CLAM Steel Research Articles

Investigation of Dynamic Tensile Behaviors of Clam Steel Under High Strain Rate and High Temperature

Investigation of Dynamic Tensile Behaviors of Clam Steel Under High Strain Rate and High Temperature

Fabricating tritium permeation barrier for PFC with a new non-coating strategy

Tritium (T) permeation through plasma-facing component (PFC) into the coolant is a major concern of fusion reactor operation. In this work, deuterium (D) permeation through CLAM steel, CLAM/CLAM and CLAM/Fe-Cr-Al samples prepared by hot isostatic pressing (HIP) are tested in a linear plasma device. Only the downstream surfaces of the samples are oxidized with controlled atmosphere to form permeation barrier. No significant effect on D diffusion and penetration can be observed for the joining interfaces, while the dense oxide layer at the downstream side plays an important role in suppressing D permeation. The downstream surface oxidization of CLAM/Fe-Cr-Al is found to effectively reduce D permeation flux by a factor up to 1000.

Enhanced D retention in RAFM steel caused by D bubbles formed inside Cr-rich surface layer

To identify the cause for an enhanced D retention in RAFM (reduced-activation ferritic/martensitic) steels with decreasing incident D fluence, ϕD, CLAM steel samples were exposed to D plasma at a low ϕD ∼ 3 × 1023 m−2 in the PISCES-A linear plasma device. At this low ϕD, a Cr-rich surface layer with a thickness of ∼ 10 nm still remains, confirmed with cross-sectional EDX (energy dispersive x-ray spectroscopy) elemental mapping. TEM (transmission electron microscopy) observations of a plasma-exposed surface reveal the formation of D bubbles inside the Cr-rich surface layer, while no D bubbles are seen in an unexposed surface. Furthermore, depth profiling using GDOES (glow discharge optical emission spectroscopy) detects D in the Cr-rich surface layer. These results demonstrate that the D retention at low ϕD is enhanced by D atoms trapped in bubbles created inside the Cr-rich surface layer.

High temperature creep behavior of friction stir welding joints for CLAM steel

The uniaxial creep tensile strength, fracture features and microstructures of friction stir welded joint with postweld heat treatment for CLAM steel have been investigated in the range of the creep applied stress from 180 MPa to 300 MPa at 823 K condition. It is found that the creep life of the FSW joints of CLAM steel increase from 1.5 h, 19.2 h and 883 h to above 6769 h respectively, when the creep stresses decrease from 300 MPa, 260 MPa and 220 MPa to 180 MPa. The inter critical heat affected zone is the weakest zone of creep rupture resistance for the FSW joint of CLAM steel, the joints mainly exhibit dislocation-controlled creep deformation mechanism and the transgranular ductile fracture mode. The microstructures of inter critical heat affected zone produce recovery and subgrain boundaries are formed in here during creep process, which result in the decrease of dislocation strengthening action; the coarser M23C6 carbides is produced or the coarser Laves phase around the M23C6 carbides is formed, which result in the reduction of precipitation and solution strengthening action, these issues are the main reasons for the deterioration of the creep performance of FSW joints. The creep fracture strength of FSW joint is estimated to be 156 MPa in the condition of 1 × 105 h creep life according to the Monkman-Grant equation, which reaches 88 % of the strength of base metal.

