GB2401115A

GB2401115A - Refurbishing corroded turbine blades involving aluminising

Info

Publication number: GB2401115A
Application number: GB0310046A
Authority: GB
Inventors: Adrian Kempster; Tony Smith
Original assignee: Diffusion Alloys Ltd
Current assignee: Diffusion Alloys Ltd
Priority date: 2003-05-01
Filing date: 2003-05-01
Publication date: 2004-11-03
Anticipated expiration: 2023-05-01
Also published as: GB2401115B; GB0310046D0

Abstract

The process for refurbishing the corroded turbine blade or vane having at least one cooling hole having an untreated internal surface which is coated with products of corrosion and has a depletion zone containing further products of corrosion comprises the following steps:-firstly removing at least part of the products of corrosion from the internal surface of the at least one cooling hole, secondly aluminising the internal surface of the at least one cooling hole obtained from the first step to form an aluminide layer of sufficient depth to enclose substantially all of the products of corrosion and at least part of the depletion zone by filling the at least one hole with an aluminising pack containing 10-65 wt% of aluminium, heating the aluminising pack for a sufficient time at a sufficient temperature to form the aluminide layer and removing the aluminising pack and thirdly removing the aluminide layer together with substantially all of the products of corrosion and at least part of the depletion zone. The turbine blade may be of a superalloy of nickel and aluminium. In the first step the products of corrosion may be removed by blasting; in the third step the aluminide may be removed by chemical means.

Description

2401 1 5 Refurbishing corroded turbine blades This invention relates to

the refurbishing of corroded turbine blades and particularly to the treatment of the internal surfaces of cooling holes in corroded turbine blades.

Turbines are made up of a compressor for supplying air, a combustion chamber, a hot section and an exhaust. The hot section contains a number of stages of turbine blades attached to a number of rings which are themselves attached to a shaft. In between the rings of turbine blades are vanes. Examples of turbines include stationary gas turbines, marine turbines and turbines present in aeroengines. Gas turbines employ oil or natural gas which is burnt to produce hot gases which act upon the turbine blades to drive a motor. Gas turbines usually employ four stages of turbine blades with the stages nearest the inlet being exposed to the highest temperatures. Gas temperatures typically range from 750-1300 C. Hot gases produced by burning oil or natural gas produce aggressive conditions in which oxygen and nitrogen present in the hot gases react with the metals present in the turbine blades to produce oxides and nitrides. In addition, contaminants present in the oil or natural gas, such as sulphur, sodium and vanadium, also react with the metals present in the turbine blades.

l urbine blades are thus made of materials which combine corrosion resistance and strength at high temperature. Typically, turbine blades are made from stainless steel or, more commonly, from so-called superalloys which are themselves made from various formulations including iron, nickel, cobalt and chromium as well as lesser amounts of tungsten, molybdenum, tantalum, niobium, titanium and aluminium. Such superalloys are well known in the art and are exemplified by Rene 80, GTD 111 and CMSX - 4.

In order to maintain corrosion resistance at these elevated temperatures, turbine blades arc' provided with protective coatings. At the lower end of the temperature ranges, 3() such as those found towards the rear of the gas turbine hot section, diffused chromium may be applied to the turbine blades. II,wever, at higher temperatures, such as near to the gas inlet, turbine blades are coated with alloys, such as nickel-cobalt chron1innl-aluminium-yttrium alloys. However, even turbine blades with protective coatings are subject to corrosion on their exposed surfaces and require refurbishment in order to maintain a sufficiently long service life, typically about 100,000 hours.

Turbine blades are detachable from their rings and are generally refurbished about every 25,000 hours. Refurbishing (or reconditioning) turbine blades involves the removal of corrosion products from the exposed surfaces of the blade.

As well as the blades, the vanes are also detachable Prom the body of the hot section of the gas turbine. The vanes are made from similar materials to the blades and may also be refurbished as and when required.

