Failure analysis of a 304 steel component aged at 623 K

A sodium leak was observed in a reducer in the carbon meter circuit of a dynamic sodium loop used for high temperature sodium testing of materials. The reducer in the carbon meter circuit had been exposed to a temperature of 623 K for 6600 h during testing. Detailed metallurgical characterization was undertaken to understand the cause of failure. Based on micro chemical and carbon analysis the structural material of the reducer was identified as 304 stainless steel containing 0.08 wt% of carbon. Cracks were observed at a distance of 4 mm from a weld interface where the microstructure appeared to be fully sensitized in nature. Electron microscopy investigations confirmed the presence of (Fe, Cr)23C6 carbides in the grain boundary. Cause of the failure was concluded as low temperature sensitization followed by intergranular stress corrosion cracking. It is recommended that 304LN stainless steel may be used as the structural material for components operating in this temperature regime for prolonged duration.