Reduced Activation Ferritic Martensitic Steel Research Articles

Irradiation Effects on Tensile Properties of Reduced Activation Ferritic/Martensitic Steel: A Micromechanical-Damage-Model-Based Numerical Investigation

The tensile properties of reduced activation ferritic/martensitic (RAFM) steels are significantly influenced by neutron irradiation. Here, a mechanism-based model taking account of the typical ductile damage process of void nucleation, growth, and coalescence was used to study the temperature and irradiation effects. The elastic–plastic response of RAFM steels irradiated up to 20 dpa was investigated by applying the GTN model coupled with different work hardening models. Through a numerical study of tensile curves, the GTN parameters were identified reasonably and satisfying simulation results were obtained. A combination of Swift law and Voce law was used to define the flow behavior of irradiated RAFM steels. The deformation localization could be adjusted effectively via setting the nucleation parameter εn close to the strain where necking occurs. Because εn changed with uniform elongation, εn decreased with the testing temperature and rose with an irradiation temperature above 300 °C. The nucleation parameter fn increased with the testing temperature for RAFM steels before irradiation. For irradiated RAFM steels, fn barely changed when the irradiation temperature was below 300 °C and then it rose at a higher irradiation temperature. Meanwhile, the ultimate strength of the simulated and experimental curves showed good agreement, indicating that this method can be applied to engineering design.

Low cycle fatigue of EUROFER97 investigated for test blanket modules of ITER: An interlaboratory study

Several reduced-activation ferritic-martensitic (RAFM) steel grades have been proposed for structural applications in fusion reactors since the 1990s by several countries. The European reference RAFM steel EUROFER97 has been produced in four batches since 1998. It is intended for fabrication of Test Blanket Modules (TBM) for ITER as well as the blanket first wall and divertor cassettes of DEMO.RCC-MRx is the design code developed for high-temperature, research and fusion reactors by AFCEN. Its latest edition contains a provisional section dedicated to EUROFER97, aggregating properties of the first two batches, whereas a full qualification and final inclusion of a material requires three batches minimum. The completion of the code is required for the design assessment of TBM units in the frame of the nuclear licensing process.The EUROfusion project coordinates efforts to broaden the knowledge of EUROFER97 properties, which will be used for closing the database gaps in RCC-MRx. Low cycle fatigue (LCF) tests are a significant part of the EUROfusion test programme, since the material fatigue properties are indispensable for the component assessment owing to the cyclic nature of a tokamak operation.In this work we report the results of interlaboratory LCF tests on EUROFER97 batch 3 performed in a temperature range relevant for its application in ITER (RT–550 °C), wide selection of total strain ranges (0.3–1.5 %) and two values of strain ratio Rε (0 and –1). Along with the estimated fatigue life and cyclic behaviour, a fractographic analysis done with scanning electron microscopy (SEM) is presented.

Short communication: Complete dissolution of MX-phase nanoprecipitates in fusion steels during irradiation by heavy-ions

New RAFM steels, with intragranular VN precipitates for enhanced creep strength, were irradiated with 2MeV Fe+ ions at 600 °C to explore MX-type precipitate response to extreme end-of-life conditions in breeder blanket components. Utilizing TEM, Analytical-STEM, and APT, the steels were characterized in the unirradiated and irradiated states. Unirradiated grains contained needle-like VN precipitates (∼1 × 1022/m2). However, throughout the 40–100 dpa irradiation damage layer, VN precipitates were completely annihilated. APT confirmed no fine-scale VN clustering, indicating re-solution with the matrix, suggesting instability. Further investigations via reactor irradiations are crucial to assess this instability under breeder blanket conditions.

