Ta Steel Research Articles

Strengthening mechanisms of reduced activation ferritic/martensitic steels: A review

This review summarizes the strengthening mechanisms of reduced activation ferritic/martensitic (RAFM) steels. High-angle grain boundaries, subgrain boundaries, nano-sized M23C6, and MX carbide precipitates effectively hinder dislocation motion and increase high-temperature strength. M23C6 carbides are easily coarsened under high temperatures, thereby weakening their ability to block dislocations. Creep properties are improved through the reduction of M23C6 carbides. Thus, the loss of strength must be compensated by other strengthening mechanisms. This review also outlines the recent progress in the development of RAFM steels. Oxide dispersion-strengthened steels prevent M23C6 precipitation by reducing C content to increase creep life and introduce a high density of nano-sized oxide precipitates to offset the reduced strength. Severe plastic deformation methods can substantially refine subgrains and MX carbides in the steel. The thermal deformation strengthening of RAFM steels mainly relies on thermo-mechanical treatment to increase the MX carbide and subgrain boundaries. This procedure increases the creep life of TMT(thermo-mechanical treatment) 9Cr-1W-0.06Ta steel by ∼20 times compared with those of F82H and Eurofer 97 steels under 550°C/260 MPa.

Computational Modeling of Compressive Behavior of Wire-Reinforced Bulk Metallic Glass Matrix Composites

The effect of wire type and volume fraction on the compressive behavior of wire-reinforced bulk metallic glass (BMG) matrix composites were investigated using the finite element method. The composites and test procedure were simulated in the ABAQUS software, and analyses on stress propagation and extracted compressive stress–strain curves were performed, which showed an acceptable agreement with experimental results. The results exhibited that Ta, Mo, W and Stainless Steel type 302 as the reinforcement wires significantly improved the plastic strain of the composites. Meanwhile, W wires sustained the composite strength in the same level of monolithic BMG, without sacrificing noticeable plastic strain of the composite in comparison with other reinforcements. The different volume fractions of W wires showed that the trend of plastic strain progress with increase in the second phase reaches a maximum at 65% of reinforcements. This behavior is caused by the fracture mechanism of the composites which is in its optimum condition when the reinforcement volume fraction is equal to 65%. The mechanical properties of the wire-reinforced BMG composites can be modeled by using the iso-strain Voigt model, which allows the prediction of the mechanical properties of a composite from the volume-weighted averages of the components properties. The above-mentioned findings corroborate the experiments elsewhere and may be considered as a valuable starting point to lead the future researches.

Effect of tempering on microstructure and mechanical properties of a Ta-added 9%Cr steel with high B and low N contents

Ta was added to a modified 9%Cr steel with high B and low N content to achieve an optimal dispersion of second phase particles. The effect of tempering on the microstructure and mechanical properties of an Fe-0.1C-9Cr-1.8W-0.6Mo-3Co-Nb-V-0,0013B-0,007N-0,085Ta steel was investigated. Typical martensite lath structure was observed after normalization at 1323 K for 0.5 h. The M23(C,B)6 and complex (Nb,Ta)(C,N) particles formed during tempering at 1023-1053 K. An increase in the hardness after tempering at 1038 K was attributed to the precipitation of the fine (Nb,Ta)(C,N) particles. The impact toughness depended significantly on tempering temperature. The impact toughness increased from 25 to 181 J/cm2 with increasing the tempering temperature from 1038 to 1053 K.

