Abstract
Alloy 709 was oxidized at 700 °C for 500 h in a steam environment. A microstructural analysis of the oxide scale is reported. Modern techniques of advanced electron microscopy were used to characterize the morphology of the oxide scale and recognize its single components. The material developed a complex, multilayered oxide scale. The outermost layer consisting of Fe2O3. Fe2NiO4 tI28 spinel was detected underneath. An internal oxidation zone is present in the innermost layer. High quality SEM-EDS maps give insight into a larger area of the oxide scale at a relatively low magnification.
Highlights
A lot of effort has been made in order to improve Fe-based alloys to increase their operation temperature in power plants, which would allow significant savings of hard coal or lignite in energy production
The 9–12% Cr martensitic steels have been very widely used. They are relatively inexpensive due to the lack of Ni and have a smaller thermal expansion coefficient, which is important in running up and shutting down cycles. These steels can operate below 620 ◦ C [1,2], which is insufficient in modern power generation systems
The SEM-EDS investigation already gives plenty of information about the oxide scale, the determination of the phases present in the sample was performed by the method of scanning transmission electron microscopy (STEM), including high resolution imaging (HR) and selected area electron diffraction (SAED)
Summary
A lot of effort has been made in order to improve Fe-based alloys to increase their operation temperature in power plants, which would allow significant savings of hard coal or lignite in energy production. It would induce a decrease in CO2 emission to the atmosphere. Alloy 709, due to its reasonable chemical composition is used in power plants It is foreseen as a good candidate to work at temperatures higher than those possible for martensitic steels. The prepared samples were oxidized in steam at 700 ◦ C up to 500 h and further investigated by mains of advanced scanning and transmission electron microscopy techniques which allow one to obtain knowledge about behavior of the steel
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