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 Cr2O3 scale posted high exfoliation resistance during cyclic oxidation tests.