Identification of tantalum precipitates in CLAM steel

Tantalum is an essential alloying element in reduced-activation ferritic/martensitic (RAFM) steels being a promising candidate structural material for fusion reactor. Tantalum has an important influence on the microstructure and mechanical properties of RAFM steels by forming tantalum precipitates. The tantalum precipitates in CLAM steel, a type of RAFM steels, normalized for 30 min at 980 °C and tempered for 90 min at 760 °C were investigated using transmission electron microscope. In addition to tantalum-rich M(C,N) carbonitrides and MC carbides with a face centered cubic crystal structure and a typical metallic element composition of about 80Ta, 5-10Cr, 2-5Fe, and 4-5V in atomic pct, tantalum-rich M3(C,N)2 carbonitrides and M3C2 carbides, which are not known to have been reported previously in steels including RAFM steels, were also identified in the steel. Tantalum-rich M3(C,N)2 and M3C2 precipitates have a typical metallic element composition of about 55-60Ta, 25-30Cr, 5-15Fe, and 1-2V in atomic pct, and a spherical morphology, as well as small sizes ranging from 15 to 40 nm in diameter. The experimentally observed tantalum-rich M3(C,N)2 and M3C2 precipitates exhibit a relatively small size, and appear to have a smaller amount compared with the tantalum-rich M(C,N) and MC precipitates observed. The effect of identified precipitate phases on the creep properties of the steel was also discussed.

Investigation on low cycle fatigue properties of CLAM steel electron beam welded joint at 550 °C

The low cycle fatigue (LCF) properties of structural materials at high temperatures are important for operational safety of fusion reactors. In this work, the investigation on LCF properties of CLAM steel joints welded by electron beam was conducted at strain amplitudes varying from 0.2 % to 0.7 % at 550 °C. It is found that the fatigue life and cyclic stress response of welded joints are lower than those of base metal. Continuous cyclic softening is observed in welded joints and base metal during LCF test. A majority of the fatigue cracks initiated at the surface of the the welded joint specimen with several initiation sites and propagated in a transgranular mode. The area of the crack propagation region decreased and the fatigue striation spacing increased with increasing strain amplitude. The cyclic softening is attributed to the coarsening of martensite laths and the reduction in dislocation density.

Preparation of Fe-Al alloy layer on CLAM steel

Reduced activation ferritic/martensitic steel is one of the candidate materials for tritium breeding module in the fusion reactor. In order to control the permeability of tritium in an acceptable range, coating with low hydrogen isotope permeability, known as tritium permeation barrier, is usually prepared on the surface of such structural materials. The FeAl/Al<sub>2</sub>O<sub>3</sub> is the first choice of tritium permeation barrier for many countries, because of its fine performance of high permeation reduction factor, corrosion resistance and high-temperature resistance. The surface morphology and microstructure of Fe-Al infiltrated layer have important influence on the quality of Al<sub>2</sub>O<sub>3</sub> coating. In this study, Al coating on the surface of CLAM steel is prepared by electroplating of aluminum from AlCl<sub>3</sub>-EMIC. Then the Fe-Al infiltrated layer is obtained by diffusion between Al and substrate by annealing. The effects of annealing time and temperature on the microstructure of Fe-Al infiltrated layer are studied by X-ray diffraction, scanning electron microscope and energy dispersive spectrometer. The results show that 20-μm-thick aluminum coating is obtained on the CLAM steel surface by electroplating. The Al coating is uniform and compact, and the size of its surface columnar grain decreases with electroplating current density increasing. Annealing results show that neither hole nor gap is observed between the Fe-Al infiltrated layer and the substrate. In addition, the infiltrated layer is found to be tightly bound to the substrate with a thickness ranging from 7 μm to 45 μm, depending on the annealing parameters. At the initial stage of annealing, Cr enriched Fe-Al alloy is formed evidently. However, such a Cr enrichment disappears at higher annealing temperature or longer annealing time due to diffusion. The surface of infiltrated layer changes from aluminum-rich phase to aluminum-poor phase, and its thickness increases with annealing time or temperature rising. The temperature dependence of the growth rate of Fe-Al infiltrated layer can be described by Arrhenius equation. At this time, the Arrhenius activation energy of aluminization on CLAM steel is calculated to be 78.48 kJ/mol. At 640 ℃ and 760 ℃, the growth of Fe-Al infiltrated layer is controlled by the grain boundary as well as the volume diffusion. When the reasonable thickness and microstructure of Fe-Al alloy layer are used and annealing time or temperature keeps as low as possible, the optimal annealing temperature and time are 700 ℃/10 h, respectively.