Creep Property of Boron Added 9Cr Heat Resistant Steels after Welding

Although welding results in premature failure by type IV fracture under high temperature creep conditions, the alloy design of light elements such as boron addition and nitrogen reduction enhances the creep lifetime of 9Cr heat resistant steel. In particular, the simulated heat affected zone (SHAZ) sample of new 9Cr steel (called TA steel) shows about 10 times longer creep lifetime than that of the standard Gr. 91 steel. The welded TA steel is thus expected to exhibit good creep properties because its SHAZ sample has coarser grains and suppresses type IV fracture. The preservation of base metal’s microstructure after welding results from the precipitate morphology, such as high grain boundary coverage by precipitates and low amount of MX being nucleation sites of ferrite grains during the a-g phase transformation. In addition, the increase of stability of M23C6 affects high pinning pressure toward grain boundary migration upon rapid heating during welding. First-principles calculations confirm the increased stability when boron is absorbed by M23C6. Moreover, the calculations reveals that boron decreases the coherency between matrix and M23C6, suppressing grain coarsening during creep tests in TA steel. It is concluded that the increased microstructural stability during welding and long high temperature exposure generates the elongated creep lifetime in welded TA steel including about 0.01 wt% boron and less than 0.01 wt% nitrogen.

3D Structural Analysis of Selected High-Temperature Materials

Abstract Research and development projects on materials for applications exposed to high temperatures and radiation include comprehensive mechanical and metallographic examinations as well as analysis methods based on electron microscopy investigations using SEM (scanning electron microscope) and TEM (transmission electron microscope). In addition to 2D EDS and EBSD analyses in the μm range, the imaging 3D analysis method has become more and more important in the past few years. Particularly, the combination of SEM images with simultaneously obtained EDS and EBSD data during 3D analyses is a test method which complements previous material testing and produces clear results since much larger data sets are used in order to determine the composition of a material and the phases present in its microstructure. Since 2014, our institute has been working with a workstation (FIB/SEM) from the company Zeiss, a crossbeam AURIGA 40 with 3D analysis. This paper will present unparalleled results of the investigations carried out on the structural and high-temperature material 12Cr0.36Ta steel, nanocrystalline tungsten and intermetallic NiAl-Cr alloys using the above-mentioned 3D technique.

Ultrahigh Cooling Rates at the Interface of Explosively Welded Materials and Their Effect on the Formation of the Structure of Mixing Zones

Explosively welded metal plates are characterized by the formation of local microvolumes at the interlayer boundaries within which there is mixing of interacting materials. These microvolumes can be arranged discretely along wavy boundaries or continuously in the form of thin interlayers along planar boundaries. Based on the results of many published works, it has been shown that the material is melted in these zones, and its subsequent solidification occurs at a high rate leading to the formation of metastable phases. In this paper, the formation of metastable phases in steel–steel, Ta–steel, Nb–Al, and Zr–Cu joints is analyzed. The cooling rates of these materials in the mixing zones is estimated. Calculations show that the cooling rate of the melts formed in the weld zones of the investigated composites is in the range 103–106 K/s. Cooling of mixing zones at such high rates results in the formation of metastable structures. In some cases, the crystallization of materials is suppressed and metallic glasses and quasicrystalline phases are formed in the melt zones.

Investigation of creep cavities in a novel 12Cr0.36Ta steel employing three-dimensional electron backscatter diffraction technique

Cavities are examined in a 12Cr0.36Ta steel specimen crept at 650°C/130MPa employing three-dimensional (3D) electron backscatter diffraction (EBSD) technique. Observation revealed that cavities are formed at the junctions where high angle grain boundaries (HAGBs) and low angle grain boundaries (LAGBs) coincide. Activation energies required for cavity formation at the junctions of four and three grains/subgrains are lower compared to nucleation in the bulk. This makes four and three grain/subgrain junctions prone to creep cavitation. Cavities have grown towards the highly misoriented grains. 3D reconstruction revealed that cavities are interconnected in reality that cannot be visualized from the two-dimensional (2D) investigations. At moderate stress level, as in present situation cavities have disc type morphology.

Overview of recent Russian materials and technologies R&D activities related to ITER and DEMO constructions

An overview is given of the activities and major achievements within recent R&D performed in Russia on materials and technologies for ITER and DEMO. In Russia, the basic materials manufacturing and technologies have been selected for ITER, for two reference DEMO breeding blanket concepts and for the related long term R&D. The review on the recent results of investigations on low activation materials (V–Ti–Cr alloys, Fe–12Cr–2W–V–Ta steel EK-181), beryllium and superconducting materials is presented. The fabrication of tubes, sheet and other forms from low activation materials is mentioned. The activity in beryllium materials both in the domestic studies and international cooperation is outlined. The progress in enhancement of the properties of superconducting materials for the ITER magnet system is presented, and the prospect of further developments in superconducting materials for DEMO magnet system is analyzed.

