Method for removing a metallic layer

Abstract: Method for removing a metallic layer. The metallic layer is cooled below room temperature (RT), leading to an embrittlement. Subsequently, the metallic layer is stripped by a blasting process of high efficiently because of the brittleness. Additionally, the substrate is prevented from damage by a comparable low blasting energy.

Description: Field of the invention This invention relates to a method for removing at least one metallic layer of a layer-system, especially for a refurbishment of coated gas turbine blades or vanes. Background of the invention In modern power generating plants used today, the high efficiency gas turbine has an increasingly vital role. Accordingly, there have been constant efforts in order to improve the efficiency of a gas turbine plant. One way of achieving this, and the most commonly employed, is to increase the turbine inlet temperatures of the combustion gases. The rotating blades and stationary vanes (both made from high strength nickel or cobalt base superalloys) in the hot section are subject to degradation by a number of attack mechanisms (e.g. oxidation, sulphidation and nitridation). Considerable efforts were made into the development of protective coatings. These coatings are designed for preventing corrosion attacks of the underlying superalloy. The oxidation and corrosion are localized on and/or within the protective layer and hence restricting damages of the vulnerable base material. Typical coatings of this type are platinum aluminide coatings, chromium diffusion coatings applied by thermochemical processes and MCrAlY coatings applied e.g. by low-pressure plasma spraying. M is a metal selected from the group iron, cobalt, nickel, Cr is Chromium, Al is aluminium and Y stands for Yttrium or another reactive element of the Rare-Earths-elements. The compositions of these MCrAlY coatings are diverse, but they all have the same objective of protecting the underlying substrate from chemical attack. The design life of a rotating blade or stationary vane in the hot section of a power generating gas turbine is generally 3 or 4 times longer than the service life. For very high temperature applications, there is often in addition to the protective coating, a thermal barrier coating provided, that has the effect of shielding the metallic parts from a hot gas. As mentioned above, the lifetime of the expensive superalloy base body exceeds that of the protective coating system. Accordingly, a refurbishment of the protective coating system can save a high amount of costs. This refurbishment operation must be capable of restoring the component within a short period. As described in the foregoing various types of chemical attack can occur at the surface of a hot section component. In particular, there is a progressive consumption of the active elements in the coating (i.e. chromium and aluminium). In some cases, although this is not desirable, there may be an attack of the base material. This may be due to the complete consumption of the active material in the coating or due to some physical detachment of the protective coating in service (e.g. foreign object damage). In this case due to their vulnerable nature, substrate attack can be very deep and involve grain boundary penetration in the case of equiaxed or directionally solidified materials. As a previous step of a refurbishment operation, any material of the old coating is removed, as well as corroded layers within the base material. Any material of this type that is not completely removed during a refurbishment operation may result in the poor adhesion of any subsequently applied new coating. This could have a marked effect on the future performance of such a coating. Also, corrosion products that are not removed during refurbishment could diffuse under operation deeper into the base material, where they may influence the mechanical properties of the base material. Mechanical stripping methods are widely used to remove consumed coatings from turbine blades or vanes. These methods include grit blasting with abrasive particles such as alumina or silica and linishing with rotary belts embedded with alumina particles. However, grit blasting may result in damages of the base body surface. Accordingly, effective high rate blasting could lead to considerable base body abrasion. In the US-PS 4,627,197 a method for removing organic surface layers is disclosed. The part with the organic surface layer is cooled, thereby increasing the brittleness of the organic layer and followed by a removal with blasting methods. The JP 07275795 A shows a similar method to remove a rubber layer by first cooling and then projecting the layer with pellets of dry ice. In the US-PS 4,527,844 a thermally insulated chamber is described for batch treatment of materials therein at low temperature. Document EP 0813930 discloses a method for removing a metallic coating of an article by blasting and exposing the article to a reducing mixture of gases. Summary of the invention In accordance with the foregoing it is an object of the invention to describe a method of removing a metallic, especially