Several techniques are known in the art for removing corrosion products from the exposed surfaces of turbine blades and vanes. See, for example, "Refurbishment Procedures for Stationary Gas Turbine Blades" by Burgel from the proceedings of the conference on "Lit'e assessment and repair" edited by Viswanathan and Allen, Phoenix, Arizona, 17-19 April 1990. Generally, the corroded parts of the exposed surfaces of the turbine blade are removed by a combination of mechanical and chemical treatments. Mechanical treatment typically involves abrasive blasting or finishing. Chemical treatment involves dissolution with acids or other suitable agents such as fluoride-containing compounds which generate hydrogen fluoride to convert aluminium and titanium oxides and nitrides into gaseous fluorides which are then 2() easily removed. 'I'here are, however, environmental and technical problems associated with the use of fluorine-containing compounds. As an alternative to tluorble-containing compounds, EP 0 525 545 discloses a process for refurbishing corroded parts loom gas turbines in which the exposed external surfaces of the parts arc cleaned, an aluminide coating is applied to the surface and the aluminide coating is then removed together with the products of corrosion.

It is known in the art that the efficiency of a gas turbine increases as the inlet temperature increases. As mentioned above, turbine blades are required to operate in gas temperatures Up to 1300 C. At these elevated temperatures, the turbine blades must be cooled. Cooling may be achieved by forcing compressed air through cooling holes in the turbine blade. An example of a compressed air-cooled turbine blade is shown in Fig. 1. Fig. Ia shows a turbine blade (or "bucket") 1, having a shank 2 with a dovetail (or "fir tree") 3 which attaches to a ring (not shown), as well as an airfoil 4 Trigs. Ib and Ic show the airfoil and dovetail ends of the turbine blade, respectively.

The cooling holes 5 are shown running through the length of the blade. Since the tip of the airfoil and the dovetail are different shapes and sizes, it is clear that the cooling holes run at varying angles to one another. Generally, a turbine blade of about 450 mm in length will have 912 cooling holes per blade having a diameter of 1-3 mm.

Although the surfaces of the cooling holes are cooled by the compressed air, the elevated temperatures, particularly at the gas inlet, cause significant corrosion of the internal surfaces. The heat produced by the hot gases generated by the burning of oil or natural gas is transferred from the external surfaces to the internal surfaces. The l O hot internal surfaces then react with the compressed air and any impurities present in the compressed air. In order to reduce corrosion, the internal surfaces are often coated with protective coatings, such as aluminide coatings. However, in many turbine blades, the internal surfaces are left uncoated and hence during their lil'etime experience significant corrosion. Unfortunately, the refurbishment techniques known l 5 in the art are unable to remove the products of corrosion from the internal surfaces of the cooling holes.

Accordingly, the present invention provides a process for refurbishing a corroded turbine blade or vane wherein the turbine blade or vane has at least one cooling hole having an untreated internal surface which is contaminated with products of corrosion and has a depletion Zone containing further products of corrosion, the process comprising the steps of (I) removing at least part ol' the products of corrosion from the internal surface of the at least one cooling hole; (ii) aluminising the internal surface of the at least one cooling hole obtained from step (i) to form an alummide layer of sufficient depth to enclose substantially all of the products of corrosion and at least part ol' the depletion 3() ;zonc by filling the at least one hole with an aluminizing pack containing 1()-65 wt% ol' aluminium' heating the aluminising pack for a sufficient time at a sufficient temperature to form the alumimde layer and removing the aluminising pack; and (iii) removing the aluminide layer together with substantially all of the products of corrosion and at least part of the depletion zone.

This process provides sufficient removal of the products of corrosion for the turbine blade or vane to be returned to service. Moreover, this process provides a sufficiently treated surface for the internal surfaces of the cooling holes to be coated with a protective coating.

