High-temperature tribological behavior of structural materials after conditioning in impure-helium environments for high-temperature gas-cooled reactor applications

Highlights: • Friction and wear for alloy 617 are lower than those for alloy 800HT • Conditioning the alloys in impure helium and in air leads to surface oxidation. • Oxidation increases the wear resistance of both alloys. • The formation of a compacted oxide layer glaze results in negligible wear. - Abstract: Incoloy 800HT and Inconel 617 have been selected as candidate structural alloys for the high-temperature gas-cooled reactor (HTGR) concept. Helium, the primary coolant, contains impurities (e.g., H₂O and CH₄) that can induce corrosion reactions at high temperatures, which in turn can affect the tribological behavior of components in sliding contact such as valves and control-rod drive systems. This paper presents results from the study of the tribological behavior of both alloys before and after conditioning them in either an impure-helium oxidizing environment or in an air environment. Both alloys were conditioned for 22 days at elevated temperatures in a once-through helium loop with 4 ppmv H₂O and tested subsequently at elevated temperatures with a pin-on-disk tribometer in an air environment - 650 °C and 750 °C for 800HT; 850 °C and 900 °C for 617 – with various applied loads - 1 N, 2 N and 5 N. SEM-EDS analysis revealed that conditioning the samples in an oxidizing environment leads to the formation of a mixed Fe/Cr-oxide on alloy 800HT and a Cr-oxide on alloy 617, both increasing the wear resistance compared to that of as-received samples. Alloy 617 exhibited lower steady-state friction coefficients compared to those of alloy 800HT. There was a significant decrease in the scatter of the steady-state friction coefficient of the conditioned samples compared to that of the unconditioned samples. The steady-state friction coefficient for alloy 800HT and 617 were found to be 0.53 ± 0.07 and 0.25 ± 0.04, respectively. The wear resistance of alloy 800HT is approximately one order of magnitude lower than that of alloy 617 in all cases that exhibited measurable wear. After sample conditioning, the wear volumes measured at low loads were undistinguishable from the unworn background, a result attributed to the formation of a compacted glaze layer under high temperatures and high contact stresses.