Experimental And Simulation Study On Lattice Strain Evolution Of P91 Steel At Room Temperature And High Temperature

The mechanical properties of high chromium tempered martensitic heat-resistant steel(such as P91)which is used as the core component of the nuclear power plant and thermal power plant have been widely focused.The in-depth study on the macro-and micro-mechanical properties and deformation mechanism of heat-resistant steel can tap the potential of engineering application,and the operation safety of core components can be ensured.The mechanical properties of these materials are extremely complex,which are largely determined by the mechanical behaviors and processes occurring on the microand nano-scale.Therefore,it is of great significance to study the micro deformation mechanism for understanding the mechanical properties of these materials.At present,the research on P91 heat-resistant steel,a kind of precipitation strengthening material,mainly focuses on the experimental characterization of the deformation mechanism and the experimental measurement of mechanical behavior.Little work has been done on the quantitative analysis of the micromechanical response related to precipitates.In this research,the macro-and micro-mechanical properties and micro-deformation mechanisms of P91 steel were studied by experiments and simulation.Based on the crystal plasticity constitutive theory,a finite element model considering the strengthening phase and crystal structure is established to simulate the uniaxial tensile process at room temperature and high temperature,and the calculation results are compared with the insitu neutron diffraction experiment results.By using two different methods,electron backscatter diffraction(EBSD)two-dimensional real-time imaging,and threedimensional virtual modeling,the representative volume element finite element models which can reflect the two-dimensional and three-dimensional structural information of materials are established,and the macroscopic and microscopic mechanical responses of the two models are compared with the experimental results.Based on EBSD and Voronoi tessellation,a representative volume element finite element model is established,which can reflect the two-dimensional and three-dimensional microstructure information of materials,respectively.The uniaxial tensile process of P91 steel at room temperature and the high temperature is simulated by using the model.The longitudinal and transverse lattice strain evolution and lattice strain distribution of four reflections are analyzed and compared with the in-situ neutron diffraction experiment results.In addition,the extreme softening(void)and extreme hardening(rigid particles)of the precipitates are considered in this model.The evolution law of lattice strain is analyzed,and the role of precipitates in micro-deformation and micromechanical response is explored.It is found that there is a strain hardening phenomenon in the longitudinal lattice strain evolution at room temperature,and the inverse Poisson effect exists in some transverse lattice strain evolution.The precipitates in P91 heat-resistant steel is the main cause of lattice strain hardening and inverse Poisson effect,which is closely related to the redistribution of stress in precipitates and matrix when the material from elastic deformation to plastic deformation at room temperature.At high temperature,due to the softening characteristics of the material itself,stress softening phenomenon appears in the longitudinal and transverse lattice strain evolution,and the inverse Poisson effect still exists in the transverse lattice strain of some reflections.The simulation results show that the stress distribution under high temperatures still plays an important role,which can slow down the stress softening phenomenon to a certain extent.Furthermore,the distribution characteristics of lattice strain in P91 heat-resistant steel are analyzed.It is found that the lattice strain follows a standard normal distribution in the elastic deformation stage,and the distribution pattern changes with the increase of deformation.The strain distribution of longitudinal lattice appears peak width broadening at room temperature,while at high temperature,the peak width will be broadened first and then remain unchanged or refined.Positive lattice strain appears in the transverse lattice strain distribution,which proves the existence of the inverse Poisson effect.After the material enters the plastic deformation stage,the inverse Poisson effect becomes more obvious with the increase of deformation.