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Synopsis : Creep tests of Ni-base heat resistant alloys ( Hastelloy XR and XRII ) were conducted at 950•Ž in four kinds of helium environments with different impurity compositions in order to examine the effect of decarburizing environment. Both alloys were decarburized in a helium environment with low partial pressure of oxygen and carbon activity. The decarburization induced degradation of creep properties such as lower creep rupture strength, higher creep rate and earlier start of accelerating creep. Whether the decarburization of the Hastelloy XR and XRII might occur or not in the primary helium coolant of high temperature gas-cooled reactor (HTGR) could be predicted by using a corrosion map of a stability diagram for chromium. Controlling impurities to maintain higher partial pressure of oxygen and carbon activity in primary helium coolant of HTGR is proposed in order to prevent degradation of creep properties of the materials caused by decarburization.

Effect of Carburizing Helium Environment on Creep Behavior of Ni-base Heat-resistant Alloys for High-temperature Gas-cooled Reactors