The present invention will now be described with reference to the accompanying 1 () drawings, in which: Figs. Ia-c show an air-cooled turbine blade suitable for use with the process of the present invention; laid. 2 shows an optical photomicrograph of a cross-section of a corroded cooling hole before treatment with the process of' the present invention; I;ig. 3 shows an optical photomicrograph of a cross-section of a corroded cooling hole after microblasting, but before aluminisation; and Figs. 4 shows an optical photomicrograph of a cross-section of a corroded cooling hole after treatment by the process of the present invention.

2() Figs la-c, as described above, show an air-cooled turbine blade suitable for treatment by the process ot' the present invention. (Although a turbine blade is shown, the present invention is equally applicable to vanes having cooling holes.) The cross- section through a cooling hole 5 which has been corroded is shown in Fig. 2. The optical photomicrograph shows a layer ot' nicker plate 6 which is used solely for preparing the metal sample. Fig. 2 shows the cooling hole 5 made up of base material 7. On the surl:ace of the cooling hole 5 is a continuous layer of oxide 8, underneath which are heavy block oxides 9. 'I'he oxides 8 and 9 are formed by reaction of metals hi the superalloy, such as aluminium, with oxygen in the hot gases, to forth aluminium oxide. Iteaction of, say, aluminium in the superalloy to form the 3() continuous oxide layer 8 and heavy block oxides 9 results in a zone of superalloy which has a r educed concentration of the metal kenning the oxide, such as aluminium.

This zone is tended a "depletion zone" 10. Within the depletion zone I O are additional oxides and nitrides 11 which have a deleterious outact on the strength Blithe superalloy. 'I'he presence of tile continuous oxide layer 8 and the heavy block oxides 9 reduces the efficecncy of the cooling process since oxide, such as aluminium oxide, effectively provides unwanted heat insulation reducing the cooling effect of the compressed air. Moreover, tle presence of the oxides 8 and 9 and the depletion zone prevents the internal surfaces from being coated with a protective coating. s

The treating of the internal surfaces of the cooling holes 5 is essentially a three-step process. In step (i) the continuous oxide layer 8 is removed from the internal surfaces of the cooling holes 5; in step (ii) the treated internal surface is aluminiscd; and in step (iii) the aluminide layer is removed.

In step (i) the continuous oxide layer 8 is removed either by chemical or physical means. Chemical means uses techniques such as aqueous acid pickling. However, the preferred method is to use physical means, such as using compressed air containing small particles of a hard ceramic material, such as aluminium oxide.

However, unlike in the treatment of'the external surf ces, it is extremely difficult to remove substantially all of the continuous oxide layer 8 from the internal surfaces. In order to target the internal surfaces of the cooling holes 5, step (i) of the present invention preferably uses a lance (or micronozzle). The lances are adapted to be inserted into the cooling holes 5 and have a sufficiently small external diameter to be inserted into the cooling holes, have a tip which is resistant to abrasion and have sut-'licient length for the entire length of the cooling holes to be reached by the ceramic particles. The lance may be a hollow metal, preferably steel, cylinder having suf'f'iciently small external diameter to be inserted into the cooling holes. The internal diameter must, of course, be sufficiently large to allow the ceramic particles to pass through the lance. The Icngtl1 must, of course, be sufficient that the compressed air containing the abrasive particles can be targeted at the entire length ol'the cooling holes. Moreover, the lance should have an abrasion-resistant tip, such as a tungsten carbide tip, to prevent abrasion by the ceramic particles. In order to clean the entire length of the cooling hole, one option is to provide a preliminary cleamng using a 3() short lance which targets the t'irst part of the cooling hole 5 nearest to the dovetail 3 and then a second' longer lance which targets the contra of the cooling hole 5 to the end furthest from the dovetail. Suitable lances arc easily manufactured and may be obtan1cd Prom companies supplying cquipn1cnt t'or abrasive blasting or general engineering suppliers.

