Reduced Activation Ferritic Research Articles

Comparative assessment of a continuum damage mechanics-based creep damage models for India-specific RAFM steel

ABSTRACT In the present investigation, a comparative assessment of Kachanov-Rabotnov (KR) and Liu-Murakami (LM) Creep Damage Models is presented for India-Specific Reduced Activation Ferritic Martensitic (IN-RAFM) steel, in the framework of Continuum Damage Mechanics (CDM) approach using ABAQUS finite element analysis software. The assessment is made based on the prediction of entire creep curve and creep rupture life of IN-RAFM steel at 823 K under uniaxial tensile loading at the stress levels of 200–260 MPa. Results disclosed that LM model accurately predicts the rupture life and creep deformation in comparison to KR model and the corresponding R-square values are 0.999 (LM model) and 0.993 (KR model), respectively. The material constant q of LM model controlled the damage evolution in tertiary creep stage. In contrast, the high value of constant ∅ in damage rate equation of KR model, led to over prediction of creep strain owing to high damage rate evolution.

Innovative Isostatic Processing Technologies for Defence and other Strategic Applications

Powder Metallurgy (P/M) involving Cold Isostatic Pressing (CIP) and Hot Isostatic Pressing (HIP) techniques is the best suited processing route for manufacture of critical components from ceramics and highly complex metallic materials. The CIP followed by sintering route was adopted for the development of fused silica radomes for application in high-speed target seeking missiles. On the other hand, the HIP, a single-step processing technique has been used for the development of First-Wall (FW) of Test Blanket Modules (TBMs) from India Specific Reduced Activation Ferritic Martensitic (INRAFM) steel. The FW is an important component required for International Thermo-nuclear Experimental Reactor (ITER) application.The details of innovative processes which have been established to overcome the specific technical challenges for the development of CIP and HIP technologies for manufacture of silica radomes and INRAFM steel first-wall respectively for intended applications are highlighted in this paper.

Mechanical behavior of ion-irradiated ODS RAF steels strengthened with different types of refractory oxides

One of the most promising candidates for constructing IV generation nuclear reactors is Oxide Dispersed Strengthening (ODS) Reduced Activation Ferritic (RAF) steel. It is known that introducing refractory oxides to the ferritic matrix makes it possible to obtain a brand-new kind of materials with an excellent set of properties. In the present work, the authors focused on verifying materials' structural and mechanical properties strengthened by three different types of refractory oxides submitted to ion-irradiation. Three materials with an elemental chemical composition of 12 %Cr, 2 %W, 0.3 %Ti, and strengthened with 0.3% Y2O3 or Al2O3 or ZrO2 were produced by mechanical alloying and subsequently consolidated by Spark Plasma Sintering technique. Manufactured materials have been submitted to high energy (500 keV) Ar-ion irradiation at room temperature with three fluences (up to 5x1015 ions/cm2). This procedure allowed to generate a thin, strongly damaged zone with approximate thickness of 230 nm. SEM/EBSD and TEM bservations, Grazing Incidence X-ray Diffraction analysis, and nanoindentation tests have been included in examination of modified layers. Implementation of these techniques allowed to reveal alteration of structural and mechanical features between unmodified material and radiation-affected layer. Obtained results showed a strong correlation between the strengthening oxide and materials' microstructural and mechanical behavior under radiation damage. It has been proved that below 1x1015 ions/cm2 mechanical properties in the modified layer of all materials are very similar. Reported behavior may be related to the efficient annealing of the radiation defect process. Above this limit, significant differences between the materials are visible. It is believed that described phenomenon is directly related to the presence of the structural features and their capacity to act as defect sinks. Consequently, type of dominant mechanisms occurring in modified layer is proposed.

