Microstructure and grain size dependence of creep and creep-fatigue properties of low carbon medium nitrogen type 316 steel

The effect of grain size on the creep and creep-fatigue behavior of low carbon medium nitrogen type 316(316FR) steel has been investigated at 550degC, compared with conventional type 316(SUS316) steel. The microstructure of ruptured specimens has been examined with electron microscope. The creep rupture strength of both steels decreased with increasing grain size. The rupture strength of 316FR was higher than that of SUS316 at any grain size. The rupture ductility of 316FR, where very small Laves precipitated on the grain boundary during creep, slightly decreased with increasing grain size, while that of SUS316, where carbide precipitated on the grain boundary, did not change with grain size. The rupture elongation of 316FR at largest grain size, however, was much higher than that of SUS316. This difference in rupture ductility is attributed to the grain boundary carbide developed during creep. The creep-fatigue life of both steels decreased with increasing grain size. Because 316FR showed longer life and smaller dependence on grain size than SUS316, 316FR had much longer life than SUS316 at large grain size. The fracture mode of 316FR changed from transgranular to intergranular with increasing grain size, while that of SUS316 was intergranular at any grain size. Although 316FR showed intergranular fracture at very large grain size, it had much longer life than SUS316. This means that the crack propagation along grain boundary of 316FR is more difficult than SUS316. Such difference in crack propagation behavior may be depends on the grain boundary precipitation, that is, Laves or carbide. (author)