Hydrogen isotope permeation and retention behavior in the RAFM steel manufactured by laser powder bed fusion

Laser powder bed fusion (LPBF) has emerged as a promising additive manufacturing technique for producing complex components across various industries. This method is particularly instrumental in fabricating reduced activation ferritic/martensitic (RAFM) steel, a candidate structural material for the tritium breeding blanket. Hydrogen transport behaviors in the conventional RAFM steel have been largely investigated; however, they are not well understood in the LPBF-RAFM steel. In this work, we studied the hydrogen isotope permeation and retention behavior in the RAFM steel fabricated by LPBF, utilizing the deuterium gas driven permeation and thermal desorption spectroscopy experiments. The results show that the hydrogen isotope permeability of the LPBF-RAFM steel is close to the conventionally fabricated RAFM steel, but the diffusion coefficient is greatly lower and the retention is markedly higher in the LPBF-RAFM steel compared to the conventional RAFM steel. The large number density of microvoids and TiC precipitates in the LPBF-RAFM steel could be the predominant factors that result in the decrease in hydrogen isotope diffusion coefficient and the increase in the total amount of deuterium retention.

In-Situ synchrotron investigation of elastic and tensile properties of oxide dispersion strengthened EUROFER97 steel for advanced fusion reactors

The augmentation of mechanical properties of reduced activation ferritic martensitic steels through the introduction of creep resistant nano-oxide particles produces a class of oxide dispersion strengthened steels, which have attracted significant interest as candidates for first wall supporting structural materials in future nuclear fusion reactors. In the present work, the effect of temperature on the elastic properties and micro-mechanics of 0.3 wt% Y2O3 oxide dispersion strengthened steel EUROFER97 is investigated using synchrotron high energy X-ray diffraction in-situ tensile testing at elevated temperatures, alongside the non-oxide strengthened base steel as a point of comparison. The single crystal elastic constants of both steels are experimentally determined through analysis of the diffraction peaks corresponding to specific grain families in the polycrystalline samples investigated. The effect of temperature on the evolving dislocation density and character in both materials is interrogated, providing insight into deformation mechanisms. Finally, a constitutive flow stress model is used to evaluate the factors affecting yield strength, allowing the strengthening contribution of the oxide particles to be assessed, and correlation between the thermally driven microstructural behaviour and macroscopic mechanical response to be determined.

Study on hot isostatic pressing conditions of ARAA for fabrication of the breeding blanket first wall

The helium-cooled concept with solid breeding materials is one of the candidates for demonstration fusion power reactor (DEMO) breeding blanket. The first wall (FW) is a core component of the breeding blanket. The FW should keep the structural integrity during the normal plasma operation which is subject to high surface heat flux, neutron wall load, and high coolant pressure simultaneously. Various fabrication methods have been studied for the development of the FW fabrication technologies. This study focuses on FW fabrication technologies with hot isostatic pressing (HIP) bonding which is considered as the most promising method for achieving such complex shape like FW having internal cooling channels. We investigated the HIP bonding conditions using Advance Reduced Activation Alloy, ARAA, which is a reduced activation ferritic-martensitic steel under development in Korea. Temperature, holding time and pressure were considered as the parameters to optimize the HIP bonding conditions. The microstructures around the HIP joints changed with respect to the HIP bonding temperature. The amount of oxide particles around the HIP joints varied depending on the degassing conditions. The tensile strength of the HIP joint was not greatly affected by the presence of the HIP bonding line and degassing conditions. On the other hand, it has been confirmed that the Charpy absorbed energy of the HIP joint was significantly affected by degassing conditions. An increase in oxide particles around the HIP joints occurred when degassing temperature and vacuum level were low, and as the quantity of oxide particles increased, the Charpy absorbed energy of the HIP joint decreased substantially.

The effect of hydrogen-helium synergism on dislocation loops in RAFM steel at different temperatures by dual-beam ion irradiation

Simultaneous Fe+H ion irradiation (denoted as Fe+H) and Fe+H ion irradiation with pre-implantation of He (denoted as He/Fe+H) were carried out on Chinese Low-Activation Martensitic (CLAM) steels at 350 ℃-550 ℃ to investigate the synergistic effect between H and He. The peak dose was 15 dpa, the He concentration in the observation area was 11 appm/dpa and the H concentration was 44 appm/dpa. The microstructure and hardening of the irradiated samples were evaluated by TEM and nanoindentation, respectively. Bubbles were not observed in irradiated CLAM steels at all temperatures, which implies that the promotion of bubbles by H may not be as strong as that by He and that H attenuates the promotion of bubbles by He. The hardening induced by He/Fe+H irradiation at 450°C is about 1.6 times that of Fe+H irradiation, the synergistic effect of H and He is most significant at this temperature. Comparison with Fe+He irradiation and single Fe irradiation reveals that Fe+H irradiation at 350°C induces lower interstitial defect damage than Fe+He irradiation, but the level of interstitial defects induced by the synergistic effect of both H and He with displacement damage is comparable at higher temperatures. The average size of dislocation loops induced by Fe+H irradiation is between that of single Fe irradiation and Fe+He irradiation, but the number density of dislocation loops induced by Fe+H irradiation is higher than that of Fe+He irradiation. The synergistic effect of He with displacement damage tends to promote the growth of dislocation loops, while the synergistic effect of H tends to promote the nucleation of dislocation loops.

