Effect of Grain Boundary Fe2Nb Laves Phase on Creep of Austenitic Heat Resistant Steel of Fe-20Cr-30Ni-2Nb in Steam Atmosphere

Abstract

Creep of carbon free Fe-20Cr-30Ni-2Nb (at.%) steels strengthened by Fe2Nb Laves phase on grain boundaries has been examined in air and steam atmospheres at 1073 K/70 MPa. Sufficiently covering grain boundaries by stable Laves phase can reduce the creep rate, resulting in a significant extension of the creep rupture life without a loss of creep ductility in steam atmosphere as well as air. This effect becomes remarkable in higher area fractions of Laves phase on grain boundaries (ρ) than 80%. The creep rupture strain slightly decrease by steam atmosphere, which would be associated with the interglanular oxidation and its related cracks propagation along grain boundaries enhanced by steam atmosphere in the creep acceleration stage. Its effect can be suppressed by covering grain boundaries by Laves phase (increasing ρ). Therefore, Grain Boundary Precipitation Strengthening by stable Laves phase is a key mechanism for use of materials for boiler tubes in the innovative advanced ultra-super critical (A-USC) thermal power plants to be operated above 973 K.