Properties of 20Cr-25Ni-Mo-Nb-N Steel Tubes Exposed in Eddystone Unit No.1 Boiler for 75,000 Hours

A 20%Cr-25%Ni-1.5%Mo-Nb-N austenitic stainless steel tube, NF709, has been developed for boiler tube applications. The calculated maximum allowable stress values for NF709 is much higher than those for other austenitic stainless steel tubes. NF709 test tubes were exposed for 75,075 hours in Eddystone Unit No.1 boiler with a steam temperature of 615°C at the superheater outlet. The removed tubes were investigated metallurgically and mechanically. No severe corrosion was observed on the outer surface. No exfoliation of the outer layer of the steam oxidation scale was observed. One percent of Mo was in solid-solution state even after long-time exposure, which is useful for improving the creep strength. Cr₂₃C₆, Nb(C,N), NbCrN and TiN were the main precipitates and the σ(Sigma) phase was slightly detected only in the flame side. Charpy impact values at 20°C of NF709 tubes after exposure was over 40 J/cm², acceptable for boiler tube use. The time to rupture for the exposed NF709 tubes became closer to that for unexposed ones at lower stresses. The above results indicate that NF709 is a promising material for superheaters and reheaters of power plants with advanced steam conditions.