Benchmarking a 9Cr-2WVTa Reduced Activation Ferritic Martensitic Steel Fabricated via Additive Manufacturing

Reduced activation ferritic (RAF) martensitic steels are promising candidates for the first wall of fusion reactors. However, current manufacturing capabilities call for these components to be made by welding wrought plates. This limits design freedom and necessitates the use of post-weld heat treatments (PWHT) in accordance with the boiler and pressure vessel code. Additive manufacturing (AM) can offer a unique solution to solve this challenge by leveraging the layer-wise deposition strategy to come up with temper bead deposition techniques to eliminate post-processing heat treatments (PPHT). However, it is necessary to benchmark the properties of RAF steels fabricated by AM with their wrought counterparts to identify the process-structure-property correlation, which is the goal of this study. The study demonstrates that while tensile properties at room temperature and high temperatures are satisfactory, the as fabricated and samples after PPHT have significant heterogeneity in tensile elongation. This has been attributed to the presence of discontinuities in the build. The as-fabricated samples have an average tensile strength of 1190 + 12 MPa and an average elongation of 15 + 5% at room temperature and 658 ± 20 MPa ultimate tensile strength (UTS) and 14 ± 7% at 600 °C. After the post-weld heat treatment, mechanical properties decrease to around 600–650 MPa and an elongation between 20–25% at room temperature to 300 MPa UTS and 25–28% elongation at 600 °C. The characterization of microstructures at various length scales demonstrates that the as-fabricated structure has a significant fraction of delta ferrite in a lath martensitic matrix. No precipitates could be identified in the as-fabricated structure. PPHT led to a decrease in the area fraction of delta ferrite and precipitation of M23C6 and MX. Detailed characterization clearly demonstrates that the lack of precipitates in the as-fabricated structure could be due to the slow tempering response of the alloy. Finally, the needs to develop new alloys to achieve the objectives stated above are articulated.

Powder Particle Size Effects on Microstructure and Mechanical Properties of Mechanically Alloyed ODS Ferritic Steels

Reduced activation ferritic (RAF) steels are expected to be widely used in challenging nuclear industrial applications under severe thermo-mechanical regimes and intense neutron loads. Therefore, actual research panorama is facing the strengthening strategies necessary to maximize both performance and endurance under these conditions. Oxide dispersion strengthened (ODS) RAF steels are leader candidates as structural materials in fusion energy reactors thanks to the reinforcement obtained with a fine dispersion of nanosized oxides in their matrix. In this study, the influence of the initial powder particle size and the selected processing route on the final material has been investigated. Two RAF ODS steels coming from atomized pre-alloyed powders with nominal particle powder sizes of 70 and 30 µm and composition Fe-14Cr-2W-0.4Ti-0.3Y2O3 (wt. %) were manufactured by mechanical alloying. Alloyed powders were compacted by hot isostatic pressing, hot crossed rolled, and annealed at 1273 K. Initial powder particle size differences minimize after milling. Both steels present an almost completely recrystallized material and similar grain sizes. The same type and distributions of secondary phases, Cr-W-rich, Ti-rich, and Y-Ti oxide nanoparticles, have been also characterized by transmission electron microscopy (TEM) in both alloy samples. The strengthening effect has been confirmed by tensile and Charpy impact tests. The two alloys present similar strength values with slightly better ductile brittle transition temperature (DBTT) and ductility for the steel produced with the smaller powder size.

A Joining Process between Beryllium and Reduced-Activation Ferritic-Martensitic Steel by Plasma Sintering.

To investigate the growth kinetics of the reaction layer and mechanical strength of joined materials, we joined beryllium and reduced-activation ferritic–martensitic steel (F82H) by plasma sintering under various conditions and characterized the joined region. Scanning electron microscopy revealed that the thickness of the reaction layer increased with an increase in the joining time and temperature. Line analyses and elemental mapping using an electron microprobe analyser showed that the reaction layer consists of Be–Fe intermetallic compounds, including Be12Fe, Be5Fe, and Be2Fe, with small amounts of chromium and tungsten. Owing to the time and temperature dependence of the reaction-layer thickness, the layer growth of Be–Fe intermetallic compounds obeys the parabolic law, and the activation energy for the reaction-layer growth was 116.2 kJ/mol. The bonding strengths of the joined materials varied inversely with the thickness of the reaction layer.

