Search

International audience; Compared to austenitic stainless steels, largely employed in the early fission reactors, high chromium Ferritic/Martenstic (FM) steels, developed since the first half of the 20th century for fossil-fuel powerplants, have a number of advantageous properties among which lower thermal expansion, higher thermal conductivity and better void swelling resistance. At the beginning of the 1970s, FM steels found their first nuclear application as wrapper and fuel cladding materials in sodium-cooled fast reactors. They are now the reference materials for in-vessel components of future fusion reactors, and are considered for in-pile and out-of-pile applications in Generation IV reactors as well as for various other nuclear systems. In this paper, after an introductory historical overview, the challenges associated with the use of FM steels in advanced reactors are addressed, including fabrication, joining and codification issues. The long term evolution of mechanical properties such as the creep and creep-fatigue behaviors is discussed and the degradation phenomena occurring in aggressive environments (lead alloys, high temperature gases, super-critical water and CO$_2$, molten salts) aredetailed. The paper also provides a brief overview of the radiation effects in FM steels. The influence of the key radiation parameters e.g. temperature, dose and dose rate on the microstructure and mechanical properties are discussed. The need to better understand the synergistic effects of displacement damage and helium produced by transmutation in fusion conditions is highlighted.

Reduced activation ferritic/martensitic (RAFM) steels are assumed as structural material of blanket design activities. Determining the design limit is essential for the RAFM steels to qualify their use in fusion blankets. This work investigates fatigue properties of a RAFM steel, F82H at elevated temperature up to 823 K in the air and vacuum condition. The tests were carried out with axial strain controlled condition using cylindrical specimen, and the results were summarized using the Manson’s modified universal slope method, which can describe fatigue lifetime as a function of tensile properties. Although fatigue lifetime of F82H decreased with temperature, the fatigue lifetime up to 723 K fell into factor of 2 of the lifetime at ambient temperature. The lifetime at vacuum condition was successfully described using universal slope method. On the other hand, the lifetime in the air was 1/3 or shorter than that in vacuum condition. It is notable that a heat of F82H prepared for IEA round robin tests demonstrated 1/5 of shorter lifetime than latest F82H. The difference is discussed with inclusion morphologies affected by steel manufacturing process and so on. This report has been prepared as an account of work assigned to the Japanese Implementing Agency within the ‘‘Broader Approach Agreement’’ between the Government of Japan and the European Atomic Energy Community.

Reduced activation ferritic/martensitic steels (RAFMs) have been developed for the structural material of fusion reactor blanket. Extensive researches were conducted using transmission electron microscopy (TEM) to define the precipitation behavior of MX (M: Metallic elements, X: C and/or N) precipitates such as vanadium and tantalum carbides and/or nitrides, since they strongly affect material properties. However, it is not completely defined yet. TEM can obtain detailed information, but the observable volume is limited and it needs a long time to obtain representative and statistically reliable microstructural information. It is fruitful to complement TEM results applying other characterization techniques together. In this work, we studied the precipitation behavior of MX in the RAFM, F82H, through the application of various techniques with larger observable volumes and compared with TEM past results. We additionally tried to find sub-nanometric MX precipitates that can be hardly observed through TEM. From the result, the TaX (X: C and/or N) phase surely varied depending on the chemical composition of alloy and heat treatment; some TaX precipitates were unstable during tempering, MX precipitates were intergranular, and sub-nanometric MX were not found in the matrix. These obtained results were helpful to define the precipitation behavior of MX in F82H.

The research and development of nuclear fusion reactors are going on conducting to acquire a next-generation energy source. It is supposed to be that the reduced activation ferritic/martensitic (RAFM) steel F82H is adopted as a structural material of the blanket module, which is the device set on the inner wall of fusion reactors. As welding for thicker plates of F82H, multi-pass welding will be adopted to the joints. In the case of multi-pass welding, hot cracking is one of serious defects in welds and is concerned in welds reheated by following weld passes. Therefore, the objectives of this study are to evaluate hot crack susceptibility in multi-pass welds of F82H and to clarify the cause of hot cracking in multi-pass welds of F82H. The hot crack susceptibility in multi-pass welds is evaluated by the longitudinal Varestraint test with double-bead and triple-bead types. From the observation of fractured surface occurred in welds of F82H after the Varestraitnt test, the crack is identified as ductility-dip crack. The cause of hot crack in multi-pass welds of F82H is clarified by Vickers hardness test, SEM microstructure observations and X-ray diffraction pattern analyses. Based on these tests, when multi-pass welding is conducted for the F82H steels with high Ta contents, it should be considered to control welding conditions for prevention of hot cracking in weld metal as well as high strength martensitic steels. These results suggest that the cause of the crack is intragranular hardening by the precipitation of TaN during welding thermal cycles.

