Abstract
The effect of grain size on high temperature mechanical properties of low carbon and medium nitrogen type 316 stainless steel (316FR) developed for fast reactor structures has been investigated, compared with conventional type 316 steel (SUS316). Tensile tests at room temperature, 550°C, 600°C, 650°C and creep rupture tests at 550°C and 600°C were conducted, using steels with grain size from 68μm to 620μm. The microstructures of creep ruptured specimens were examined with optical and electron microscopy. The tensile strength of both steels decreased with increasing grain size at all temperatures. 316FR steel showed lower tensile strength than SUS316 steel at high temperatures, while the former had the same tensile strength as the latter at room temperature. The creep rupture strength of both steels decreased with increasing grain size at both temperatures. The rupture strength of 316FR steel was higher than that of SUS 316 steel at 550°C in all grain sizes, while the former was higher than the latter only in the small grain size region at 600°C. The 10000h rupture ductility of both steels decreased with increasing grain size at both temperatures. The decrease of ductility corresponded to the increase of grain boundary fracture. The size and density of carbides on the grain boundary increased with increasing grain size.
