An experimental high-temperature rotary valve was found stuck due to growth and distortion after approximately 100 h. Gas temperatures were suspected to have been high due to overfueled conditions. Both the rotor and housing in which it was stuck were annealed ferritic ductile iron similar to ASTM A395. Visual examination of the rotor revealed unusually heavy oxidation and thermal fatigue cracking along the edge of the gas passage. Material properties, including microstructure, composition, and hardness, of both the rotor and housing were evaluated to determine the cause of failure. The microstructure of the rotor was examined in three regions....