The purpose of the present study is to clarify the instability behavior of M23C6 under irradiation, specifically the occurrence of radiation-induced amorphization (RIA). Ion irradiation of 10.5 MeV-Fe3+ at elevated temperatures from 573 to 623 K was conducted into the reduced activation ferritic/martensitic steels (F82H) and its model alloy (Fe-8Cr-0.1C). A bilayer contrast of the particle consisting of an amorphous-rim phase and inner crystalline core of M23C6 was observed in the irradiated F82H specimen, but not in the model alloy. From the high-resolution electron microscope observation, the preferential occupation site of W into M23C6 lattice was identified as 8c-site prior to irradiation in F82H specimen, which shifted to other sites due to chemical disordering upon irradiation. Evaluation of the intensity ratio between 8c and another site of M23C6, 8c/4a, then revealed that the extent of chemical disordering of W was mitigated at the amorphous-crystal interface region in comparison with the central of the particle. The hypothesis for the formation mechanism of an amorphous-rim in M23C6 was presumed as the deviation from the stoichiometric composition at the local interface due to the irradiation-enhanced diffusion and/or ballistic mixing under the current circumstances, although the efforts from experimental and/or simulation studies are still necessary to achieve a further understanding of the RIA behavior in M23C6.

The small specimen test technique and the less-specimen-consumed test method for evaluating the fracture toughness are highly required in the nuclear field. In the present study, the fracture toughness properties of the F82H steels with the size of 1/4 CT and 1/6 CT were evaluated by the master-curve approach. Comparing the 1/4 CT and 1/6 CT data with that obtained by the 1 CT specimen, it shows no substantial effect of the specimen size on the evaluation of the fracture toughness by the master-curve approach.

The acoustic emission (AE) technique was first applied to characterize the hot isostatic pressing (HIP) joint interface of reduced activation ferritic/martensitic (RAFM) steel and its applicability for the quality assurance was assessed. The preliminary evaluation identified the characteristic AE signal related to the cracking event at the HIP joint interface: loud and instantaneous AE, which is considered as a consequence of macro cracking. However it is very hard to detect any AE signals related to initial deformation prior to the maximum torque, e.g., yielding and work hardening. In contrast, it is clarified that the cumulative AE energy is closely related to the absorbed energy by torsion. Since it is recognized that the absorbed energy by torsion was dependent on the absorbed energy by Charpy impact test, a good correlation was eventually identified among them and this will be helpful in establishment of the AE technique for the quality assurance of the HIP joint interface.

Fusion DEMO reactor requires over 11,000 tons of reduced activation ferritic/martensitic steel and it is indispensable to develop the manufacturing technology for producing large-scale components of DEMO blanket with appropriate mechanical properties. This is because mechanical properties are generally degraded with increasing production volume. In this work, we focused mechanical properties of F82H–BA12 heat which was melted in a 20 tons electric arc furnace. Plates with difference thicknesses from 18 to 100 mmt were made from its ingot through forging and hot-rolling followed by heat treatments. Tensile and Charpy impact tests were then performed on plates focusing on their homogeneity and anisotropy. From the result, their homogeneity and anisotropy were not significant. No obvious differences were observed in tensile properties between the plates with different thicknesses. However, Charpy impact property changed with increasing plate thickness, i.e. the ductile brittle transition temperature of a 100 mmt thick plate was higher than that of the other thinner plates.

In the present study, friction stir welding was employed to join F82H steels and develop a potential joining technique for a reduced activation ferritic/martensitic steel. The microstructures and mechanical properties on the joint region were investigated to evaluate the applicability of friction stir welding. F82H steel sheets were successfully butt-joined with various welding parameters. In welding conditions, 100 rpm and 100 mm/min, the stirred zone represented a comparable hardness distribution with a base metal. Stirred zone induced by 100 rpm reserved uniformly distributed precipitates and very fine ferritic grains, whereas the base metal showed a typical tempered martensite with precipitates on the prior austenite grain boundary and lath boundary. Although the tensile strength was decreased at 550 °C, the stirred zone treated at 100 rpm showed comparable tensile behavior with base metal up to 500 °C. Therefore, friction stir welding is considered a potential welding method to preserve the precipitates of F82H steel.

From the creep rupture data for the reduced activation ferritic/martensitic steel F82H, the ratio between the load in the small punch (SP) creep test and the stress in the standard creep test is estimated to be 2.1 in an argon atmosphere and 2.3 in a vacuum. However, the reasons for the difference between the values of stress conversion coefficients and the physical meaning of these values have not been clarified. In this study, stress analyses of SP creep specimens were carried out for F82H by using finite element analysis to clarify the physical meaning of the stress conversion coefficient. The friction coefficient between the ball and specimen affected the SP creep properties, and an increase in the friction coefficient resulted in an increase in the rupture time. The applied load (F) and the averaged equivalent stress in the steady state (σs) correlated linearly, and the increasing friction coefficient tended to decrease σs. It was possible to replicate the test results obtained in the argon atmosphere with a friction coefficient of 0.39 and the results obtained in the vacuum with a friction coefficient of 1.3. It was also found that the SP creep test results, which were determined by σs, agree well with the standard creep test results.

