The invention deals in the field of power plant engineering and materials science. It relates to a wear-resistant and oxidation-resistant turbine blade and also to a method for producing such a wear-resistant and oxidation-resistant turbine blade.
The reduction of leakage losses in turbines has been the subject of intensive development work for several decades. During operation of a gas turbine, relative movement between the rotor and the housing is unavoidable. The resultant wear of the housing or wear of the blades has the effect that the sealing action is no longer provided. As a solution to this problem, a combination of thick coatings which can be ground away on the heat shield and abrasive protective coatings on the blade tips is provided.
Methods for applying additional coatings to blade tips or for increasing the resistance to wear by suitable modification of the blade tip have been known even since the 1970s. Various methods have likewise been proposed for simultaneously making such protective coatings resistant to frictional contacts and oxidation caused by the hot gas by a combination of abrasive particles (carbides, nitrides, etc.) with oxidation-resistant materials. Many of the proposed methods are expensive and complex to implement, however, and this makes commercial use more difficult.
One of the popular strategies therefore lies in dispensing entirely with the protection of the blade tip against wear and providing the heat shield with special porous, ceramic rub-in coatings. Owing to their high porosity, these can also be rubbed in to a certain extent by unprotected blade tips. However, considerable technical risks are associated with this method, since the porous, ceramic rub-in coatings do not ensure the same resistance to erosion as dense coatings. A further risk lies in operational changes to the porous, ceramic rub-in coatings (densification by sintering), which can have a negative effect on the tribological properties. For this reason, a combination with wear-resistant (abrasive) blade tips is expedient when using ceramic protective coatings on heat shields.
In recent decades, a plurality of methods for producing abrasive blade tips have been developed as shown in, for example, U.S. Pat. No. 6,194,086 B1. Although the use of laser metal forming (LMF) to build up abrasive blade tips has been known since the start of the 1990s (see for example DE 10 2004 059 904 A1), this method is still used rarely on an industrial scale.
The present disclosure is directed to a turbine blade for a turbine rotor. The blade has a main blade section, including a blade tip, which extends in a radial direction and is formed at the blade tip either as a crown, with an inner and outer crown edge extending in the radial direction, or as a shroud with a web, which extends in the radial direction and has lateral edges. At least portions of the surface of the main blade section are provided with at least one first protective coating of an oxidation-resistant material, the at least one first, oxidation-resistant protective coating is a metallic coating, in particular an MCrAlY coating. The first protective coating is arranged at least at the inner and/or outer crown edge or at the web edges, the first protective coating is not present at the radially outer blade tip of the turbine blade. The radially outer blade tip includes a second, at least single-layer wear-resistant and oxidation-resistant protective coating which is built up by laser metal forming. The second protective coating on the blade tip overlaps along the outer and/or inner crown edge or the web edges at least partially with the first, metallic protective coating arranged there.
In a further aspect, the present disclosure is directed to a method for producing the above turbine blade. The method includes coating, in a preceding production step, at least portions of the surface of the main blade section of the turbine blade with the oxidation-resistant, metallic protective coating, in particular the MCrAlY coating and an oxidation-resistant, ceramic thermal barrier coating is optionally applied to the protective coating. The method also includes removing the at least one oxidation-resistant protective coating on the radially outer blade tip by controlled machining, in particular grinding away, CNC milling and/or chemical coating removal. The method also includes applying the wear-resistant and oxidation-resistant protective coating to the blade tip in one layer or in a plurality of layers by laser metal forming, such that said coating overlaps along the outer and/or inner crown edge or the web edges at least partially with the first, metallic protective coating applied beforehand, but not with the ceramic thermal barrier coating optionally applied beforehand.
The invention is explained in more detail below on the basis of exemplary embodiments and with reference to
The drawings show exemplary embodiments of the invention.
a-5f show, in two variants, the production sequence for the production of a turbine blade according to the invention;
The aim of the invention is to avoid the disadvantages of the known prior art. The invention is based on the object of developing a wear-resistant and oxidation-resistant turbine blade which can be used both for producing new parts and for reconditioning (retrofitting), where the production of said turbine blade requires only minimum adaptation of the existing production process.
The special feature of the embodiment described here of such a component is the best possible compatibility with conventional turbine blades and the processes for producing the latter. This requires only a small outlay to adjust current production sequences and opens up very interesting prospects for reconditioning and retrofitting. This object is achieved in that the wear-resistant and oxidation-resistant turbine blade having a blade tip, that extends in the radial direction and is formed at the blade tip either as a crown with an inner and outer crown edge extending in the radial direction or as a shroud with a web, which extends in the radial direction and has lateral edges, At least certain zones on the surface of the main blade section are provided with at least one first protective coating made up of an oxidation-resistant material. The at least one first, oxidation-resistant protective coating is a metallic coating, in particular an MCrAlY coating (M=Ni, Co or a combination of both elements). The first protective coating is arranged at least at the inner and outer crown edge or web edge. The first protective coating is not present at the radially outer blade tip of the turbine blade, and the radially outer blade tip is made up of a second, at least single-layer wear-resistant and oxidation-resistant protective coating which is built up by laser metal forming. The second protective coating on the blade tip overlaps along the outer and/or inner crown edge or web edge at least partially with the first, metallic protective coating arranged there.
In a method for producing a turbine blade as described above, in a preceding production step, at least portions of the surface of the main blade section of the turbine blade are coated with the oxidation-resistant, metallic protective coating, in particular a MCrAlY coating. An oxidation-resistant, ceramic thermal barrier coating is optionally applied to the protective coating. The method includes the following features:
The advantages of the invention are that the basic body of the turbine blade is protected against oxidation on all critical surfaces exposed to the hot gas, and at the same time the blade tip is tolerant to frictional contacts with the heat shield, and this makes it possible to reduce the size of the hot gas breach and thus to reduce the leakage losses. The efficiency of the turbine can thereby be increased significantly.
