Metallurgical Investigation of Non-separation in SS304L NC Pyrovalve of Satellite Launch Vehicle During Functional Test

Normally closed (NC) pyrovalves made of annealed SS304L are used in the propulsion system of liquid engines in satellite launch vehicles. NC pyrovalves prevent the fluid flow when closed and upon activation (i.e. opening) allow fluids to flow. During functional testing of valve, flow did not occur upon activation. Detailed observation revealed the activation by firing of the cartridge, but the flow path did not open due to non-separation of fluid line closed ends (i.e. diaphragm closed ends). Further analysis indicated that the diaphragm material was harder than intended, surpassing both its specified hardness and the hardness of the piston material. The diaphragm material was found to be in cold worked condition in place of annealed condition. This indicates that the material to be cut (i.e. diaphragm material with an average hardness of 304 ± 5 VHN) is harder than the cutter (i.e. piston material with an average hardness of 154 ± 12 VHN). Due to the higher strength of diaphragm material as indicated by the higher hardness than expected, the mechanical force exerted by the piston on firing of explosive cartridge could not cut/separate the diaphragm and flow path did not establish leading to the failure of pyrovalve operation.