Kunio Onizawa

Bio: Kunio Onizawa is an academic researcher from Japan Atomic Energy Agency. The author has contributed to research in topics: Piping & Residual stress. The author has an hindex of 12, co-authored 82 publications receiving 554 citations.

Effects of chemical composition and dose on microstructure evolution and hardening of neutron-irradiated reactor pressure vessel steels

Abstract: The correlation of microstructure evolution and hardening was studied in two kinds of A533B-1 steel with high and low levels of Cu irradiated in a range of dose from 0.32 to 9.9 × 10 19 n cm −2 ( E > 1 MeV) under a high flux of about 1.7 × 10 13 n cm −2 s −1 using three-dimensional local electrode atom probe (3DAP), positron annihilation (PA) techniques, and Vickers microhardness. The early rapid hardening was found to be caused by mainly matrix defects such as mono- or di-vacancies ( V 1 − V 2 ) and/or dislocations indicated by the PA analysis. The 3DAP analysis showed that dense dispersion of dilute Cu rich clusters and lean distribution of Mn–Ni–Si rich clusters, which were identified to possess the same dislocation-pinning effect by applying a Russell and Brown model, were responsible for large and small hardening in high- and low-Cu steels irradiated above 0.59 × 10 19 n cm 2 , respectively.

Development of probabilistic fracture mechanics analysis codes for reactor pressure vessels and piping considering welding residual stress

Abstract: Probabilistic fracture mechanics (PFM) analysis codes for reactor pressure vessels (RPVs) and piping, called as PASCAL (PFM Analysis of Structural Components in Aging LWRs) series, have been developed. The PASCAL2 (PASCAL version 2) evaluates the conditional probability of fracture of an RPV under transient conditions including pressurized thermal shock (PTS) considering neutron irradiation embrittlement of the vessels. Recent improvements to PASCAL2 are related to the treatment of weld–overlay cladding. The results using the improved code indicate that the residual stress by weld–overlay cladding affects the fracture probability to some extent. The PASCAL-SP (PASCAL – Stress corrosion cracking at welding joints for Piping) evaluates the probabilities of failures including leakage and breaks of safety-related piping complying with Japanese regulation and rules. Effects of welding residual stress distribution as well as inspection accuracy are focused in this study. Residual stress distributions have been determined by parametric FEM analyses and incorporated into the code.

Microstructural changes of a thermally aged stainless steel submerged arc weld overlay cladding of nuclear reactor pressure vessels

Abstract: The effect of thermal aging on microstructural changes in stainless steel submerged arc weld-overlay cladding of reactor pressure vessels was investigated using atom probe tomography (APT) In as-received materials subjected to post-welding heat treatments (PWHTs), with a subsequent furnace cooling, a slight fluctuation of the Cr concentration was observed due to spinodal decomposition in the δ-ferrite phase but not in the austenitic phase Thermal aging at 400 °C for 10,000 h caused not only an increase in the amplitude of spinodal decomposition but also the precipitation of G phases with composition ratios of Ni:Si:Mn = 16:7:6 in the δ-ferrite phase The degree of the spinodal decomposition in the submerged arc weld sample was similar to that in the electroslag weld one reported previously We also observed a carbide on the γ-austenite and δ-ferrite interface There were no Cr depleted zones around the carbide

Effects of thermal aging on microstructure and hardness of stainless steel weld-overlay claddings of nuclear reactor pressure vessels

Abstract: The effects of thermal aging of stainless steel weld-overlay claddings of nuclear reactor pressure vessels on the microstructure and hardness of the claddings were investigated using atom probe tomography and nanoindentation testing. The claddings were aged at 400 °C for periods of 100–10,000 h. The fluctuation in Cr concentration in the δ-ferrite phase, which was caused by spinodal decomposition, progressed rapidly after aging for 100 h, and gradually for aging durations greater than 1000 h. On the other hand, NiSiMn clusters, initially formed after aging for less than 1000 h, had the highest number density after aging for 2000 h, and coarsened after aging for 10,000 h. The hardness of the δ-ferrite phase also increased rapidly for short period of aging, and saturated after aging for longer than 1000 h. This trend was similar to the observed Cr fluctuation concentration, but different from the trend seen in the formation of the NiSiMn clusters. These results strongly suggest that the primary factor responsible for the hardening of the δ-ferrite phase owing to thermal aging is Cr spinodal decomposition.

Study on microstructural changes in thermally-aged stainless steel weld-overlay cladding of nuclear reactor pressure vessels by atom probe tomography

Abstract: The effect of thermal aging on microstructural changes was investigated in stainless steel weld-overlay cladding composed of 90% austenite and 10% δ-ferrite phases using atom probe tomography (APT). In as-received materials subjected to cooling process after post-welding heat treatments (PWHT), a slight fluctuation of the Cr concentration was already observed due to spinodal decomposition in the ferrite phase but not in the austenitic phase. Thermal aging at 400 °C for 10,000 h caused not only an increase in the amplitude of spinodal decomposition but also the precipitation of G phases with composition ratios of Ni:Si:Mn = 16:7:6 in the ferrite phase. The chemical compositions of M 23 C 6 type carbides seemed to be formed at the austenite/ferrite interface were analyzed. The analyses of the magnitude of the spinodal decomposition and the hardness implied that the spinodal decomposition was the main cause of the hardening.

