Microstructure and deformation mode of a stainless steel rupture disc exposed to sodium–water reaction

This paper deals with microstructural studies carried out on an austenitic stainless steel rupture disc which was exposed to sodium–water reaction. The rupture disc was part of a leak simulator put in a micro leak test section which was used to study the ‘self wastage’ of steam generator tubes. During micro leak testing, the rupture disc failed exhibiting a linear crack at a much lower pressure of 10 MPa rather than bursting open at the higher designed pressure of 15 MPa. The failed rupture disc revealed different microstructural features on the inner (steam exposed) and outer (sodium exposed) surfaces. Using microstructure as the signature, the temperature experienced by the rupture disc was predicted as ≥ 1273 K. Evidence for the exposure of the rupture disc to highly exothermic sodium–water reaction was obtained in the form of sodium rich debris, microcracks and deformation bands. Detailed transmission electron microscopy revealed the nature of deformation bands as deformation twins which is not a preferred failure mode for austenitic stainless steels.