Akiyoshi Nomoto

Effects of aging temperature on G-phase precipitation and ferrite-phase decomposition in duplex stainless steel

G-phase precipitation and ferrite-phase decomposition in a cast duplex stainless steel (DSS) aged at 623–723 K for up to 8000 h were investigated using atom probe tomography (APT). Large sample volume was observed in every APT experiment, which yielded significantly statistical results. The number density of G-phase precipitates tended to be high and their sizes were small at lower aging temperatures. G-phase precipitates grew during prolonged isothermal aging. The concentrations of nickel, silicon, manganese and molybdenum in G-phase precipitates tended to increase as the precipitates grew. Heterogeneous distributions of alloying elements within G-phase precipitates were observed. An interesting positional relationship of G-phase precipitates with dislocations was revealed. Regarding the ferrite-phase decomposition, local chromium concentrations in the ferrite phase varied fast at higher aging temperatures. Good correlation between the variation of local chromium concentrations and aging conditions was revealed, which indicates that the variation can be estimated for arbitrary aging conditions. Representative distances between chromium-enriched and chromium-diluted regions were long at higher aging temperatures. Time exponent of the representative distances of ferrite-phase decomposition as well as the size of G-phase precipitates increased with aging temperatures.

Microstructural Characterization of RPV Materials Irradiated to High Fluences at High Flux

Understanding the embrittlement of reactor pressure vessel (RPV) steels at high fluence region is very important for the long term operation of nuclear power plants. In this study, extensive microstructural analyses were performed on the RPV steels irradiated to very high fluences beyond 1020n/cm2, E>1 MeVat high fluxes under the Pressurized Thermal Shock and Nuclear Power Plant Integrity Management projects in Japan. Three dimensional atom probe analyses were performed to characterize the solute atom cluster formation in these materials. The effects of fluence, flux, and chemical compositions on the characteristics of clusters were analyzed. The formation of dislocation loops was identified in the transmission electron microscopy analyses of high and low Cu steels, and the changes in loop size and number density with fluence were studied. P segregation on grain boundaries was also studied by surface analyses as well as grain boundary chemical analyses. We found that nonhardening embrittlement due to grain boundary fracture is not a major contributor to the embrittlement in these materials and irradiation conditions. The correlation of the microstructural changes and the Charpy transition temperature shifts was studied. The volume fraction of solute atom clusters has an excellent correlation with the transition temperature shifts. The Orowan model calculations of the contributions of dislocation loops to the transition temperature shifts show that in low Cu materials, dislocation loops may be a major contributor, but in Cu containing materials its contribution is weak. Root-sum-square of the contributions of solute atom clusters and dislocation loops seems to be a reasonable model to describe the total ΔRTNDT.

Accurate determination of the number density of G-phase precipitates in thermally aged duplex stainless steel

G-phase precipitation in the ferrite phase in thermally aged duplex stainless steel (DSS) was investigated. A needle-shaped sample of DSS aged at 673 K for 5000 h was observed by transmission electron microscopy (TEM), and subsequently by atom probe tomography (APT). The precipitates of the G-phase observed by TEM corresponded well to clustering atoms observed by APT. On the other hand, regarding the precipitates of the G-phase that formed in an earlier stage of aging, the present study suggests that not all the precipitates can be detected by TEM. A large area of DSS aged at 673 K for 5000 h containing both the ferrite and austenite phases was observed. The number density of precipitates of the G-phase in the ferrite phase was small in the vicinity of the phase boundary and increased with the distance between the phase boundary and each field of view. The number density reached an almost constant value at a distance of approximately 4 µm from the phase boundary. The suppression of G-phase precipitation in the vicinity of the phase boundary is discussed in terms of the depletion of alloying elements that comprise the G-phase.

Embrittlement Correlation Method for the Japanese Reactor Pressure Vessel Materials

A new embrittlement correlation method developed for the Japanese reactor pressure vessel (RPV) steels is presented. The Central Research Institute of Electric Power Industry and the Japanese electric utilities conducted a project to develop a new embrittlement correlation method for the Japanese RPV steels based on the understandings on the mechanisms of the RPV embrittlement. In addition to the information from the literatures, we generated new information by characterizing the microstructural changes in the surveillance materials of the Japanese commercial reactors. We found that in low Cu materials, solute atom clusters containing little or no Cu atoms are formed at relatively low fluence of 3×1019 n/cm2, E>1 MeV. The volume fraction of the solute atom clusters has a good correlation with the Charpy transition temperature shift regardless of the Cu content. We also found that the microstructure of the boiling water reactor surveillance material is different from that of the archive material irradiated in material testing reactor. The understandings on the RPV embrittlement mechanisms were formulated using a set of rate equations, and the coefficients of the equations were optimized using the ΔRTNDT values of the Japanese surveillance database. This method considers the effect of neutron flux. Only one set of coefficients was developed, and they are independent of the product form. Predictions of the new embrittlement correlation method were compared with those of the recent U.S. correlation method as well as the U.S. surveillance data. The comparison shows the characteristics of the present method.

Effects of solute elements on hardening and microstructural evolution in neutron-irradiated and thermally-aged reactor pressure vessel model alloys

ABSTRACT Nanometer-sized Cu-enriched solute clusters containing Mn, Ni, and Si atoms are considered as the primary embrittling feature in reactor pressure vessel steels. In order to understand the effects of solute atoms Mn, Ni, and Si on hardening and cluster formation, reactor pressure vessel model alloys FeCu, FeCuSi, FeCuNi, and FeCuNiMn were irradiated at 290 °C in a research reactor. Thermal ageing at 450 °C was also carried out to compare with the results in the neutron irradiation. The addition of Mn resulted in larger hardening and higher cluster number density in both thermal ageing and neutron irradiation. In FeCu0.8NiMn alloy, the size distribution of Cu-enriched clusters formed in 62-h thermal ageing (almost peak hardening) was very similar to that formed in the neutron irradiation, indicating they are on a similar growing stage. But the average Ni and Mn composition in clusters formed in neutron irradiation was higher. A good linear relationship between hardening and the square root of cluster volume fraction for both neutron irradiation and thermal ageing data was found.