In a preliminary study carried out for evaluating creep properties with the small punch (SP) creep testing method, creep tests were performed using reduced activation ferritic/martensitic steel (RAFMs) F82H specimens both in an argon atmosphere and in vacuum (less than 10−3 Pa). The results showed that the rupture time was approximately 2.5 times longer in the argon atmosphere compared to that in the vacuum. Changes in the deflection rate with the deflection and time and changes in the minimum creep deflection rate with the rupture time were almost independent of the test atmosphere, although the minimum creep deflection rate decreased as the rupture time increased. After comparing the SP creep test results, which were sorted by using the Larson-Miller parameter, and the standard creep test results, the ratio (F/σ) between the load (F) in the SP creep test and the stress (σ) in the standard creep test was determined to be 2.1 and 2.3 for the argon atmosphere and vacuum, respectively. Consequently, it was found that the ratio, namely, the load/stress conversion coefficient, depended on the test atmosphere. This result implied that the stress and/or strain distribution on the SP specimen varied with the type of atmosphere. This change with the test atmosphere seemed to be closely associated with changes in the friction between the specimen and the ball.

Understanding the effects of helium on microstructures and mechanical properties of reduced-activation ferritic-martensitic steels is important to use of these steels in fusion reactor structures. The 9Cr-2WVTa steels were doped with 58Ni and 60Ni isotopes at 2 weight percent to control the rate of transmutation helium generation. The samples were irradiated in the High Flux Isotope Reactor to ~24 displacements per atom at nominal temperatures of 300, 400, and 500°C, producing 228 and 7 atomic parts-per-million helium in the 58Ni- and 60Ni-doped samples, respectively. Transmission electron microscopy revealed a variety of precipitates and the radiation-induced dislocation loops and cavities (voids or helium bubbles). Tensile tests of the irradiated samples at the irradiation temperatures showed radiation-induced hardening at 300°C and radiation-induced softening at 400°C. Analysis indicates that the hardening primarily originated from the loops and cavities. The 58Ni-doped samples had greater strengthening contributions from loops and cavities, leading to higher hardening with lower ductility than the 60Ni-doped samples. The greater helium production of 58Ni did not show pronounced reductions in ductility of the samples.

Reduced activation ferritic/martensitic (RAFM) steels are promising candidate for structural material of tritium breeding blanket in a fusion reactor. Database accumulation and definition of design criteria for RAFM have been intensively studied together with the progress of blanket design activities. As a part of database accumulation for fusion blanket, a RAFM steel, F82H was fatigue-tested at 573 and 673 K in the air. Axial strain-controlled fatigue tests were carried out with a cylindrical specimen with 8 mm of diameter with -1 of strain ratio condition in accordance with Japanese Industrial Standard, JIS Z 2279, “Method of high-temperature low cycle fatigue testing for metallic materials.” For high cycle tests, the maximum test cycles exceeded 106 cycles. Fatigue lifetime of F82H at temperatures ranging from room temperature to 673 K fell into the factor of 2 of the empirical fitting curve for 673 K. It was studied using experimental results that fatigue-related design limit based on RCC-MRx, such as fatigue design curves, half-life cyclic curves, and related coefficients.

Tungsten (W) is the primary candidate material as a plasma facing material in fusion devices, as for its high melting temperature, good thermal conductivity and low sputtering rate, and vacuum plasma spray (VPS) technique is preferred as it is applicable for large area without brittle interlayer, but the thermal conductivity of W layer is very poor, and easy to detach, mainly caused by its porous structure. W Friction stir processing (FSP) was applied on VPS-W to improve these poor properties, and it was suggested that FSP can contribute to significant improvement in both mechanical and thermal properties of the VPS-W coating.

The present study investigated tensile properties of F82H steel after aging at 400–650 °C for 100,000 h (100 kh), and discusses the mechanisms for change in tensile properties due to the long-term aging. Tensile strength was degraded during the aging. The degradation of yield strength and ultimate tensile strength was 50 MPa or less at 550 °C and below in both room temperature and high temperature tests. Above 550 °C, yield strength and ultimate tensile strength decreased with increasing aging temperature. On the other hand, no degradation of ductility was detected after the aging. Most of the degradation of the tensile strength could be attributed to loss of solid solution hardening by W due to precipitation of Laves phase (Fe2W) at 550 °C. However, other mechanisms, such as coarsening of martensite structure and recovery of dislocations, should be taken into account to explain additional degradation above 550 °C.

Fusion DEMO reactor requires over 11,000 tons of reduced activation ferritic/martensitic steel. It is necessary to develop the manufacturing technology for fabricating such large-scale steel with appropriate mechanical properties. In this work, we focused fundamental mechanical properties and microstructures of F82H-BA12 heat which was melted using a 20 tons electric arc furnace followed by electroslag remelting process. Its raw material of iron was blast furnace iron, because the production volume of electrolytic iron which has been used in former heats, is limited. After melting and forging, this F82H-BA12 heat was heat-treated in four different conditions to consider their fluctuations and to optimize them, and tensile and Charpy impact tests were then performed. The result of these mechanical properties were comparable to those of former F82H heats less than 5 tons which were melted applying vacuum induction melting.