Influence of thermomechanical treatment on mechanical properties of CLAM steel

To enhance the creep resistance of CLAM steel and thus elevate its maximum service temperature, in our prior research, a superior thermomechanical treatment process was developed based on thermal simulation experiments conducted. This study employed forging to achieve the desired thermal deformation in accordance with the superior thermomechanical treatment process, followed by mechanical property testing post tempering. Comparative analysis with normalized and tempered CLAM steel (N + T CLAM steel) revealed notable improvements in yield strengths ranging from 31–55 %, ultimate tensile strengths from 30–46 %, and a reduction in total elongations by 12–21 % at both room temperature and elevated temperatures. Additionally, the ductile-to-brittle transition temperature (DBTT) increased from -65 °C to -27 °C. Creep behavior of N + T CLAM steel exhibited similarities to other RAFM steels, with stress exponents measured at 500 °C, 550 °C, 600 °C, and 650 °C recorded as 16.5, 15.0, 10.2, and 7.8, respectively. The creep activation energy was determined to be 556 kJ/mol, closely aligning with results from Eurofer 97 steel and F82H steel, and approximately twice the self-diffusion activation energy of iron. The Larson-Miller parameter method was employed to forecast stress-rupture limits under various conditions, with a predicted result of 120 MPa under typical design life conditions (550 °C / 100,000 h) for the cladding module, comparable to Eurofer 97 steel and F82H steel. TMT CLAM steel exhibited a stress exponent of 16.7 at 550 °C, with a creep activation energy of 578 kJ/mol. Under typical design life conditions (550 °C / 100,000 h) for the cladding module, the predicted stress-rupture limit increased to 176 MPa, representing a 47 % improvement over N+T CLAM steel. Moreover, under the same conditions (550 °C / 230 MPa), the rupture time of TMT CLAM steel was 54 times longer than that of N+T CLAM steel.

Enhanced disperse Ta(C, N) precipitation in reduced activation ferritic/martensitic steels by optimized two-step austenitizing

High-density MX precipitates block the movement of dislocations during creep at high temperature and increase sink strength to point defects under irradiation. Therefore, the precipitation control of MX phase is crucial for the advancement of reduced activation ferritic/martensitic (RAFM) steels. In this paper, an optimized two-step austenitizing strategy was proposed to enhance Ta(C, N) precipitation in RAFM steels, and the temperature effects of the first and second austenitizing on the Ta(C, N) precipitation enhancement were studied. During the first austenitizing, Ta(C, N) particles with a small radius (∼12 nm) and a high number density (∼8.5 × 1019 m−3) precipitated by heterogeneous nucleation on the M23C6 phase boundaries retained by the comparatively low temperature (900 °C). Then in the second austenitizing, enhanced Ta(C, N) precipitates kept good size and thermal stability below 1050 °C. Compared to RAFM steels treated by conventional normalizing and tempering, the strength and creep performance were significantly improved by the pinning effect of the enhanced dispersed Ta(C, N) particles. Furthermore, the optimized two-step austenitizing strategy was validated in the other two RAFM steels, showing broad application prospects in RAFM and heat-resistant steels.

