Reduced Activation Ferritic/martensitic Research Articles

Microstructure of the tungsten and reduced activation ferritic-martensitic steel joint brazed with an Fe-based amorphous alloy

The effective joining of tungsten (W) and reduced activation ferritic-martensitic (RAFM) steels is crucial to fabrication of the divertors and first wall of future fusion reactors. In the present work, a low-activation Fe-based amorphous alloy of Fe67.8Cr11.5Si2.1B18.6 (at. %) is designed as the filler metal for brazing W and RAFM steels. Crack-free joint has been achieved with the amorphous alloy filler and a vanadium interlayer metal by short-time vacuum brazing at 1270 °C. Layer structures composed of intermetallic phases and single solid-solution phases, respectively, are alternately formed in the W/RAFM steel joint, exhibiting alternating hard and soft mechanical characteristic. Long-distance diffusion of W atoms to the steel substrate is blocked by the formation of FeW2B2 and Fe3B phases, and Si and B elements are confined within the main bonding seam of the joints. Microstructure recovery for the steel is realized following the standard heat treatment procedures. The present results suggest a promising way of making strong and tough W/RAFM joints with low-activation Fe-based amorphous alloys.

Evolution of Dislocation Loops Induced by Different Hydrogen Irradiation Conditions in Reduced-Activation Martensitic Steel.

Hydrogen can be induced in various ways into reduced-activation ferritic/martensitic (RAFM) steels when they are used as structural materials for advanced nuclear systems. However, because of the fast diffusion of hydrogen in metals, the effect of hydrogen on the evolution of irradiation-induced defects was almost neglected. In the present work, the effect of hydrogen on the evolution of dislocation loops was investigated using a transmission electron microscope. Specimens of reduced-activation ferritic/martensitic (RAFM) steels were irradiated with hydrogen ions to 5 × 1020 H+ • m−2 at 523–823 K, and to 1 × 1020 H+ • m−2 − 5 × 1020 H+ • m−2 at 723 K. The experimental results reveal that there is an optimum temperature for dislocation loop growth, which is ~723 K, and it is greater than the reported values for neutron irradiations. Surprisingly, the sizes of the loops produced by hydrogen ions, namely, 93 nm and 286 nm for the mean and maximum value, respectively, at the peak dose of 0.16 dpa under 723 K, are much larger than that produced by neutrons and heavy ions at the same damage level and temperature. The results indicate that hydrogen could enhance the growth of loops. Moreover, 47.3% a0 <111> and 52.7% a0 <100> loops were observed at 523 K, but a0 <111> loops disappeared and only a0 <100> loops existed above 623 K. Compared with the neutron and ion irradiations, the presence of hydrogen promoted the formation of a0 <100> loops.

An investigation of microstructural evolution in electron beam welded RAFM steel and 316LN SS dissimilar joint under creep loading conditions

Microstructural evolution in creep tested electron beam welded Reduced Activation Ferritic Martensitic (RAFM) steel and 316LN stainless steel dissimilar weld joints has been studied at 823 K under different stress levels. The rupture life of the weld joints spanned between 17,596 and 190 h with variation in applied stress from 160 to 220 MPa. Failure of the weld joints occurred in the RAFM steel side at all stress levels. Microstructural examination showed that the failure location shifted from the unaffected base metal region of the RAFM steel to the ‘soft region’ within the heat affected zone (HAZ) in the RAFM steel side with decrease in applied stress. The strength mismatch between various regions within the HAZ of RAFM steel and the microstructural instability of ‘soft region’ in the RAFM steel are the two important factors which dictate the failure location and rupture life of the joints.

Effects of yttrium on microstructure and properties of reduced activation ferritic-martensitic steel

ABSTRACTThis study investigates the inclusions, microstructure, tensile properties, and impact toughness of reduced activation ferritic/martensitic (RAFM) steels with different Y contents (0.006, 0.034 and 0.071 wt-%). Owing to the pinning of the grain boundary to the Y inclusions (which refines the original austenite grain size) and an existing Fe–Cr–Ta–Y–S–O phase, the tensile strength at both room and high temperatures increased with increasing Y content (below 0.071%). Increasing the Y content to 0.034 wt-% decreased the ductile-to-brittle transition temperature (DBTT). However, when the Y content reached 0.071 wt-%, the DBTT increased as the Y precipitated into blocky yttrium-rich inclusions. The microstructure, tensile properties, and impact toughness of the RAFM steel were optimised at a Y content of approximately 0.034 wt-%.