Creep rupture tests near the tempering temperature were performed, and the creep behavior at high temperatures and the structures of fracture specimens were investigated. Three kinds of F82H test specimens were used: IEA-heat, mod.3, and BA07. The time-to-rupture of the BA07 specimens was the longest under all the test conditions. This was because the minimum creep rates of BA07 were smallest, and a large quantity of fine precipitates of MX from the ESR treatment were considered to be effective in providing creep resistance. Although mod.3 specimens showed a high creep resistance under high stress, the time-to-rupture of mod.3 and IEA-heat were almost the same at low stress. This was because the fine tempered martensitic structure was weakened by being subjected to a high temperature for a long period. Therefore, it is considered that a large quantity of fine MX precipitates are effective for creep resistance near the tempering temperature.

Reduced activation ferritic/martensitic steels (RAFMs), such as F82H steel, are designed to enhance the high-temperature strength by formation of MX-type nanometer-scale precipitates, mainly TaC. However, their instability under irradiation was recently reported. The purpose of this work, therefore, is to clarify the mechanism employing simultaneous observations under electron irradiation at elevated temperature in a high voltage electron microscope. In this work, Fe-0.2 wt.% TaC was fabricated as a model alloy of F82H steel. The instability of the precipitates was observed under electron irradiation at 1 MeV or above. The remarkable shrinkage and disappearance were clearly observed under irradiation with 1.5 MeV and above. On the contrary, the precipitates were mostly stable below 0.75 MeV. Two kinds of mechanism of the irradiation-induced instability were deduced from the electron-energy dependence. One is the dissolution and diffusion of tantalum from precipitates in ferrite matrix. The other is the displacements of tantalum in precipitates that introduce dissolution of Ta into matrix.

Reduced-activation ferritic/martensitic steels (RAFMs) are the primary candidate structural materials for fusion reactor blanket components. He bubbles, which formed under 14 MeV neutron irradiation, is considered to cause some mechanical property changes. In a previous study, Hasegawa et al. investigated the fracture behavior using Charpy impact test of He implanted F82H by 50 MeV α-particles with cyclotron accelerator, and the ductile brittle transition temperature (DBTT) was increased and intergranular fracture (IGF) was observed. However, the cause of the IGF was not shown in the previous study. To clarify the cause of the IGF of the He implanted F82H by 50 MeV α-particles with cyclotron accelerator, the microstructure of the He implanted F82H was investigated. After Charpy impact test at 233 K, the brittle fracture surface of the He implanted specimen was observed by SEM and TEM. By SEM observation, grain boundary surface was clearly observed from the bottom of the notch to a depth of about 400 μm. This area correspond to the He implanted region. On the other hand, at unimplanted region, river pattern was observed and transgranular fracture occurred. TEM observation revealed the He bubbles agglomeration at dislocations, lath boundaries, and grain boundaries, and the coarsening of precipitates on grain boundaries. IGF of the He implanted F82H was caused by both He bubbles and coarsening precipitates.

The present study reports the corrosion behavior of a reduced-activation ferritic martensitic steel F82H exposed to high temperature pressurized water for 28 and 100 h using a rotating disk apparatus at rotation speeds of 500 and 1000 rpm at a temperature of 573 K under a water pressure of 15 MPa with

Reduced activation ferritic/martensitic (RAFM) steels have been developed as the structural material for the fusion demonstration reactor, DEMO. These steels contain elements that produce radioactive isotopes and decay to low levels in timeframe required by the waste management scenario. Developments within the past quarter-century suggest a practical limit to the removal of undesired impurities such as Co, Cu, Ni, Mo and Nb. The concentrations of elements essential for the mechanical properties of RAFM steels, such as Al and N, required a compromise between the waste disposal scenario and performance demand. The limits and potential of RAFM steel pertaining to reducing the activation levels after service are discussed based on the actual achievements of F82H, Japanese RAFM steel, and numerical analyses of the activity. It was found that in order to achieve the shallow land burial limits 100 years after a reactor shutdown, Ni is the most significant impurity that must be removed (Mo in the case of the first wall). Limiting N below concentrations of 100 ppm will not be possible for a large scale melt, but concentrations of Al up to the maximum amount that has been achieved present no problems.

The material properties, focusing on the properties used for design analysis were re-assessed and newly investigated for various heats of reduced activation ferritic/martensitic steel, F82H. Moreover, irradiation effects on those properties were studied in this work. Most of physical properties of unirradiated F82H are insensitive to the heat-to-heat variation, and more preferable to Grade 91 specified in the ASME code. Therefore numerical analysis using data of Grade 91 can be a conservative evaluation for F82H. The change in these properties of F82H is less than 6%. Therefore the irradiation effects on thermal stress and variation of electromagnetic force in plasma disruption could be quite small in the irradiation conditions studied.

A DEMO reactor requires over 3500 tons of reduced activation ferritic/martensitic steel. To prepare such a large quantity of the material with appropriate mechanical properties, it is important to study the effect of various factors in the manufacturing process for mass production. In our work, we focused on the effects of hot forging and the cooling method after normalizing, which have not been previously studied. Plates with three different thicknesses were fabricated from slabs with two different hot forging reduction ratios, and the tensile and Charpy impact properties were evaluated for each of these plates. The plates made using a lower hot forging reduction ratio had different tensile properties, and inhomogeneity and anisotropy were observed in the Charpy impact test results. These results indicate that the hot forging operation to which the ingot is initially subjected must be sufficiently high to ensure that the appropriate mechanical properties are achieved. To test the effect of the cooling method, plates cooled in air and those quenched in water after normalizing were prepared, and tensile and Charpy impact tests were performed on these plates. No significant differences were observed indicating that air cooling is sufficient to obtain the appropriate mechanical properties.