Tempering Process and Precipitation Behavior of 8%Cr–2%WTa Steel

In order to clarify the stability of precipitates in 8%Cr-2%W-0.09%Ta steel at high temperatures, tempering and precipitation behavior of the steel has been examined precisely. Tempering was performed mainly at 740°C for 0.5-1 000 h. Hardness peak was found in a tempering curve at 650-740°C for 3.75-8 h. Discontinuous changes in X-ray diffraction intensity of the matrix, apparent lattice strain and apparent crystalline particle size, which were estimated from integral breadth, the amount of extracted residue, and the Cr and Ta content in the residue were observed at 740°C for 2-12 h tempering. These discontinuous changes correspond to the rotation of preferred orientation or the generation of subgrains accompanying the dissolution and the subsequent re-precipitation of M 23 C 6 and TaC. The re-precipitation of fine M 23 C 6 particles is responsible for the hardness peak. Three kinds of TaC with different chemical composition and different size are found. The amount and the size of the coarse and medium TaC particles are affected by the discontinuous changes in microstructure. Fine particles of TaC grow very slowly, which is caused by the continuous dissolution of TaC due to the annihilation of precipitation site, dislocations, and re-precipitation on dislocations. Particle sizes of TaC and M 23 C 6 estimated using X-ray diffraction peak coincide with those observed in transmission electron microscopy.

In situ phase characterization in tempering and aging of Fe–Cr–W steels

The in situ measurements of hardness and X-ray diffraction have been performed at high temperature for Fe–Cr–W steels. The effect of tungsten concentration was investigated by comparing Fe–9Cr–2W–V–Ta steel and Fe–9Cr–3W–V–Ta steel. The tungsten addition increases hardness below 873 K. An aging test for 3600 s at 873 and 1073 K showed no clear change of hardness. A structural change from bcc to fcc and tungsten carbide coarsening are observed by X-ray diffractometry.

Effects of precipitation morphology on toughness of reduced activation ferritic/martensitic steels

Reduced activation ferritic/martensitic steels (RAFs) are leading candidates for structural materials of D-T fusion reactors. It is reported that 9Cr–2W–V, Ta steel (JLF-1), one of the RAFs, has superior phase stability, swelling resistance and mechanical properties against high-fluence neutron irradiation. Recently 9Cr– xW–V, Ta steels ( x=2.5, 3.0 and 3.5, JLS-series hereafter) were developed for use at higher temperatures. In this work, JLF-1 and JLS-series were thermal-aged at 823 and 923 K. Charpy impact tests were performed before and after thermal-aging. Microstructural features were observed using transmission electron microscope with energy dispersive X-ray spectrometer. From the results of Charpy impact tests, the ductile to brittle transition temperature was found to increase both by thermal-aging and by increasing tungsten content. This behavior was consistent with microstructural evolution of intergranular precipitates such as M 23C 6 and Laves phase coarsening.

Void swelling in reduced activation ferritic/martensitic steels under ion-beam irradiation to high fluences

Swelling behavior of a 9Cr–2.0W–V,Ta reduced activation ferritic/martensitic steel (JLF-1) and a tungsten-enriched 9Cr–2.5W–V,Ta steel (JLS-1) under ion-beam irradiation was studied. A technique of dual-beam ion irradiation utilizing a pair of electrostatic accelerators was employed to achieve high fluence levels and a helium production rate that are relevant to fusion power reactor blanket environments. Under single ion-beam irradiation at 743 K, a cavity structure was formed only in regions of more than 40 dpa. Cavities were in the martensite lath structure. Cavities did not form at the displacement peak about 1400 nm. Under dual ion-beam irradiation at 743 K, a cavity structure was formed at dual ion-beam irradiated regions. A bi-modal size distribution of faceted voids and spherical helium bubbles of diameter up to 2 nm were observed.

Relation between Creep Rupture Strength and Substructure of Heat Resistant Steel.

