The Effect Of Thermal Aging On The Microstructure And Impact Property Of RPV Model Steel

Nuclear power has the features of low pollution and high efficiency. It eases theenergy crisis, and reduces the carbon emissions at the same time. Reactor pressurevessel （RPV） is nonreplaceable major component of the pressurized water reactor（PWR） in power plants. RPVs are usually made of low alloy ferritic steels and A508class3（A508-Ⅲ） steel is one type of these materials. After long-term service underthe neutron irradiation, the RPV steel becomes embrittled and the shift in theductile-to-brittle transition temperature （DBTT） which is the main parameter used tomeasure the embrittlement degree of RPV increases. The irradiation-inducedembrittlement of RPV steel is presently the main impact factor of ensuring theoperation safety and assessing operation lifetime of nuclear power plants. Theembrittlement effect of RPV steel is mainly correlated with the formation of highnumber density Cu-rich nanophases induced by the crystal defects in theneutron-irradiated steel.If the experiments were carried out in the way of neutron irradiation to investigatethe precipitation of Cu-rich nanophases in RPV steel, besides the high costs, it wouldbe very inconvenient to operate the experiments since the irradiated specimens havestrong radioactivity. The Cu-rich nanophases can also precipitate in RPV steelthermally aged at290℃if the content of residual Cu element is less than0.08wt.%,but it will take too much time more than20years. In order to reduce theexperimental period and simultaneously accelerate the precipitation of Cu-richnanophases, experiment was performed by using RPV model steel containing higherCu content than commercially available A508-â…¢ steel. The specimens of RPVmodel steel were tempered at660℃for10h followed air cooling after heattreatment at880℃for30min and water quenching, then they were isothermallyaged at370℃for different time. Several techniques （such as conventional transmission electron microscopy, high resolution transmission electron microscopy,energy spectrum, atom probe tomography and so on） were used to investigate themicrostructure, the precipitation process and structural evolution of the Cu-richnanophases, the interfaces segregation of solute or impurity atoms as well as theeffect of the precipitation of Cu-rich nanophases on the DBTT in PRV model steelduring thermal aging at370℃temperature. The following conclusions can bedrawn:（1） It is the first time to observe that the Cu atoms can segregate on the {110}planes of the α-Fe matrix in a period of two or three layers during the nucleationprocess of Cu-rich nanophase. The periodical segregation also causes the internalstress and the lattice distortion to be anisotropic. It is also observed that the Cu-richregions undergo a transformation from bcc structure to multi-twined9R structure bymeans of a shear along the {110} plane of bcc structure, while the interface betweenthe Cu-rich nanophase and the α-Fe matrix is coherent. It is firstly observed that thesame Cu-rich nanophase consists of bcc,9R orthorhombic structure and9Rmonoclinic structure segments. The coexistence of multi-structure within the sameCu-rich nanophase suggests that the structural evolution of Cu-rich nanophase is verycomplicated. Finally, the Cu-rich nanophases transformed to fcc or fct single structurefrom9R structure.（2） The segregation of solute or impurity atoms at the grain boundaries wascharacterized. The sequence of segregation tendency for different atoms from strongto weak is C> P> Si> Mn> Mo> Ni, whilst Cu atoms were clearly depleted at thegrain boundaries. There is a competitive interaction between C and P atoms that thesegregation amount of P and C atoms is inversely correlated with each other. Si atomsalso segregate to the grain boundaries, but it depends on the characteristic of the grainboundaries. The segregation of Si atoms to some grain boundary is obvious, but itdoes not to others. The C segregation range at grain boundaries is the widest.According to the width of the composition profiles at the half intensity for different atoms at the grain boundaries, the segregation range of C atoms is1.5times widerthan that of Mn. Ni and Mo atoms.（3） It is observed that the segregation characteristics of the solute or impurity atomsat the phase interfaces are different from that at the grain boundaries. Ni, Si, P atomssegregate on the interfaces between cementites and the α-Fe matrix, and Mn, Mo, Satoms enrich in the cementites. Cu atoms are ejected from the cementites, but the Cusegregation on the interface is not detected. Ni and Mn atoms evidently segregate tothe interfaces between the Cu-rich phase and the α-Fe matrix, while C, P, Mo, Siatoms prefer to segregate towards the α-Fe matrix near the interfaces, but theirsegregation amount at the interfaces of Cu-rich phase and the α-Fe matrix is less thanthat at the grain boundaries.（4） The precipitation of the Cu-rich nanophases indeed can lead to the shift ofDBTT towards higher temperature for the RPV model steel by thermal aging. TheCu-rich nanophases precipitate on dislocations in the specimen aged at370℃for3000h, and the clusters become a little coarsened when the aging time is extended to13200h. For the specimens aged for1150h, Cu-rich nanophases were on thenucleation stage assessed by TEM as well as APT analysis, and they did not have aneffect on the DBTT of the RPV model steel. For the specimens aged for3000h,Cu-rich nanophases precipitated with an average equivalent diameter of1.5nm and anumber density of4.2×10²²m^-3. and it results in the increase of the DBTT from-100℃to-60℃. For the specimens aged for13200h, Cu-rich nanophasesslightly coarsened to2.4nm of the average equivalent diameter, while the numberdensity is similar to that of the specimens aged for3000h. In this case the DBTTrose to-45℃. The Cu concentration in the α-Fe matrix for the specimen aged for13200h is still more than the limitation of Cu solubility in the α-Fe matrix at370℃.It means that the precipitation of Cu-rich nanophases does not reach the equilibriumstate. The precipitation of Cu-rich nanophases induced by thermal aging reveals asmaller impact on the DBTT than that by neutron irradiation. From the thermal aging aspect, the much lower number density of Cu-rich clusters and the absence of thedefects induced by neutron irradiation in the matrix could account for thisphenomenon.（5） The Cu-rich precipitations with the diameter larger than3nm could beextracted from the α-Fe matrix by carbon replica using the etchant of4%nitric acidalcohol solution. As to the specimens aged for different time, a different crystalstructure of bcc,9R and fcc can be observed for Cu-rich nanophases with differentsize and different Cu contents, but the crystal structure of the Cu-rich nanophasesdoes not directly depend on the size and Cu content of them. The Cu-richnanophases contain Fe, Mn and Ni with varying degree while the Cu-richnanophases are all homogeneously solid solution determined by high resolutionlattice image, which is not like the multi-phases in the bulk materials ofCu-Fe-Mn-Ni alloys.