Effects of aging in high temperature helium environments on room temperature tensile properties of nickel-base superalloys

Abstract

The influence of high temperature aging treatment on room temperature tensile properties of wrought nickel-base superalloys Alloy 617 and Haynes 230 was investigated. A significant decrease in elongation was observed for Alloy 617 exposed to a heavily oxidizing and decarburizing condition because of coarsening of grain boundary carbides and extensive inter-granular oxidation. On the other hand, Haynes 230 showed much lower ductility when exposed to a heavily carburizing condition, especially at 1000°C because extensive carburization occurred due to a reaction with tungsten. Considerable loss of ductility for Alloy 617 and Haynes 230 was also observed in He–H2–H2O–CO–CO2–CH4 and He–H2O–CO–CO2 environments, which were the slightly oxidizing and decarburizing conditions. Loss of ductility was predominantly associated with brittle inter-granular cracking, while the extent of loss of ductility decreased depending on the decarburization depth. Decarburization was observed more extensively in helium with H2–H2O–CO–CO2–CH4 than helium with H2O–CO–CO2, and for Alloy 617 than for Haynes 230. Finally, the role of H2 in accelerating decarburization is discussed.