METHOD FOR REPAIRING TURBINE COMPONENTS

BACKGROUND OF THE INVENTION

The present invention relates to the technology of gas turbines. It refers to a method for repairing turbine components, especially turbine blades, by welding.

PRIOR ART

Typical gas turbine blades, an example of such a blade 10 is illustrated in Fig. 1 , comprise an airfoil 1 1 , which extends in radial direction from a shaft 12 and ends with a blade tip 14. Below the shaft 12 is a root section 13 (foot of the blade) for mounting the blade 10 in a respective groove of the rotor (not shown). Part of the blade tip 14 is a crown with a crown plate 14' and a rim (squealer 14"). Those tip features are often referred as squealer tips and are designed to minimize the tip leakage flow, protect the blade tip from the full impact of high temperature leakage gases, when the tip cavity is used as a cooling system and protect the blade against incidental rubs. An additional feature of the blade tip 14 is a so called letterbox 15 (details see below). Gas turbine components, such as turbine blades, are often made of nickel-based alloys. Precision casting of such alloys is widely used as a primary shaping technology process. Precision casting has some typical technology related limitations, which must be considered already during design of the components manufactured by this process. Some of the design features implemented for manufacturability by precision casting of a component might reduce overall component lifetime, e.g. having these design elements which have a negative influence on oxidation lifetime or lead to reduced high cycle or low cycle fatigue lifetime.

Typical examples are inserts, closure plates or letterboxes, which close the casting related openings in the component, here the turbine blade, in order to ensure the correct internal cooling path. For precision casting these openings are needed for removal of the ceramic cores by chemical leach/etching process. During the new- part manufacturing process chain these openings (letterboxes) are later closed by brazing in closure plates. During service these locations suffer significantly and the parts show often a high degree of distress after de-installation.

For reconditioning and second lifecycle of such components a restoration of the original design is necessary. This includes so far a replacement of the brazed letterbox, but subsequently a high fall-out rate is observed.

To re-braze a new letterbox, a certain amount of healthy base material, this means material which is not being already influenced by previous brazing, is needed due to material and mechanical integrity reasons. This restriction leads to a certain fallout rate (scrap) during reconditioning. For example, through going cracks above the letterbox are such scrap reasons. Furthermore, a certain wall thickness below the letterbox repair is needed in order to avoid braze erosion, which also leads to fall-out. In addition, a wrong positioning of the electrode during EDM (Electrical Discharge Machining) could also lead to scrap, for example if the letterbox hasn't been eroded out completely, the crown plate is too thin etc. Instead of the described and well known re-brazing of a turbine tip it is also possible to use a standard weld strategy for closing the component, for example closing a casting hole of a removed letterbox simply by weld filler. But such a method results in a non-optimal inner geometry and leads to weld-related defects, like suck-backs and voids, pores and cracks of the welded repair, which were outside the allowable quality limits for mechanical integrity reasons.

As a state-of-the-art solution for the closing of blade tips usually tip cap welding is performed as described for example in documents EP 2 071 124 A2, WO

2013/1 69747 A1 or WO 2013/169754 A1 and which illustrated in Fig. 2a, Fig. 2b and Fig. 2c (mainly taken from the prior art document WO 2013/1 69754 A):

A new closure plate or tip cap 48 is placed above a previously grinded or machined (repair surface 46) blade tip (Fig. 2a) and welded in place (Fig. 2b), e.g. with different weld fillers. The closure of the blade resp. internal cavity 34 is realized by the closure plate/tip cap 48 itself. The finished assembly structure consists according to Fig. 2c of the blade itself with pressure and suction side walls 22, 24, the new tip cap 48 and one or several welded elements, e.g. weld joint 54, peripheral portion 56 and replacement squealer tip 62. This is the typical repair approach for the case, where a blade tip requires a complete or partial opening into its internal cavity 34 (cooling air side).

There are three joined material interfaces (blade, tip cap and weldings) which is a disadvantage because each interface is usually a source of defects. The disadvantage and therefore a limitation of the standard "tip cap" approach consists typically in significant weld defects in the area marked "C" in Fig. 1 c and often in an not sufficient bonding between the blade, the weld filler and the tip cap.

It is also known in prior art to keep the repair zone outside of the internal cavity (limited repair).

Welding with a more plastic and forgiving weld filler or increasing the joined length by a special geometry between blade, tip cap and weld filler could bring some improvement. But for cases where additionally the local wall-thickness of blade close to the critical area C is low, this might be not sufficient from mechanical integrity point-of-view.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an efficient method for repair of a worn component, especially a gas turbine blade, with an improved quality. An optimal inner geometry as well as only defects diminished to an allowable extend should be reached.

