304,316系ステンレス鋼のクリープ破断伸びと破断時間の関係

抄録

Creep rupture elongation of type 304 and 316 austenitic stainless steels has been investigated at 550°C and 600°C. Rupture elongation of conventional 304 (SUS 304) steels decreases with increasing time-to-rupture (TR) and their rupture elongation decreases to about 10% after 5 × 10⁴ h at 600°C. Although conventional 316 (SUS 316) steels show slight decrease in rupture elongation with increasing TR in a relatively short-TR region, their elongation recovers with TR in a long-TR region. It has been concluded that such differences between SUS 304 and SUS 316 steels are related to the difference in phase stability. In SUS 304 steels, G phase and α phase precipitate on grain boundaries during creep resulting in decreased elongation. On the contrary, in SUS 316 steels, the amount of G phase on grain boundaries is smaller than that of SUS 304 and α phase does not precipitate. In order to increase rupture elongation, low carbon (0.01%) and medium nitrogen (0.07%) steels have been developed (304 MN, 316 MN). Although there is little improvement in rupture strength and elongation between SUS 304 and 304 MN, 316 MN has higher rupture strength and elongation than those of SUS 316. In the case of type 304, phase instability, which is the main cause of elongation loss at a long-TR region, can not be improved by such modification of compositions. Reducing carbon content and increasing nitrogen in type 316 suppress precipitation of carbides, which is the main cause of loss in strength and elongation during creep.