Design of Reduced Activation Ferritic/Martensitic Steels with Improved Creep Resistance by Thermodynamic Modelling

An approach based on the systematic use of thermodynamic modelling has been worked out and applied for the development of reduced activation ferritic/martensitic (RAFM) steels with improved creep resistance by composition and temperature tuning. Initially, a series of Thermo-Calc® calculations was performed to construct a database of phase equilibria. Based on this database and constraints deduced from literature, heat treatment temperatures and compositions, potentially leading to better creep resistance, were identified. The search algorithm was then fine-tuned by adding new constraints to find compositions and accompanying heat treatments that should result in the anticipated improved mechanical properties. An experimental heat resembling an optimized alloy according to the model was characterized and found to show improved creep resistance compared to the reference steel, EU97-2, thereby validating the model.

Grain size reduction strategies on Eurofer

One of the options currently taken into account for the realization of the first DEMO reactor is the “water-cooled blanket”. This option implies a minimum irradiation temperature for the blanket material in the range of 280–350 °C. In addition to the DBTT (Ductile to Brittle Transition Temperature) shift due to the DPA (displacement per atom) damage under irradiation, also the issue of the increased embrittlement due to He production must be taken into account. This issue appears even more detrimental and less manageable because the DBBT shift due to the Helium production does not saturate with the dose, as it results from previous works reported in literature. The experimental results and the difference in behaviour between ODS (Oxide Dispersion Strengthened Steels) RAFM (Reduced Activation Ferritic Martensitic) and other FM (Ferritic Martensitic) alloys (EM10, P91) showed that it is possible to improve the resistance to He embrittlement by both intra-granular precipitation of Y-Ti oxides and by decreasing the grain size at the same time. Nevertheless, anyway, the multiplication of the grain boundaries increases the dilution of He on grain surface, delaying the formation of He bubbles on grain boundaries and, therefore, the susceptibility to the He embrittlement. Several grain size reduction strategies have then been investigated on EUROFER both at the austenitization stage, on the PAGS (Prior Austenite Grain Size), and at the tempering stage, on the tempered martensite. The microstructural observations have been carried out by means of SEM (Scanning Electron Microscopy). Also the effect of grain size reduction on the toughness of the material will be taken into account; The DBTTs resulting from impact tests on KLST specimens will be shown. The outcomes of the microstructural observations, as well as the preliminary mechanical characterization (impact tests) will be discussed in this paper.

The correlation among microstructural parameter and dynamic strain aging (DSA) in influencing the mechanical properties of a reduced activated ferritic-martensitic (RAFM) steel

The additional effect of the dynamic strain aging (DSA) on the microstructural parameters in influencing the yield strength was analyzed through comparing the difference between the experimental yield strength and the calculated value at room temperature (RT) in 9Cr-1.7W-0.1C reduced activated ferritic-martensitic (RAFM) steel samples treated under two different heat treatments. Different heat treatments leaded to the variation of the microstructural parameters of this steel. Then the contributions of these microstructural parameters and DSA on the tensile properties at elevated temperatures were studied with the modified Crussard–Jaoul (C–J) analysis based on the Swift equation.

Ion-irradiation hardening of Ti/Ta-added reduced activation ferritic-martensitic steel evaluated with a new nanopillar fabrication technique

Radiation-induced hardening of newly developed Ti/Ta-added reduced activation ferritic-martensitic (RAFM) steel is evaluated by combining self-ion irradiation and cross-sectional micropillar compression tests. 500-nm micropillars were fabricated on a cross-section polished surface using argon ion beam at locations of the ion-irradiated and unirradiated layers. The compressed micropillars showed that the deformation mode and flow behavior were changed because of ion irradiation. The radiation-induced increase in yield strength was evaluated by considering the specimen size effects manifested in the micropillar compression tests. The evaluated radiation hardening of the Ti/Ta-added RAFM steel at the dose of ∼2 dpa was found to be comparable to that of the reference RAFM steel with a composition similar to Eurofer97. The size and the number density of radiation-induced dislocation loops are analyzed, and the difference between the two RAFM steels is discussed.

