Patent Application
20070217903
The present invention relates to the chemical composition of steel used for rotors of turbines, and more particularly to the chromium content of the steel bearing surfaces of the rotors.
It is common for rotors of turbines to be forged from steel containing chromium in excess of nine percent. Such rotors are well suited for most requirements of turbine rotors including chemical resistance and stress loading, for example. One problem, however, that this high chromium content has historically shown to cause “wire-wooling.” Wire-wooling is a process that damages bearings that ride against high chromium metals. It is therefore not uncommon for bearings to experience wire-wooling failures while riding on vertical bearing thrust surfaces as well as on horizontal cylindrical surfaces where these surfaces exhibit high chromium content. One method employed to reduce wire-wooling is to apply an intermediate metal material with chromium content of less than 2.2 weight percent between the turbine rotor and the bearing.
A process that has been used to apply an intermediate layer to cylindrical surfaces, is to shrink a cylindrical alloy sleeve containing less than 2.2 weight percent chromium over the cylindrical surfaces. Though well suited for cylindrical portions of a turbine subjected to moderate stresses this method cannot be applied to highly loaded or flat surfaces such as those used for thrust control. The shrunk-on approach is only a mechanical attachment and could come loose during operation especially in high stress locations.
For flat thrust surfaces one conventional method of applying an “overlay” of low chromium steel involves welding low chromium steel over the rotor to create a surface of steel containing less than 2.2 weight percent chromium. To minimize the welding time required, a submerged arc welding (SAW) process is employed since it allows for very high rates of metal deposition. The SAW process, however, permits molten metal to pool resulting in dissolution of some chromium from the high chromium steel into the low chromium overlay potentially resulting in a bearing surface with chromium content in excess of the 2.2 weight percent maximum desired level. To prevent this dissolution, a few layers of gas tungsten arc weld (GTAW), also known as tungsten inert gas (TIG) welding, may be applied prior to the SAW process. The lower welding temperature of the GTAW permits very little chromium dissolution to take place and creates a barrier that prevents dissolution from the base material into the SAW layers.
Another drawback of the large pool of molten metal generated with the SAW process may occur while welding onto a vertical surface. During vertical welds the force of gravity acting on the molten metal can cause it to flow downward away from the desired location. Since by design thrust bearing surfaces are orthogonal to the rotational axis of the rotor, and it is not practical to position the rotor axis in orientations other than horizontal; the thrust surfaces are oriented vertically.
A method developed to overcome the running of the molten metal during the SAW process is to machine the thrust surface to a conical angle of typically 22.5 degrees from vertical. Such an angle provides enough of a horizontal component to prevent the molten metal from running. However, use of the 22.5-degree angle weld preparation requires machining a large volume of the rotor material only to replace it with a larger volume of weld metal. A significant portion of this weld metal must then be machined away to return the thrust-bearing surface to a vertical orientation. This process, though effective is very time consuming and costly.
An additional concern created by this process is the degradation that results in the material properties of the original base metal of the rotor. Of specific concern is the fillet radius formed between a cylindrical bearing surface journal) concentric to the rotational axis of the rotor and the thrust-bearing surface orthogonal to it. After welding the 22.5-degree or greater angle the fillet radius is completely covered by welded on metal that must be machined away to reform the fillet radius. During the welding process temperatures of the base metal of the rotor are elevated in what is commonly known as a heat-affected zone (HAZ). Some of the base metal in the HAZ undergoes material property changes that result in a weakening of the metal. Since the fillet radius is known to be a highly stressed area during operating conditions involving high levels of thrust any weakening of the base metal in this area may contribute to premature rotor failures.
Additionally, the SAW process frequently leaves defects embedded within the welded metal that, left undetected and corrected, can also lead to premature rotor failures. Ultrasonic testing (UT) is typically used to detect such defects. UT is most effective on layers that have a uniform thickness and a weld line that is perpendicular to the outer surface on which the UT transducer is placed; neither of which conditions exist with the conical angle that the SAW process described above is performed on.
Accordingly there is a need in the art for a more efficient and less defect prone method of applying a low chromium content layer to the thrust bearing surfaces of a turbine rotor without weakening base material properties in specific locations.
Disclosed herein is a method that relates to preparing a rotating member to enhance bearing durability comprising, applying a metal material to a surface of a rotor orthogonal to a longitudinal axis of the rotor, the material having a chromium content less than a chromium content of the rotating member, and surfacing said metal material orthogonal to the axis of the rotor.
Further disclosed herein is an apparatus that relates to an enhanced bearing durability rotating member, comprising, a rotor with a metal material surface orthogonal to a longitudinal axis of the rotor having less chromium content than the rotor, and a boundary layer between the metal material surface and the rotor orthogonal to the axis of the rotor.
Referring now to the figures, which are exemplary embodiments, and wherein like elements are numbered alike:
A detailed description of several embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures.
Referring to
It is common for the turbine rotor 10 to be forged of high chromium content steel, typically in excess of nine weight percent chromium, and more specifically in the range of 9.5 to 13 weight percent chromium. To prevent wire-wooling failures and to thereby enhance bearing durability, an embodiment of the invention applies layers 12 of lower chromium content metal material than the rotor 10, and more specifically less than 2.2% chromium, to the rotor 10.
One embodiment of the invention utilizes a GTAW process to weld a low, about 0.5%, chromium layer 12 onto rotor thrust surfaces 24 (see
Another advantage stemming from the lower heat input available with the GTAW process is the ability to weld on a vertical surface without gravity causing the molten metal to run downward. A vertical weld however does require that the weld beads 40 as shown in
The processing time required to complete the weld upon the thrust surface 24 depends, partially, upon how thick the welded layer 12 must be. There are two main characteristics to consider when determining what weld depth 52 is required. First is the distance from the weld line 26 required to assure that a chromium content of less than 2.2% is achieved, and second the machining depth 50 necessary to create a smooth bearing surface 18, 22 from the rough welded surface 58. To determine a chromium content minimum depth 54, we analyze
Referring now to
Some embodiments of the invention may include the following advantages: no weld machining preparation is required, welding time and material requirements are minimized, the finished bearing surface resists wire-wooling problems of associated bearings and has a chromium content of less than 2.2%, and base metal material properties, in a highly stressed fillet radius area, are unaffected by the welding process.
While the embodiments of the disclosed method and apparatus have been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the embodiments of the disclosed method and apparatus. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the embodiments of the disclosed method and apparatus without departing from the essential scope thereof. Therefore, it is intended that the embodiments of the disclosed method and apparatus not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the embodiments of the disclosed method and apparatus, but that the embodiments of the disclosed method and apparatus will include all embodiments falling within the scope of the appended claims.
