Influence of a Simulated HTGR Environment on the Mechanical Properties of a Commercial Ni-Cr-Mo-Fe Alloy (Hastelloy Alloy X)

摘要

The influence of a simulated advanced-reactor helium environment, containing 500 mu atm H sub 2 /50 mu atm CH sub 4 /50 mu atm CO/approx. 1 mu atm H sub 2 O, on the mechanical properties of two heats of Hastelloy Alloy X is discussed. Simultaneous exposures in air and controlled-impurity helium at temperatures in the range of 650 exp 0 to 1000 exp 0 C for times of 3000 h or more were performed. A combination of tensile testing, Charpy V-notch impact toughness testing, and creep testing was used to study the effects of reactor helium/metal interactions on the mechanical behavior of this alloy. Carburization was identified as the primary corrosion phenomenon. Increasing exposure time and temperature were observed to increase the depth of carburization. The increase in carbon concentration in the carburized zone suppressed the additional formation of M sub 6 C, which is observed in air-aged specimens, and resulted in the precipitation of M sub 23 C sub 6 , a chromium-rich carbide variant. The precipitation of M sub 23 C sub 6 in the carburized zone occurred primarily along grain and twin boundaries; however, matrix precipitation was also observed, the degree of which depended on exposure temperature. Strength and impact toughness properties were found to be controlled primarily by thermal aging reactions, with only a small effect related to the carburization. Although tensile and creep ductilities were decreased as a result of carburization, substantial ductility remained. Variation was observed between the two heats, the finer-grained heat appearing to be weaker in the high-temperature creep tests and also possibly more susceptible to a loss of creep strength as a result of carburization. (ERA citation 05:013898)