Designing a high Si reduced activation ferritic/martensitic steel for nuclear power generation by using Calphad method

A high Si reduced activation ferritic/martensitic (RAFM) steel for nuclear structure application is successfully designed by using Calphad method. The main designed chemical composition is C 0.18–0.22%, Cr 10.0–10.5%, W 1.0–1.5%, Si 1.0–1.3%, V+Ta 0.30–0.45%, and Fe in balance. High Si design brings excellent corrosion resistance, while low activation is advantageous in the nuclear waste processing. The experimental results indicate that the newly designed high Si RAFM steel had full martensitic structure and uniformly distributed fine second phase particles, and exhibited excellent mechanical properties and corrosion resistance. Compared to the P91 steel, this new RAFM steel designed by Calphad method is expected to be a promising candidate used in nuclear power generation, which also provides a new and effective approach to the development of RAFM steel for nuclear application.

Influence of W and effect of loading mode on the substructural evolution of reduced activation ferritic/martensitic (RAFM) steels

Influence of tungsten (W) on the evolution of substructure during creep-fatigue interaction (CFI) and effect of loading mode in terms of continuous cycling (CC) and CFI at 823 K of reduced activation ferritic/martensitic (RAFM) steels has been investigated. Increase in W from 1.4 to 2 wt% resulted in an improvement of 1.6 times of CFI life. Electron back scatter diffraction (EBSD) analysis of CFI deformed specimens demonstrated that the subgrain size was 1.8 times smaller and boundary dislocation density was 3 times higher in case of 2 wt% W steel compared to 1.4 wt% W. It can be deduced that W addition resulted in decrease in the recovery rate and enhancing the CFI life. In context to loading mode, the subgrain size was 1.5 times higher and dislocation density was 2 times lower in case of CC tested specimen with respect to CFI tested. Under CFI, the number of cycles to failure were drastically reduced due to the synergistic damage of cyclic loading and creep deformation.

Surface morphology of F82H steel exposed to low-energy D plasma at elevated temperatures

Targets of Reduced Activation Ferritic Martensitic (RAFM) steel F82H were exposed to low-energy (200 eV), high flux (about 1022 D/m2s) deuterium (D) plasma at 623–773 K to various D fluences in the range from 1 × 1025 to 2.5 × 1026 D/m2. The surface morphology of the plasma-exposed targets was examined with a field-emission scanning electron microscope. Cross-sectional observations of nano-structures formed on the F82H target surfaces were performed using a transmission electron microscope equipped with an energy dispersive X-ray spectrometer. It has been shown that nano-sized fiber-like layers are formed on the target surfaces under D plasma exposure. Micro-sized surface morphology pattern depends on the D fluence. As the D fluence increases, clusters of the fiber-like layers begin to be formed and organized into ordered structure.

Microstructure and tensile properties of laser engineered net shaped reduced activation ferritic/martensitic steel

A laser additive manufacturing technique, laser engineered net shaping (LENS), was successfully applied to manufacture a reduced activation ferritic/martensitic (RAFM) steel with nominal composition of Fe-9Cr-0.11C-1.5W-0.4Mn-0.2V-0.12Ta (wt%). The as-deposited LENS-RAFM steels were normalized and tempered. The microstructures of as-deposited and heat treated LENS-RAFM steels were characterized by using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The tensile tests of as-deposited and heat treated samples in different directions were carried out at room temperature and 873 K. The results showed that columnar dendrites grow epitaxially along the direction of deposition in vertical section (YOZ) and a mixture of equiaxed and columnar grains appears in horizontal section (XOY). No precipitates are observed in the as-deposited sample while Cr-rich M23C6 and Ta-rich MX type carbides appear in the heat treated sample. The as-deposited sample showed anisotropic tensile properties which could be eliminated by heat treatment. The tensile strength of the LENS-RAFM steel is similar to conventional RAFM steels such as EUROFER 97 and CLAM.

