| Abstract |
Operators of geothermal resource production and power generation facilities face a large number of technological challenges, not the least of which is optimizing the performance of the various materials of construction. With regard to metallic materials, degradation mechanisms include pitting corrosion, general corrosion, erosion, various forms of stress corrosion cracking, embrittlement, fatigue, and potential for exposure to large strains from seismic activity. In many cases, the welded connections represent a possible “weak link” in the chain of mechanical integrity. In other cases, welding represents a form of “life jacket” that can cost effectively increase the service life components by either reducing the rate of degradation or restoring the integrity of components that are already degraded. This paper describes the ways in which welding can inadvertently promote accelerated degradation of welded fabrications in geothermal service and approaches toward minimizing the susceptibility to those forms of welding-related degradation. The approaches include optimizing the selection of weld filler metals, careful control of welding procedure parameters including heat input, and using beneficial surface treatments or postweld heat treatments in selected locations. Methods of using welding to extend the serviceability of degrading components include the use of welded cladding, including conventional metallurgically-bonded weld overlays of corrosion resistant alloys, and techniques originally pioneered by other industries, including use of corrosion resistant alloy “wall paper” lining and direct deposition welding. Finally, this paper describes advances in weld cooling rate modeling that have resulted in the development of user-friendly software that allows staff to optimize welding procedures intended for use on in-service, pressurized piping. The models allow users to evaluate susceptibility to both burning though of thin wall pipe caused by excessive heat input, and the susceptibility to hydrogen cracking associated with the formation hard heat affected zones of welds made under conditions that promote fast cooling rates. |