degraded, layer effectively, thereby leaving the underlying substrate or other layers unaffected. According to the invention, the metallic degraded layer reveals at a lower temperature than room temperature a higher low temperature embrittlement than the underlying substrate or layers, because a brittle material is more easily removed by a blasting process. This effect is the so called embrittlement-effect. By having the metallic layer or layers cooled, wherein the material of these layers show a higher embrittlement compared to the substrate, it or they can be removed easily by an abrasive blasting process like sand blasting, blasting of ceramic particles or even non abrasive blasting processes like water jet blasting or blasting by pellets of dry ice. The layer-system with the metallic layer can be effectively cooled in liquid gases like nitrogen, oxygen, or carbon dioxide, because of the good heat transfer in a liquid to a layer-system of solid state. The layer-system can be cooled by partly cooling the metallic layer or by cooling the whole layer-system. Preferably the method causes another effect, by which the removal of the metallic layer is performed even more easy, the so called temperature-tension-effect. The metallic layer reveals at a low temperature below room temperature a higher thermal contraction than the substrate or underlying layers. This is because of a higher thermal expansion coefficient at least partly from room temperature to low temperature and/or because a temperature difference between the substrate or underlying layers and the metallic layer. This different thermal contraction leads to a mechanical tension between the degraded layer and the underlying layers or substrate. Only by a small input of energy, caused by mechanical impact, the tension is increased beyond a spallation threshold, resulting in a spallation of the metallic layer. No abrasive process for removing the layer is needed. The method according to the invention can use both the temperature-tension-effect and the embrittlement-effect. Preferably, this method is used for the refurbishment of vanes or blades of a gas turbine, which have a superalloy base body. The metallic layer is a so called MCrAlY-layer, which is degraded, especially by corrosion products. The method can be further improved by following steps or properties: (a) the blasting process is an abrasive process, (b) the blasting process is a non abrasive process, (c) the blasting process is a sand blasting process, (d) dry ice particles are used for the non-abrasive blasting process, (e) the layer-system is cooled in liquid nitrogen, (f) the metallic layer is stronger cooled than the substrate, (g) the metallic layer reveals at a temperature (LT) below room temperature a higher low-temperature embrittlement than the substrate or an additional layer between the substrate and said metallic layer, (h) the metallic layer has at a low temperature (LT) below room temperature a higher thermal expansion coefficient compared to the substrate or an additional layer between the substrate and said metallic layer , thereby leading to a mechanical tension between the said metallic layer and the additional layer or the substrate when the cooling is performed, (i) the removal of the at least metallic layer is controlled, (j) stripping residual parts of the at least metallic layer with a mechanical or chemical process, (k) the at least metallic layer comprises a brittle material, (l) the substrate is made from a superalloy, (m) the metallic layer comprises an alloy of the group MCrAlY, (n) the cooled part of the layer system is at least partly reheated. Also all combinations of a) to n) lead to an improvement of the inventive method according claim 1. Brief Description Of The Drawings: Figure 1a shows a layer-system with a metallic layer at room temperature with known material parameters, Figure 1b shows a degraded surface in detail, Figure 2 shows a cooled layer-system with material parameters at low temperature, and Figure 3 shows a cooled layer-system with two layers, and Figure 4 a to f shows several steps of the inventive method. Detailed Description Of The Invention The invention may be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these illustrated embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art Figure 1a shows a layer-system 1, which consists of e.g. a metallic substrate 7 and a metallic surface layer 4, which can be degraded (Fig. 1b). The layer-system 1 can reveal another layers underlying the surface layer 4 (Fig. 3). The layer-system 1 is for instance designed for a turbine blade, wherein the substrate 7 e.g. is a superalloy base material (e.g. Ni-Cr with Ni 3 Al) and the metallic surface layer 4 is a so called MCrAlY coating, wherein M stands for a metal of Fe, Co, Ni, Cr is chromium, Al is aluminium and Y yttrium or any rare-earth element or a mixture of rare-earth elements. Figure 1b shows a metallic degraded layer 4 with corrosions products 5 at a surface 3 or in a certain depth from the surface 3. In order to conserve the expensive base body of the turbine blade, i.e. the substrate 7, the degraded layer 4 needs to be at least partly