Patent Application
20090193656
The subject invention relates to steam turbines. More particularly, the invention relates to erosion protection of steam turbine last stage buckets.
Last stage buckets of steam turbines can be exposed to a volatile environment where airfoils, particularly the leading edges of the airfoils, of the bucket are eroded due to moisture. Currently, one method of increasing the durability and erosion resistance of the bucket involves fixing inserts formed from an erosion resistant material, such as cobalt-based stellite, to the leading edges of the bucket airfoils. This method includes precision machining of an insert to match the bucket airfoil, and then fixing the insert to the bucket via electron-beam welding.
This method does not allow the material composition of the insert to be easily adjusted to, for example, increase the alloy composition of the insert, or to add refractory metals such as chromium, molybdenum, tungsten, nickel, tantalum, and/or vanadium as well as refractory metal carbides to further enhance durability and erosion resistance. Further, the electron-beam welding process currently utilized is costly and time-consuming, and must be performed in a vacuum environment. The use of the vacuum environment limits the opportunity to utilize shielding gases for the purpose of controlling the chemistry of the final alloy. Additionally, the electron-beam welding process cannot be utilized to weld a stellite insert to a bucket whose parent alloy is titanium based.
A method for forming a bucket for a steam turbine includes forming a leading edge on at least one airfoil. The leading edge is comprised of an erosion resistant material and is bonded to the airfoil.
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawing.
The material, in one embodiment, is deposited, and a laser cladding process achieves the fusion bond. As illustrated in
Utilization of the laser cladding process allows the material deposited on the forward face 14 to be adjusted as needed. The composition of the material may be enhanced as needed to provide a desired combination of metallurgical properties by adding various alloying elements into the stellite alloy material. Such additional alloying elements may include, for example, refractory metals such as chromium, molybdenum, tungsten, nickel, tantalum, and/or vanadium as well as refractory metal carbides in desired proportions to enhance resistance to heat, wear, and/or corrosion of the deposited material. It is to be appreciated that elements other than those listed above may be added to the stellite material and still be within the scope of the invention.
Further the composition of the deposited material can be modified for particular a particular operating environment or for a particular bucket material. For example, for applications such as nuclear wet stream where cobalt is restricted, a material deposit that results in a cobalt-free leading edge 18 with enhanced erosion resistance can be provided. Further, a material deposit that is base-matched to titanium can be provided when an erosion resistant leading edge 18 is desired for a bucket 10 formed of a titanium-based alloy.
While embodiments of the invention have been described above, it will be understood that, both now and in the future, various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.