Effect of thermomechanical treatment on the microstructure and mechanical properties of Si-containing oxide dispersion strengthened reduced activation ferritic steel synthesized via mechanical alloying and spark plasma sintering

Abstract The present work describes the effect of thermo-mechanical treatment on the microstructure, and its subsequent effect on the mechanical properties of Si-containing oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel. ODS-RAF steel powder mixture with a nominal composition of Fe-14Cr-2W-0.3Ti-1Si-0.3Y2O3 was mechanically alloyed up to 50 h and subsequently consolidated via spark plasma sintering (SPS) at 900 °C followed by thermo-mechanical treatment (TMT) at 850 °C. Microstructural study revealed that the sintered sample consisted of ultrafine grains (0.35 µm) along with nano-size particles (Cr2TiO4, SiO2, and Y2Ti2O7) distributed in the matrix. This sintered sample exhibited excellent mechanical properties such as Vickers hardness (600 VHN), compressive strength (1795 MPa), and total elongation (21%), which is ascribed to the presence of ultrafine grains and nano-size particles. The thermo-mechanically treated sample exhibited an increase in the density by 2.7% and a reduction in the grain size by 23%. Consequently, the Vickers microhardness, compressive strength, and failure elongation of the sintered sample improved by 13%, 37%, and 19%, respectively, upon thermo-mechanical treatment.

Effect of Si addition on the microstructure and mechanical property of nanostructured oxide dispersion strengthened ferritic steel synthesized via mechanical alloying and spark plasma sintering

Nanostructured oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel powders of two different nominal compositions, Fe-14Cr-2W-0.3Ti-0.3Y2O3 and Fe-14Cr-2W-0.3Ti-1Si-0.3Y2O3, wt.%, were synthesized by mechanical alloying and consolidated via spark plasma (SPS) sintering at different sintering temperatures (900, 1000 and 1050 °C). The effect of Si addition on the sinterability, phase composition, microstructure, and mechanical properties viz. microhardness, nanoscale hardness, and elastic modulus were investigated. The addition of 1 wt.% Si caused reduction in milling time and resulted in finer (5 ± 1.2 μm) and uniform particles. The addition of Si also led to faster reduction in crystallite size as compared to the Si-free sample. TEM analysis revealed formation of Y2Ti2O7 and Cr2O3 nano-sized particles in the Si-free ODS steel. However, formation of Cr2TiO4 and SiO2 along with Y2Ti2O7 particles were observed in Si-containing ODS steel. Hardness and Elastic modulus of Si-containing ODS steel sample sintered at 900 °C was higher than that of the samples sintered at higher sintering temperature because of fine microstructure and absence of α-Fe to γ-Fe phase transformation in 900 °C sintered sample.

Activation characteristics of candidate structural materials for a near-term Indian fusion reactor and the impact of their impurities on design considerations

Activation analyses play a vital role in nuclear reactor design. Activation analyses, along with nuclear analyses, provide important information for nuclear safety and maintenance strategies. Activation analyses also help in the selection of materials for a nuclear reactor, by providing the radioactivity and dose rate levels after irradiation. This information is important to help define maintenance activity for different parts of the reactor, and to plan decommissioning and radioactive waste disposal strategies. The study of activation analyses of candidate structural materials for near-term fusion reactors or ITER is equally essential, due to the presence of a high-energy neutron environment which makes decisive demands on material selection. This study comprises two parts; in the first part the activation characteristics, in a fusion radiation environment, of several elements which are widely present in structural materials, are studied. It reveals that the presence of a few specific elements in a material can diminish its feasibility for use in the nuclear environment. The second part of the study concentrates on activation analyses of candidate structural materials for near-term fusion reactors and their comparison in fusion radiation conditions. The structural materials selected for this study, i.e. India-specific Reduced Activation Ferritic−Martensitic steel (IN-RAFMS), P91-grade steel, stainless steel 316LN ITER-grade (SS-316LN-IG), stainless steel 316L and stainless steel 304, are candidates for use in ITER either in vessel components or test blanket systems. Tungsten is also included in this study because of its use for ITER plasma-facing components. The study is carried out using the reference parameters of the ITER fusion reactor. The activation characteristics of the materials are assessed considering the irradiation at an ITER equatorial port. The presence of elements like Nb, Mo, Co and Ta in a structural material enhance the activity level as well as the dose level, which has an impact on design considerations. IN-RAFMS was shown to be a more effective low-activation material than SS-316LN-IG.

