Effect Of Hot Working Parameters On Microstructure And Properties Of Commercial Pure Zirconium

Zirconium has a very small thermal neutron absorption cross-section,corrosion resistance,low thermal expansion coefficient,good thermal conductivity and strength,plasticity.It is widely used in the manufacture of key structural materials for the nuclear industry and chemical industry.Due to the harsh environmental conditions,the performance requirements for industrial pure zirconium materials are very high.China's zirconium resources are very abundant,but the production and processing technology of industrial zirconium products is not mature.Based on the previous research results and the characteristics of pure zirconium and the environmental requirements of the product,this paper studies the behavior of pure zirconium under different thermal deformation conditions and provides a theoretical basis for the development of pure zirconium hot rolling process.First,it is necessary to determine the thermal properties of the R60702 industrial pure zirconium plate at high temperatures,such as thermal expansion coefficient and thermal conductivity.The phase transition temperature and CCT curve are determined and referenced for the setting of hot compression and hot rolling processes.The thermal simulation of the industrial pure zirconium R60702 under 500?950? and 0.1?20 s^-1deformation condition was carried out.The flow stress of pure zirconium increases due to a decrease in temperature and an increase in strain rate under high temperatures conditions.The constitutive model of industrial pure zirconium high-temperature deformation is established.The correlation coefficient value of the calculated model is 0.9467,and the absolute average error is 8.8895,the stress value calculated by the prediction model is well fitted to the experimental value.The apparent activation energy of high-temperature thermal deformation of pure zirconium is 209 k J/mol.By analyzing flow stress and combining the microstructure under different deformation conditions,the optimal thermal deformation processing window of commercially pure zirconium is determined as deformation temperature 700?900?,strain rate 0.1?0.15s^-1,and the maximum power dissipation coefficient is reached under the 800?,0.1 s^-1condition.Combined with the microstructure observation,it is found that the main softening mechanism of industrial pure zirconium at 800? and above is geometric dynamic recrystallization.Unstable microstructures occur when the deformation temperature is below 700?.The two-stage compression test of industrial pure zirconium was carried out,and it was found that the softening degree of pure zirconium increased with the increase of residence time and deformation temperature.The static softening rate curves at different intervals of 600? and 700? are S-shaped.With the inkcrease of residence time,the rate of change of static softening rate increases first and then decreases.The softening rate at800? is higher and the microstructure after deformation has substantially completed recrystallization within 5 s.Different hot rolling process parameters and different heat treatment conditions have varied effects on the properties of pure zirconium.Combined with the microstructure and mechanical properties of the microstructure after deformation,the optimal hot rolling process parameters are as follows:heating temperature is 750?800?,the finishing temperature is 550?,the degree of deformation is 60%.The complete recrystallization is related to temperature and deformation reduction.At 540? the grains did not completely recrystallize,and when the temperature was 580?,complete recrystallization occurred in a short time but caused grain coarsening and reduced tensile properties of the plate.At 60%reduction,the optimum annealing process is 560? for 60 minutes.A recrystallization diagram of industrial pure zirconium was drawn by measuring the grain size of the microstructure under different heat treatment conditions.The recrystallized grain size is affected by the increase of the deformation amount.The larger the deformation,the weaker the influence.If the temperature is too low,the recrystallization cannot be completed in time.Excessive grain growth affects the final properties of the material.Both the heat treatment temperature and the degree of rolling deformation need to be controlled within a reasonable range to control product performance.