On account of the inaccessibility of the internal surfaces of the cooling holes, it has been found that cleaning the internal surfaces only removes the continuous oxide layer 8, the heavy block oxides 9 remaining. The treatment in step (i) does, however, remove a sufficient amount of the continuous oxide layer to allow aluminisation in step (ii). Preferably, substantially all of the continuous oxide layer 8 is removed. lTig.

3 shows a cooling hole 5 which has been cleaned by mieroblasting with a lance in accordance with the present invention. This optical photomierograph shows a eross- section of the cooling hole 5 and specifically the absence of the continuous oxide ] O layer 8, but the remaining presence of the heavy block oxides 9 and the depletion zone I (), giving way to the base alloy 7. Surprisingly, despite the remaining presence of the heavy block oxide 8, by modifying step (ii) the internal surfaces of the cooling holes 5 1 may still be aluminized.

In step (ii) the internal surfaces of the cooling holes are aluminised to encapsulate the remaining heavy block oxides 9 as well as the depletion zone 10. The present applicant has found that conventional methods for removing the products of corrosion, such as hydrogen fluoride, abrasive putties and aluminising agents, such as those disclosed in EP 0 525 545, are unable to remove the products of corrosion in 2() such narrow cooling holes 5, particularly given the presence of heavy block oxides 9 on the interior surl:aces. I-lowever, it has been found that an aluminising peek containing 10-65 wt% aluminium will aluminise the cleaned internal surfaces of cooling holes 5 in the presence of a sufficient quantity of moderator to allow diffusion of the aluminium into the surface ol the cooling hole. The aluminising pack of the present invention contains aluminium, a moderator, an energizer and a diluent.

l or aluminization, an aluminium halide is generated in situ. Accordingly, in the alumimsing pack of a present invention, aluminium is present at 10-65wt%, more preferably 20-40 wt%, more preferably 25-40 wt% and most preferably 30-40 wt%, based on the total weight of the aluminising pack A moderator, such as chromium, nickel or iron, may be present at 1-2() wt%, more preferably 2-1 () wt%, based on the total weight of the aluminising pack. The moderator absorbs the aluminium halide vapour providing a reduced vapour pressure of aluminium halide vapour at the surface of the cooling hole which encourages clift'usion n1to the cooling hole rather than deposition of a layer of aluminium on the surface ot' the cooling hole. 'I'he amount of moderator must be sufficient to provide diffusion rather than deposition. However, since diffusion is temperature controlled, as the temperature increases, diffusion is favoured and hence less moderator is required. The ratio of aluminium to moderator is typically]: I to 4: 1.

The energizer used for the aluminising process generally contains a halide element such as chloride or fluoride. The preferred halides are of sodium and ammonium and 1() ammonium chloride is particularly preferred. The energiser is generally present at ().05-10 wt%, preferably 0.1-5 wt%, based on the total weight of the aluminising pack.

The diluent is generally a refractory oxide powder that makes up the balance of the ingredients in the aluminizing pack. The diluent is preferably Al2O3 (alumina), 'i'i()2(litania), MgO or Cr2O3. 'I'he most preferred refractory diluent is alumina.

During the aluminization process the aluminising pack should preferably be protected from attack by atmospheric oxygen. Protection may involve an inert atmosphere, whiel1 may be produced by ammonium salts present in the pack which decompose at elevated temperatures. Alternatively, protection may be provided by a reducing atmosphere, such as hydrogen or a hydrogencontaining gas mixture.

()nec the cooling holes 5 have been filled with the aluminizing pack, the turbine blade 1 is heated t'or a sul'fieient time and at a sut'fieient temperature for the aluminide layer to term. 'I'ypieally, heating will be carried out t'or from 1-1() hours, preferably 2-5 hours, more prel'erably 2-3 hours, at a temperature of 800-1000 C, preferably 850- t)50 C, more prct'erably 850-')0() C. The aluminizing will typically be carried out until an aluminidc layer is formed to a depth sufficient to encapsulate substantially all of the products of corrosion from the internal surfaces and the further products of corrosion in the depletion zone, as well as at least part ol' the depletion zone, preferably to a depth of 3()-10() ,um, more preferably 4()-80,um, most preferably 45-65 Sum.