Creep rupture tests were performed at around 650°C on a low carbon steel, a W-bearing low carbon steel, a low carbon steel containing 0.1 % of TaN and a 0.1 %C-8%Cr-2%W-0.2%V-0.04%Ta steel (F-82H), structural material for fusion reactors. The equivalent obstacle spacing for mobile dislocations is calculated using only rupture data according to the creep theory developed by some of the authors. Thin films taken from the ruptured specimens were examined under a transmission electron microscope (TEM). The observed sub-grain size or the calculated inter-particle distance roughly coincides with the equivalent obstacle spacing. This indicates that the microscopic variable, i.e. the obstacle spacing for mobile dislocations, such as sub-grain size or inter-particle distance, can be directly calculated from only the macroscopic variable, i.e. time to rupture or creep rate.

Mechanical properties and microstructure of advanced ferritic–martensitic steels used under high dose neutron irradiation

Some results of the study of mechanical properties and structure of ferritic–martensitic chromium steels with 13% and 9% chromium, irradiated in the BOR-60 reactor up to different damage doses are presented in this report. Results concerning the behaviour of commercial steels, containing to molybdenum, vanadium and niobium, and developed for the use in fusion reactors, are compared to low-activation steels in which W and Ta replaced Mo and Nb. It is shown that after irradiation to the dose of ∼10 dpa at 400°C 0.1C–9Cr–1W, V, Ta steels are prone to lower embrittlement as deduced from fracture surface observations of tensile specimens. Peculiarities of fine structure and fracture mode, composition and precipitation reactions in steels during irradiation are discussed.

Influence of tantalum and nitrogen contents, normalizing condition and TMCP process on the mechanical properties of low-activation 9Cr–2W–0.2V–Ta steels for fusion application

For clarifying the reason of good creep and mechanical properties and improving the mechanical properties of the low-activation 9Cr–2W–0.2V–Ta steels (JLF-1 steels), the influence of Ta, N contents, normalizing condition and thermo-mechanical controlled processes (TMCP) on the mechanical properties of the 9Cr–2W–0.2V–Ta steels was investigated in contrast with conventional 9Cr–1Mo–0.2V–Nb steels. The creep strength of the 9Cr–2W–0.2V–Ta steels increases with increasing amount of Ta and N contents irrespective of the manufacturing processes. An increase in the normalizing temperature or the application of TMCP processes can also improve creep strength. It is hypothesized that the increase in creep strength due to an increase in normalizing temperature or the application of TMCP processes is attributed to the uniform distribution of fine (Ta,V)(C,N) precipitates. Furthermore, the 9Cr–2W–0.2V–Ta steels demonstrate good toughness mainly due to a fine austenite grain size. The 9Cr–2W–0.2V–Ta steels have equivalent or better creep strength and better toughness compared with 9Cr–1Mo–0.2V–Nb steels.

Electrocatalytic synthesis of polyaniline on non-noble metal electrodes

Electrochemical synthesis (ES) of polyaniline (PAn) at the surface covered by an oxide film is realized at a significantly higher voltage than is needed for the formation of a nonoveroxidized polymer. The involvement of catalytic amounts of the salts of transition metals in ES of PAn essentially facilitates the process at Ti, Ta, Pb, Al and stainless steel electrodes. IrCI2−6 anion was found to be an effective catalyst. During ES, IrCI2−6 forms a complex with aniline, where IrIY oxidizes aniline to a radical cation. The formation of radical cation is a limiting stage ES PAn. It is realized in a moment in the presence of IrCI2−6, thus providing a significant decrease of the induction period at a potentiostatic synthesis and the lowering of a working potential at a galvanostatic synthesis. It is shown that the composition of the complex includes IrIII and 2 aniline molecules, which form a paramagnetic dimer inside a coordination sphere. © 1997 John Wiley & Sons, Ltd.

