Characterization Of Creep Behavior Of P91 Steel Using EBSD Technique

As the primary material of key components in ultra-supercritical thermal power plants for ages,the heat-resistant P91 steel with tempered martensite is subjected to elevated temperature and high pressure over a long period.Under these conditions,creep damage of these components with the evolution of microstructures inevitably occurs,which resulting in the deterioration of structural integrity of components.Therefore,it is very necessary to study the creep damage mechanism,in order to predict the residual creep life accurately and ensure the safe operation of ultra-supercritical thermal power plants.In this paper,the principal alloy was P91 steel.Firstly,its mechanical properties were obtained by the uniaxial tensile testing at 600℃and 620℃,respectively.Then,the creep data and damage specimens were prepared by the interrupted and creep rupture tests under the conditions of 600℃/165 MPa and620℃/145 MPa.Consequently,the Electron Back-Scattered Diffraction（EBSD）technique and hardness measurement were employed to investigate the damage behavior of P91 steel.The main achievements can be seen as follows:The ultimate tensile strengthσ_b,Young’s modulus E,yield strengthσ_0.2.2 and elongationδof P91 steel at 600℃and 620℃were obtained by the uniaxial tensile tests.The complete creep curve,creep rupture time and creep rupture strain were obtained under the conditions of600℃/165 MPa and 620℃/145 MPa by the elevated temperature creep rupture tests.The creep rupture times were 219 h and 110 h,and the creep rupture strain were 0.296 and 0.332.The damage specimens under different creep states were prepared by the interrupted creep tests.The crystal orientation distribution maps,grain boundary distribution maps and the density of geometrically necessary dislocations（GNDs）were determined by the EBSD technique.Grain orientation indicates that the grain has a certain degree of plastic deformation during creep.The number of small-angle grain boundaries rise first and then decline due to the influence of strain accumulation and creep mechanism.Besides,the density of GNDs increases rapidly from the primary creep to the beginning of the secondary creep and then reaches a peak when the creep rate is minimum.Subsequently,it decreases slowly and then remains constant until the creep rupture,which is consistent with the trend of the number of small-angle grain boundaries.Systematic analysis was obtained by combining the relevant EBSD damage assessment parameters with hardness measurements.The effective grain size represents the grain coarsening during creep.The local misorientation expresses the change of the micro plastic strain during mechanical damage.The hardness shows the variation of the mobile dislocation density,the coarsening of precipitates and the change of material’s mechanical properties.It is found that during creep,the effective grain size increases monotonously.The local misorientation first increases and then decreases,while the hardness decreases monotonously during creep.In addition,the relationship between hardness and efficient grain size follows the Hall-Petch formula.