Oxide-dispersion-strengthened (ODS) steels are attractive materials for the fuel cladding of fast reactors and the first-wall material of fusion blanket. High-chromium ferritic ODS steels have better corrosion-resistance properties, but they have poor material workability and anisotropy, making their practical application difficult. In contrast, low-chromium ferritic/martensitic ODS steels have better workability and their anisotropy can be reduced through martensitic transformation. However, their corrosion-resistance properties are poor, compared to high-chromium ferrtic ODS steels. In this work, we developed a corrosion-resistant coating technique for 8Cr ferritic/martensitic ODS steel. The ODS steel was coated with 304 or 430 stainless steel by changing the canning material from mild steel to stainless steel in the conventional material processing procedure and using it as a coating material., 著者所属: 日本原子力研究開発機構(JAEA)

A fusion reactor is expected as one of the new electric power sources in next generation. Reduced activation ferritic/martensitic steel F82H is planned to be used as a structural material for the blanket modules set on the inner wall of the reactor. However, especially in the case of laser beam welding (LBW), the weldability of the steel was not completely clarified. On the other hand, although post weld heat treatment (PWHT) should be conducted for the welds of the steel in accordance with general standards for chrome steels, the heat treatment conditions were uncertain. Therefore, adaptability of LBW as a joining method for the steel and the applicable PWHT conditions for the welded joints were investigated in this study. The effect of LBW conditions on weld penetration behavior were ascertained by observation of cross sections in the welds. The adequate PWHT conditions were confirmed in consideration of both hardness distributions measured in welds and ductile-brittle transition temperatures (DBTT) evaluated using Charpy impact test. Full penetration without weld defects such as hot cracking, porosity etc. was obtained for plates with the thickness of 4mm of the steel by control welding conditions. That means laser beam is one of useful welding heat sources to realize sound weld joints of the steel. In addition, due to select appropriate PWHT conditions, the hardness in welds was suppressed to the level of base metal and the toughness in the welded joints was improved to a practical level without the damage to base metal.

Steel plates were friction-stir-welded together under conditions in which samples were first heated above the lower critical temperature of the alloy and subsequently cooled at approximately 100 K s−1. This method produced joints with an excellent balance between tensile strength and ductility. Severe plastic deformation during friction-stir-welding stabilized the austenite phase in the steel joints. The austenite phase was subsequently transformed through deformation into the martensite phase when the joints were actually used.

The 8CrODS ferritic steel is based on J1-lot developed for the advanced fusion blanket material to increase the coolant outlet temperature. A hot-rolling was conducted at the temperature above Ar3 of 716 °C, and its effect on the microstructure and tensile strength in 8CrODS ferritic steel was evaluated, comparing together with normalized and tempered specimen. It was confirmed that hot-rolling leads to slightly increased fraction of the ferrite and highly improved tensile strength. This ferrite was formed by transformation from the hot-rolled austenite during cooling due to fine austenite grains induced by hot-rolling. The coarsening of the transformed ferrite in hot-rolled specimen can be attributed to the crystalline rotation and coalescence of the similar oriented grains. The improved strength of hot-rolled specimen was ascribed to the high dislocation density and replacement of easily deformed martensite with the transformed coarse ferrite.

This work investigates irradiation response in the joints of F82H employed for a fusion breeding blanket. The joints, which were prepared using welding and diffusion welding, were irradiated up to 6 dpa in the High Flux Isotope Reactor at the Oak Ridge National Laboratory. Post-irradiation tests revealed hardening in weldment (WM) and base metal (BM) greater than 300 MPa. However, the heat affected zones (HAZ) exhibit about half that of WM and BM. Therefore, neutron irradiation decreased the strength of the HAZ, leaving it in danger of local deformation in this region. Further the hardening in WM made with an electron beam was larger than that in WM made with tungsten inert gas welding. However the mechanical properties of the diffusion-welded joint were very similar to those of BM even after the irradiation.

Several types of reduced activation ferritic/martensitic (RAFM) steel have been developed over the past 30 years in China, Europe, India, Japan, Russia and the USA for application in ITER test blanket modules (TBMs) and future fusion DEMO and power reactors. The progress has been particularly important during the past few years with evaluation of mechanical properties of these steels before and after irradiation and in contact with different cooling media. This paper presents recent RAFM steel results obtained in ITER partner countries in relation to different TBM and DEMO options. (C) 2012 Elsevier B. V. All rights reserved.

Extraction residue analysis was conducted on reduced activation ferritic/martensitic steels, such as F82H-BA07 heat, F82H-IEA heat and JLF-1 JOYO heat. M23C6 type precipitates, TaC precipitates and Fe2W Laves phase were identified in the present analyses. Precipitation was enhanced in gauge section of creep specimens tested at 550 C under 162 MPa for 43.7 kh, and at 650 C under 35.3 MPa for 37.5 kh. Z phase (CrVN) was not detected after the creep tests and after an additional aging at 700 C for 100 h for the creep specimens.

