Effect of Specimen Thickness on the Degradation of Mechanical Properties of Ferritic-Martensitic P91 Steel by Direct-fir
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TRODUCTION
SUPERCRITICAL CO2 (sCO2) power cycles are gaining considerable interest due to their higher thermodynamic efficiency compared to conventional steam Rankine power cycles.[1–3] Other advantages of sCO2 power cycles include low capital cost due to compact turbomachinery and less negative impact on environment. An sCO2 power cycle is classified into two categories: indirect-fired and direct-fired. In an indirect-fired sCO2 power cycle, a relatively pure CO2
SAJEDUR R. AKANDA and O¨MER N. DOG˘AN are with the National Energy Technology Laboratory, 1450 Queen Ave SW, Albany, OR 97321. Contact e-mail: [email protected] RICHARD P. OLEKSAK and KYLE A. ROZMAN are with the National Energy Technology Laboratory and also with the Leidos Research Support Team, 1450 Queen Ave SW, Albany, OR 97321. REYIXIATI REPUKAITI is with the National Energy Technology Laboratory and also with the School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, OR 97331. Manuscript submitted May 20, 2020; accepted October 13, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
working fluid is indirectly heated by the heat source such as nuclear, geothermal, fossil fuel or concentrated solar power. On the other hand, in a direct-fired sCO2 cycle, fossil fuel is burnt in pure O2 (oxyfuel combustion) and the resulting CO2-rich stream is expanded in a turbine.[4,5] The combustion introduces additional impurities such as H2O and O2 in the working fluid stream in a direct-fired sCO2 cycle.[6] Like in a steam power cycle, an important concern about structural materials applications in a CO2-based power cycle is the materials oxidation by high temperature CO2-rich environment. However, due to the presence of C in the CO2-rich environment, an additional concern is the materials carburization by C diffusion from the environment into the materials. Alloys containing high levels of Ni such as austenitic stainless steels and Ni-based superalloys are usually resistant to both oxidation and carburization in high-temperature CO2-rich environments.[7–9] However, these materials are also expensive. The use of less expensive materials such as 9Cr ferritic-martensitic steels, up to an established maximum use temperature, could represent a considerable cost savings in construction of future supercritical CO2 power plants.
The issue of simultaneous oxidation and carburization is of utmost importance for 9Cr ferritic-martensitic steels. For example, many studies have shown that exposure of these steels to high-temperature CO2 results in simultaneous growth of an Fe-rich oxide scales on the surface of the steel and carburization of the underlying substrate.[7,10,11] While growth of these Fe-rich oxides is relatively fast, they still obey parabolic kinetics and therefore acceptable quantities of metal thickness loss remain possible over long-term exposures at moderate temperatures.[7] However, at somewhat higher temperatures and/or long exposure times the oxidation mechanism can change, where parabolic growth kinetics switch
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