| The 10%Cr ferritic heat-resistant steel is often used as rotors and other essential parts for supercritical and ultra-supercritical steam turbines,which is of excellent performance,such as high temperature and pressure resistance.X12Cr Mo WVNb N10-1-1?X12?ferritic heat-resisting steel is a typical material of 10%Cr steel.Germany,Japan and other countries have conducted extensive research on the material composition,process preparation and properties of X12 steel.China has also studied it systematically.Today,our country is implementing a positive nuclear power strategy.The installed capacity of nuclear power is 28.31 million kilowatts.And The 13th Five-Year Plan states that our country will have 58 million kilowatts of nuclear power by 2020.So that nearly 30 million kilowatts is needed.Therefore,there is a great demand for core components such as nuclear steam turbine rotors.However,many important parts including nuclear turbine rotor is prone to internal stress,inclusions,cracks and other defects the surface in high-temperature forging production,due to large forgings,large internal temperature gradient.Therefore,it is necessary to study the plasticity of 10%Cr steel at elevated temperature,which can provide a theoretical basis for the forgings.It was analyzed that the influence of temperature and strain rate on the flow stress behavior of X12 ferritic heat-resistant steel in this paper.The high temperature constitutive model of X12 steel was established and modified in order to reproduce the forging process of nuclear thermal power by numerical simulation.The specific research contents can be showed as follows:?1?The temperatures and strain rates are very important to X12 ferritic heat-resistant steel.The compression experiments of different temperature?950-1250??at different strain rates(0.001-1s-1)were carried out by Glebble-3800 thermal mechanical simulator.The results showed that:At the same strain rate and strain,the flow stress of X12 steel decreases greatly with the increase of temperature.For example,when the strain rate is 0.1s-1and strain is 0.35,the peak stress of X12 steel decreases from 123.6MPa at 1050?to51.7MPa at 1250?,and the peak stress decreases by half.At the same temperature and strain,the flow stress of X12 steel increases rapidly with the strain rate increasing.When the engineering strain is 0.25 and the temperature is 1150?,the peak stress of X12 steel increases from 57.6MPa at the strain rate of 0.01 s-1to 113.8MPa at 1 s-1.?2?A two-stage model was established and the parameters were calculated.According to the characteristics of flow stress in X12 steel,the two-stage model included dynamic recovery and dynamic recrystallization were analyzed at high temperature,respectively.Based on the constitutive model proposed by Laasraoui,a two-stage hyperbolic sine model was established.And the parameters of each variable in the model were calculated by using the linear fitting and inverse analysis.In order to verify the accuracy of the model established,the two-stage constitutive model of X12 steel was compared with the experimental data.The average relative error of the value is 0.0995.This results show that the two-stage model can accurately characterize the high temperature stress curve of X12steel.?3?The ideal technological parameters were determined for high temperature forging of X12 steel and the correctness of the model was verified by numerical simulation.The macro mechanical properties and microscopic grain structure indicate that the optimal forging process of X12 steel is at strain of 0.25 and strain rate of 0.1s-1at 1150?.And the microscopic grains are distributed uniformly,the grain refinement is obvious and the average grain size is 15um at this processing condition.The average error is 0.075 between the load displacement curve obtained from the constitutive model and the experimental data.So that the two-stage model can be used to describe the constitutive model of X12steel at high temperature.And it could be also applied for the finite element simulation analysis. |
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