The key issue for DEMO application is that Reduced activation ferritic/martensitic (RAFM) steels fabrication technologies has to be highly assured, especially with respect to high availability, reliability and reduced activation capability on the DEMO level fabrication, which requires not a few tons but thousand tons RAFM fabrication. One of the key technical issues of RAFM fabrication is the control of Ta, and deoxidation of the steel with a limited amount of Al addition. The series of F82H (Fe–8Cr–2W–V, Ta) melting revealed that Ta have tendency to form oxide on melting process, and this will have large impact on reliability of the steels. Al is also the key elements, as it is commonly used for deoxidation of steels, and achieving lower oxygen level is essential to obtain good mechanical properties, but the maximum concentration of Al is limited in view of reduced activation capability. These tendency and limitation resulted in the Ta oxide agglomeration in the middle of plate, but the remelting process, ESR (electro slag remelting), was found to be successful on removing those Ta oxides.

For developing the fatigue life evaluation method using small specimen, the effect of specimen size and shape on the fatigue life of the reduced activation ferritic/martensitic steels (F82H-IEA, F82H-BA07 and JLF-1) was investigated by the fatigue test at room temperature in air using round-bar and hourglass specimens with various specimen sizes (test section diameter: 0.85–10 mm). The round-bar specimen showed no specimen size and no specimen shape effects on the fatigue life, whereas the hourglass specimen showed no specimen size effect and obvious specimen shape effect on it. The shorter fatigue life of the hourglass specimen observed under low strain ranges could be attributed to the shorter micro-crack initiation life induced by the stress concentration dependent on the specimen shape. On the basis of this study, the small round-bar specimen was an acceptable candidate for evaluating the fatigue life using small specimen.

Reduced activation ferritic/martensitic steels (RAFMs), such as F82H steels, have been developed as candidates of structural materials for fusion. In the design of a fusion reactor, cooling channels are built in the first wall of the blanket. One large issue is to determine how to join rectangular tubes to thin panels to fabricate the first wall. Hot Isostatic Pressing (HIPing) is a solution to solve the issue. Because of the thin HIPed walls of the channels, the specimen size for inspection of HIPed interface is limited. In the present research, Small Specimen Test Techniques (SSTT) are screened for the destructive toughness investigation technique of HIPed F82H joints. 1/3 size Charpy V-notch impact (1/3 CVN) and small punch (SP) tests are employed for the present research. The toughness of the HIPed joints is strongly affected by various surface finishing of specimens treated previous to the HIPing. In the present research, several kinds of HIPed joints were surface finished by different methods and investigated by 1/3 CVN impact test. The HIPed F82H joints had different toughness ranging from 20% to 70% of the toughness of the F82H base metal. The SP test is also available for the investigation of toughness change by the HIPing. The sensitivity of 1/3 CVN impact test against toughness change was better than the SP test, it revealed that the SP test has some limitations.

Long-term rupture data for 79 types of heat-resistant steels including carbon steel, low-alloy steel, high-alloy steel, austenitic stainless steel, and superalloy were analyzed, and a constant for the Larson–Miller (LM) parameter was obtained in the current study for each material. The calculated LM constant, C, is approximately 20 for heat-resistant steels and alloys except for high-alloy martensitic steels with high creep resistance, for which \( C \approx 30 \). The apparent activation energy was also calculated, and the LM constant was found to be proportional to the apparent activation energy with a high correlation coefficient, which suggests that the LM constant is a material constant possessing intrinsic physical meaning. The contribution of the entropy change to the LM constant is not small, especially for several martensitic steels with large values of C. Deformation of such martensitic steels should accompany a large entropy change of 10 times the gas constant at least, besides the entropy change due to self-diffusion.

Hot isostatic pressing (HIP) is a key technology used to fabricate a first wall with cooling channels for the fusion blanket system utilizing a reduced-activation ferritic/martensitic steel. To qualify the HIPped components, small specimen test techniques are beneficial not only to evaluate the thin-wall cooling channels containing the HIP joint but also to use in neutron irradiation studies. This study aims to develop the torsion test method with special emphasis on providing a reasonable and comprehensive method to determine interfacial shear properties of HIP joints during the torsional fracture process. Torsion test results identified that the torsion process shows yield of the base metal followed by non-elastic deformation due to work hardening of the base metal. By considering this work hardening issue, we propose a reasonable and realistic solution to determine the torsional yield shear stress and the ultimate torsional shear strength of the HIPped interface. Finally, a representative torsion fracture process was identified.