Identification of precipitate phases in CLAM steel

Precipitate phases in a reduced activation ferritic/martensitic steel CLAM (HEAT-0912) were observed and analyzed using transmission electron microscopy and energy dispersive spectroscopy. The steel samples were normalized at 980 °C for 30 min and then tempered at 760 °C for 90 min followed by an air cooling. Through the selected-area electron diffraction pattern analyses and energy dispersive X-ray data, eight kinds of precipitate phases were identified to be Ta-rich MX carbonitride with a f.c.c. crystal structure, Ta-rich M3X2 carbonitride with a base-centered monoclinic crystal structure, Ta-rich MSi2 phase with a hexagonal structure, V-rich MX carbonitride with a f.c.c. crystal structure, V-rich M2C carbide with an orthorhombic-simple crystal structure, Cr-rich M23C6 carbide with a f.c.c. structure, Cr-Ta-rich M7C3 carbide with an orthorhombic-simple crystal structure, Cr-rich M6C carbide with a f.c.c. structure, respectively. There appear to have no previous reports of the Ta-rich MSi2, Ta-rich M3X2, V-rich M2C and Cr-rich M6C phases identified in reduced activation ferritic/martensitic steels.

Investigation of prior austenite grain and delta ferrite in CLAM welded joints after different diffusion annealing processes

Abstract CLAM steel is one of the structural material used in test blanket modules as a barrier or blanket for fusion reactors. Different diffusion annealing processes were employed in order to investigate the effect of δ-ferrite elimination and prior austenite grain coarsening. The results show that both alloying elements diffusion and microstructures coarsening during the diffusion annealing treatment influence the contents and morphologies of δ-ferrite. The polygonal blocky δ-ferrite and the coarse grains were determined as the main factors deteriorating the impact toughness for the CLAM welded joint. A CLAM welded joint with good impact toughness and appropriate microstructure in terms of grain size and δ-ferrite content was obtained after diffusion annealing process at 1100 °C for 10 h.

Effects of electroslag remelting process and Y on the inclusions and mechanical properties of the CLAM steel

Abstract Y-containing CLAM steels were melted via vacuum induction melting and electroslag remelting. In this study, the evolution, microstructure, and mechanical properties of the alloy inclusions (ESR-1 (0 wt.% Y), ESR-2 (0.016 wt.% Y) and ESR-3 (0.042 wt.% Y)) were investigated. Further, the number of inclusions in ESRed steel was observed to obviously decrease, and the distributions were more uniform. The fine Y–Al–O inclusions (1–2 μm) were the main inclusions in ESR-2. The addition of Y affected the prior austenite grain size (PAGZ), increasing the tensile strength at test temperature. Low ductile–brittle transition temperature (DBTT) was obtained because of the fine PAGZ and dispersive inclusions. For the ESRed CLAM steel with 0.016 wt.% Y, the yield strengths were 621 MPa at 20 °C and 354 MPa at 600 °C in air. Further, the uniform elongation and elongation of the ESR-2 alloy were 5.5% and 20.1% at 20 °C, respectively. Meanwhile, the DBTT tested using full-size Charpy impact specimen (55 cm × 10 cm × 10 cm) was reduced to −83 °C.

Influence of LBE Temperatures on the Microstructure and Properties of Crystalline and Amorphous Multiphase Ceramic Coatings

An Al2O3–TiO2 amorphous composite coating with a thickness of 100–120 μm was fabricated on China low activation martensitic steel (CLAM steel) by oxygen acetylene flame spraying technology and the laser in-situ reaction method. We investigated the microstructures and mechanical properties of the coating after liquid lead-bismuth eutectic (LBE) alloy corrosion under different temperatures for 300 h and found that the corrosion temperature of the LBE had no observable effect on the microstructure and chemical phase of the Al2O3–TiO2 amorphous composite coatings. However, the mechanical properties (micro-hardness and shear strength) of the Al2O3–TiO2 multiphase coating deteriorated slightly with the increase in the immersion temperature of the LBE. As a result of oxygen acetylene flame spraying and laser in-situ reaction technology, it was found that the Al2O3–TiO2 amorphous composite coating exhibits an excellent LBE corrosion resistance, which is a candidate structural material for the accelerator-driven subcritical system (ADS) to handle nuclear waste under extreme conditions.

