Environmental deterioration analysis of heat resisting alloys for the mono-propellant thruster

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L-605 alloy has been used for the mono-propellant structure and screen materials because of its excellent combination of high strength and high oxidation resistance. However, deterioration of L-605 alloy in a high temperature environment with nitrogen and ammonia has been reported. Therefore, the composition, microstructure, hardness and tensile properties of L-605 alloy were evaluated with and without heat treatment in a nitrogen gas atmosphere in order to investigate the degradation mechanism of the alloy. The chromium nitride and the tungsten carbide were precipitated in the alloy after the heat treatment. These precipitates were increased with increasing heat treatment duration. Tensile strength and elongation to fracture of L-605 alloy were decreased with heat treatment at 1173K for up to 86.4ks in nitrogen gas atmosphere. These results indicate that the chromium nitride and tungsten carbide formations are causal factors of deterioration of L-605 alloy.