Impact of trace silicon on irradiation hardening and embrittlement of RAFM steel subjected to neutron irradiation

The content of silicon (Si) element in reduced activation ferritic/martensitic (RAFM) steels varies significantly among different candidates of fusion reactor structural materials. Si is not intentionally added to the RAFM steel but rather remains as a residue during the deoxidation process in steel fabrication. To control and reduce the Si content, removing processes and special treatments are required. The additional refining processes significantly increase the cost of steel fabrication. Currently, the role of residual or trace Si in neutron irradiation of steel remains unclear. In the presented study, two 9Cr-2 W RAFM steels with Si contents of 0.1 wt.% and 0.01 wt.% were fabricated and subjected to a neutron irradiation at 563 K with a neutron fluence of 4.8 × 1023m−2. The main results show that 0.1 wt.% Si can contribute to the suppression of irradiation hardening and embrittlement, as evidenced by the reduction in irradiation-increased hardness and yield strength, while maintaining preferable elongation. Microscopic analysis suggests that an appropriate content of Si helps refine the grains in RAFM steel, increases the ductile fracture region of the fracture surface, and effectively reduces the density of dislocation loops induced by neutron irradiation. The findings evidence that retaining a certain amount of trace Si should be an informed and rational choice in the design of RAFM steel.

Microstructure refinement and enhanced mechanical properties of wire arc additively manufactured RAFM steel via post heat treatments

Reduced activation ferritic/martensitic (RAFM) steel with high strength and neutron radiation damage resistance is deemed one of the promising structural materials for fusion blankets. In this study, RAFM steel tube was prepared through wire arc additive manufacturing, and the evolution of phase composition, microstructural features and mechanical properties during the as-built and heat treatment processes were investigated. The results show that the as-built state is predominantly composed of α-Fe, and the microstructure's morphology from bottom to top consists of alternating columnar grain regions, equiaxed coarse grain regions, and fine grain regions. Normalizing and tempering treatment eliminates the heterogeneous microstructure, resulting in the presence of M23C6 carbides enriched with Cr and W, as well as MX phases enriched with Ta and V. Partitioning heat treatment results in the formation of ultrafine martensitic, quenched martensite and bainitic microstructures. The mechanical properties of wire arc additively manufactured RAFM steel are influenced by variations in microstructure. The coarse-grained region formed in the as-built state represent weak links. Normalizing and tempering treatment reduces the strength while improving ductility. Partitioning heat-treated RAFM steel exhibited a high strength of 1.1 GPa and an elongation of approximately 10.9%. This study provides a method of combining additive manufacturing with heat treatment to enhance the strength and ductility of RAFM steel simultaneously.

Experimental determination of the surface rate constants of protium and deuterium in Eurofer97

The EUROFER reduced activation ferritic-martensitic (RAFM) steel is meant to be a firm candidate as structural material to be part of future fusion reactors. The correct definition of the transport parameters of hydrogen isotopes in these materials turns out to be a key issue in this field, as long as it can affect the operation of the reactor itself when quantifying the fuel inventory as well as the radiological safety of the whole facility.Traditionally the diffusion limited regime (DLR) of these kind of RAFM steels has been experimentally tested and values for the permeability and diffusivity have been published, showing some differences depending on the specific composition of the steel, the thermal treatment during the manufacturing processes, etc. However, these RAFM steels will have to operate also under conditions of pressure and temperature that will lead to surface limited regimes (SLR). In this case, there is much less information available and data for the adsorption and recombination constants is not so extensive.This work shows the results for a new set of experimental permeation measurements carried out within the EUROfusion program in order to determine the SLR. The surface rate constants of protium and deuterium were experimentally obtained for Eurofer97-3 at two different facilities (PermRIG at UPV/EHU and Thermoperm at CIEMAT), with consistent results that show a good agreement.

Diffusion bonding of RAFM steels: Evolution of interfacial oxide layer with pressure and microstructure and mechanical property after post bonding heat treatment

The effect of elevating the bonding pressure from 10 to 20MPa(5MPainterval)at 1050?C for 60 minutes on the diffusion bonded joint of reduced activation ferritic/martensitic(RAFM)steel was studied. The results indicate that as the bonding pressure increases, the joint quality correspondingly improves. The oxide layer at the bonding interface underwent an evolution process from continuous to discontinuous and ultimately disappeared with the increase of bonding pressure. The optimal joint was achieved at the pressure of 20 MPa in this work. Considering the exploration of joint reliability in engineering applications, the optimal joint was subjected to a post bonding heat treatment(PBHT) of 750?C for 90 minutes. The diffusion bonded sample subjected to PBHT displays microstructural characteristics and tensile properties similar to those of the base metal. Notably, tensile fracture does not occur at the bonding interface, but in the base metal far from the interface. This work can provide a feasible method for the removal of oxides at the diffusion bonding interface of RAFM steel, and also contribute to the establishing and optimization of diffusion bonding processes, thereby improving the quality of diffusion bonded joint.

