Hastelloy Alloy X - Status Report on the Behaviour in Simulated HTR Helium

Creep and corrosion tests on four casts of Hastelloy alloy X have been conducted in simulated HTR helium and in air at temperatures in the range 750 to 1000°C. The creep data reviewed are from an accumulated test time of approximately 4.4 x 10⁵ specimen hours. The creep and rupture strengths tended to be below those predicted from the manufacturer's data. This apparent shortfall in strength is not taken as indicating any potential or inherent problem with the alloy, but merely as a reflection qf the effect of test time on creep strength. The result emphasises the need for extreme care if short term data are to be extrapolated to long times. From the creep data available so far there is no indication of any effect of test environment on the creep strengths observed. This statement stands even though up to 45% of the cross-section of a specimen may be affected by a visible microstructural change caused by carburisation from the environment. The carburisation which has been seen in tests to date does not seem to lead to any serious ductility losses over and above those expected from normal air ageing and accordingly, subject to the provisos and assumptions given in Section 8.4., it would seem that a lifetime of 100000 hours could be achieved for this alloy at temperatures up to 885°C. In its present form Hastelloy alloy X shows sufficient promise for use at high temperatures 800°C) in HTR environments that considerable effort on improving its properties still further would be justified. (author)