Research On Microstructure Evolution Of Reactor Pressure Vessel Model 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 to about 0.35 wt.%,while changed the content of Mn and Ni elements.This steel is generally referred to as the pressure vessel(RPV)model steel.Smelted three different kinds of chemical 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 and the evolution process of Cu-rich phase in different crystal structures 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)During the quenching process,the addition of Mn element can effectively prevent the formation of bainitic ferrite in RPV model steel.Manganese has a significant retardation effect on formation temperature of bainitic ferrite.The higher Mn content of RPV model steel will form bcc structure of Cu cluster,and in the dark field image of periodic stripes perpendicular to the beam has the characteristics of <112> crystal orientation.The growth direction of the elliptical bcc Cu clusters are identified to be parallel to <112> directions on {110} planes of the ferrite matrix,at the same time,it is also find untransformate retained austenite with acicular in ferrite.While the lower Mn content of the RPV model steel,the B2 Cu clusters are formed,the growth direction of the needle-like Cu clusters are identified to be parallel to <001> directions on {110} planes of the ferrite matrix.The morphology and concentration of both bcc Cu and B2 Cu clusters are related to the characteristics of the initial dislocations.Furthermore,the characteristics of these Cu clusters are clearly different from those of discrete Cu precipitates which result from homogeneous precipitation.(2)Using the APT technology and getting the Mn and Ni atoms are discharged at the interface between the Cu-rich cluster and the ferrite matrix,and the Mn and Ni atoms which are discharged out of the row that are enriched in the region to form a partial polymer,while the segregation area of Mn and Ni atoms will become new nucleation sites for Cu-rich cluster.Cu-rich cluster of new enrichment formed in the growth process as well as continue to exclude Mn and Ni atoms outward,resulting in alternating high and low distribution of Cu content of Cu-rich cluster formed,and Mn or Ni atoms which excluded at the interface between the Cu-rich cluster and the ferrite matrix,that the content increases gradually with the growth of the Cu-rich cluster,further formation of the "shell" structure surround the Cu-rich cluster,at the same time,there is segregation region of Mo atoms at the Mn and Ni atoms segregation region that tend to be in the side of the ferrite matrix,which may be one of the important reasons to prevent the Cu-rich clusters grew up.(3)The morphology of the Cu-rich cluster,which are precipitated in the grain boundary,are mostly spherical or ellipsoidal,and the Cu-rich cluster precipitated in the grain are mostly rod-like by APT technology,this may be due to the precipitation of Cu-rich cluster with different ways of nucleation and growth in grain and grain boundaries.At the same time,there are a lot of parallel dislocation in the triple grain boundary,nucleation and growth of Cu-rich cluster at parallel dislocation or the region of high Mn,Ni content,which near the dislocation in the triple grain boundary,that the morphology are mainly spherical or ellipsoidal.Also,there are other coarse clusters in the triple grain boundaries and precipitation of Cu-rich cluster in the central region of these coarse clusters,it is possible that the Cu-rich cluster in this position with first nucleation,and in the process of growing up,there are other element atoms constantlysegregation in its periphery,then also formed the "shell" structure that surround the Cu-rich cluster,which result in the growth of the Cu-rich clusters is prevented.(4)The evolution of the Cu-rich phase between the different crystal structures are not dependent on its size.(5)RPV model steel aged for 11000 h at 400? and observed the Cu-rich phase precipitated begin with the bcc structure in the ferrite matrix,with the continuous growth of Cu-rich phase,the crystal structure of bcc structure is not directly transformate into the fcc structure,but the priority into the twinned 9R structure,then the 9R Cu transformed to twinned fcc structure by the glide of ±a/3 [100]9R shockley-type partial dislocations during growth of the Cu-rich phase with twinned 9R structure,finally,twins in fcc Cu precipitates disappeared to form stable fcc Cu particles by lattice relaxation,which shows relationship of K-S with matrix.(6)In addition,RPV model steel aged for 11000 h at 400? are also observed the common structure of Cu-rich phase with twinned 9R of monoclinic,but there is a serious lattice distortion and the number of stacking faults in the twinned 9R structure,which leads to higher energy in this region,and then result in local area can not maintain the typical "fringe distribution fishbone" of the Cu-rich phase,and have the trend of transiforming to multiple twinned 9R structure.It is also observed that another Cu-rich phase with twinned monoclinic 9R structure,which have untwinned monoclinic 9R structure in local region,which may be caused by the serious lattice distortion and the number of stacking faults that produce higher energy within the twinned 9R structure.(7)RPV model steel aged for 3000 h at 370? and observed the bcc structure of Cu-rich phase can be transformed into an orthogonal 9R structure,and then transformed into the orthogonal 9R structure with multiple twin relationships in the subsequent growth process.It also can be transformed directly to the lower energy of the monoclinic 9R structure.In addition,it can be transformed into the orthogonal 9R structure,and then transformed into the monoclinic structure of 9R.The new twins formed as well as the twinned boundary in the middle part of the Cu-rich phase is disconnected,when the structure of Cu-rich phase with single twinned 9R transformate to multiple twinned 9R structure.And twinned boundary will be accompanied by migratting,which leads to the change of the width of the twinned band beside the twinned boundary,and twinned boundary migration to narrow one side of twinned band,the angle between the closepacked(009)and the twinned boundary(11-4)is also changed during the process of transformation.