stripped. The degraded layer 4 consists of ductile and brittle materials. This removal has to be performed in such a manner, that substantially no corrosion products 5 are present. The corrosion products 5 form only a part of the surface layer 4, and their depth into the layer 4 can vary along the surface 3. The corrosion products 5 are often oxides and can be removed by a blasting process even at room temperature. However, after such a conventional blasting process there still remain metallic areas 6 at the surface which are ductile and are difficult removed by the conventional processes. By now cooling the metallic layer, the low temperature brittleness leads to a surprisingly improved efficiency of the blasting process for metals made brittle by the method. For an uniform stripping of a layer 4, it is necessary to remove an uniform thickness of or totally the metallic layer 4. As an additional advantage, the invention enables a stripping process with comparable low impact energy in the blasting particles. Conventional blasting needs high energies for removal of ductile material, leading to significant danger of damage of the substrate. This is substantially reduced by the embrittlement of the cooling step. For the material of the substrate 7, some material parameters like the strength (σ) and a parameter, which indicate the brittleness of the material, for instance the fracture toughness (K), are shown (Fig. 1a) as well as for the metallic surface layer 4. A high K-value corresponds to a ductile material. There must be no essential relationship between the material parameters of the substrate 7 and the surface layer 4 at room temperature (RT). σ S RT indicates the strength of the substrate (s) at room temperature, σ S LT indicates the strength of the substrate (s) at a low temperature, σ L RT indicates the strength of the layer (L) at room temperature, σ L LT indicates the strength of the layer (L) at a low temperature, K s RT indicates the fracture toughness of the substrate (s) at room temperature (RT), K S LT indicates the fracture toughness of the substrate (s) at a low temperature (LT), K L RT indicates the fracture toughness of the layer (L) at room temperature, K L LT indicates the fracture toughness of the layer (L) at a low temperature. Figure 2 shows the relationship of the material parameters between the substrate 7 and the metallic surface layer 4 at a low temperature (LT), e.g. about 77K (liquid nitrogen), compared to room temperature (RT). At the low temperature (LT), the material of the surface layer 4 shows a low temperature embrittlement, that means that the strengths of the surface layer 4 is eventually increased, but the K-value is decreased, leading to a more brittle material. The K-value of the substrate 7 is higher than or equal to the K-value of the surface layer 4. With a more brittle surface layer 4, the surface layer 4 can be easily removed by a blasting process, e.g. abrasive blasting processes or non abrasive blasting processes. The method might be applied not only by to cooling the whole layer-system 1, but also by cooling only or mainly the metallic surface layer 4, for instance by dipping the surface layer 4 in liquid nitrogen, wherein the substrate 7 is not in contact with the liquid nitrogen. If only the metallic surface layer 4 was cooled and surface layer 4 is removed, the substrate 7 can be further processed immediately. If the whole layer-system 1 was cooled and the surface layer 4 is removed, the substrate 7 is heated up again to room temperature before further processing. According to the reduced blasting energy, the design and geometry of the substrate remains unaffected. This is of particular importance for blades or vanes since the aerodynamic properties are critical and easily influenced by the geometry and shape. With that so called heat tinting is possible to assure the total removal of the metallic surface layer 4 or corrosion products 5. Small areas of the metallic surface layer 4 or corrosion products 5 can still be removed by other chemical or mechanical means. Another effect, which supports to the removal of the metallic layer 4 of the layer-system 1 is, that during cooling a tension can occur between the metallic surface layer 4 and the substrate 7. This is caused by different thermal expansion coefficients of the substrate 7 and the surface layer 4. In this case no low-temperature embrittlement of the metallic surface layer 4 is needed. Surprisingly, the method can be applied even for layer-systems which are designed to stand (high-temperature) thermal shocks and mechanical impacts during use, for example turbine blades of gas turbines, made of superalloys, coated with a MCrAlY layer and a thermal barrier coating. The method can even be applied for metallic coatings 4, having materials with different ductility on metallic substrates 7. The metallic surface layer 4 may comprise metallic material and a ceramic material, like corrosions products 5. Figure 3 shows a turbine blade 1, which was first coated with a metallic coating 11, for example a MCrAlY-layer 11, and subsequently coated with an outer layer 10, for example a ceramic thermal