The metallurgical background to rejuvenation heat treatments and weld reparability procedures for gas turbine sheet metal components

Sheet materials for hot gas path components in modern land based gas turbines demand high strength over the temperature range 650-950°C and freedom from serious in-service embrittlement. This is particularly critical where a gas turbine is subject to cycling since thermal stresses can lead to the cracking of such components. It is also highly desirable that if cracking does occur, components can be repaired safely and easily. Haynes 230, a modern alloy, is relatively immune to in-service embrittlement, particularly in comparison to some older materials, but may require a rejuvenation heat treatment to facilitate repairs after in-service exposure. A rejuvenation heat treatment at 1177°C for 0.5 hour was shown to restore stress rupture and weld ductility to those required by AMS 5878A and Section IX of the ASME Vessel and Boiler Code. Stress rupture results for the aged samples indicated that lives were in excess of those for materials in the as-received condition. This was ascribed to grain boundary precipitation.