The 316MN steels with improved creep rupture properties were developed for structural materials of fast breeder reactors. To develop weld materials for the 316MN steels the effects of C, N, Mo, and delta ferrite on creep rupture properties of weld metals were investigated. Creep rupture strength and rupture ductility were improved by reducing carbon and increasing nitrogen content. Delta ferrite, which is necessary for preventing hot weld cracking, reduces the rupture strength but increases rupture ductility. On the basis of these test results, low carbon and medium nitrogen type 316 and 16-8-2 weld materials were developed. Creep rupture strength of bot hweld metals is equivalent to that of base metal. Rupture ductility is also comparatively high and independent of rupture time. Some intermetallic compounds such as Laves and sigma phases, etc., which precipitate in delta ferrite during creep or high temperature aging, do not affect creep rupture properties, but the sigma phase seriously reduces the Charpy impact toughness. The loss of toughness of type 16-8-2 weld metal is less than that of type 316 weld metals.