Recovery of He, H and D implanted Ta, Nb, W and AISI 316L steel samples in annealing

The annealing behaviour of Ta, Nb, W and AISI 316L stainless steel samples implanted with 20 keV helium ions and either with 15 keV hydrogen or 12 keV deuterium ions was studied. By observing the release of trapped hydrogen and deuterium, it was concluded that the helium associated hydrogen trapping defects recover at a temperature of 488 K in steel, 583 K in W, 648 K in Ta, and 553 K in Nb. The observed recovery temperatures of hydrogen irradiation induced traps are lower than the recovery temperatures of traps trapping solid soluble hydrogen migrating from the metal bulk after the helium implantation, deduced in an earlier study [P. Haussalo, J. Keinonen, U.-M. Jäske, and J. Sievinen, J. Appl. Phys. 75, 7770 (1994)]. This indicates that the hydrogen implantation changes the characteristics of the helium associated defects. The proposed trapping mechanism is binding at the surface of helium bubbles, as suggested in the above mentioned study, affected by the properties of the ion-implantation induced damage.

Effects of Ta and Nb on microstructures and mechanical properties of low activation ferritic 9Cr-2W-0.2V steel for fusion reactor.

In order to develop low activation high chromium ferritic steels for fusion reactors, the effects of 0.05-0.16 wt% Ta and 0.05 wt% Nb alloyings, and heat treatments on elevated temperature strength, toughness and microstructures of 0.1C-9Cr-2W-0.2V-0.04N steels were investigated. Normalizing and tempering conditions were determined as heating at 1050°C for 30 min and at 800°C for 1 h through observing the dissolution of carbonitrides of Ta and As temperatures, respectively. The optimum amount of Ta in consideration of creep-rupture strength and toughness was approximately 0.10 wt%. Delta-ferrite formed in 0.16 wt% Ta steel of which Cr-equivalent was approximately 10 wt%.The steels alloyed with Ta showed only slightly lower creep-rupture strength than that of the steel alloyed with the optimum amount of Nb, 0.05 wt%. Toughness of the steels alloyed with Ta was superior to that of the steel alloyed with Nb. Hence, Ta-addition can be recommended over Nb-addition in view of the improved toughness.

Impact behavior of Cr-W steels

Chromium-molybdenum ferritic (martensitic) steels are leading candidates for the structural components for fusion reactors. However, irradiation of steels containing molybdenum or niobium in a fusion environment will produce long-lived radioactive isotopes that will lead to difficult waste disposal problems. To alleviate the waste disposal problem, ferritic steels are being developed that are analogous to conventional Cr-Mo steels, but with molybdenum replaced by tungsten and niobium replaced by tantalum. Experimental steels containing 0.1% C, 2 1/4 to 12% Cr, 0 to 2% W, 0 or 0.25% V, and 0 or 0.07% Ta were produced. Charpy impact properties were determined. A 5Cr-2W-0.25V steel and a 9Cr-2W-0.25V-0.07Ta steel had the best impact properties. The impact properties of these two steels as well as those of a 9Cr-2W-0.25V and a 12Cr-2W-0.25V steel were as good or better than the properties of similarly heat treated conventional 9Cr-1MoVNb and 12Cr-1MoVW steels.

Heat treatment behavior and tensile properties of Cr−W steels

Ferritic steels containing Cr, W, and V are being developed for fusion reactor applications. These steels would be alternatives to the Cr−Mo steels that are being considered for structural components. Eight experimental steels were produced. Chromium concentrations of 2.25, 5, 9, and 12 pct were used. Steels with these chromium compositions and with 2 pct W and 0.25 pct V were produced. To determine the effect of tungsten and vanadium, 2.25Cr steels were also produced with 2 pct W and no vanadium, and with 0.25 pct V and zero and 1 pct W. A 9Cr steel containing 2 pct W, 0.25 pct V, and 0.07 pct Ta was also studied. For all alloys, carbon was maintained at 0.1 pct. Tempering behavior of the steels was similar to that of the Cr−Mo steels being considered. Tensile studies indicated that the 2.25Cr−2W−0.25V and 9Cr−2W−0.25V−0.07Ta steels had the highest strengths with properties similar to those of the 9Cr−1MoVNb and 12Cr−1MoVW steels, which are the strongest of the Cr−Mo steels of interest.