Comparison of electron-beam-welded joints manufactured by rolled and selective laser melted reduced activation ferritic/martensitic steel

As the core component of China Fusion Engineering Test Reactor (CFETR), the forming and manufacturing of the first wall faces severe challenges due to the constraints of materials, service environment and geometric structure. The microstructure and mechanical properties of welded joints obtained by rolled and laser selective melted (SLMed) reduced activation ferritic/martensitic (RAFM) steel were investigated in depth. The microstructure analysis results showed that the weld metal (WM) was composed of coarse lath martensite and δ-ferrite, while the microstructure were mainly coarse grains in heat affected zone (HAZ) which was easy to break. The average grains size of SLMed RAFM steel (S-RAFMs) was smaller than that of rolled RAFM steel (R-RAFMs), since the rapid melting and solidification of laser in the additive manufacturing process prevented grains to grow. The microstructure of R-RAFMs joint was relatively more uniform and there was no obvious texture and preferred orientation, while the WM of S-RAFMs joint had a weak cubic texture. The results of mechanical properties showed that the S-RAFMs joints exhibited higher strength (UTS = 697.46 MPa) and the R-RAFMs joints exhibited better plasticity (EL = 13.49 %) under the same tensile conditions. The tensile and impact fracture morphologies showed that the fracture mechanism of two joints was ductile-brittle mixed fracture, and the dimples of R-RAFMs joints were finer and denser. It was clearly observed that the hardness of WM (∼390–430 HV0.3) was higher than that of the base metal (BM) (∼190–230 HV0.3) from the hardness distributions along the width directions. The average hardness of BM in R-RAFMs joint (227 HV0.3) was lower than S-RAFMs joint (197.3 HV0.3), which was mainly related to the fine grain strengthening effect during SLM forming. The above results may provide theoretical guidance for the high-performance manufacturing of the first wall and promote the application of SLM and EBW technology in the field of nuclear fusion.

Thermal hydraulic optimization on the First Wall of COOL blanket for CFETR

The supercritical carbon dioxide (S-CO2) cOoled Lithium-Lead (COOL) blanket is under development for China Fusion Engineering and Test Reactor (CFETR). The S-CO2 is adopted to cool the structural components, in which the key component First Wall (FW) dominates the total heat power due to the large plasma facing heat flux. In this paper, referring to the response of material temperature, coolant velocity and pressure drop, the thermal hydraulic optimization is firstly performed on the structure design (i.e. cross area and pitch of channel) using the single variable control method, and the optimization principle is that the outlet temperature remains as the same when the optimization variables are changed. Finally, the optimal structure design of FW is obtained, namely cross area of 12 × 12 mm2 and channel pitch of 5 mm. On basis of this, the comparative analyses are carried out using the different cooling system, structural material, i.e. Reduced Activation Ferritic Martensitic (RAFM) steel and Oxide dispersion-strengthened alloy (ODS) ferritic steel, as well as helium as coolant. Furthermore, the satisfied designs are obtained by referring to the constraints of coolant pressure drop and maximum temperature on the structural steel. Then, the thermal hydraulic designs of FW are continuously matching with the Power Conversion System (PCS) to ensure that the heat power removed by the S-CO2 on the primary side can be transferred to the secondary side. Finally, the achievable thermodynamic design are obtained, including the parameters of high operating pressure, turbine inlet temperature, split ratio as well as the thermoelectricity conversion efficiency. The results show that the high pressure of PCS should be larger than 25 MPa when the RAFM steel is used as the structure material, due to the limitation on the outlet temperature of FW. On the contrary, the ODS can raise the outlet temperature, which is beneficial for matching with the PCS, and the thermoelectricity conversion efficiency can reach up to 47.4 % at the high pressure of 25 MPa. The analyses results will provide useful guidance on the system design both for the Primary Heat Transfer System (PHTS) and PCS of CFETR.