Aiming at understanding thermo-mechanical loads in the first wall of DEMO: Stress–strain evolution in a Eurofer-tungsten test component featuring a functionally graded interlayer

For the future fusion demonstration power plant, DEMO, several blanket designs are currently under consideration. Despite geometric and operational differences, all designs suggest a first wall (FW), in which tungsten (W) armour is joined to a structure made of Reduced Activation Ferritic Martensitic (RAFM) steel. In thermo-mechanical analyses of breeding blankets, this joint has received limited attention. In order to provide a basis for better understanding of thermally induced stresses and strains in the FW, the thermo-mechanical behaviour of a water-cooled test component is explored in the current contribution. The model aims at providing a simple geometry that allows straightforward comparison of numerical and experimental results, while trying to keep boundary conditions as realistic as possible. A test component with direct RAFM steel-W joint, and a test component with a stress-redistributing, functionally graded RAFM steel/W interlayer in the joint is considered in the current contribution. The analyses take production- and operation-related loads into account. Following a detailed analysis of the evolution of stress components and strain in the model, a parameter study with respect to geometric specifications and loads is presented.The analyses show that, even in a small test component, a direct RAFM steel-W joint causes enormous plastic deformation. The implementation of a functionally graded interlayer reduces stresses and strains significantly, but vertical normal stresses at the joint's circumference remain considerable. With the component geometry considered here, the graded interlayer should be at least 1 mm thick and contain 4 sublayers to appropriately redistribute stresses. Beyond a component width of 14 mm, stresses increase strongly, which may pose a risk to the applicability of large-scale FW components, too.

Bubble-loop complexes induced by helium irradiation and effect of pre-irradiation and post-irradiation of hydrogen in reduced-activation ferritic/martensitic steel

ABSTRACTIn the present work, the synergistic effect of high concentration hydrogen and helium on the dislocation loops and bubbles as well as their correlations in reduced-activation ferritic/martensitic (RAFM) steels is investigated. Such an effect was transmuted from 14 MeV neutron irradiation and has been one of the most challenging issues for RAFM steels for future fusion reactors. After low dose (0.18 dpa) high concentration (5000 appm) single-ion helium irradiation at 723 K, very large dislocation loops were observed, and the majority of bubbles were inside dislocation loops, forming bubble-loop complexes. These bubble-loop complexes defects were also present in hydrogen/helium and helium/hydrogen sequential-ion irradiated steels. Pre-irradiated hydrogen ion effectively inhibited the later growth of loops induced by helium post-irradiation, and the higher the ratio of hydrogen to helium fluence, the greater the effect of inhibition. At high fluence of hydrogen pre-irradiation, the structure of bubble-loop complexes disappeared. On the other hand, hydrogen post-irradiation promoted the growth of loops induced by helium pre-irradiation, and the higher the ratio of hydrogen to helium fluence, the greater the effect of promotion. The mechanisms for hydrogen/helium synergistic effects are discussed.

Evolution of creep behavior of CLAM steel after thermal aging at 550 °C

As the primary candidate structural materials for fusion energy systems, reduced activation ferritic/martensitic (RAFM) steel must to withstand high temperature for a long periods of time. The microstructure of RAFM steel would change due to thermal aging in service, and that would resulting in the change of creep behavior. The aim of this paper is to investigate change of creep behavior of CLAM steel after thermal aging at 550 °C for 2000 h and 4000 h. The results showed that the creep property degenerated obviously after thermal aging. After aging for 4000 h, the creep-rupture time of CLAM sample decreased from 202 h to 111 h and the minimum creep rate increased from 3.61 × 10−4 s−1 to 6.84 × 10−4 s−1, compared to un-aged sample. The reason for creep property degeneration was inferred to be that the solute atom in CLAM matrix decreased during the process of second phase precipitated and coarsened after thermal aging. The analysis indicated that the maximum operating temperature and rupture life of CLAM sample was decreased by the effect of thermal aging.

