Internal Friction Study On Precipitation And Re-dissolution Mechanism Of MnNiSi Phase In Reactor Pressure Vessel Steel

As a novel type of clean energy which can replace fossil energy,nuclear energy has been widely concerned by countries around the world.Vigorously developing nuclear energy has become the most promising and powerful means to solve the world energy shortage crisis.However,during the nuclear reactor operating,the reactor pressure vessel has been exposed to various harsh environment such as high temperature,high pressure,fluid erosion and long-term irradiation of high-energy particles.As the only irreplaceable important component in the nuclear reactor,the service life of the reaction pressure vessel(RPV)steel determines the safety,life and economic benefits of the whole nuclear power plant.At present,it has been shown that the precipitation of Cu-rich(CRPs)or Mn-Ni-Si-rich(MNPs)induced by irradiation/aging are the most important factors leading to the embrittlement of RPV steels.But the microphysical mechanism related to the precipitation and development of CRPs/MNPs is still controversial.Because of the diversity and complexity of the composition of industrial RPV steel,it is impractical to directly study the precipitation behavior of the second phase under the aging condition.Therefore,in this experiment,Fe-1.6% Mn-1.0% Ni-0.2% Si-0.1% Cu(wt.%)and Fe-1.6% Mn-1.0% Ni-0.2% Si(wt.%)alloys(CB and CF steels respectively)were used to systematically study the precipitation and evolution of CRPs/MNPs.In this paper,the changes of microstructure and mechanical properties of the materials after aging at different temperatures were analyzed by means of internal friction(IF),supplemented by transmission electron microscope(TEM),tensile test,Vickers hardness,nano-indentation and other testing techniques.In Chap.3,the metallography of CF steel and the phase of CB steel were selectively analyzed.With the increase of aging temperature(RT→400 oC),the grain size of CF steels grew from 55 μm and 61μm to 100 μm and 114μm respectively,then it basically stabilized at about 100~145μm and 114~140 μm.And there is no obvious phase transformation in the RPV simulated steel during thermal aging.The temperature dependenced internal friction test results show that the same grain boundary relaxation internal friction peak P1 related to the diffusion of Mn,Ni and Si atoms appears at about 645 °C in CB and CF steels.With the increase of aging temperature,the corresponding position and relaxation characteristics of P1 peak remain unchanged,while the height decreases gradually(400 °C to 500 °C),until it disappears completely(600 °C).Finally,when the aging temperature increased to 700 °C,the P1 peak reappeared.Combined with the techniques of TEM and EDS,there is no obvious CRPs/MNPs appeared in the original state.With the aging temperature increased from 400 °C to 600 °C,CRPs and MNPs particles/clusters gradually precipitated in the matrix of CB and CF steels,the size and concentration of the second phase in CB steel are obviously higher than those in CF steel.After annealing at 700 °C for 200 h,the CRPs and MNPs at the grain and grain boundaries of CB and CF steel dissolve into the matrix again.In Chap.4,the effects of thermal aging-induced precipitation and re-dissolution of CRPs/MNPs on the mechanical properties of CB and CF simulated steel are discussed.For CB steels,with the precipitation of CRPs and MNPs particles in the grain and grain boundary,the yield strength,Vickers hardness and nano-hardness in the grain show an obvious upward trend,that is,obvious hardening occurs.In CF steel,the distribution of MNPs is mainly concentrated in the grain boundary region,the yield strength increases obviously,while the nano-hardness in the grain decreases.When two kinds of RPV simulated steels were aged and annealed at 700 °C for 200 h respectively,the mechanical properties were restored to a certain extent due to the re-dissolution of CRPs/MNPs.