The blade according to the invention can be produced by an inexpensive and simple method.
The increased resistance to wear of the turbine blade with respect to frictional contacts makes it possible to apply relatively dense ceramic coatings to the heat shields. Good rub-in behavior can thus be combined with the requisite long-term erosion resistance of the ceramic coatings on the heat shields.
It is particularly advantageous that the turbine blade can be embedded in the rotor of the turbine directly following the laser metal forming (LMF step) without further heat treatment, and can thus be used for turbine operation.
Further advantageous refinements are described in the dependent claims.
By way of example, the metallic protective coating can be covered by a ceramic thermal barrier coating, and the second, oxidation-resistant and wear-resistant protective coating which is applied by laser metal forming overlaps at least partially only with the metallic protective coating, but not with the ceramic thermal barrier coating. As a result, optimum protection against oxidation is provided and the integrity of the TBC is not impaired, i.e. spalling of the TBC is prevented.
Furthermore, it is advantageous if the wear-resistant and oxidation-resistant protective coating is comprised of an abrasive material, which is preferably cubic boron nitride (cBN), and of an oxidation-resistant metallic binder material, in particular having the following chemical composition (amounts in % by weight): 15-30 Cr, 5-10 Al, 0.3-1.2 Y, 0.1-1.2 Si, 0-2 others, remainder Ni, Co.
Moreover, it is advantageous if the proportion of abrasive material in the wear-resistant and oxidation-resistant multi-layer protective coating increases outward in the radial direction, because this ensures optimum adaptation to the load conditions.
The abrasive coating can be used for all types of turbine blades. In the case of blades without a shroud, the abrasive coating is applied to the crown (or to part of the crown). In the case of blades with a shroud, the method can be used to provide improved protection of the shroud web against wear.
The described embodiment of the turbine blade can be used both for producing new parts and for reconditioning (retrofitting). Here, only minimum adaptation of the existing production process is required.
A particularly interesting commercial potential is the retrofitting or reconditioning of existing blades. These blades can be modified by the disclosed method in order to achieve reduced leakage losses and thus improved efficiency of the turbine when they are refitted. For this option, it is not necessary beforehand to remove a protective coating which may already be present on the main blade section, and this makes a simplified production method possible.
The protective coating 5 is comprised of an abrasive material 6, which is preferably cubic boron nitride (cBN), and an oxidation-resistant binder material, which preferably has the following chemical composition (amounts in % by weight): 15-30 Cr, 5-10 Al, 0.3-1.2 Y, 0.1-1.2 Si, 0-2 others, remainder Ni, Co. A particularly suitable binder material which is actually used is, for example, the commercial alloy Amdry995.
This can be seen particularly well in
The special feature of the approach described here is the special design of such a wear-resistant protective coating 5. The single-layer or multi-layer coating 5 is applied such that it at least partially overlaps with other, existing protective coatings 4. By way of example, the existing protective coatings 4 are MCrAlY coatings known from the prior art (M=Ni, Co or a combination of both elements) which, in the case of most turbine blades subjected to high levels of loading, protect the surfaces of the main blade section against oxidation and corrosion. Furthermore, a ceramic thermal barrier coating (TBC) may additionally be applied to said MCrAlY coating on the main blade section, and the integrity of this thermal barrier coating is not impaired by the proposed method.
Owing to the overlapping with the existing protective coatings, the proposed embodiment of an oxidation-resistant abrasive coating on the blade tip ensures that the surfaces of the blade tip which are exposed to the hot gas are efficiently protected. Application of this wear-resistant coating by the LMF method also makes it possible to schedule this coating operation as the last production step in the production process. The following technical problems are thereby avoided:
The above-mentioned problems are avoided if, as described here, the abrasive coating is applied by laser metal forming as the last step in the process chain. A simple and inexpensive implementation lies in completely removing the radially outer MCrAlY (if appropriate, also TBC) coating(s) by milling away or grinding away or by chemical processes by a defined amount. The wear-resistant coating is then applied by LMF to the then exposed basic material. A decisive factor here is the locally very limited action of the laser beam, which, if the process is carried out in a controlled manner, keeps the effects on the adjacent regions of the blade to a minimum. It is thus possible to apply such a wear-resistant coating in the immediate vicinity of a TBC protective coating without damaging the latter (see, for example,
In contrast to conventional (e.g. electrodeposition) coating methods, those surfaces of the turbine blade 1 which are not to be coated (e.g. the blade root) do not have to be protected by a masking method. The LMF process is a welding method and produces a stable, metallurgical bond with the basic body of the blade without additional diffusion heat treatment. Owing to the small local introduction of heat, the local hardening is kept to a minimum despite the rapid solidification process. The component can thus be installed immediately after the wear-resistant protective coating has been applied, without further, subsequent steps.
As already described above, it is possible, in a further preceding production step, to provide the blade tip with an additional thermal barrier coating 4a. In the design variant shown in
A further exemplary embodiment is shown in
The invention can be used manifoldly for shroud-less turbine blades, but also for components having a shroud. The service life of the abrasive coating, which is dependent on the respective operating conditions (temperature, fuel), must be taken into consideration. The service life is optimized by good distribution and complete embedding of the abrasive particles in the oxidation-resistant binder matrix. Nevertheless, the main aim is to protect the turbine blade tip above all during the run-in phase. This corresponds to a duration of several dozen to several hundred operating hours.
It goes without saying that the invention is not restricted to the exemplary embodiments described.
Number | Date | Country | Kind |
---|---|---|---|
102009051661.1 | Nov 2009 | DE | national |