The effect of Ar-ion irradiation on nanomechanical and structural properties of ODS RAF steels manufactured by using HIP technique

The influence of low energy ion irradiation on mechanical and structural properties of Oxide Dispersion Strengthened (ODS) Reduced Activation Ferritic (RAF) steels were investigated using Nanoindentation (NI), High Resolution Scanning Electron Microscopy (HR SEM) and Energy Dispersive X-ray Spectroscopy (EDS) techniques. ODS samples were manufactured by means of High Isostatic Pressing (HIP) technique and irradiated at room temperature with 320 keV Ar2+ ions up to fluences reaching 1 × 1016 cm−2. The results revealed fast increase of nanohardness in the function of two dependencies: (i) vanadium addition and (ii) ion fluence range. Furthermore, along with increasing vanadium content, the increase in the number of ∼100 nm sized precipitates consisting of Cr-O-V and Cr-C-V with small amounts of alloying elements (Ti, Al, W) have been observed. Simultaneously, the presence of nano-precipitates, with average size below 10 nm has been recorded. Conducted research confirms that their number decreases with increasing V content. The reported hardening effect is most probably related to: (i) quantity and size of precipitates identified as chromium-vanadium oxides and carbides with addition of alloying elements and (ii) level of damage created by ion irradiation.

Overview of LLCB TBM design and R&D activities in India

The Lead-Lithium Ceramic Breeder Test Blanket Module (LLCB-TBM) is the Indian TBM representing the DEMO breeding blanket, to be installed in ITER radial port no-2 for testing in ITER. The conceptual design of LLCB TBM systems and their engineering design are under progress. The neutronic analysis and engineering design of LLCB TBM set (TBM+TBM Shield) is undergoing optimization. The LLCB TBS auxiliary systems; Helium cooling systems, Lead-Lithium Cooling System and Tritium Extraction Systems design are conceptualized and detail design is under progress. The system arrangements in port cell area, Tokamak Cooling Water System (TCWS) vault and Tritium building along with interface requirements have been worked out within the allocated space. LLCB TBS related R&D activities are under progress at Institute for Plasma Research (IPR), Gandhinagar in association with Bhabha Atomic Research Centre (BARC), Mumbai and Indira Gandhi Centre for Atomic Research, Kalpakkam. The major research and development areas are liquidmetal technologies, lithium ceramic pebbles, lead-lithium eutectic alloy, India specific Reduced Activation Ferritic Martensitic Steels (IN-RAFMS) development and fabrication technologies development. This paper will highlight the current LLCB TBM set and axillary systems design and status of R&D activities in various areas.

Thermomechanical Fatigue Behavior of a Reduced Activation Ferritic-Martensitic Steel

Thermomechanical and isothermal fatigue behaviour of a Reduced Activation Ferritic−Martensitic (RAFM) steel have been investigated under in−phase (IP) and out−of−phase (OP) conditions using different temperature ranges. OP TMF yielded the lowest lives compared to both IP TMF and isothermal fatigue (IF) cycling at the maximum temperatures (Tmax) in all the three temperature ranges. It was thus observed that the traditional design practice that relies on IF testing at the Tmax is bound to be nonconservative for the above steel, which could lead to adverse consequences on structural integrity. The results obtained are rationalised and discussed based on the detailed microstructural observations.

