Kazuya Miyahara

Tensile and Creep Properties of an Oxide Dispersion-Strengthened Ferritic Steel

The tensile and creep properties of two oxide dispersion-strengthened (ODS) steels with nominal compositions of Fe–12Cr–0.25Y2O3 (designated 12Y1) and Fe–12Cr–2.5W–0.4Ti–0.25Y2O3 (12YWT) were investigated. Optical microscopy, transmission electron microscopy, and atom probe field ion microscopy studies indicated that the 12YWT contained a high density of extremely fine Y–Ti–O clusters, compared to the much larger oxide particles in the 12Y1. The fine dispersion of particles gave the 12YWT better tensile and creep properties compared to commercial ODS alloys and ferritic/martensitic steels that would be replaced by the new ODS steel.

Defect and void evolution in oxide dispersion strengthened ferritic steels under 3.2 MeV Fe+ ion irradiation with simultaneous helium injection

Abstract In an attempt to explore the potential of oxide dispersion strengthened (ODS) ferritic steels for fission and fusion structural materials applications, a set of ODS steels with varying oxide particle dispersion were irradiated at 650°C, using 3.2 MeV Fe + and 330 keV He + ions simultaneously. The void formation mechanisms in these ODS steels were studied by juxtaposing the response of a 9Cr–2WVTa ferritic/martensitic steel and solution annealed AISI 316LN austenitic stainless steel under the same irradiation conditions. The results showed that void formation was suppressed progressively by introducing and retaining a higher dislocation density and finer precipitate particles. Theoretical analyses suggest that the delayed onset of void formation in ODS steels stems from the enhanced point defect recombination in the high density dislocation microstructure, lower dislocation bias due to oxide particle pinning, and a very fine dispersion of helium bubbles caused by trapping helium atoms at the particle–matrix interfaces.

Aging phenomena before the precipitation of the bulky laves phase in Fe-10%Cr ferritic alloys

The detailed study through microstructural observation on the initial stage of precipitation behavior of the Laves phase in the 9--12%Cr ferritic steels is slightly difficult, because the matrix phase is martensite containing a high number density of dislocations and, secondly, the similar size and shape of carbides are formed with the Laves phase during aging treatments. In the present research, the precipitation behavior of the Laves phase, particularly, focusing on an initial stage of it, was investigated using sample Fe-10%Cr ferritic alloys.

Effect of Oxide Species and Thermomechanical Treatments on the Strength Properties of Mechanically Alloyed Fe - 17% Cr Ferritic ODS Materials

The effects of several oxide species, such as Y203, Zr02 and MgO, and the thermomechanical treatment (TMT) after the mechanical alloying (MA) process on the strength properties of Fe-17%Cr ferritic ODS (oxide dispersion strengthening) MA materials were investigated. Y20, showed the most uniform dispersion of the finest particles among me above oxides, but the microstructural evolution during the TMT had a larger effect on the strengthening of the alloys than the fine and uniform dispersion of the Y2O3 particles had.

Effects of ε martensite and nitrogen on the damping property of high strength Fe–Cr–Mn alloys

Abstract The damping and strength property of Fe–12 mass% Cr–6 to 30 mass% Mn alloys has been investigated intending to develop a material which has high strength and high damping property. The effect of α′ (including α) and e martensite phase and γ matrix phase, which constitute the microstructure of cold rolled Fe–Cr–Mn alloys, on the damping property at room temperature has been investigated. A 50% cold worked Fe–12% Cr–22% Mn alloy indicated the largest volume fraction of e martensite phase and the highest damping property with high ultimate tensile strength of more than 1.2 GPa and 0.2% proof strength of about 0.9 GPa. Nitrogen effect on the damping property of the Fe–12% Cr–22% Mn alloys has been also investigated.

