Role of Niobium on stress corrosion cracking of SUS 347 stainless steel in high temperature water

Role of Niobium on stress corrosion cracking (SCC) of SUS347 stainless steel was investigated in high temperature water at 562K. SCC test was carried out using a slow strain rate testing technique (SSRT) in dissolved oxygen of 8 ppm. The precipitation behavior of NbC and M23C6 was evaluated by transmission electron microscopy and intergranular corrosion test. All the specimens were firstly stabilized at 1173 K for 3.6 ks, in order to precipitate more NbC in the specimen. The specimen was solutionized at 1273 K to 1573 K for 1.8 ks, and then sensitized at 923 K for 72 ks. The solution-treated specimen up to 1273 K occurred general corrosion and had low pitting potential, and SCC in the high temperature water hardly occurred, because of the presence of NbC. The amount of NbC decreased with increasing solution treatment temperature in the range of 1473 K to 1573 K. M23C6 precipitated at the grain boundary, and intergranular corrosion occurred by the sensitization treatment at 923 K. The intergranular stress corrosion cracking (IGSCC) occurred in the range above 1473 K, because of the precipitation of M23C6 at the grain boundary. (author)