Long-Term Creep Properties of Hastelloy XR in Simulated High-Temperature Gas-Cooled Reactor Helium

Abstract

Creep tests of Hastelloy XR (a modified version of the conventional Hastelloy X) were carried out in simulated high-temperature gas-cooled reactor (HTGR) helium at 800, 900 and 1,000°C. The test results up to about 50,000 h showed no significant degradation in creep properties. The creep-rupture strength obtained through long-term tests was above the level corresponding to the design allowable creep-rupture stress of the High-Temperature Engineering Test Reactor. The values of the stress exponent were 4.5 to 5.7 when the stress dependence of the steady-state creep rate was expressed in terms of the Norton equation. It is judged that dominant creep process is dislocation creep. Rupture lives could be estimated with sufficient accuracy using Larson- Miller parameter. Carburization during creep in simulated HTGR helium did not degrade creep properties of Hastelloy XR. Internally formed cracks were initiated at sites of precipitates at grain boundaries, growing nearly perpendicular to the stress axis. Two phases, Cr-rich carbide and Mo-rich carbide, co-existed in specimens after long-term creep tests.

KEYWORDS:

• Hastelloy XR
• creep
• helium atmosphere
• HTTR
• rupture life
• rupture ductility
• steady-state creep rate
• design allowable stress
• Norton equation
• Larson-Miller parameter
• carburization
• grain boundary
• precipitates
• carbides