Influence of cold-work and phosphorus content on neutron-induced swelling of ternary Fe-Cr-Ni alloys

Abstract Phosphorus additions can either increase or decrease void swelling of simple ternary Fe-Cr-Ni alloys during neutron irradiation, depending on the irradiation temperature, phosphorus level and cold-work level. Phosphorus is shown in these simple alloys to exert its primary influence while in solution. Phosphide precipitation appears to play only a secondary role in void swelling. The role of cold-work is not always to suppress swelling in Fe-Cr-Ni and Fe-Cr-Ni-P alloys, however, particularly at relatively higher irradiation temperatures, where cold-working often increases swelling substantially. Coldworking also appears to alter somewhat and suppress formation of phosphide precipitates.

Improvement of high temperature strength and low temperature toughness of high manganese-chromium austenitic steels

Abstract High Mn-Cr austenitic steels are still considered to be an important high temperature structural material from the point of view of fast-induced radioactivity decay (FIRD) and non-magneticity. The objective of the present study is to investigate the mechanical properties of 12% Cr-15% Mn austenitic stainless steels and to compare these properties with those of the reference materials of JPCAs and JFMS, which are being investigated for the development of fusion reactor structural materials in Japan. The effects of the alloying elements V, Ti, Ta, etc. were investigated to determine the improvement of mechanical properties. Tiny precipitates of VN and Ti(C, N) raised the high-temperature strength considerably. Content of 0.1 to 0.2% C, however, formed very coarse precipitates of M 23 C 6 type carbide on the grain boundaries, which deteriorated low temperature toughness inducing intergranular fracture. Microstructural evolution during long-term aging was also investigated.

Influence of tungsten, carbon and nitrogen on toughness and weldability of low activation austenitic high manganese stainless steels

The effect of alloying elements of tungsten, carbon and nitrogen on high temperature strength, toughness and weldability of Fe12Cr15Mn alloy has been investigated. The high temperature stregth of Fe12Cr15Mn0.2C0.1N at 873 K increases with the addition of 2–300W without affecting ductility. The toughness as estimated by Charpy tests, is also not influenced by the addition of 2–3%W, while the increase of carbon content decreases the absorbed energy. The transition temperature shifts to higher temperature by aging at 873 K for 3600 ks, but it is still lower than room temperature. The degradation of tougheness after aging is considered to be related to the precipitation of M23C6 on grain boundaries. The weldability evaluated by hot cracking susceptibility is not affected by alloying of tungsten and carbon in this alloy system. It is noted that the alloys studied show less hot cracking susceptibility than commercial AISI 316L stainless steel.

Microstructures of neutron-irradiated Fe12CrXMn (X = 15–30) ternary alloys

Abstract The FeCrMn alloy system is being studied as an alternative to the FeCrNi system because of the desire to reduce long-term radioactivation in fusion power devices. In this study four Fe12CrXMn (X = 15, 20, 25, 30 wt.%) alloys were irradiated in FFTF to 20 dpa at 643 K and 40 dpa at 679, 793 and 873 K to investigate the influence of manganese content on void swelling and phase stability. These results confirm and expand the results of earlier studies which indicate that the FeCrMn system is relatively unstable compared to that of the FeCrNi system, with alpha and sigma phases forming as a consequence of thermal aging or high temperature irradiation.

Effects of W and Mn on the mechanical properties, microstructure and aging behaviors of 10% Cr ferritic steels considered as reduced radio-activation materials

Abstract The effects of W and Mn alloying elements on the microstructure, mechanical properties and aging behavior of 10% Cr ferritic stainless steels were investigated. Tensile strength and hardness after tempering and/or aging treatment increased gradually with increasing Mn content, but the effect of W additions was very small. Mn and W, however, had large effects on the Charpy absorbed energy. The absorbed energy after tempering and/or aging treatment decreased with increasing Mn content. But as for the effect of W, Charpy absorbed energy had maximum peaks at 1% W addition for all 10Cr-0 to 5 Mn (mass%) steels. The fracture mode of the impact test at room temperature was complex. Martensitic materials showed intergranular brittle fracture after tempering but ductile fracture after aging treatment. Dual phase material which contains a ferritic phase and a martensitic phase indicated ductile fracture after tempering, but transgranular cleavage fracture occurred after aging. It was concluded that 1% W addition would be most appropriate to 10Cr-0 to 5Mn-0.08C ferritic steels from the points of view of hardness, tensile strength, Charpy absorbed energy and fracture mode.