Research On The Evolution Of Microstructure Of Alloy 690 In Different Processing States And Its Influence On Thermal Conductivity

The 690 alloy,a low carbon high chromium nickel base corrosion resistant alloy,is used for the manufacture of pressurized water reactor nuclear power plant steam generator heat transfer tube.The manufacturing process of 690 alloy heat transfer pipe includes cold or hot deformation and heat treatment processes such as forging,hot rolling,cold rolling,solid solution heat treatment,desensitization heat treatment.This transformation of processing state has an important i mpact on the mechanical properties and corrosion resistance of the material.A lot of systematic research work has been done in transformation of processing state,however,the influence of processing state on its thermal conductivity is rarely reported.Therefore,this paper takes 690 alloy as the research object,using Thermo-Calc thermodynamic calculation software,scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),metallographic microscope(OM),physical and chemical analysis,X-ray diffraction(XRD),etc.The influence of the microstructure evolution of 690 alloy on its thermal conductivity is expected to provide a process reference for the 690 alloy tube with excellent thermal conductivity.The main findings are as follows:(1)The cold rolled 690 alloy has the lowest thermal conductivity,the second forging state and the highest thermal conductivity in hot rolling state.During cold rolling,the increased defects in crystals caused by the higher the volume fraction of grain boundaries and dislocation density,and the enhanced the lattice distortion effect affected by the precipitation of fine and continuous M23C6 are the main factors for the decline of thermal conductivity of Alloy 690.Correspondingly,the decrease of the solid solution atomic amount caused by the precipitation of the large-sized M23C6 phase in the hot-rolled state 690 alloy structure weakens the scattering of phonons and electrons by the solid solution atoms,which is the main mechanism for the increment in thermal conductivity of the hot-rolled 690 alloy.(2)The influence of solution temperature on the thermal conductivity of 690 alloy is mainly the change of solute atomic quantity caused by the dissolution of the secondary phases.In addition,crystal defects such as volume fraction of grain boundary,dislocation density,and morphology of annealed twin crystal have less impact on its thermal conductivity.(3)After thermally treated at 715 °C,the thermal conductivity of the alloy first decreased then increased.This is because the M23C6 phase is in a coherent relationship with the matrix,and the lattice distortion is large.The thermal conductivity of the alloy decreased with the precipitation of increased amount of M23C6 phase.However,when the fine M23C6 phase become coarsened,the overall distortion is reduced and the thermal conductivity of the alloy is increased.(4)Considering the current manufacturing process and use status of the 690 alloy,it is recommended to adopt a comprehensive proce ss of controlling larger second phase sizes during hot rolling,lowering solid solution temperature and longing the time of thermally treated to obtain higher 690 alloy thermal conductivity.