Thermal aging properties of reduced activation ferritic/martensitic steel F82H was researched. The aging was performed at temperature ranging from 400 °C to 650 °C up to 100,000 h. Microstructure, precipitates, tensile properties, and Charpy impact properties were carried out on aged materials. Laves phase was found at temperatures between 550 and 650 °C and M 6 C type carbides were found at the temperatures between 500 and 600 °C over 10,000 h. These precipitates caused degradation in toughness, especially at temperatures ranging from 550 °C to 650 °C. Tensile properties do not have serious aging effect, except for 650 °C, which caused large softening even after 10,000 h. Increase of precipitates also causes some degradation in ductility, but it is not critical. Large increase in ductile-to-brittle transition temperature was observed in the 650 °C aging. It was caused by the large Laves phase precipitation at grain boundary. Laves precipitates at grain boundary also degrades the upper-shelf energy of the aged materials. These aging test results indicate F82H can be used up to 30,000 h at 550 °C.

As one of the reduced activation ferritic/martensitic steels, the weldability of thick F82H plate was experimentally examined using new heat sources in order to minimize the total heat input energy in comparison with TIG welding. A full penetration of 32 mm thick plate could be produced as a combination of a 12 mm deep first layer generated by a 10 kW fiber laser beam and upper layers deposited by a plasma MIG hybrid welding with Ar + 2%O shielding gas. Also, the effect of mechanical restraint on the weldability under EB welding of thick F82H plate was studied by using FEM to select an appropriate specimen size for the basic test. The appropriate and minimum size for the basic test of weldability under EB welding of 90 mm thick plate might be 200 mm in length and 400 mm in width where the welding length should be about 180 mm.

The status and key issues of reduced activation ferritic/martensitic (RAFM) steels R&D are reviewed as the primary candidate structural material for fusion energy demonstration reactor blankets. This includes manufacturing technology, the as-fabricated and irradiates material database and joining technologies. The review indicated that the manufacturing technology, joining technology and database accumulation including irradiation data are ready for initial design activity, and also identifies various issues that remain to be solved for engineering design activity and qualification of the material for international fusion material irradiation facility (IFMIF) irradiation experiments that will validate the data base.

Oxide dispersion strengthened (ODS) steels show a high anisotropy in their creep behavior because of the δ-ferrite grain elongated in the hot-rolled direction and the characteristic formation of creep cavities. In this work, the relationship between the δ-ferrite grain and the growth of creep cavities in 8Cr-ODS steels was investigated. The samples of two ODS steels with different δ-ferrite volume fractions were machined parallel and perpendicular to the hot-rolled direction. Creep rupture tests and interrupted tests were performed at 700 °C and about 197 MPa. Cavities formed in the martensite along δ-ferrite grains during creep deformation. The area fraction of the cavities of all specimens increased in proportion to the cube root of test time. When the volume fraction of δ-ferrite was high and δ-ferrite grains elongated parallel to the load direction, δ-ferrite then obstructed the propagation of cracks. However, when the volume fraction of δ-ferrite was low and δ-ferrite grains elongated perpendicular to the load direction, δ-ferrite grains had little effect on crack propagation.

The effect of specimen shape on the micro-crack growth behavior of F82H was investigated by the low cycle fatigue test at room temperature in air at the total strain range of 0.4–1.5% using three types of the miniature hourglass-type specimens with different stress concentration factors. In case of the high strain range conditions above 0.8%, almost no effect of the specimen shape on the fatigue life, micro-crack initiation life and the crack growth rate was observed. In case of the low strain range conditions below 0.6%, the effect of specimen shape on the fatigue life was observed. The fatigue life was considered to be dependent on the stress concentration factor. Based on the micro-crack growth analysis, the difference of the fatigue life under the low strain range conditions was attributed not to the crack growth rate but to the micro-crack initiation life dependent on the specimen shape.

Extraction residue analysis was conducted on reduced activation ferritic/martensitic steels, such as F82H-BA07 heat, F82H-IEA heat, JLF-1 JOYO heat and CLAM steel. M23C6 type precipitates, TaC precipitates and Fe2W Laves phase were identified in the present analyses. M23C6 precipitates were coarsened in F82H-BA07 compared with the other steels at as-normalized and tempered (NT) condition. TaC precipitate formation was enhanced in JLF-1 and CLAM compared with F82H-BA07 and F82H-IEA at as-NT condition. Laves phase were detected in F82H-IEA after aging above 550 °C, where solid solution W was significantly decreased. F82H-IEA exhibited hardening after aging at 400 and 500 °C for 100 khr, whereas softening at 600 and 650 °C. This behavior is similar to JLF-1 and CLAM, and can be understood by precipitation of TaC and Laves phase.

A hot isostatic press (HIP) process is a key technology to fabricate a first wall with cooling channels of the blanket system utilizing a reduced-activation ferritic/martensitic (RAFM) steel such as F82H. A primary objective of this study is to characterize interfacial properties of HIPed F82H joints by torsion to identify the feasibility of this test method for the inspection of the HIPed F82H joints. It is apparent that the absorption energies of the HIP joints varied by the processing conditions, although the maximum shear strength was not much different. The fracture surface observation study indicated that the reduction of the absorption energy obtained in the torsion tests was primarily due to the oxide formed on the interface of the HIP joint. Of particular importance, these features were very consistent with the results of Charpy impact tests. Preliminary test results then suggest that the torsion test method with a miniature specimen has a potential to evaluate the shear properties of the HIPed joint interface.

