Creep Behavior And Life Prediction Of High Chromium Heat Resistant Steel At Elevated Temperature

In order to meet the requirement of high operation parameters for equipment in the fields of thermal power generation,energy and chemical industry,the heat-resistant steels with high chromium have been widely used in superheaters,reheaters,high temperature headers and steam pipes of ultra-supercritical boilers,due to their excellent high temperature tensile strength,high temperature oxidation corrosion resistance,high thermal conductivity and low thermal expansion coefficient.Under the conditions of elevated temperature and complex stress,creep damage occurs inevitably,and performance deterioration may seriously threaten the safe and reliable operation of these components.Therefore,it is of great importance to study the creep behavior of the heat-resistant steel with high chromium systematically and predict its long-term creep life accurately.In this work,the deformation behavior of P91 steel during creep has been investigated.Then,microstructural evolution of precipitates,dislocations and substructures has been evaluated by various methods.On this basis,microstructure evolution based models have been established to predict the long-term creep life for P91 steel.The specific research contents and main conclusions are as follows.Firstly,the mechanical properties of P91 steel were obtained at 580,600 and 620?by high temperature uniaxial tensile test.Then the creep curves were obtained by continuous creep rupture tests under the initial stress around.Besides,the crept specimens with different damage states were prepared by a series of interrupted creep tests.Based on the analysis of deformation and rupture behavior during three creep stages?deceleration stage,steady-state stage and acceleration stage?,creep deformation and damage mechanisms of P91 steel were clarified.The results show that creep deformation of P91steel is closely related to the dislocation movement,and creep damage is ascribed to the microstructural degradation under the present creep conditions.The microstructure evolution of P91 steel was analyzed systematically by using OM,SEM,EBSD and TEM techniques.The results indicate that the multi-scale tempered martensite structures exhibit different evolution behaviors during creep process.For instance,the size and distribution of MX carbonitrides have not changed significantly during short-term creep due to their high thermal stability.In contrast,the M₂₃C₆ carbides located along boundaries have been coarsened obviously.Besides,coarsening of carbides and substructures becomes more obvious with the decrease of applied stress and increase of creep time.Additionally,the creep fracture surface exhibits typical characteristic transgranular ductile fracture mode.The microstructure degradation of P91 steel during creep was evaluated with X-ray diffraction,microhardness and longitudinal ultrasonic mearsurements.The variation of mobile dislocation density during creep process was quantitatively analyzed with X-ray diffraction analysis by introducing the half-width parameter and W-H model.The variation of microhardness during creep was investigated by Vickers hardness measurement.Then,the correlation between ultrasonic attenuation coefficient,velocity and creep life fraction was investigated by longitudinal wave ultrasonic measurement.The results show that the mobile dislocation density decreases monotonously,especially during the primary creep and then keeps stable.Vickers hardness decreases monotonously during the whole creep.Ultrasonic attenuation coefficient increases first,then decreases and then increases during the creep process,and the change of ultrasonic velocity increases rapidly during the acceleration stage.Combining with the analysis of microstructure evolution,it indicates that the variations of these selected parameters are closely related to the evolution behavior of dislocations during primary creep,while the variations during secondary and tertiary creep are closely related to the evolution behavior of M₂₃C₆ carbides and substructures.Based on the above experimental results and theoretical analysis,the MG relation and precipitate coarsening based model and physically based CDM model were established to evaluate the long-term creep life.Compared with the traditional power law constitutive equation,the MG relation and precipitate coarsening based model quantifies the interaction between precipitates and dislocations by introducing the internal stress.In the meanwhile,the effects of dislocation,precipitates and work hardening on creep deformation are comprehensively considered by introducing several damage factors into the CDM model.The results show that both of the models can give reliable predictions of long-term creep life for P91 steel.In particular,the physically based CDM model can provide useful insights into the underlying creep mechanisms and simulate creep curve of material.Therefore,the CDM model has great potential in both theoretical analysis and engineering application of long-term creep life assessment.