barrier coating 10 or formed by corrosions products of the layer 11. The ceramic coating 10 is a brittle material. During application of the method for stripping the metallic layer 11, the ceramic material of the outer layer 10 remains on the layer 11 and is stripped first or jointly. The following steps are performed : The layer 11 and the outer layer 10 are cooled, leading to embrittlement of the layer 11. Subsequently the brittle materials can easily removed by a blasting process by first blasting particles onto the surface of the outer layer 10 and after removing the outer layer 10 on the layer 11. The ceramic outer layer 10 can also be removed by a blasting technique in a first step at room temperature without cooling. The layer-system 1 can also comprise other layers below the layer 11. Figure 4a shows as a layer system 1 a turbine blade or turbine vane 13. Additionally a tank 15 is provided which is filled with a cooling liquid 17, e.g. liquid nitrogen. The turbine blade or vane 13 will be cooled in this liquid 17. The turbine blade or vane 13 can also be cooled in a cooling apparatus 19 without having contact with a liquid (Fig. 4b). Figure 4c shows how the turbine blade or vane 13 is cooled in the liquid 17. For simplification only a part (area surrounded by dotted line in Fig. 4a) of the vane or blade 13 is shown in the following. The layer system 1, 13 is shown in a cross-sectional view showing its substrate 7 and the layers 11, 10. One possibility to perform the inventive method is to cool the layer 11 by dipping the layer system 1 in the liquid 17 so that the layer 11 is in contact with the cooling liquid 17. The layer 11 is cooled and effects the mechanical properties as described to in Figure 2. The outer layer 10 is also cooled. It is also possible to cool the whole layer system 1 by dipping the layer system 1 totally in the liquid 17, so the substrate and the layers 10, 11 are in contact with the cooling liquid 17 (fig. 4d). If the desired change in mechanical properties is reached the layer system 1 is exposed to particles 24 which are blast on the surface 3 of the outer layer 10 (Fig. 4e). The particles 24 are supplied by a blasting apparatus 21. The particles 24 can be particles of dry ice or ceramic. The outer layer 10 and the layer 11 are exposed to the blasting particles 24 as long as the layer 11 is almost or totally removed . Fig. 4f shows a layer system 1, 13 with has no layers 10, 11 anymore.

Claims: Method for removing a metallic layer (4) of a layer-system (1), said layer-system (1) comprising a substrate (7) coated with said metallic layer (4), the method which comprises the following steps: stripping at least partly the metallic layer (4) by a blasting process, characterized in that , the layer-system (1) is cooled at least partly before the blasting process. Method according to claim 1, wherein said blasting process is an abrasive process. Method according to claim 1, wherein said blasting process is a non abrasive process. Method according to claim 2, wherein a said-blasting process is a sand blasting process. Method according to claim 3, wherein dry ice particles are used for the non-abrasive blasting process. Method according to claim 1, wherein the layer-system is cooled in liquid nitrogen. Method according to claim 1, wherein said metallic layer (4) is stronger cooled than the substrate. Method according to claim 1, wherein said metallic layer (4) reveals at a temperature (LT) below room temperature a higher low-temperature embrittlement than the substrate (7). Method according to claim 1, wherein said metallic layer (4) reveals at a temperature (LT) below room temperature a higher low-temperature embrittlement than an additional layer between the substrate (7) and said metallic layer (4). Method according to claim 1, wherein the said metallic layer (4) has at a low temperature (LT) below room temperature a higher thermal expansion coefficient compared to an additional layer between the substrate (7) and said metallic layer (4), thereby leading to a mechanical tension between the said metallic layer (4) and the additional layer when the cooling is performed. Method according to claim 1, wherein the said metallic layer (4) has at a low temperature (LT) below room temperature a higher thermal expansion coefficient compared to the substrate (7) thereby leading to a mechanical tension between the said metallic layer (4) and the substrate (7) when the cooling is performed. Method according to claim 1, wherein the metallic layer (4) comprises corrosions products (5). Method according to claim 1, applied for refurbishment of a gas turbine blade (13) or vane (13). Method according to claim 1, wherein the substrate (7) is made from a superalloy. Method according to claim 1, wherein the metallic layer (4) comprises an alloy of the group MCrAlY. Method according to claim 1, wherein the cooled part of the layer system (1) is at least partly reheated.

Inventors: ESSER WINFRIED DR, REICHE RALPH, ESSER, WINFRIED, DR., REICHE, RALPH
Applicants: SIEMENS AG, SIEMENS AKTIENGESELLSCHAFT
Application year: 2001
Main classification: F01D5
IPC codes: C23C4, F01D25, F01D5, B24C1