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9-12% Cr ferritic-martensitic stainless steels are widely used as high temperature construction materials in power plants due to their excellent creep and oxidation resistance. The growing share of renewable energy sources in power generation forces many of these plants into more flexible operation with frequent load shifts or shutdowns. These cyclic operation profiles constitute a major lifetime issue.
The present contribution reports on current findings obtained in a multidisciplinary project which combines cyclic mechanical and cyclic oxidation testing with detailed microstructural analyses. Mechanical analyses are carried out on P92 and P91 steel grades to give an overview of softening phenomena and lifetimes obtained in isothermal cyclic loading (low cycle fatigue, LCF), non-isothermal cyclic loading (thermo-mechanical fatigue, TMF), and service-like combinations of creep and fatigue periods (creep-fatigue interaction).
Oxidation testing focuses on the grades P92 and VM12 with the intention of clarifying the impact of frequent passes through intermediate temperature levels on the kinetics of steam-side oxidation and the characteristics of the evolving oxide scales. An attempt is made to evaluate their composition, strength, integrity and adhesion after up to 250 temperature cycles. Flat coupons as well as curved tube sections are tested to assess the mutual influence of geometry on oxide scale integrity.
Complementary microstructural investigations by scanning and transmission electron microscopy plus EBSD are used for phase identification and substrate/oxide interface characterisation. The evolutions of grain size and dislocation density under different test conditions are quantified.
In recent years, the performance of heat resistant ferritic-martensitic steels under cyclic oxidation and cyclic mechanical loading has gained considerable attention. The growing contributions of renewable energy sources to electricity generation have triggered a shift from continuous (baseload) operation of conventional power plants towards cyclic or “flexible” operation, which is needed to stabilize the electric grids by balancing the highly variable renewable’s input.
Compared to the impressive amount of data on isothermal oxidation/corrosion and static
mechanical loading (creep) that was compiled over decades, only little and sometimes
contradictory information is so far available on the reaction of these alloys to cyclic
conditions. The latter may involve frequent transients or holds at intermediate temperatures, as well as shut-downs and start-ups with high temperature rates which result in cyclic mechanical loads (thermo-mechanical fatigue, TMF) and thermo-cyclic oxidation at variable temperature.
Our contribution will report on recent findings obtained within the framework of a junior
research group which investigates the oxidation and fatigue of the ferritic-martensitic grades P91, P92 and VM12 SHC under thermo-cyclic conditions. Cyclic oxidation tests are carried out in steam using different sample shapes (flat coupons, U-segments and rings taken from heat exchanger tubes). Special attention is paid to changes in the kinetics and the integrity and possible delamination/spalling of the oxide layers. Mechanical tests are carried out on material from real steam pipes with a focus on the softening behaviour and lifetime issues resulting from combined creep/TMF loadings. All experiments are carried out in the 300-620°C regime with temperature and load profiles that resemble typical loading scenarios in power plants and are complemented by detailed microstructural characterisation.