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 degrees 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-H(2)-H(2)O-CO-CO(2)-CH(4) and He-H(2)O-CO-CO(2) 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 H(2)-H(2)O-CO-CO(2)-CH(4) than helium with H(2)O-CO-CO(2), and for Alloy 617 than for Haynes 230. Finally, the role of H(2) in accelerating decarburization is discussed. (c) 2010 Elsevier B.V. All rights reserved,