Advanced type stainless steel 316FR for fast breeder reactor structures

Low carbon and medium nitrogen type 316FR stainless steel has been developed for structural materials of fast breeder reactors (FBR). In order to elucidate the relationship between creep rupture properties and microstructures, the microstructural changes of 316FR steel have been investigated in comparison with conventional medium carbon and low nitrogen type SUS316 steel. Creep rupture tests were conducted at 550 and 600degreesC. The microstructures of ruptured specimens were examined with an electron microscope and the solute concentrations were assessed by analyzing extracted residues. The 316FR had higher rupture strength and ductility than SUS316 for long-term creep. Laves phase precipitated mainly on the grain boundary at 550 and 600degreesC and also in matrix after 10 000 h creep at 600degreesC in 316FR. Laves phase on the grain boundary did not decrease rupture ductility. The solid solution hardening by nitrogen was effective for a long period of time, because of the extremely small amount of nitrides precipitated during creep. On the other hand, large amount of carbides precipitated on the grain boundary and in the matrix of SUS316. They caused loss of the rupture strength due to the decrease in solute carbon content. The precipitation of carbides on the grain boundary resulted in loss of ductility because of the grain boundary embrittlement. In conclusion, the reason why 316FR had higher rupture strength and ductility than SUS316 was due to the higher phase stability during high temperature exposure. (C) 2003 Elsevier Science B.V. All rights reserved.