Heat Treatment Process Simulation And Creep Life Prediction Of High Chromium Heat Resistant Steel

Under the dual pressure of energy crisis and environmental pollution,thermal power units have been forced to develop in the direction of elevated and high pressure in order to improve power generation efficiency.For example,when supercritical conditions（530～540°C/18MPa）are increased to ultra-supercritical（above 600°C/30MPa）,the efficiency can be increased from 30～35%to 42～47%,and the carbon dioxide emissions can be reduced by 30%.Due to its excellent comprehensive performance,high chromium heat-resistant steel has become the material of choice for key components in thermal power unit equipment.However,these heat-resistant steel components will inevitably undergo creep deformation under high temperature and high pressure for a long time,until the equipment can not be used.Therefore,it is very important to study the creep behavior of heat-resistant steel.In this paper,the common high chromium heat-resistant steel P91 is used as the object of study.Firstly,its equilibrium phase diagram was obtained by thermodynamic calculation,and the optimal heat treatment temperature was analyzed.Then,through the simulation calculation of the heat treatment process,the morphology and distribution of the precipitates in the P91 steel after heat treatment were investigated.On this basis,the creep life was quantitatively predicted by using the above related parameters.The specific content is as follows:Firstly,high temperature uniaxial tensile tests were conducted on P91 steel to obtain high temperature tensile curves and related mechanical parameters such as elasticity modulus E,yield strength_0.2 and ultimate tensile strength₍（7） at 600°C.Secondly,the creep stress was selected near 0.67_0.2,and the creep endurance tests were carried out at different stress levels of 145,150,160,165,175,and 190 MPa at 600°C respectively,and the complete creep curve and relevant creep parameters were obtained.The equilibrium phase diagram of P91 steel was calculated using Mat Calc software to obtain the relationship between the phase composition and temperature of P91 steel as well as the theoretical₁ and₃ temperatures.The results show that the larger austenite phase region allows P91 steel to be normalized in a wider range,which provides a reliable basis for the optimization of relevant temperature parameters in the subsequent heat treatment process,and also provides a powerful tool for the analysis of precipitate types and elemental composition after heat treatment.After the heat treatment temperature of P91 steel was obtained from the above calculation,the heat treatment process was simulated by Matcalc software.The optimized heat treatment process was selected as follows:normalizing at 1040°C for 20min,and tempering at 770°C for 90min.The size and distribution of the precipitation phases in the P91 steel after the heat treatment were obtained by calculation;then the original samples after the heat treatment test were observed by scanning electron microscopy,and the size of precipitated phase was statistically analyzed by the software.It is found that the simulation results are consistent with the experimental results,which further verified the validity of the simulation results of the heat treatment process.Based on the strengthening mechanism and coarsening behavior of precipitate phase,the creep life prediction model of P91 steel was established based on microstructure evolution.On this basis,combined with the precipitation phase size and distribution characteristics obtained above,the model was numerically calculated and extrapolated to the condition of low stress.The results show that the predicted life under low stress conditions has a high degree of coincidence with the long-term creep life in databases such as ECCC and NIMS,and it is still very accurate even when the creep life reaches 10⁵h.