The distribution and forming of tritium in two types of China reduced activation ferritic-martensitic steels

A definitive assessment on the distribution and forming of tritium in RAFM steels which would be considered as the primary candidate structural material for fusion reactor is a key safety requirement for selecting effective decommissioning process of nuclear facilities. In this study, the acid etching is used to determine tritium concentration and distribution of CLAM steel and CLF-1 steel exposure to the mixed gas of 90% deuterium and 10% tritium (1.2 MPa mixed gas, 18 h at 896 K) and later aging for 4 years. The bulk distribution of tritium concentration in both steels is calculated, based on the diffusion equation and finite difference method. In addition, the samples are soaked in distilled water to evaluate the tritium-hydrogen exchange before etching. The results show that the tritium on the metal surface could be effectively removed. There are two forms of tritium distributed in both steels: “soluble tritium” and “non-soluble tritium”. And the former is usually less than the latter. Compared with CLAM steel, tritium content in CLF-1 steel is higher.

Development of a micro-scale Y–Zr–O oxide-dispersion-strengthened steel fabricated via vacuum induction melting and electro-slag remelting

Abstract In this paper, the CLAM steel strengthened by micro-scale Y–Zr–O was prepared by vacuum induction melting followed by electroslag remelting (VIM-ESR). Yttrium (Y) and zirconium (Zr) were easy to aggregates into massive yttrium-zirconium-rich inclusions in the steel melted by vacuum induction melting (VIM), which would interrupt the continuity of the matrix and reduce the mechanical properties of steel. Micron-sized Y–Zr–O inclusions would be produced with the removal of original blocky Y–Zr-rich inclusions and the submicron-sized inclusions smaller than 0.2 μm could be retained in the steel. The small grain size and the better refinement and distribution uniformity of Y–Zr–O inclusions after remelting would be responsible for the better yield strength and toughness. For VIM-ESR alloy, the ultimate tensile strength is 749 MPa and the yield strength is 642 MPa at room temperature, meanwhile they are 391 MPa and 367 MPa at 600 °C, respectively. Meanwhile, the ductile-brittle transition temperature (DBTT) reduced from −43 °C (VIM) to-76 °C (VIM-ESR).

Tritium-permeation-barrier properties of erbium oxide (TPB) coating on CLAM steel

Erbium oxide (Er2O3) coatings are considered as the candidate for tritium permeation barrier (TPB) because of its low permeation reduced ratio. The enhanced adhesion between TPB layer and metal substrate is one of the key issues for the high reliability. In this work, erbium coatings were firstly prepared by radio frequency (RF) magnetron sputtering and then annealed to generate Er2O3 TPB coatings, accompanied with Er2Fe17 due to inter-diffusion between Er and steel substrates. The coatings are compact and smooth if the annealing temperature is above 873 K. The samples are insulator at room temperature, and meet the requirement of TPB in blanket. The adhesion of coatings could be improved with elevating annealing temperature. According to the AES profile, a thickness controllable TPB multi-layered model of Er2O3/Er2O3+Er/Er/Er2Fe17/Fe is proposed.

