Effects of microstructures on the creep-rupture properties and fracture mechanisms in austenitic heat-resistant steels

Both rupture life and creep ductility increase with increasing grain size in the SUS304 steel, in which a mixed mode of transgranular fracture and intergranular fracture occurs. This was correlated with the increased proportion of transgranular fracture with grain size. Rupture life increases and creep ductility decreases with grain size, especially in the range from 8.4 to 99 μm, in the 21Cr–4Ni–9Mn steel, in which only intergranular fracture occurs. The difference in the fracture mechanism led to the different grain size dependence of creep ductility between these steels. These experimental results were supported by the size distribution of creep cracks and the results of the fractal analysis of fracture surface profiles in both steels.