Heating apparatus for a welding operation and method therefor

#High temperature cobalt and nickel-based superalloys are widely used to form certain components of gas turbine engines, including combustors and turbine vanes and blades. While high-temperature superalloy components are often formed by casting, circumstances exist where superalloy components are preferably or are required to be fabricated by welding. For example, components having complex configurations, such as turbine midframes and shroud support rings, can be more readily fabricated by welding separate castings together. Because the cost of components formed from high-temperature cobalt and nickel-based superalloys is relatively high, restoring and repairing these components is typically more desirable than replacing them when they become worn or damaged. As a result, welding is also widely used as a method for restoring blade tips, and for repairing cracks and other surface discontinuities in superalloy components caused by thermal cycling or foreign object impact. To improve yields, superalloy components are often welded while maintained at an elevated temperature, e.g., in excess of about 1500° F. (about 815° C.)

#Superalloy components of gas turbine engines must generally be thermally stress-relieved before welding to relax residual stresses present from engine service, and then stress-relieved after welding to relax residual stresses induced during cool down from the welding operation. Heat treatment also provides stress relief by dissolution of a portion of hardening gamma prime (γ′) in γ′-strengthened nickel-base superalloys. Generally, the heat treatment parameters will vary depending on the alloy of interest, the amount of residual stress relief and dissolution required, furnace design, component geometry and many other factors. The ramping rates, soak temperatures, hold times and cooling rates for stress relief and dissolution heat treatments are critical in order to obtain the desired stress relief without adversely affecting the superalloy and its properties.

#In the past, pre-weld and post-weld heat treatments have been performed in large batch heat treatment furnaces to ramp and hold a group of components at a suitable heat treatment temperature. Drawbacks to the use of batch heat treatment processes include long heat treatment times due in part to the mass of the large batch furnace and the mass of the typically large number of components being heat treated. Additionally, long queuing times occur while batches are assembled as individual components are repaired. Therefore, use of batch furnace pre-weld and post-weld stress relief heat treatments represent a time delay to the flow of components through a welding line, and is an inefficient method to metallurgically condition components for welding.

#FIG. 1 is a schematic representation of a combination heat treatment and welding apparatus in accordance with this invention.

#The present invention is generally directed to superalloy articles that undergo a welding operation during their fabrication, restoration or repair. While the advantages of this invention will be described with reference to components of gas turbine engines, the invention is also applicable to a variety of applications in which an article is to be heat treated before and after welding in a manner that will not degrade the properties of the article.

#An apparatus 10 for performing the heat treatments and welding operation of this invention is schematically depicted in FIG. 1. The apparatus 10 includes an enclosure 12 specially adapted for performing a welding operation such as tungsten inert gas (TIG) or plasma transferred arc (PTA) welding in a controlled atmosphere. The apparatus 10 is also shown to include an induction heating coil 14 of a type known in the art. The coil 14 is preferably sized and shaped to closely surround a very localized region of an article being welded so that uniform and rapid heating of the localized region occur. The enclosure 12 also includes an inlet 18 through which an inert gas (such as argon) is fed from below to the interior of the enclosure 12, so that an inert atmosphere envelopes the article while at the elevated processing temperatures of this invention. A one-way flow control valve 32 is shown attached to one wall of the enclosure 12, through which the inert gas and fumes can be exhausted from the enclosure 12 while preventing air from being drawn into the enclosure 12. Finally, the apparatus 10 is shown to include a welder 20. The welder 20 is preferably a TIG or PTA welder, though it is foreseeable that other welding systems could be used. The filler material may be in the form of a superalloy wire whose composition is metallurgically compatible with that of the article and appropriate for the operating environment of the article. Notably absent from the apparatus 10 is an access opening to the interior of the enclosure 12, and an exhaust hood for removing fumes generated during the welding operation.

#According to the invention, the apparatus 10 further includes a pair of gloves 30 attached to one of the walls of the enclosure 12 so that the article being welded and/or the welder 20 can be manipulated within the enclosure 12. The gloves 30 eliminate the prior requirement for an opening in the enclosure 12 to permit access to the welder 20 and the article being welded. As a result, with the exception of the one-way valve 32, the enclosure 12 remains sealed while the article is at an elevated temperature to prevent air surrounding the enclosure 12 from being drawn into the enclosure 12 due to severe thermal gradients or other potential causes.