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Thermal-mechanical fatigue of oxide scales in cyclic water vapor oxidation of 9-12 % Cr steels
(2017)
Ferritic-martensitic steels with 9-12 % Cr-content are widely used as high temperature construction materials in power plant due to their high temperature properties such as creep resistance, strength, and favorable oxidation resistances. However, typical operating conditions include cyclic thermal/mechanical loading, and as a result the oxide scale might fail; cracks are formed and the oxide scale is spalled off. The oxide spalls can cause steam tube blocking and local thermal stresses, which might provoke shorter component service life.
This work investigates the scale spallation behavior of ferritic-martensitic steels under cyclic load condition between 320°C and 620°C for up to 210 cycles in pure steam. All samples were isothermally pre-oxidized at 620°C for 500 h in water steam with 30 bar pressure to create an initial magnetite layer with low porosity. Different sample shapes, rectangular coupons, full rings and U-shape segments, are investigated to evaluate the impact of the geometry on these conditions. After oxidation experiments the oxidation kinetics in terms of mass change, metal loss and thickness scale is determined. In addition, the integrity of the oxide layer as well as its adherence to the matrix are characterized by modified hardness tests and scratch tests. Breakage of the layers at lower applied forces indicates a substantial loss of adhesion strength at intermediate cycle numbers.
This work investigates the distribution of Chromium and Cobalt as well as other elements and their effect on the corrosion scale formation of ferritic-martensitic alloys under single and dual conditions. For this purpose, coupons of technical VM12-SHC alloy and model Fe-13Cr alloy were annealed at 600°C for 960 h in artificial flue gas. This kind of experiment represents the classical single condition corrosion test. To realize dual condition, an experimental setup was designed in such way that each face of the specimens could be in contact with a different atmosphere.
In this presentation the comparison between single and dual conditions will be evaluated in terms of phase composition of the scale and material loss. Preliminary results show clear deviations between both conditions in the distribution of chromium, as well as the diffusion outward of cobalt in the technical alloy.