Influence of flowing water vapor containing environment on high-temperature behavior of 9Cr creep-resistant steels

Abstract

Steam superheaters are used to increase the temperature and transport the flowing superheated water vapor before it enters the steam turbine. Creep-resistant steels with required high resistance to high-temperature oxidation are used for production of superheaters. The elimination of the protective oxide layer formation on the steel surface limits their upper application temperature in a water vapor containing environment. In this paper, the oxidation behavior of MarBN (MARtensitic 9Cr steel strengthened by Boron and MX Nitrides) steel exposed for 1000 h at 600 and 650 °C in a mixed atmosphere of air + 10% water vapor was analyzed. The oxidation mechanism was discussed on the basis of oxidation kinetics, morphological observations, microscopic and XRD analysis of the oxide layer. The oxidation kinetics was governed by the parabolic law. SEM analysis and XRD analysis showed that the outer steel oxide layer after 1000 h exposure at 600 °C was mainly composed of hematite Fe2O3 and a small proportion of Cr2O3. The inner oxide layer was composed of (FeMnCr)2O4 and Cr2O3. The steel surface after 1000 h of oxidation at 650 °C, the outer oxide layer consisted of hematite Fe2O3, Cr-rich phase and magnetite Fe3O4, and the inner oxide layer was mainly iron-chromium-manganese spinel (FeMnCr)2O4.

Keywords

• High temperature corrosion
• 9Cr steels
• Oxide layer
• SEM
• XRD