Research On Microstructure Evolution Of Low-alloy Ferritic Steel

On the basis of the content of reactor pressure vessel steel?A508-III?which is widely used in the current international.Increased Cu content of RPV model steels,and quenched for 0.5 h at 880?,then tempered for 10 h at 660?,finally aged for different time at 370?and 400?.The precipitation process of Cu-rich phase,the effect of alloying elements on the precipitation behavior of Cu-rich phase were researched in detail by field emission scanning electron microscope?SEM?,transmission electron microscope?TEM?,high resolution transmission electron microscopy?HRTEM?,energy dispersive spectrometer?EDS?and three dimensional atom probe?APT?technique.Obtained the following conclusions:?1?After aging 4000 h at 400?,the modulated structure formed by spinodal decomposition,and spinodal decomposition promoted the precipitation of?Fe,Mo?₂C carbide and Cu-rich clusters.The high Mn steel?the content of Mn is 1.4 at.%?samples had only spinodal decomposition and does not appear dissimilar atom segregation of short-range ordering;and the low Mn steel?the content of Mn is 0.66at.%?samples exist spinodal decomposition and atom segregation of short-range order two instability in the process of model transformation;occurrence of spinodal decomposition to form a C/Mo rich region,C/Mo depleted zone of lamellar alternately arranged metastable structures of residual austenite in the bainitic ferrite phase boundary C rich distribution in the aging process?Fe,Mo?C carbides and eventually into a stable structure?Fe,Mo?₂C;?2?After aging 4000 h at 400?,the impurity Cu in steel will be precipitated in the form of Cu-rich clusters,and the Cu cluster will precipitate in the lamellar?Fe,Mo?₂C carbides.And in the high Mn steel,Cu atom number density is less than the number density of Cu atom in low Mn steel,but the size of Cu-rich clusters of high Mn steel less than the high Mn steel;there is obvious segregation of Ni and Mn atoms in the carbides distributed in the lamellae alternately distributed at the interfaces of bainitic ferrite phases,besides the precipitation of Cu rich clusters.In high Mn steel,Mn and Ni atoms are mainly concentrated inside the Cu rich clusters,while in low Mn steels,Mn atoms are still concentrated inside the Cu clusters,while Ni atoms tend to segregate at the interface between Cu rich clusters and matrix.?3?Using APT technology,it is found that after high Mn Steel Aging at 370?for 4000 h,there are rich Cu clusters at the phase boundaries,and there are no Cu rich clusters in the matrix.That Cu rich clusters preferentially at the interface precipitation of cementite and grows,and the nucleation and growth driven by the phase at the interface of energy fluctuation during;rising in temperature at any time,the ageing samples of 400?in the 4000 phase boundaries have precipitated carbides and Cu rich clusters,and the formation of a"shell"structure and carbide precipitates at the interface between the Cu rich clusters with carbide,inhibit the growth of Cu rich clusters.?4?Mn rich and Ni rich clusters around Cu rich clusters of Cu rich phase nucleation point,with the aging process,Cu rich clusters in the segregation process of continuous Ni and Mn atoms at the center outward out,will eventually form rich in Mn and Ni clusters wrapped Cu rich clusters of"shell"structure.