Thermo-hydraulic optimization study for a high heat flux unit of the K-DEMO divertor target

Exhaust the very demanding incident heat flux of about 10 ∼ 20 MW/m2 concentrated on the narrow area of the divertor target is one of the most challenging requirements in the fusion DEMO reactor. The divertor has to remove the plasma power within allowable temperature ranges of the comprising materials such as armor, heat sink, and coolant. To satisfy the requirement, the optimized design of the divertor is significant as well as the selection of the comprising materials. In the K-DEMO (Korean fusion demonstration reactor), the target peak heat flux of 10 MW/m2 was set in the steady-state operation and a water-cooled divertor concept employing the tungsten monoblock has been primarily considered. RAFM (reduced activation ferritic martensitic) steel has been considered the primary candidate as heat sink material and CuCrZr has been the second best plan in the K-DEMO divertor. The purpose of this study is to derive optimum design for two design options: RAFM and CuCrZr. To carry out the design optimization based on thermo-hydraulic and statistical analyses, input parameters such as top thickness and lateral thickness of monoblock and thickness of coolant tube and output parameters such as maximum temperatures of monoblock and coolant tube are defined. In the parameter correlation analysis, regression equations defining the relation between the input and output parameters were derived with statistical values. Finally, the optimized designs for CuCrZr and RAFM models satisfying objectives and constraints were derived by the response surfaces optimization. The optimized designs applying the derived design parameters showed the designs satisfy the thermal requirement.

Hot crack susceptibility in multi-pass welds of reduced activation ferritic/martensitic steel 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.

Effect of Ti addition on hardness change during tempering in reduced activation ferritic/martensitic (RAFM) steels

The specificities of hardness change during tempering in Ti-containing reduced activation ferritic/martensitic (RAFM) steels were found while developing new RAFM steels with improved mechanical properties. Hardness decline during tempering of RAFM steel where Ti replaced Ta (Ti-RAFM) and RAFM steel with both Ta and Ti (TaTi-RAFM) was greater than that in conventional RAFM steel with Ta (Ta-RAFM). It is believed that Ti addition tend to accelerate the growth of MX precipitates and thereby accelerated diffusion of carbon from the martensite matrix, leading to a large decrease in hardness of martensite in Ti-containing RAFM steels during tempering.

Erosion-corrosion resistance of Reduced Activation Ferritic-Martensitic steels exposed to flowing liquid Lithium

In the framework of the IFMIF (International Fusion Materials Irradiation Facility) project, a lab scale plant named Lifus 6, designed and constructed in ENEA Brasimone Centre (Italy), has been employed for the evaluation of the resistance of Reduced Activation Ferritic-Martensitic (RAFM) steels to the erosion-corrosion mechanism exerted by flowing liquid Lithium. In this experimental campaign, Eurofer 97 and F82H specimens have been subjected to three different tests, defined by a Lithium temperature of 330 °C, a Nitrogen concentration in Lithium ≤30 wt ppm, a Lithium speed on the specimens surface of 15 m/s and by a duration respectively of 1222, 2000 and 4000 h. The assessment of the tests outcome has been made on the basis of the registered mass variation of each specimen, as well as its roughness alteration and its metallographic and compositional investigation. The analyses indicate the good resistance behavior of both the materials, which are characterized by corrosion rates <0.2 μm/year, very few and isolated corrosion items and only a small Chromium and Tungsten depletion in the alloy composition at the surface.

Effect of microstructures to tensile and impact properties of stir zone on 9%Cr reduced activation ferritic/martensitic steel friction stir welds

The present study reports the microstructural evolution and mechanical properties of the stir zone (SZ) in friction stir welding (FSW) reduced activation ferritic/martensitic (RAFM) steel. The effects of the grain size, retained austenite (RA), and precipitates on the SZ tensile properties and impact toughness are discussed. The SZ of the weld exhibits a good balance of strength and ductility at 600 °C temperatures and a slightly decrease of impact toughness comparing with the base metal (BM). The RA plays a critical role in effecting the ductility and impact toughness of SZ. The influence of RA to the ductility of SZ becomes more effective as the carbon concentration of RA is increased to 1% or above where the thermal and mechanical stabilities are improved. The highly dispersed and refined MX carbonitrides and M3C carbides during FSW are good for the ductility and impact toughness of SZ as well. It indicates the SZ of FSW RAFM steel weld has the potential to have good mechanical properties.