Damage Mechanism And Crack Prediction Of 12% Cr Steel During Hot Deformation

As the core basic component of clean and efficient coal-fired power generation technology,the high-quality production of large rotor forgings of ultra supercritical units plays a vital role in saving resources,reducing emissions,protecting green ecology and improving energy utilizate efficiency.As a "bottleneck" problem,the design and core manufacturing technology of forgings has not been fully realized made in China.The common material of high and medium pressure rotor forgings is 9%-12% Cr heat-resistant steel,which is characterized by complex composition,high alloy composition,high deformation resistance,poor plasticity and narrow forging temperature window.As a result,the refinement of microstructure and cracking control in the blank making and forging process become the key problems to be solved urgently.Based on the current manufacturing problems of high and medium pressure rotor,this paper takes the ultra supercritical rotor material with 12% Cr heat-resistant steel as the research object.Based on the combination of physical simulation,numerical simulation,microstructure characterization and process test,the hot deformation characteristics of the material are studied.The grain growth model,dynamic and static recrystallization model of the material are established to clarify the refining mechanism of coarse grains in the deformation process and find out the control measures The effects of process parameters on crack initiation and crack evolution was studied by hot tensile test,the mechanism of hot deformation cracking was explored,the critical threshold of crack initiation and fracture under different deformation conditions was determined,and the crack initiation and fracture model of thermal,mechanical and microstructure ternary coupling was established.Through the above research,it is expected to make a breakthrough in the formation mechanism and process control of hot forging defects of heat-resistant steel,and provide theoretical support for the independent manufacturing of ultra supercritical rotor forgings in China.The hot deformation mechanical behavior and deformation mechanism of12% Cr ultra supercritical rotor steel under simple loading path were studied by hot simulation compression test.Different true stress-strain curves of 12%Cr steel in cast and forged state were obtained.Based on these curves,the thermodynamic parameters related to cast and forged 12%Cr steel were calculated in uniaxial isothermal compression test The two-stage physical constitutive model of rotor steel in work hardening dynamic recovery(hw-drv)and dynamic recrystallization(DRX)was established and modified.Based on the hot tensile test and microstructure observation,the stress-strain curves of as forged and as cast 12% Cr ultra supercritical rotor steel at different high temperature tensile strain rates were obtained.The effects mechanism of process parameters on the tensile curves was analyzed,and the factors influencing the crack initiation and propagation of the steel were discussed.By means of TEM(transmission electron microscope)analysis,the precipitated phase structure in 12% Cr steel was investigated,and the mechanism of crack initiation and propagation was clarified.Combined with thermal simulation experiment,numerical simulation and the shrinkage ratio,temperature and strain rate on the thermal deformation are analyzed the influence of the damage behavior,based on the strain accumulation peak strain and fracture strain of 12% Cr steel calculation formula,process of setting up the damage model,established the damage evolution model,the thermal deformation of 12% Cr steel in combination with numerical simulation and the shrinkage ratio test,The critical shape variables of damage and cracking of cast and forged 12Cr% steel were determined in the actual forging condition.The model was verified by upsetting experiments under different deformation conditions.The prediction accuracy of the model for cracking under high temperature deformation of 12%Cr steel was verified,which provides a theoretical basis for cracking prediction and control in practical production.