Investigation on different oxides as candidates for nano-sized ODS particles in reduced-activation ferritic (RAF) steels

Future generation reactor concepts are based on materials that can stand higher temperatures and higher neutron doses in corrosive environments. Oxide dispersion strengthened steels with chromium contents ranging from 9 to 14wt.% – produced by mechanical alloying – are typical candidate materials for future structural materials in fission and fusion power plants.Y2O3 has proven to be a good candidate for addition to ferritic steels during mechanical alloying to form nano-sized dispersion oxide particles during compacting of the material. These oxide particles have many positive effects on the material such as improved high-temperature properties and higher corrosion resistance. However, there is potential for improvements by choosing different oxides.In this present work, four different oxides (MgO, La2O3, Ce2O3 and ZrO2) are selected by looking at their thermal stabilities and Gibbs free enthalpies of various chemical compositions. These oxides are mixed and mechanically alloyed with ferritic steel powder (Fe13Cr1W0.3Ti) and compared to a reference material produced with Y2O3 (Fe13Cr1W0.3Ti+Y2O3). The materials were characterized in terms of their mechanical properties and detailed microstructural investigations by transmission electron microscopy and electron backscatter diffraction.All further results of the mechanical testing and microstructural characterizations are analyzed, compared, and discussed in this paper.

Mechanical properties and thermal stability of nanostructured ODS RAF steels

The main goal of the research reported in this paper was to evaluate the microstructure and mechanical properties of an oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel after a long-term and high temperature annealing. The mechanically alloyed ODS RAF steel powder with a composition of Fe–14Cr–2W–0.3Ti–0.3Y2O3 was consolidated by hot isostatic pressing at 1150°C for 3h followed by a high speed hydrostatic extrusion at 900°C. The samples were subsequently exposed to various heat treatments, such as annealing at 850°C–1350°C for 1h in argon and to a long-term isothermal annealing at 750°C up to 10000h in air. The results indicate that due to the Zener pinning of the nanoparticles at the grain boundaries, the ODS RAF steel is thermally stable up to high annealing temperature of 1250°C. However, annealing at 1050°C applied after the hydrostatic extrusion of the steel significantly improves its ductility. Further increase in the temperature up to 1350°C resulted in nanoparticles coarsening (size ∼8.0nm, number density 2.5×023m−3) accompanied by an increase in the volume fraction of the coarse grains and a reduction of microhardness from 460 to 330HV0.1. Long-term annealing at 750°C up to 10000h brought about a systematic loss of both strength and ductility. However, the microhardness increases up to 500HV0.1. TEM observations revealed that the grain size after ageing remains almost unchanged. On the other hand, the number of dislocation free larger grains and of larger precipitates with a size of ∼25nm located at grain boundaries significantly increase.

Improvement of Impact Toughness and Creep Properties in Reduced Activation Ferritic Steels by Consumable Electrode Remelting

The effect of smelting processes on mechanical properties and microstructure of reduced activation ferritic steels was studied. Creep properties and impact toughness of reduced activation ferritic steels were obviously improved by vacuum induction melting followed by consumable electrode remelting process in comparison with the conventional vacuum induction melting process. The difference of impact toughness and creep properties between both steels mainly depended on the aspect ratio and mean size of precipitates. Decreasing the aspect ratio of carbides makes development of a shear band more difficult, which could increase impact energy and creep resistance.

Structure of complex oxide nanoparticles in a Fe–14Cr–2W–0.3Ti–0.3Y2O3 ODS RAF steel

One of the most crucial steps in the development of oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steels is the engineering of their microstructure, which includes control of the type and size of oxide nanoparticles.In this work, the composition and crystal structure of oxide particles grown in the Fe–14Cr–2W–0.3Ti–0.3Y2O3 ODS RAF steel were characterized using advanced spectroscopic and microscopic techniques. The electron energy loss spectroscopic mapping has shown presence of numerous fine Y–Ti–O oxides but also larger Cr–Ti–O and Cr–N particles among those extracted from the bulk samples. In addition, atom probe tomography of the as-compacted ODS RAF samples revealed a uniform spatial distribution of fine oxides containing mainly Y, Ti, and O. The orthorhombic YTiO3, having distorted perovskite structure, was identified in all analyzed oxides using HR-STEM and diffraction pattern analysis.

