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.
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ASME 2007 Pressure Vessels and Piping Conference
July 22–26, 2007
San Antonio, Texas, USA
Conference Sponsors:
- Pressure Vessels and Piping Division
ISBN:
0-7918-4287-8
PROCEEDINGS PAPER
Properties of 20Cr-25Ni-Mo-Nb-N Steel Tubes Exposed in Eddystone Unit No.1 Boiler for 75,000 Hours Available to Purchase
Hiroyuki Mimura,
Hiroyuki Mimura
Nippon Steel Corporation, Tokyo, Japan
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Tetsuo Ishitsuka
Tetsuo Ishitsuka
Nippon Steel Corporation, Hikari, Yamaguchi, Japan
Search for other works by this author on:
Hiroyuki Mimura
Nippon Steel Corporation, Tokyo, Japan
Tetsuo Ishitsuka
Nippon Steel Corporation, Hikari, Yamaguchi, Japan
Paper No:
CREEP2007-26753, pp. 197-202; 6 pages
Published Online:
August 20, 2009
Citation
Mimura, H, & Ishitsuka, T. "Properties of 20Cr-25Ni-Mo-Nb-N Steel Tubes Exposed in Eddystone Unit No.1 Boiler for 75,000 Hours." Proceedings of the ASME 2007 Pressure Vessels and Piping Conference. Volume 9: Eighth International Conference on Creep and Fatigue at Elevated Temperatures. San Antonio, Texas, USA. July 22–26, 2007. pp. 197-202. ASME. https://doi.org/10.1115/CREEP2007-26753
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