Relationship between phase stability and creep rupture ductility of type 304 and 316 stainless steels

Metallographic studies were conducted on creep ruptured specimens of Type 304 and 316 stainless steels, which had been tested up to about 100,000 hours at 550 and 600oC. Although the rupture ductility of conventional 304 (SUS304) steels decreased monotonically with increasing rupture time, in SUS316 steels ductility was recovered given longer rupture times. It has been concluded that such differences are caused by the difference in phase stability. In SUS304 steels, coarse G phase and α phase precipitated at grain boundaries, and cavities generated at interphase interfaces during creep, resulting in decreased ductility. In SUS316 steels, the amount of G phase at grain boundaries was smaller than that in SUS304 and the α phase never precipitated. In order to increase rupture ductility, low carbon (0.01%) and medium nitrogen (0.07%) steels had been developed (304MN, 316MN). The rupture strength and ductility of 304MN is comparable to that of SUS304. In the case of Type 304, phase instability, which is the main cause of ductility loss at longer rupture time, could not be improved by such a modification of composition. On the other hand, reducing carbon content and increasing nitrogen content in Type 316 suppressed precipitation of carbides, which is the main cause of loss in ductility during creep. Thus, higher rupture ductility is obtained in 316MN steels. (author)