A Monel Series 3800 Pressure Relief Valve in oxygen service was destructively evaluated to determine the cause of failure. The piston reportedly seized the first time it was used after installation. Visual examinations found deep gouges in the mating surfaces of the piston and cylinder, along with surface defects in the piston that were not associated with gouges. There was no evidence of foreign material in either the defects or gouges.
Metallurgical evaluations identified large inclusion stringers in the bulk piston material that consisted of both gas porosity and non-metallic inclusions. These inclusion stringers resulted from unknown issues during melting and casting of the piston material. Comparable metallurgical evaluations of the piston surface defects also identified a combination of gas porosity and non-metallic inclusions associated with the defects. Machining of the piston during fabrication exposed the inclusion stringers at the surface where spalling of the Monel metal introduced debris into the valve. The cause of valve seizure was Monel material that was ejected from the piston surface resulting in interference, galling and gouging during valve operation.