Application of master curve method to the evaluation of fracture toughness of F82H steels

Fracture toughness data was obtained for reduced-activation ferritic (RAF) steels with different sizes of specimens (1 compact tension(CT), 1/2 CT and 1/4 CT) using the master curve (MC) method in the transition temperature region. Considering the size adjustment by ASTM E1921, effects of specimen size on the fracture toughness are not observed and the reference temperature (T0) is around 164K which is similar to those (154K) of other previous studies. However, the data are not well represented by a MC, showing a rather large number of data below the lower boundary curve. Our proposed new MC was derived within the framework of the ASTM E1921 standard to apply the MC method to F82H steel. This new MC analysis can be applied to RAF steels to estimate T0 with a better description of the data scatter in the transition temperature region of fracture toughness than that of the conventional MC analysis.

Microstructural Modifications Due to Tungsten and Tantalum in 9Cr Reduced Activation Ferritic Martensitic Steels on Creep Exposure

Replacement of Mo and Nb by W and Ta in modified 9Cr-1Mo class of steels has been significantly useful for achieving reduced activity and improved mechanical properties. The addition of W and Ta strongly influences microstructure which comprises of the substructural changes and precipitation kinetics under both thermal and stress exposure. This study focusses on the effect of W and Ta on microstructural changes on thermal and creep exposure at 823K in 9Cr-W-Ta-0.2V- 0.1C steels, with W and Ta concentrations varying from 1 to 2% and 0.06 to 0.14% respectively. Increase in W from 1 to 1.4 or 2%, was beneficial to retain lath structure and retard the coarsening of M23C6 during long term thermal aging. Detailed microstructural investigation of the steesl after creep deformation showed extensive subgrain formation and coarsening of M23C6 carbides than just thermal exposure. Influence of W on subgrain size was more pronounced than Ta. Increase in W was found to be useful to control the coarsening rate of M23C6 carbides, even under stress, while MX precipitate size showed no significant change.

Characterization of precipitates in nano structured 14% Cr ODS alloys for fusion application

Oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steels, have been considered as promising materials for application in fusion power reactors up to about 750°C. Four ODS RAF steels, with compositions of Fe–13.5Cr–2W–(0–0.2–0.3–0.4)Ti–0.3Y2O3 (in wt.%) were produced by powder metallurgy technique. For the different Ti-contents, the correlation between microstructure and mechanical properties was analyzed by means of scanning electron microscope (SEM) and transmission electron microscope (TEM) equipped with energy- dispersive X-ray spectrometer (EDX) and electron energy loss spectrometer (EELS). A bimodal grain size distribution was observed in all as-hipped Ti-containing ODS alloys. These alloys consisted of coarse grains typical ranging from 1μm to 8μm and fine grains well below 1μm in diameter. The addition of Ti resulted in the formation of spherical Ti oxides rather than Cr oxides owing to the stronger affinity of Ti. The influence of Ti on particle size refinement was striking and the optimum effect was obtained when adding 0.3% Ti. Generally the hardness increased consistently with increasing in Ti content. The ODS alloying with 0.3% Ti exhibit the highest strength due to the optimum refinement of mean ODS particle size.

Effects of specimen size on fracture toughness of phosphorous added F82H steels

Reduced-activation ferritic (RAF) steels are promising candidate structural materials for fusion reactor blanket components. However, a large amount of He on grain boundaries over 550–700 appm may cause non-hardening embrittlement (NHE) on the RAF materials. Therefore, understanding the fracture toughness on the RAF steels are critical in design and operation of the reactor. In the present study, constitutive data and widely obtained fracture toughness data sets at transition temperatures of F82H are investigated to establish the master curve method for RAF steels from viewpoints of fracture mechanics. For effective production of irradiation database, small specimen tests is applied to evaluate fracture toughness properties for F82H steels with different levels of phosphorous addition so that it can simulate the effect of fracture mode change from cleavage to intergranular fracture on the shift of transition curve. In this study, effects of specimen size and phosphorous on fracture toughness are also evaluated using the different size specimen (1 CT, 1/2 CT and 1/4 CT). As a result, phosphorous added steels show intergranular fracture in a brittle manner even in temperatures of transition region. The master curve approach is used here for evaluation of fracture toughness change by the degradation of grain boundary strength based on the ASTM E1921.