Influence of tensile stress on hysteresis loop of Reduced Activation Ferrite & Martensitic steel

The influence of tensile stress on hysteresis loop of Reduced Activation Ferrite & Martensitic steel (RAFM steel) in elastic region was quantitatively evaluated through experiments in this paper. In view of in vessel structures of the RAFM steel in the Tokamak devices, the variation of magnetic material properties caused by forces or mechanical deformation may lead to unexpected influences on their dynamical behaviors, even safety problem. In this paper, tensile experiments of a RAFM steel were conducted under magnetic field up to 60 kA/m to measure its hysteresis loops under different tensile stress. Tensile specimens of a RAFM steel developed in China (CLAM steel) were fabricated and measured with a multi-field coupling experimental system. The maximum relative permeability under different experimental conditions was extracted from the measured hysteresis loops. Experimental results show that a clear variation of the maximum magnetic permeability was observed when the external tensile stress was enlarged. In addition, based on a linear theory of magneto-solid coupling mechanics, a model of constitutive relation between tensile stress and maximum magnetic permeability was put forward and validated based on the experimental results.

Study on creep fatigue of heat sink in W/RAFM divertor for CFETR

The divertor target is one of the main components enduring the high heat load in fusion reactor. The life time and integrity of the divertor are severely limited by the failure of the target heat sink especially in operation of high temperature and long operating period of China Fusion Engineering Test Reactor (CFETR). Therefore, the study of the failure mechanism of heat sink under long pulse and high-power H-mode operation is one of the bases to ensure the reliability of CFETR.In this paper, the finite element method is used to analyze the structural response of the heat sink to the thermal load of the W/RAFM(Reduced Activation Ferritic–Martensitic) divertor target under CFETR design operating conditions. Then the numerical creep-fatigue damage accumulation model is built to explore the fatigue failure mechanism of divertor target heat sink in high power / long pulse operation during which the stress relaxation is taken into account. The preliminary creep-fatigue assessment of CLAM steel as divertor target heat sink in CFETR design operation phase is presented and provides important theoretical basis for life prediction of heat sink for future fusion reactor.

Simulation of impact toughness with the effect of temperature and irradiation in steels

Abstract One of the important requirements for the application of reduced activation ferritic/martensitic steel is to retain proper mechanical properties in irradiation and high temperature conditions. In order to simulate the impact toughness with the effect of temperature and irradiation, a simulation model based on energy balance method consisted of crack initiation, plastic propagation and cleavage propagation stages was established. The effect of temperature on impact toughness was analyzed by the model and the trend of the simulation results was basicly consistent with the previous experimental results of CLAM steels. The load-displacement curve was simulated to express the low temperature ductile-brittle transition. The effect of grain size and inclusion was analyzed by the model, which was consistent with classical experiment results. The transgranular-intergranular transformation in brittle materials was also simulated.

Experimental investigation on deuterium effects on the tensile performance of two types of China Reduced Activation Ferritic-Martensitic steels

The present work is aimed to study on the effect of deuterium on mechanical properties of two martensitic steel, i.e. CLAM steel and CLF-1 steel. Tensile tests were operated at room temperature before and after deuterium charging. The results showed the ultimate tensile stress and area reduction coefficient of both steels were decreased after deuterium charging. The shrinkage rates of hydrogen induced reduction of area of CLAM steel and CLF-1 steel were 18.65% and 45.98%, respectively, which showed poor resistance to hydrogen embrittlement. This may be related to a lot of lath martensite phase in both steels. And the mechanism of even more sensitive to hydrogen damage for CLF-1 steel is probably due to its finer prior austenite and lath martensite phase, higher contents of carbide particles, as well as more hydrogen content and lower diffusivity.

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.

Corrosion behavior of as-weld CLAM steel after laser shock peening process and tempering in static molten lithium lead

China low activation martensitic steel was welded by gas tungsten arc welding, after that laser shock peening and tempering treatment are applied to as-weld material, its compatibility with molten lithium lead are studied. Tempering treatment weight loss is 1.32 mg/cm2 compared to LSP is 1.91 mg/cm2 after 3000 h. Tempered martensite is more excellent than coarse martensite, as-weld surface corrosion grooves are serious. Passivation layers protect the internal metal from destroy in molten metal and it firstly appeared in defective areas during LSP process. Main elements markedly reduced through dissolving into molten metal, microstructure and phase transformation took place along the whole corrosion surface.