Characterization Of Creep Behavior Of P91 Steel Using Electrochemical Technology

P91 steel has been widely applied to structural components in ultra-supercritical thermal power plants,which resulting from the excellent properties,such as elevated-temperature creep resistance,impact toughness,elevated-temperature oxidation resistance and thermal stability.Under the conditions of elevated temperature and high pressure,creep of these components will inevitably occur and lead to the final fracture.Thus,it is very necessary to study the microstructural evolution behavior of P91 steel during the elevated-temperature creep,in order to ensure the safe operation of above components.Electrochemical technology has been used to analyze the performance degradation of in-service metals due to its advantages of simple equipment,convenient operation and fast analysis.However,there are few reports on the study of electrochemical measurements of P91 steel after creep.In the present study,the elevated-temperature creep testing of P91 steel was firstly performed to prepare the specimens with different microstructures.Then,the microstructure evolution of P91 steel during creep were investigated by scanning electron microscopy?SEM?,X-ray diffractometer?XRD?and micro-Vickershardnesstesting,respectively.Inaddition,theelectrochemical measurements of these specimens were carried out to characterize the microstructure change using some related parameters.The main contents and results can be seen as follows:?1?P91 steel was subjected to a high-temperature uniaxial tensile test at 600?by using a high-temperature creep testing machine to obtain relevant high-temperature mechanical property parameters.The complete creep curve and creep rupture time were obtained by the high temperature creep rupture test of P91 steel at 600?with 165MPa.Meanwhile,the interrupted creep tests were carried out in different creep stages to prepare specimens with different creep damages,respectively.?2?The dislocation density,precipitates distribution and hardness of above specimens were quantitatively characterized by XRD,SEM and micro-Vickers hardness tester,respectively.The results show that the dislocation density and hardness decrease monotonically with creep time.In contrast,the number of precipitates initially increases,and then decreases with creep time,while the area fraction of precipitates increases monotonically.?3?The polarization behavior of P91 steel in different non-neutral?acidic and alkaline?electrolytes was studied by electrochemical method.The results present that in10%HNO₃ solution,there are two passivation platforms of the polarization curves and the peak current density in the transpassive region increases monotonously with creep time,which is mainly attributed to the continuous increase of precipitates area fraction.On the other hand,the polarization curves exhibit a unique passivation platform in 5.4%KOH solution.The critical passivation current density decreases first and then rises with creep time,which is caused by by dislocation density,precipitates and grain boundary.?4?In 3.5%NaCl solution,the polarization curve and impedance spectroscopy of above specimens were measured.The results illustrate that there is no passivation platform in the polarization curve,while the self-corrosion current density increases first,then decreases and finally rises again with the creep time.Based on above results,it can be concluded that during the creep of P91 steel,the values of electrochemical characterization parameters is strongly dependent on the internal microstructures.As a result,the electrochemical technology offers a potential approach for quantitative characterization of the creep behavior of P91 steel.