Evolution Of Matrix Microstructure And Carbides During Tempering Of Fe-C-W Alloy Under High Magnetic Field

The material problem is the key to realize controllable fusion energy application.Low activation martensite/ferritic steel is widely considered as the preferred structural material for future fusion demonstration reactors and the first commercial fusion power station.Its service conditions are high temperature（325～550℃）and high magnetic field（2.2～6.5 T）.The evolution of microstructure（martensite,dislocation and carbide）under the extreme service conditions of high magnetic field will seriously affect the service life of steel and the safety of fusion reactor.In this paper,a variety of microstructure detection methods（scanning electron microscopy,transmission electron microscopy,X-ray diffraction,electron backscattering diffraction,small angle neutron scattering）were used to study the tempering of low activation test steel（Fe-C-W steel）by high magnetic field at different temperatures（200℃,500℃,700℃）for different time（10 min,1 h or 2 h）influence of microstructure and alloy carbide evolution during the process.Theoretically,the modified Williamson-Hall method was extended by introducing the magnetic field factor,and the change rule of dislocation density was obtained qualitatively.When tempering at 200℃for 2 h,the high magnetic field affects dislocation density and carbide precipitation,and promotes martensite recovery.The dislocation density decreased from（2.115±0.011）x10¹⁴/m² to（1.125±0.007）x10¹⁴/m² due to the high magnetic field promoting atomic migration in the matrix and weakening the effect of Koch air mass on dislocation pinning.On the other hand,the nucleation rate of Fe₃C increased due to the high magnetic field.Its thermodynamic nature was that the magnetic field affects its magnetic free energy,thus reducing the total free energy of nucleation and the nucleation potential barrier,resulting in the number density of carbide increased from（1.13±0.02）×10⁶/mm² to（1.64±0.05）×10⁶/mm².When tempering at 500℃for 10 min and 2 h,the recovery of martensite was promoted by high magnetic field.Dislocation disassembles and merge to form sub-grain boundaries,split martensitic lath,and eventually transform into ferrite.After tempering for 10 min,lattice constant and dislocation density decreased,carbide precipitation increased,and the average grain size did not change significantly.After tempering at moderate temperature for 2 h,the lattice constant and dislocation density change trend of Fe-C-W sample was consistent with that of isothermal temperature for 10 min.The volume fraction of carbide increased from 0.16%to 0.26%,and the average radius increased from23.44 nm to 30.42 nm.When tempering at medium temperature for different times,the change of carbides with enhanced magnetic field was consistent with that of low temperature tempering.The magnetic free energy of carbides decreased with the application of high magnetic field,which led to the increase of nucleation rate and promoted the precipitation of carbides.When tempering at 700℃for 1 h and 2 h,high magnetic field to promote martensite occur recovery and recrystallization of martensite.The average grain size increases and martensite changes to ferrite structure.The edge dislocation can obtain enough energy to produce climbing,and the dislocation wall with a certain misorientation along the direction perpendicular to the slip plane can be formed by the high-density dislocation in the laths through the movement,resulting in the subcrystal with polygonal structure,and the dislocation density in the grain decreases.At the same time,dispersed carbides were precipitated in the matrix,and the size of carbides increases and the number density per unit area decreases under the application of magnetic field.When tempering at high temperature for 1 h,the grain size of carbide tends to be polarized and its growth rate was different.The small grain size grown slowly while the large grain size grown rapidly.