Reduced activation ferritic/martensitic (RAFM) steels are recognized as the primary candidate structural materials for fusion blanket systems. Because of the possibility of creating sound engineering bases, such as a suitable fabrication technology and a materials database, RAFM steels can be used as structural materials for pressure equipment. Further, the development of an irradiation database in addition to design methodologies for fusion-centered applications is critical when evaluating the applicability of RAFM steels as structural materials for fusion-neutron-irradiated pressure equipment. In the International Fusion Energy Research Centre (IFERC) project in the Broader Approach (BA) activities between the EU and Japan, R&D is underway to optimize RAFM steel fabrication and processing technologies, develop a method for estimating fusion-neutron-irradiation effects, and study the deformation behaviors of irradiated structures. The results of these research activities are expected to form the basis for the DEMO power plant design criteria and licensing. The objective of this paper is to review the BA R&D status of RAFM steel development in Japan, especially F82H (Fe–8Cr–2W–V, Ta). The key technical issues relevant to the design and fabrication of the DEMO blanket and the recent achievements in Japan are introduced.

The dissimilar butt welded joint of reduced-activation ferritic/martensitic steel (RAF/M) F82H and austenite stainless steel (AISI304 (SUS304)) were studied by friction stir welding. The effect of the position of the steels and tool plunging was considered in order to prohibit the mixing of the F82H and SUS304. When the dissimilar butt welding was performed such that the F82H plate was on the advancing side and the tool was plunged on the F82H side, defect-free joints could be successfully fabricated. Optical microscopy and EDX analysis were used to characterize the dissimilar joint microstructures and the interface. It was confirmed that the dissimilar joint formed no mixed structure and inter-metallic compounds.

Chemical composition analysis using inductively coupled plasma spectroscopy and phase identification using X-ray were performed on the extracted residues of 8 pct Cr martensitic steel strengthened by nanoscaled complex oxides, which consist of yttrium, titanium, and oxygen. Some Y2Ti2O7 particles, which were stable during normalizing, decomposed into Y2O3 and Ti2O3 during tempering. This reaction reversibly occurred between normalizing and tempering. Y2Ti2O7 particles formed in the steel had other constituents in solid solution as compared to the completely stoichiometric Y2Ti2O7 particles synthesized artificially in air. As for the mechanism of the decomposition of Y2Ti2O7 particles in the steel, segregation of oxygen to dislocations induced by normalizing caused the decomposition of Y2Ti2O7 during tempering. In addition to that, the interfacial strain between Y2O3 particles or Ti2O3 particles within Cr23C6 carbides, which are formed by tempering, was lower than the strain between Y2Ti2O7, which precipitated in Cr23C6 carbides or the matrix. This difference in interfacial strain could also promote the decomposition of the Y2Ti2O7 particles in the steel.

The effect of specimen size and shape on the fatigue life of reduced activation ferritic/martensitic steel “F82H IEA-heat” was investigated to develop a fatigue life evaluation method using a small specimen for testing neutron-irradiated materials. The fatigue test was carried out at room temperature in air using three kinds of round-bar specimen (minimum diameter cross sections of 1, 4, and 7 mm) and one hourglass specimen (minimum diameter cross section of 1.25 mm). The effect of specimen size on fatigue life from a level of 103 to 104 cycles was almost negligible for the round-bar specimens. The shorter fatigue life at the total strain range below 0.6% and the slightly longer fatigue life at the total strain range above 0.8% of the hourglass specimen relative to that of the standard specimen were observed from a level of 102 to 104 cycles.

ITER test blanket modules are the most important components to validate energy production and fuel breeding for future fusion demonstration reactors. Reduced activation ferritic/martensitic steel is recognized as one of the promising structural materials for the breeding blanket systems. Beryllium is a primary candidate plasma facing materials for ITER blanket. In this work, the interfacial properties of Be/reduced activation ferritic/martensitic steel (RAF/Ms) joints were investigated for the first wall of an ITER test blanket module (TBM). The joints were produced by the solid-state hot isostatic pressing (HIP) method. Chromium (Cr) was used as a diffusion barrier with a thickness of 1 μm or 10 μm, formed by plasma vapor deposition on the Be surface. The HIPping was conducted at 1023 K and 1233 K with 160 MPa of static pressure. The temperatures are standard normalizing and tempering temperatures of F82H. EPMA showed the Cr layer effectively worked as a diffusion barrier at 1023 K. However, for the F82H/Be interface which underwent HIP at 1233 K followed by tempering a Be rich layer was formed. Bend tests revealed that a thin Cr layer and low temperature HIP is preferable. The joint with a thick Cr layer suffer from brittleness of Cr itself.

The effects of small amount (1 or 2 wt.%) of Ni additionson the irradiation hardening of the reduced-activation ferritic/martensitic steel, F82H, used as fusion reactor blanket structural materials were investigated by means of Fe-ion irradiation experimental test method and nano-indentation technique. The ion-irradiation hardening of Ni-added F82H is larger than that of the steel without Ni addition. The methodology to derive the irradiation hardening of ion-irradiated F82H steel was proposed from the results of hardness depth profile.