Steam turbines generally include a number of rotors coupled together, with each rotor including a shaft having an integrally-formed wheel that is equipped with multiple blades (buckets) attached to the perimeter or rim of the wheel. A conventional technique for securing the blades to the rotor is to form the rim to have a dovetail cross-section, with each blade having a root portion formed with a complementary dovetail feature that interlocks with the dovetail region of the rim to secure the blade to the rotor.
Because steam turbine rotors must operate at high rotational speeds in a thermally-hostile environment, rotor materials have been selected in part on the basis of creep-rupture strength. For this reason, the prior art has favored CrMoV and martensitic stainless steel alloys as rotor materials for the high pressure first stage and first stage reheat turbine rotors of steam turbines, while NiCrMoV and NiMoV low alloy steels have been favored for the low pressure rotors of steam turbines. Although rotors made from these alloys have exhibited a long service life, it is possible that wear, erosion, corrosion, shock, fatigue and/or overstress may occur, necessitating repair or replacement of a rotor. In addition, the dovetail region at the rotor rim may be subject to stress corrosion cracking, resulting from a combination of high stress and either a highly corrosive event or long exposure to mildly corrosive elements. The dovetail region is particularly susceptible to cracking as a result of the rim being subjected to higher stresses.
In the past, repairs of rim dovetail regions have been performed by welding, in which the damaged portion of the rim is removed and a steel weldment is built up in its place. In many cases, the repair requires the compromising of certain properties due to the limitations of the weld alloy or process, particularly if the service temperature of the rotor will exceed about 500.degree. C. (about 950.degree. F.). For example, weldments formed from alloys typically used in the prior art, such as CrMo alloys, exhibit rupture strengths of less than the CrMoV base alloy for the rotor. Though repairs using a CrMoV alloy would yield a weldment with improved strength at room temperature, strength is marginal at higher temperatures due to weldments formed from this alloy having a reduced rupture strength. Though steel repair materials can offer the benefit of higher rupture strengths, such improvements are at the expense of lower toughness (embrittlement). Furthermore, prior art repair methods using steel repair materials have generally required significant preheating, moisture control, and a post-weld stress relief operation.
Finally, full normalizing and temper heat treatments, which are generally required to develop properties similar to that of the original steel alloy of the rotor, would undesirably alter the precise dimensions of the rotors. Other alloys having desirable mechanical and environmental properties are often incompatible with the rotor base material. For example, martensitic chromium stainless steels disclosed in U.S. Pat. No. 4,710,103 to Gaber et al. and U.S. Pat. Nos. 4,897,519 and 4,958,431 to Clark et al., exhibit mean coefficients of thermal expansion that differ significantly from the base CrMoV material (e.g., about 11.6.times.10.sup.-6 /.degree.C. for 12Cr steels as compared to about 14.4.times.10.sup.-6 /.degree.C. for CrMoV steels at 540.degree. C.). Consequently, repairs made with such materials are susceptible to thermal fatigue.
Accordingly, what is needed is a method for repairing a steam turbine rotor which entails a relatively uncomplicated process that employs a repair material whose mechanical, thermal and environmental properties are both compatible and at least comparable with, preferably superior to, traditional rotor materials.