Ternary Fe-Cu-Ni many-body potential to model reactor pressure vessel steels: First validation by simulated thermal annealing

Abstract: In recent years, the development of atomistic models dealing with microstructure evolution and subsequent mechanical property change in reactor pressure vessel steels has been recognised as an important complement to experiments. In this framework, a literature study has shown the necessity of many-body interatomic potentials for multi-component alloys. In this paper, we develop a ternary many-body Fe–Cu–Ni potential for this purpose. As a first validation, we used it to perform a simulated thermal annealing study of the Fe–Cu and Fe–Cu–Ni alloys. Good qualitative agreement with experiments is found, although fully quantitative comparison proved impossible, due to limitations in the used simulation techniques. These limitations are also briefly discussed.

Microstructure and Local Brittle Zone Phenomena in High-Strength Low-Alloy Steel Welds

Abstract: This study is concerned with a correlation between the microstructure and the local brittle zone (LBZ) phenomena in high-strength low-alloy (HSLA) steel welds. The influence of the LBZ on toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests. Micromechanical processes involved in microvoid and cleavage microcrack formation were also identified using notched round tensile tests and subsequent scanning electron microscopy (SEM) analyses. The LBZ in the HAZ of a mUltipass welded joint is the intercritically reheated coarse-grained HAZ whose properties are strongly influenced by metallurgical factors such as an effective grain size and high-carbon martensitic islands: The experimental results indicated that Charpy energy was found to decrease monotonically with increasing the amount of martensitic islands, confirming that the martensitic island is the major microstructural factor controlling the HAZ toughness. In addition, microvoids and microcracks were found to initiate at the interface between the martensitic island and the ferrite matrix, thereby causing the reduction in toughness. These findings suggest that the LBZ phenomena in the coarse-grained HAZ can be explained by the morphology and the amount of martensitic islands.

Current understanding of radiation-induced degradation in light water reactor structural materials

Abstract: Current phenomenological knowledge and understanding of mechanisms are reviewed for radiation embrittlement of reactor pressure vessel low alloy steels and irradiation assisted stress corrosion cracking of core internals of stainless steels. Accumulated test data of irradiated materials in light water reactors and microscopic analyses by using state-of-the-art techniques such as a three-dimensional atom probe and electron backscatter diffraction have significantly increased knowledge about microstructural features. Characteristics of solute clusters and deformation microstructures and their contributions to macroscopic material property changes have been clarified to a large extent, which provide keys to understand in the degradation mechanisms. However, there are still fundamental research issues that merit study for long-term operation of reactors that requires reliable quantitative prediction of radiation-induced degradation of component materials in low-dose rate high-dose conditions.

Atom probe tomography

Abstract: Atom probe tomography (APT) provides three-dimensional compositional mapping with sub-nanometre resolution. The sensitivity of APT is in the range of parts per million for all elements, including light elements such as hydrogen, carbon or lithium, enabling unique insights into the composition of performance-enhancing or lifetime-limiting microstructural features and making APT ideally suited to complement electron-based or X-ray-based microscopies and spectroscopies. Here, we provide an introductory overview of APT ranging from its inception as an evolution of field ion microscopy to the most recent developments in specimen preparation, including for nanomaterials. We touch on data reconstruction, analysis and various applications, including in the geosciences and the burgeoning biological sciences. We review the underpinnings of APT performance and discuss both strengths and limitations of APT, including how the community can improve on current shortcomings. Finally, we look forwards to true atomic-scale tomography with the ability to measure the isotopic identity and spatial coordinates of every atom in an ever wider range of materials through new specimen preparation routes, novel laser pulsing and detector technologies, and full interoperability with complementary microscopy techniques. This Primer on atom probe tomography introduces the fundamentals of the technique and its experimental set-up, describes recent developments in specimen preparation, highlights aspects of data reconstruction and analysis, and showcases various applications of atom probe tomography in the materials sciences, geosciences and biological sciences.

Microstructure evolution and impact fracture behaviors of Z3CN20-09M stainless steels after long-term thermal aging

Abstract: Z3CN20-09M steels of primary circuit piping in Daya Bay Nuclear Power Plant were studied on the microstructure evolution and fracture behaviors after thermal aging at 400 °C for up to 20,000 h. The impact toughness of aged materials decreases a lot with aging time, and the impact fracture features change from ductile dimples into brittle cleavages in ferrite and tearing in austenite. Nano-indentation tests indicate that hardness in ferrite continuously increases with aging time. After long-term aging, ferrite decomposes into coherent Cr-rich and Fe-rich domains, and extensive G-phases precipitate homogeneously in ferrite, but no precipitate in austenite. Spinodal decomposition in ferrite leads to the thermal aging embrittlement in CASS. G-phase, with the Fm-3m space group and the lattice parameter of 1.14 nm, adopts a cube-on-cube orientation relationship with the ferrite matrix.