After heating, the reacted aluminising pack is removed from the cooling holes. The reacted aluminising peek may have to be forced out using, for example, compressed air.

Alter the internal surfaces of the cooling holes 5 have been aluminized, the aluminide coating is removed using a suitable treatment in step (iii). Preferably the aluminide layer is removed using chemical means, such as soaking in an aqueous acid solution, such as a 20% hydrochloric acid solution. Soaking should be carried out until the aluminide layer has been removed and will typically be for 10-18 hours, preferably 12-14 hours, although the time may be reduced if the temperature is elevated. In order to allow fresh acid solution to enter the cooling holes, the turbine blade I should be removed every, say, thirty minutes and replaced into the aqueous acid solution.

During chemical treatment, the exposed external surfaces of the turbine blade are preferably protected to prevent degradation ot'the surfaces in the acid solution using conventional techniques.

Fig. 4 shows the optical photomicrograph of the cooling holes 5 shown in Figs. 2 and 3 after removal of the aluminide layer. Fig. 4 shows that the heavy block oxides 9 have been completely removed as well as the depletion zone 10 and the oxides and nitrides I 1 within the depletion zone. The surface Blithe cooling hole 5 shown in Fig. 4 is suitable for application ova conventional protective coating.

I'he refurbished turbine blade may then be reused. However, in an optional step (iv), a protective coating is applied to the internal surl:aces of the cooling holes using conventional techniques. Suitable coatings include aluminide coatings, see for example Mevrel R et al in "Pack Cementation Processes" Material Science l'echnology March 19X6, volume 2, 2()1-206.

IIxamplc 3() I'llis example relates to nickel-based superalloy firststage industrial gas turbine blades, calf approximately 450 mm overall length and contaimng eleven radial cooling holes. Tile superalloy is IN 738 1,c having the following composition carbon 0.1%.

X

chromium 16.0% cobalt 8.5% molybdenum 1.75% tungsten 2.6% the balance being nickel.

I'he approximate diameters of these holes are shown in the Table. s

Table. Approximate diameters of turbine blade radial cooling holes Radial cooling hole no. Diameter - 2 3 4 5 6 7 8 9 10 b (mm) _ Dovetail 7.0 6.0 6.0 6.0 6.0 6 0 6.0 6.0 4.0 I'm cavity 5 5 3. 0 30 3.0 3 0 3 0 3 0 3 0 3.0 2.5 D Adjacent to leading edge I' adjacent to trailing edge These turbine blades had been used in service without a protective internal aluminide i coating present in the radial cooling holes. As a consequence degradation of the base; material at and adjacent to the surface of the cooling holes had occurred as shown in the optical photomicrograph in Fig. 2. This degradation included oxide and nitride formation accompanied by the formation of a depletion zone.

In order that protective internal aluminising of the radial cooling holes in these blades could be performed during refurbishment the region of base material degradation in each hole was l'irst removed by subjecting the blades to the following three-step 2() process in accordance with the present invention.

Step (I) - F31asting of radial cooling holes This first step was to at least substantially but pret'erably completely remove the continuous oxide layer on the surl'acc of the radial cooling holes to facilitate step (ii).

All blasting was carried out t'rom the dovetail cNd using an aluminum oxide blasting medians. 'I'he cooking holes were blasted with 180 grit at a pressure ol' 30 psi for about 2 minutes using a steel lance with a carbide tip. The blasting was carried out twice per hole.

Step (ii) - Internal aluminising of radial cooling holes In order to encapsulate the region of base material degradation within an aluminiumrieh "3-phase NiAI nickel aluminide layer, the radial cooling holes in each turbine blade were internally peek aluminised at 900 C for 3 hours under a hydrogen atmosphere using an aluminising compound having the following composition: aluminium 35 wt%, chromium 20 wt%, ammonium chloride 0.1 wt%, the balance 1 () being caleined alumina. 'I'he resulting aluminium penetration depth was 45-65 m.