Effect of Additional Irradiation at Different Fluxes on RPV Embrittlement

The effect of the neutron flux at high fluence on the microstructural and hardness changes of a reactor pressure vessel (RPV) steel was investigated. An accelerated test reactor irradiation of a RPV material, previously irradiated in commercial reactors, was carried out at the lowest possible neutron fluxes in order to obtain neutron fluences up to approximately 1×1020 n/cm2 (E>1MeV). State-of-the-art experimental techniques such as three-dimensional atom probe were applied to carry out advanced quantitative characterization of defect features in the materials. Results for the same material irradiated in both high and low flux conditions are compared. For neutron fluences above 6×1019 n/cm2 (E>1MeV) the difference in the neutron fluence dependence of the increase in hardness is not seen for any neutron flux condition. The volume fraction of solute atom clusters increases linearly with neutron fluence, and the influence of neutron flux is not significant. The component elements and the chemical composition of the solute atom clusters formed by the irradiation do not change regardless of the neutron fluence and flux. The square root of the volume fraction of the solute atom clusters is a good correlation with the increase in hardness.Copyright

Effect of Neutron Flux at High Fluence on Microstructural and Hardness Changes of RPV Steels

The effect of the neutron flux at high fluence on the microstructural and hardness changes of reactor pressure vessel (RPV) steels was investigated in succession to the previous study [1]. An accelerated test reactor irradiation of copper containing RPV materials, previously irradiated in commercial reactors, was carried out at the lowest possible neutron fluxes in order to obtain neutron fluences up to approximately 1×1020 n/cm2 (E>1MeV). State-of-the-art experimental techniques such as three-dimensional atom probe were applied to carry out advanced quantitative characterization of defect features in the materials. Results for the same materials irradiated in both high and low flux conditions are compared. For neutron fluences above 6×1019 n/cm2 (E>1MeV) the difference in the neutron fluence dependence of the increase in hardness is not seen for any neutron flux condition. The number densities and the diameters of solute atom clusters for the low flux irradiation materials tend to be lower and larger, respectively, than that for the high flux irradiation materials, while the volume fraction of solute atom clusters increases linearly with increasing neutron fluence, and the effect of neutron flux is not significant. The component elements and the chemical composition of the solute atom clusters formed by irradiation for the same material do not change regardless of the neutron fluence and flux. The square root of the volume fraction of the solute atom clusters provides a good correlation with the increase in hardness.Copyright

Characteristics of the New Embrittlement Correlation Method for the Japanese Reactor Pressure Vessel Steels

Neutron irradiation embrittlement of reactor pressure vessel steels is an important ageing issue for the long term operation of light water reactors. A new embrittlement correlation method was developed by CRIEPI and the Japanese electric utilities in 2007. This method is primarily based on the fundamental understandings on the embrittlement mechanisms: i.e. microstructural changes were modeled by the mathematical form of rate equations, and the predicted microstructural changes were further correlated with the mechanical property changes in transition temperature region. The coefficients of the rate equations were optimized using the Japanese surveillance data of RPV embrittlement. This method was adopted as the revision of the Japanese code, JEAC 4201–2007, in 2007. In this paper, after a brief explanation on the new correlation method, the predictions of the new method will be investigated through comparisons with the previous correlation, JEAC4201–2004, and the US surveillance data in order to identify the characteristics of the new method.Copyright

Microstructural Changes Related to Through-Wall Attenuation of Neutron Irradiation Embrittlement

The through-wall attenuation of neutron fluence of reactor pressure vessel (RPV) steels is often expressed using an exponential decay function based on some estimate of displacements per atom (dpa). In order to verify this function, an irradiation project was performed in which 18 layers of Charpy specimens and one central temperatue control layer were stacked in a block to simulate a 190 mm thick RPV wall. Three western-type RPV steels (medium and low copper plates and a high copper Linde 80 flux weld) were irradiated in this project. Mechanical property tests of these materials have been performed under a consortium of EPRI, CRIEPI, NRI-Rez and ATI Consulting to fully characterize the mechanical properties in terms of Charpy transition temperature and upper-shelf energy, as well as reference fracture toughness using the Master Curve. Some results have been reported at previous PVP conferences. In this paper, we report the results of microstructural characterization using three-dimensional atom probe tomography (APT) of the medium copper plate and the high copper weld metal. The microstructures obtained by APT reasonably explain the changes in mechanical properties of these materials, and the difference in the response of these materials to irradiation was also identified. The mixed effect of fluence/flux/spectrum is discussed from the microstructural point of view.Copyright

Dislocation Dynamics Simulation of Grain Boundary Effects on Yield Behavior of Metals

This paper describes dislocation dynamics simulation of grain boundary effects on yield behavior of metals, such as α-Fe bcc metal. Since the stress field arising from the grain boundary has not been well understood yet, the geometrical effect of the grain boundary can be handled in the simulation by the use of rigid boundary condition. The dislocation pileups can be observed near the grain boundary in the result of the DD simulation. And the yield stress in the crystal having the grain boundary becomes larger than that in the crystal having free surface. This result tells us that the Hall-Petch effect can actually describe well the effects of the grain boundary on the yield behavior of metals.