinhole apertures 12 are provided. In use, radial cracks 13 form due to operating conditions such as high temperatures and large forces. The method described hereinbelow provides a novel and inventive alternative to existing repair techniques, and includes a number of steps as described with reference to Figure 3, as well as Figures 4(a) to 4(f).

Figure 2 shows a part of a turbine rotor disc 10 with a radial crack having formed at one of the locating holes 12. The crack is typically identified by way of visual inspection or some other form of non-destructive testing during routine maintenance.

Figure 4(a) shows the original aperture 12 having a radial crack 13 (seen in Figure 2) extending therefrom. The aperture 12 has a first open end 14.1 and a second open end 14.2. In this example the crack is located in one of the apertures located in a central flange of the turbine rotor disc 10. It will, however, be appreciated that the method can also be applied in cases where the crack is located in the upper or the lower flange of the turbine rotor disc 10.

Once the crack 13 has been identified, the affected aperture is drilled out to a diameter exceeding the radial extent of the radial crack, as is shown in Figure 4(b). Should the crack be located at a hole in the central flange, one of the adjacent and axially aligned apertures in the upper or lower flange will also have to be drilled out and repaired afterwards, so as to allow access to the affected aperture.

The next step, shown in Figure 4(c), is to secure a backing plate 30 in the gap immediately below the drilled out aperture 14, in order for the backing plate 30 to cover the first opening 14.2 of the drilled out aperture 14. The backing plate 30 includes a recessed zone 31 which is in use aligned with the drilled out aperture 14. The backing plate 30 (also shown in Figure 3) is typically bolted to the flange using the holes adjacent the holes to be repaired.

Once the backing plate 30 has been secured in position, a consumable tool 20 is used to deposit material on the backing plate and the rotor disc. Using friction welding processing, the consumable tool (also known as stud) is rotated and, with the application of axial force, friction is generated. The friction creates heat that piasticizes the tool material, and allows it to be bonded to the rotor material. After sufficient material from the consumable tool has been deposited onto the rotor disc, the rotation is stopped while an axial force, referred to as the forge force, is maintained. The forge force is applied for a predetermined time to consolidate the plasticized material, in so doing forming a plugged zone 40. The backing plate and filling material that protrudes from the surface of the fiange is then machined flush with the upper 11.1 and lower 11.2 surface of the flange, resulting in the plugged structure shown in Figure 4(e), The final step is then to drill a new hole 50 according to a desired specification, as is shown in Figure 4(f).

The backing plate 30 may be held in position by way of one or more supporting plates (not shown). The backing plate may for example be sandwiched between two supporting plates, or may be housed in a window formed in a proximai zone of a continuous support plate.

During practical experimentation a number of preferred process parameters have been identified as being particularly important in the successful implementation of the new method. Although not exhaustive, a list of these parameters are provided below: Axial Force - The axial force to which the consumable tool is subjected during processing.

Rotational Speed - The speed at which the consumable tool is rotated during processing.

Forge Force - The axial forces to which the consumable tool is subjected after processing to consolidate material.

Forge Time - Time that the Forge Force is applied.

Consumable Length ~ The distance that the consumable tool is moved axiaily during processing.

Volume Fill - The volume of consumable tool material deposited into the hole vs the original hole volume. More than 100% deposition is possible due to compression of molecular structure of the consumable tool and part being processed.

The following values have been found to be suitable for the successful implementation of the new method, but it should be appreciated that the method is not limited to these values:

The experiments referred to above confirmed the feasibility of using FHPP as a repair technique for turbine disc blade-locating holes. The preliminary process parameter window was developed using mild steel (AISI 1018) to identify the effects process parameters such as axial force, forging force, rotational speed and consumable length have on the welds. The preliminary welds were quantified with respect to the bonding percentages in the area of concern and the width of the heat affected zone. The results obtained from the AISI 1018 welds were used in the development of a test matrix for 26NiCrMoV145. Process parameters such as axial force, rotational speed, consumable length and forging force were investigated, as well as the variation in tool chamfer angle in 26NiCrMoV145. High bonding percentages identified acceptable process parameters. The effects of pre-heating and post weld heat treatment were furthermore quantified in terms of micro- hardness.

Results obtained by FHPP of AISI 1018 revealed that the most significant parameter is the total clearance between the hole and consumable tool. An excessive total clearance did not produce any good welds regardless of process parameter selection. It was found that a suitable total clearance is in the order of a ratio of between 5 and 10% (clearance as a ration of hole diameter). In one example it was found that a 1mm clearance in a 15mm hole worked well.

The most significant process parameter, when using a suitable total clearance, is axial force followed by rotational speed. Axial forces below 9 kN and rotational speed below 5000 RPM produced welds with lower bonding percentages. Primary and secondary flash formation shape can be a good indication of weld quality and unacceptable weld process parameters.

Rotor material (26NiCrMoV145) welds showed that bonding percentages at the repair of the hole were above 95% for axial force between 12 and 21 kN at a rotational speed of 5000 RPM. At the bottom base of the hole a decrease of 11.3% bonding was found with a change of chamfer angle.

Forge force had no effect on the bonding percentage but reduced void volume size at the last shear plane. Pre-heating improved base bonding percentage but did not affect the rotor bonding percentage. Post weld heat-treatment of 740°C resulted in a decrease in the weld nugget Vickers micro-hardness from ± 550HV to ± 300HV, which is approximate parent material.

It will be appreciated that the above is only one embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention.

Claims

CLAIMS:

1. A method of repairing a radially cracked aperture extending into a metal body, the method including the steps of:

- plugging the aperture by way of a frictional welding process in which a consumable welding tool is introduced into the aperture in order to form a plugged body; and

- drilling a new aperture having a predetermined diameter in the plugged body.

2. The method of claim 1 in which the aperture extends through the metal body, with the aperture having a first open end and an opposite second open end.

3. The method of claim 2 including the step of locating a backing plate adjacent the first open end of the aperture and inserting the consumable welding tool into the aperture through the second open end, in order for an end of the welding tool in use to abut the backing p!ate at the onset of the plugging process.

4. The method of c\aim 3 in which the backing plate is in the form of a consumable made of the same material as the metal body, and part of which is consumed during the friction welding process.

5. The method of claim 3 or 4 in which the backing plate is at least partially recessed in a zone immediately below the first open end of the drified out aperture, in order for the friction welding process to start below the metal body thereby increasing the processing temperature at the bottom of the aperture and reducing the risk of a lack of fusion at or towards the first open end of the drilled hole.

6. The method of any one of the preceding claims including the step of machining the remainder of the backing piate protruding from the first open end, as well as the plugging protruding from the second end, flush with upper and lower surfaces of the body.

7. The method of any one of the preceding claims in which there is a clearance between the outer surface of the welding too! and the inner surface of the aperture.

8. The method of claim 7 in which a ratio of the clearance to the diameter of the welding tool is between 0.05 and 0.1.

9. The method of any one of the preceding claims in which the metal body is in the form of a turbine rotor disc.