Force-saving Forming Method And Microstructure Controlling Of 1Cr12Ni3Mo2VN Extra-large Blade Of Nuclear Power

The ultra-large last-stage blade is the key component of the third generation nuclear power island,the greater the exhaust area of the blade,the higher the efficiency of the turbine.The high requirement of ultra-large blade is guaranteed by the inner quality,such as material quality,grain structure,forging flow line and so on.The large blades that manufactured by the forging process can meet these requirements.Therefore,the research and development of ultra-large blade forming technology is the guarantee of producing high quality blades,and it is one of the keys to the high-tech development of the steam turbine.Based on the force-saving forming method of ultra-large blade,the thermal deformation behavior of1Cr12Ni3Mo2 VN nuclear steel was studied,and the constitutive equation,dynamic recrystallization critical strain model and Yada model of dynamic recrystallization were established,and these are the bases for multi-field coupling simulation of the ultra-large blade force-saveing forming technology.An iterative algorithm of billet size for closed roll forging was proposed and the effect of multi-heating forging on microstructure was studied.The principle of force-saving forming technology was defined,and the force-saving forming method of ultra-large nuclear power blade was proposed and the technological process was determined.In the new technology,the blade body is forged by near-net-shape roll forging and the left part(root,convex and crown)were forged by die forging,and force-saving purpose is achieved by reducing the projection area of each step.The true stress increases with the increase of the strain rate when the deformation temperature is constant and this reveals that 1Cr12Ni3Mo2 VN steel is a strain rate sensitive material,the critical strain value of dynamic recrystallization increases with the increase of strain rate.At constant strain rate,the critical strain value increases as the temperature decreases.When deformed at low strain rate,the recrystallized structure is a mixed grain and inhomogeneous,and with the increase of strain rate,the recrystallization structure transfers to homogeneous and equiaxed grain.When designing the forging process,appropriate strain rate should be adopted to obtain finer and uniform recrystallized structure.At low strain rates,recrystallization nucleation takes place through grain boundary bow mechanism,and at high strain rate,recrystallization nucleation is via subgrain aggregation.Using the data at strain 0.2,the material parameters of Arrhenius constitutive equation with hyperbolic sine were calculated by linear regression method;the critical strain model and Yada model of the dynamic recrystallization were constructed by dynamic recrystallization heat compression test.The principle and the characteristics of closed roll forging of blade were analyzed.These characteristics determine that the metal flows in the length direction,and no metal flows in the width direction.To ensure that the blade does not undergo lateral bending during roll forging,the condition of equal velocity of metal flowing through each characteristic cross section is introduced,an iterative algorithm was proposed to calculate the billet size of roll forging,and the billet size was calculated by using this algorithm.The principle of choosing the diameter of the round steel bar of the large blade was determined by considering the sectional area of the root,crown and the convex of the blade.Each step of the force-saving near-net-shape technology of ultra-large blade was simulated by the multi-field coupling finite element method,and the feasibility of each step and grain structure were analyzed,the results show that the ultra-large blade can be produced by force-saving forming technology by using smaller tonnage forging device,and the blade’s size and grain structure also meet the requirements.The effect of multi-heating forging on the grain structure of 1Cr12Ni3Mo2 VN steel was studied by thermal compression.The validity and reliability of multi-field coupling simulation of the technology were verified;although the microstructure of multi-heating forging has some heredity,the final grain structure of multi-heating forging is mainly determined by the final deformation,and the experimental results show that the grade of grain at the body is 6.5and at the root is 6,which meets the requirement of grade 4.The final process deformation temperature can not be lower than 1050℃,the range of 1100~1150℃ is appropriate,the deformation should be greater than 20%.Field experiments have proved that the upsetting process and the die preforming process are feasible.The existing equipment can meet the requirements of upsetting and die preforming process,and the semi-open upsetting process will be adopted in the production.