Influence Of Thermal Ageing On Corrosion Behavior Of Ferritic-martensitic Steel P92 In Supercritical Water

P92 steel has been widely applied to heating pipes in ultra-supercritical units of coalfired power plants.Its microstructure changes significantly in the course of prolonged service in coal-fired power plants,which will lead to a reduction in various performance and service life.The majority of current research concerning the effect of microstructure degradation on the performance of P92 steel focuses on mechanical properties and creep life,but the influence of microstructure ageing on its oxidation behaviour under the environment of supercritical water has not been fully explored,which may rusult in underestimation of remaining life of boiler pipes.First of all,the high temperature oxidation reaction mechanism between Fe-Cr binary alloy and H2O was investigated through reaction force field molecular dynamics.Further more,the defect properties like defect formation energy of vacancies and intersititials of Fe-Cr spinel and magnetite were calculated by the first principle method.These calculations provide preliminary theoretical foundation for the later oxidation experiments in super-critical waterThen,based on the theory of alloy thermodynamics,short-time thermal ageing experiments at 800℃ were carried out to accelerate the ageing process of P92 steel.P92 steel specimens with distinguished ageing microstructure were obtained.The accuracy of thermal ageing experiments was verified by calculating the size of carbides through semiempirical method.The experimental results showed that during thermal ageing process,the martensitic slats coarsened,merged and were then gradually replaced by equiaxed subgrains.In the mean time,the average grain size of carbides increased initially but decreased when thermal ageing time reached 400 hours.Finally,the oxidation behaviour of P92 steel in supercritical water after thermal ageing was investigated through experiments in autoclave.Corrosion experiments up to 1500 hours were carried out at 600℃/25MPa in supercritical water.After oxidation experiments,the gain weight during exposure was acquired throug electronic balance.Then the surface along with cross-sectional morphology of the oxide film were observed by scanning electron microscope and the composition of the phase were obtained by X-ray diffraction.After that the influence of microstructure ageing on its corrosion behaviour was fully discussed.The results showed that the oxidation kinetic curves all lie between parabolic and cubic laws despite the time of thermal ageing and the oxidation weight gain increases with increasing thermal ageing time.A two-layered structure of oxides composed of Fe3O4 and(Fe,Cr)3O4 were observed.Compared with the specimens without thermal ageing,those after ageing experiments were more prone to cracking and exfoliation.This paper investigated the microstructure ageing characteristics of P92 steel at high temperature and its effect on the oxidation behaviour of P92 steel in supercritical water through experiments.The interaction mechanism between thermal ageing and oxidation has been fully discussed,which is important for the correct assessment of the life of thermal power station heating surfaces.