Spent compound was mechanically removed prom the radial cooling holes of each turbine blade after aluminising.

Step (iii) - Aqueous acid pickling Next, aqueous acid pickling was used to remove the aluminide layer in each radial cooling hole encapsulating the region of' base material degradation. Pickling was carried out at 50 C. The pickling solution in the radial cooling holes was replenished at 30 minute intervals and pressure washing of the holes was carried out at 2 hour intervals. Total pickling time was typically 14 hours. The end of the required pickling was signalled by the cessation of gas evolution.

Tlle optical photomierograph in Fig. 4 shows a typical region of the surface of a radial cooling hole in one of the turbine blades on completion ol'the three-step process. As is evident loom the photomicrograph, the region ol'base material degradation has been successt'ully removed and the radial cooling holes in the blades are hence in a suitable condition for application of the final protective internal alumblide coating. 1()

Claims

Claims 1. A process for refurbishing a corroded turbine blade or vane

wherein the turbine blade or vane has at least one cooling hole having an untreated internal surface which is coated with products of corrosion and has a depletion zone containing further products ol'corrosion, the process comprising the steps of: ; (i) removing at least part of the products of corrosion from the internal surface of the at least one cooling hole; (ii) aluminising the internal surface of the at least one cooling hole obtained from step (i) to form an aluminide layer of sufficient depth to enclose substantially all of the products of corrosion and at least part of the depletion zone by filling the at least one hole with an aluminising pack containing 10-65 wt% of aluminium, heating the aluminising pack for a sufficient time at a sufficient temperature to form the aluminide layer and removing the aluminising pack; and; (iii) removing the aluminide layer together with substantially all of the 2() products of corrosion and at least part of the depletion zone.
2. A process as claimed in claim 1, wherein the turbine blade or vane is made of superalloy, preferably a superalloy containing nickel and aluminium.
3. A process as claimed in claim I or 2, wherein the products of corrosion! removed from the internal surface in step (i) is a continuous oxide layer.
4 A process as claimed in claim 3, wherem the continuous oxide layer is aluminium oxide.
process as claimed in any precedhlg claim, wherein the products of j corrosion removed in step (I) are removed by physical means. 1 1
6. A process as claimed in claim 5, wherein the products of corrosion are removed in step (i) are removed by blasting with ceramic particles.
7. A process as claimed in claim 6, wherein blasting is carried out using one or S more lances adapted to be inserted into the at least one cooling hole.
8. A process as claimed in claim 7, wherein the lance has a sufficiently small external diameter to be inserted into the at least one cooling hole, a tip which is resistant to abrasion and sufficient length for the entire length of the at least one cooling hole to be reached by the ceramic particles.
9. A process as claimed in any preceding claim, wherein the further products of corrosion in the depletion zone are oxides and nitrides.
10. A process as claimed in any preceding claim, wherein the aluminising pack used in step (ii) contains 20-40 wt% of aluminium.
11. A process as claimed in any preceding claim, wherein the aluminising pack in step (ii) is heated for 1-10 hours.
12. A process as claimed in any preceding claim, wherein the aluminising pack is heated in step (ii) at 8()0-1 ()00 C.
13. A process as claimed in any preceding claim, wherein the aluminide layer formed in step (ii) penetrates the internal surface of the at least one cooling hole to a depth of 30-80 m.
14. A process as claimed in any preceding claims, wherein the aluminide layer is removed In step (fit) by chemical means. 3()
A process as claimed in claim 14, wherein the external surfaces of the turbine blade or vane are protected prior to removal of the aluminide layer.
16. A process as claimed in claims 14 or IS, wherein the aluminide layer is removed by soaking the turbine blade or vane in an aqueous acid solution.

17 A process as claimed in any preceding claim, wherein the process is applied to a turbine blade.