Improving thermal conductivity of reduced-activation ferritic/martensitic steel using three-dimensional woven copper fiber

Reduced-activation ferritic/martensitic (RAFM) steel is a primary candidate for blanket modules in nuclear-fusion reactors. Blanket modules transfer thermal energy generated by nuclear-fusion reactions to the coolant; therefore, blanket materials must have high thermal conductivity. RAFM steel is suitable for fusion reactors owing to its high strength, toughness, and resistance to neutron irradiation; however, its thermal conductivity is lower than that of other metals. This study fabricates RAFM steel–copper composites using three-dimensional (3D) woven copper fibers with different volume fractions as a preform to improve the thermal conductivity of RAFM steel around the coolant pipes. The microstructural, mechanical, and thermal properties of the developed composites and the correlations between them are experimentally investigated. Finite element simulations were also conducted on the composites comprising 3D woven copper fibers to evaluate the effective thermal conductivity. The factors that improve the effective thermal conductivity of RAFM steel–copper composites are discussed through a comparison with copper-powder composites with comparable volumes. This study demonstrates the manufacturing of composites with excellent thermal and mechanical properties using various 3D woven structures as preforms.

Microstructure of tungsten coatings effect on deuterium plasma-driven permeation through RAFM steel

Tungsten (W) coatings with different morphologies were deposited on CLF-1 steel by changing the sputtering time and discharge power using the magnetron sputtering method. Deuterium (D) plasma-driven permeation (PDP) experiments through W-coated CLF-1 were then carried out in the temperature range of 583–833 K. The obtained results showed that dense W coatings reduced steady-state permeation flux and prolonged the time of permeation flux reaching steady-state. Due to the existence of high-density intrinsic defects, D effective diffusivities of the coating are lower than that of bulk W. The increase of coating thickness reduced permeation flux. In addition, by changing the bias voltage and studying the influence of incident ion energy on D-PDP, it is found that the introduction of W coating affected the overall D transport by re-balancing the bulk diffusion and surface recombination processes. The thickness and microstructure of W coatings played a key role in determining the D steady-state permeation flux at varying temperatures.

Study on the Improved Mechanical Properties of RAFM Steel After Dual Equal-Channel Lateral Extrusion: Reflected by Quantified Microstructure

Study on the Improved Mechanical Properties of RAFM Steel After Dual Equal-Channel Lateral Extrusion: Reflected by Quantified Microstructure

Effect of nitrogen on microstructural evolution and mechanical properties of low carbon reduced activation ferritic/martensitic steel with boron microalloying

In this work, the microstructural evolution and mechanical properties of 0.01 wt% boron microalloying low carbon reduced activation ferritic/martensitic (RAFM) steel with various nitrogen content were studied. The results show that nitrogen could greatly reduce the dimension and volume fraction (fV) of M23C6 carbides and increase the fV of MX carbonitrides. Fine BN inclusions with the dimension of ∼200 nm could form with the addition of 0.01 wt% nitrogen, while clustered BN inclusions with the dimension of ∼6 μm could be found by 0.02 wt% nitrogen added. The higher nitrogen could result in the higher fV and number density of MX carbonitrides, which contribute to the higher dislocation density and dislocation strengthening. However, the strength and toughness of RAFM steel deteriorated by clustered BN inclusions.

Microstructure and mechanical properties of multi-component nano oxide-dispersion strengthened RAFM steel fabricated via laser powder bed fusion

Y4Zr3O12 multi-component nanoparticles prepared by sol–gel method were adsorbed on the surface of gas-atomized reduced activation ferritic/martensitic (RAFM) steel powder. This mixture powder was then utilized in the fabrication of oxide-dispersion strengthened (ODS) RAFM steel through laser powder bed fusion. The microstructure of the ODS RAFM steel comprised a combination of columnar ferrite grains and martensitic laths. The Y4Zr3O12 nanoparticles were homogeneously distributed in the matrix, with an average size of 36.3 nm and a high density of 2.49 × 1020/m3. The results of the mechanical properties tests revealed that the additively manufactured ODS RAFM steel exhibited a good balance of strength and elongation.