According to independent claim 1 the method for repairing a precision casted turbine blade, which comprises an airfoil with a pressure and a suction side with a wall thickness and internal cooling channels, a tip having a tip crown with a crown plate and a squealer and at least one letterbox which was closed by brazing during new-part manufacturing, wherein the letterbox area was damaged during operation in the first life cycle of the blade, comprises the following steps. a) Removing the damaged letterbox area together with the appropriate tip crown; then

b) Building-up the pressure and suction side of the airfoil by welding to a sufficient height; then

c) CNC machining of the airfoil to the height of a bottom side of the crown plate; then

d) Electrical discharge machining (EDM) of the inner contour of the weld build-up including an exact opening geometry of an internal cavity for incorporation of a closure plate; then

e) Inserting an exactly fitting closure plate into the opening machined in step d) and fixing the closure plate by spot welding; then

f) Selecting a weld filler dependent on the required properties and over- welding completely the closure plate such, that the closure plate itself is re-melted and absorbed into the liquid weld pool mainly formed by the selected weld filler, thereby building-up the crown plate; then

g) Welding the squealer tip and finally

h) Restoring the inner and outer contour of the blade by grinding.

The core of the present application is to place a closure plate into the cavity of the blade opening instead of placing it above the machined level. In the final assembly structure this closure plate does not function as the closing element of the blade, but it is merely a process aid for the subsequent over-welding process.

The closure plate has the function of a weld pool fixation, avoiding that in the critical area "C" the liquid weld pool solidifies with a significant geometrical imperfection (called "suck-backs" or "undercuts"). The closure plate itself is re- melted and absorbed into the liquid weld pool formed mainly by the selected weld filler. After welding, the closure of the blade is realized by the welding itself rather than by any "foreign" tip cap. As an advantage, this allows to limit the number of joined material interfaces from three (blade, tip cap and weldings) to two (blade and weldings incl. dissolved closure plate). Because each interface is usually a source of defects, this is a significant advancement over the standard state-of-the- art process.

In a preferred embodiment removing in step a) comprises machining or grinding out the damaged letterbox area completely, including the braze-influenced zone from new-part manufacturing and the local damage from operation during the first life cycle of the blade. The depth of the cut-out is preferable up to 5 mm below the today's letterbox and an opening angle for the cut-out is in the range of 100-120 ° with a radius R3.

In one embodiment the building-up in step b) comprises a weld build-up of the pressure side and suction side walls with an increasing wall thickness along the build-up so that the wall thickness at the level of the spot welded closure plate in step e) is higher than at the level of the machined/grinded blade in step a), it is an advantage if the thickening of the wall thickness from the ex-service thickness up to a thickness which equals 2 times the new-make design nominal wall thickness at this area, because then a shift of the critical area "C" towards the middle of the blade, that means away from the load carrying side walls, can be reached. This has the advantage that the size of the closure plate shrinks (limited influence on the weld pool) and the quality of the bonding between the blade and the squealer tip increases by reducing the weld imperfections.

Other preferred embodiments are described in the additional dependent claims, for example when an automated laser welding with an optical scanner for the laser beam is used for adjusting the weald bed thickness and thus the local wall thickness of the weld build-up during the process the repaired component has an extreme high quality and therefore a long second lifetime.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings. Fig. 1 shows a photograph of a typical gas turbine blade in a perspective view;

Fig. 2a-c show several steps for closing of the blade tip according to the prior art (tip cap welding);

Fig. 3 shows a photograph of an ex-service gas turbine blade with

removed letterbox and surrounded material (identified by the circle in the figure) after step a) of the present invention; Fig. 4 shows schematically the tip of the blade with a gauge designed for better control of the inner contour of welding; Fig. 5a-b show schematically embodiments of the building-up of one of the airfoil sides walls by welding with an increase of the wall thickness after step b) of the present invention ; Fig. 6 shows a photograph of the airfoil after step c) of the present

invention ;

Fig. 7a-c show photographs with several steps for closing the cavity with an inserted exactly fitting closure plate, the spot welded plate and completely over welded plate (after steps e) and f) of the present invention.

DETAILED DESCRI PTION OF DI FFERENT EMBODIMENTS OF THE INVENTION

In the following detailed description of a preferred embodiment reference is made to the accompanying drawings that form a part hereof and which is shown by way of illustration, and not by way of limitation. It is to be understood that other embodiments may be utized and that changes may be made withgout departing from the scope of the present invention.

As an example, the applicant's GT1 3E2 Blade 1 (precision casted new blade 1 0) has two letterboxes 1 5 (in the tip area 14 with a tip crown), which are closing the casting related core holes. During service these locations suffer significant and the parts show often a high degree of distress after deinstalation.

For reconditioning and a second life cycle of such a component, so far a restoration of the original design, including a replacement of the brazed

letterboxes 1 5, is done, which causes a relative high fall-out rate. According to main idea of the invention the original precision casting driven design feature of the letterbox 15 is not restored during reconditioning, but entirely removed and replaced by a special welded structure.

According to independent claim 1 the disclosed method for repairing that precision casted turbine blade 10 comprises the following steps:

a) Removing the damaged letterbox 1 5 area together with the appropriate tip crown 14'; then

b) Building-up the pressure and suction side 22, 24 of the airfoil 1 1 by

welding to a sufficient height; then

c) CNC machining of the airfoil 1 1 to the height of a bottom side of the crown plate 14'; then

d) Electrical discharge machining (EDM) of the inner contour of the weld

build-up including an exact opening geometry of an internal cavity 34 for incorporation of a closure plate 1 6; then

e) Inserting an exactly fitting closure plate 1 6 into the opening machined in step d) and fixing the closure plate 1 6 by spot welding; then

f) Selecting a weld filler 54 dependent on the required properties and over- welding completely the closure plate such, that the closure plate 1 6 itself is re-melted and absorbed into the liquid weld pool mainly formed by the selected weld filler 54, thereby building-up the crown plate 14'; then g) Welding the squealer 14" of the tip 14 and finally

h) Restoring the inner and outer contour of the blade 10 by grinding.

