Study of the failure of a duplex stainless steel valve

A study has been performed to determine the reason for the failure of a group of duplex stainless steel valves after 15 years in service. Chemical analysis did not agree with the required quality. Metallographic analysis revealed some small pits with corrosive products inside which were blamed to be the origin of long cracks and consequently of the failure. Pitting and cracks progressed along the ferrite-austenite interface where carbides and brittle phases are precipitated. This precipitation drags a significant content of chromium whose percentage in the surrounding matrix is depleted losing its pitting resistance being attacked by the circulating fluid. The use of a steel with higher pitting resistance and avoid working in the temperature range where both carbides and brittle phases are formed is recommended.