Improvement of oxidation resistance of 9 mass% chromium steel for advanced-ultra supercritical power plant boilers by pre-oxidation treatment

Significance Statement

Developing new materials for advanced ultra-supercritical coal-fired power plants operating at a maximum of 700 °C or beyond has been done to realize high efficiency in several countries. The development implements the replacement of 9-12 mass % chromium steel, denoted as 9 to 12Cr steel, with nickel based alloys for boiler or turbine components subjected to high temperatures. However, the nickel-based alloys bear a steep cost and the best way to lessen this would be to apply the martensitic 9-12 mass % chromium steel.

The implementation of the 9 to 12 mass% chromium for high temperature applications in boilers necessitates improvement of long-term creep strength, oxidation resistance and weld joints. Generally, a high chromium content enhances resistance to oxidation through the formation of a protective Cr2O3. However, the 9 to 12 mass% chromium is insufficient for Cr2O3 formation, but this can be enhanced through the addition of some elements such as aluminum, silicon and through grain refinement. Therefore, with the formation of the Cr2O3 scale is also possible for the 9Cr steel.

Dr. Fujio Abe and colleagues at National Institute for Materials Science in Japan realized the formation of protective scale on 9Cr steel surface through a pre-oxidation treatment in argon with small traces of oxygen. Steels cannot oxidize in argon as it is inert, but the small amount of oxygen was responsible for the oxidation. Their work is published in Corrosion Science.

The authors considered 9Cr (mass%) steel with varying silicon concentrations and three versions of martensitic 9Cr steel namely, MARN, MARB1 as well as MARB2. The three versions were 9 mass% Cr steel strengthened by nitrides and 100 and 200 ppm boron respectively. These versions were synthesized with an aim of improving creep strength.

The authors obtained sheet test pieces from already heat treated bulk materials and then exposed to pre-oxidation treatment at 500-700 °C in argon gas containing small amounts of oxygen. The researchers performed the oxidation test in steam at 650 °C for more than 20000 h. The sheet test pieces were set in ceramic crucibles that were placed in the testing furnace and steam was introduced when the furnace temperature exceeded 200 °C.

The authors weighed the samples by a balance before and after oxidation. They examined polished parts of the samples by scanning transmission electron microscope and by an optical microscope.

The authors achieved the formation of the Cr2O3 scale on 9Cr steel by pre-oxidation treatment in argon gas with approximately 0.1ppm to 1vol % oxygen. This was responsible for the oxidation resistance recorded at 650 °C. The team observed that the gain in weight of the 9Cr steel after oxidation reduced with increase in pre-oxidation time and temperature. This observation was retained with increasing silicon concentration in steel. Lower oxygen concentration enhanced Cr2O3 scale formation.

The developed thin Cr2O3 scale was found to be stable for subsequent oxidation in steam at 650 °C for more than 20000 h. The researchers did not observe any breakaway in the weight gain curves for the selected test pieces with pre-oxidation treatment. The thin Cr2Oscale posted high exfoliation resistance during cyclic oxidation tests.

About the author

Dr. Fujio Abe is a Senior Scientist at National Institute for Materials Science (NIMS), Tsukuba, Japan. He has been working at NIMS (formerly National Research Institute for Metals, NRIM) since 1978. Scientific interests of Dr. Abe focus on creep strength, microstructure and oxidation behavior of heat resistant steels and alloys for power plants. He received August-Thum Gedenkmedalle from Darmstadt University of Technology, 2005, Carl von Bach Commemorative Medal from MPA Stuttgart, Germany, 2014, etc. He is a Member of Section II on Materials, Codes and Standards Committee, American Society of Mechanical Engineers (ASME).

About the author

Dr. H. Kutsumi is a Director of Technical Department at Kamijima Heat Treatment Co., Ltd., Tokyo, Japan. He was a Research Fellow at NIMS from 2007 to 2016. Scientific interests of Dr. Kutsumi focus on oxidation behavior of heat resistant steels and physical metallurgy of steels.

About the author

Mr. H. Haruyama is a Researcher at Materials Research Group of Thermal Power Systems Research Department, Mitsubishi Hitachi Power Systems, Ltd, Hitachi, Japan. He was a Research Fellow at NIMS from 2002 to 2004. Scientific interests of Mr. Haruyama focus on oxidation behavior of heat resistant steels and coating technologies of steels and alloys for power plants.

About the author

Mr. Hiroshi Okubo is an assistant researcher at NIMS. He has been working at NIMS since 2000. Scientific interests of Mr. Okubo focus on oxidation behavior of heat resistant steels, martensitic transformation of ferritic steels under magnetic field and alloy design of fuel cell materials. He received a Best Paper Award at the 8th Japan International SAMPE Symposium, November 2003, Tokyo, Japan. His presentation title was “Effects of A New Thermomechanical Heat Treated Process on Creep Properties of High Cr Ferritic Heat Resistant Steels”.

Reference

Fujio Abe1, H.  Kutsumi2, H.  Haruyamac3, and H.  Okubo4. Improvement of oxidation resistance of 9 mass% chromium steel for advanced-ultra supercritical power plant boilers by pre-oxidation treatment. Corrosion Science, volume 114 (2017), pages 1–9.

Show Affiliations
  1. Structural Materials Division, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan
  2. Formerly at National Institute for Materials Science (NIMS) and at present with Kamijima Ltd., 2-23-13 Naka-ikegami, Ohta-ku, Tokyo 146-0081, Japan
  3. Formerly at National Institute for Materials Science (NIMS) and at present with Mitsubishi Hitachi Power Systems, Ltd., 3-1-1 Saiwai-cho, Hitachi, 317-8585, Japan
  4. National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan

 

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