By grinding out the damaged letterbox 15 area completely, the local damaged zone from operation of the first life cycle as well as the braze-influenced zone fro the new-part manufacturing is removed (see Fig. 3). In a preferred embodiment the depth of the cut-out is up to 5 mm below the originally letterbox 15 and the opening angle for the cut out is 100 to 120 ° with a radius R3.

After that first step, the airfoils 1 1 at the pressure side 24 and the suction side 22 (as well as the intermediate wall in this embodiment) are weld build-up. In present embodiment the weld-build-up is realized with an increasing wall thickness w along the build-up. A gauge designed for better control of the inner contour of the welding could be used with advantage (see Fig. 4).

The thickening of the local wall below the closure plate level from a value w1 , which is the local wall-thickness of the grinded / machined blade to a higher value w2 at the level of the closure plate 1 6 makes it possible to shift the critical area "C" (see Fig. 5) towards the middle of the blade 10, thus away from the load carrying side walls. On one hand it is now possible to shrink the size of the closure plate 16 (limited influence on the weld pool), and on the other hand the quality of bonding between the blade 10 and the squeeler tip 14 above is increased by reducing the weld imperfections. It is preferred when the thickening of the wall thickness from ex-service thickness up to a thickness which equals 2 times the new-make design nominal wall thickness in this area. After step b), which means after welding the airfoil sides 22, 24 up to a sufficient height the component is CNC machined to the height of the bottom side of the crown plate 14'. The correct height of the CNC-grinding ensures correct local wall thickness w, correct weight distribution and thus correct loading situation for the crown tip 14 (Fig. 6).

Then electrical discharge machining of the inner contour of the weld build-up including the exact opening geometry of an internal cavity 34 for incorporation of a closure plate 1 6 follows. By doing so, the amount of weld filler 54 (weight = load) and the inner contour of the weld build-up can be controlled for mechanical integrity reasons. The correct inner contur can be assured by the shape of the EDM electrode.

The next step is essential - an exactly fitting closure plate 1 6 preferrably produced by laser or water-jet cutting is spot welded into the cavity 34 produced by the EDM process (see Fig. 7a and Fig. 7b) and afterwards the entire closure plate 1 6 is over-welded /(see Fig. 7c). During that close weld process this closure plate 1 6 serves as a weld pool fixation, securing a stable weld pool and thus a controlled overall weld process including inner geometry of the weld. This has the advantage that typical weld defects such as like geometrical suck-bucks, sharp notches and /or cracks at edges of the inner geometry are minimized. The material for the closure plate 1 6 is with advantage Hastelloy X, Haynes 230 or a similar Ni-base superalloy. The thickness sof the closure plate 1 6 is preferable 1 .0 to 1 .8 mm. The size of the closure plate 1 6 is optimized for each component.

Relative to the outer airfoil geometry there is a preferred innerwards shift for 2.0 to 4.0 mm. The interface design between the closure plate 16 and the machined part of the blade is in a preferred embodiment "butt fit" (see Fig. 5a) or as an alternative design with a shelf (see Fig. 5a). It is an advantage to use one or combinations of several different weld filler materials optimized for high strength, for high oxidation resistance or for high weldability.

A buffer weld could be realized with different properties compared to the crown plate 14' or the squealer 14" tip weld. This depends on the repaired component and its distress mode. The weld process is realized by manual or automated TIG (Tungsten Inert Gas) welding, MAG (Metal Active Gas) welding or laser welding (LMF- Laser Metal Forming). Preferred is an automated laser welding including an optical scanner for the laser beam with the possibility of adjusting the weld bead thickness and thus the local wall thickness of the weld build-up during the process.

After over-welding the complete closure plate 1 6, additionally the squealer tip 14', 14" is being welded according to the known state of the art. Finally the inner (tip cavity) and the outer (airfoil) contour of the blade 10 are restored by grinding, finishing the repair of the component.

The disclosed weld repair is embedded in an overall process chain including restore cooling features, redrilled cooling holes, recoating etc. Of course, the disclosed process is not limited to the described embodiments. It could be used for all components in which during repair a cavity has to be closed.

LIST OF REFERENCE NUMERALS

10 blade, turbine blade

1 1 airfoil

12 shaft

13 foot

14 blade tip

14' crown plate

14" squealer

15 letterbox

16 closure plate

18 airfoil outer wall

22 pressure side wall

24 suction side wall

34 internal cavity

46 repair surface

48 closure plate / tip cap

54 weld joint (weld filler)

56 periphal portion

62 replacement squealer portion

w wall thickness

w1 wall thickness at the level of the machined blade

w2 wall thickness at the level of the welded closure plate