Influence of hot rolling and high speed hydrostatic extrusion on the microstructure and mechanical properties of an ODS RAF steel

An argon gas atomized, pre-alloyed Fe-14Cr-2W-0.3Ti (wt.%) reduced activation ferritic (RAF) steel powder was mechanically alloyed with 0.3wt.% Y₂O₃ nano-particles in an attritor ball mill and consolidated by hot isostatic pressing at 1150 °C under a pressure of 200 MPa for 3 h. In the aim to improve its mechanical properties the ODS steel was then submitted to a thermo-mechanical treatment (TMT): hot rolling (HR) at 850 °C or high speed hydrostatic extrusion (HSHE) at 900 °C, followed by heat treatment (HT).Transmission electron microscopy (TEM) observations of the ODS alloys after TMT and heat treatment revealed the presence of elongated grains in the longitudinal direction, with an average width of 8 μm and an average length of 75 μm, and equiaxed grains, a few microns in diameter, in the transverse direction. Two populations of oxide particles were observed by TEM: large Ti-Al-O particles, up to 250 nm in diameter, usually located at the grain boundaries and small Y-Ti-O nanoclusters, about 2.5 nm in diameter, uniformly distributed in the matrix. Charpy impact tests revealed that the HSHE material exhibits a larger upper shelf energy (5.8 J) than the HR material (2.9 J). The ductile-to-brittle transition temperature of both alloys is relatively high, in the range of 55-72 °C. Tensile mechanical properties of both ODS alloys were found satisfactory over the full range of investigated temperatures (23-750 °C). The HSHE material exhibits better tensile strength and ductility than the HR material. These results indicate that HSHE can be considered as a promising TMT method for improving the mechanical properties of ODS RAF steels.     

Effect of thermo-mechanical treatments on the microstructure and mechanical properties of an ODS ferritic steel

The Fe-14Cr-2W-0.3Ti-0.3Y₂O₃ oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel was fabricated by mechanical alloying of a pre-alloyed, gas atomised powder with yttria nano-particles, followed by hot isostatic pressing and thermo-mechanical treatments (TMTs). Two kinds of TMT were applied: (i) hot pressing, or (ii) hot rolling, both followed by annealing in vacuum at 850 °C.The use of a thermo-mechanical treatment was found to yield strong improvement in the microstructure and mechanical properties of the ODS RAF steel. In particular, hot pressing leads to microstructure refinement, equiaxed grains without texture, and an improvement in Charpy impact properties, especially in terms of the upper shelf energy (about 4.5 J). Hot rolling leads to elongated grains in the rolling direction, with a grain size ratio of 6:1, higher tensile strength and reasonable ductility up to 750 °C, and better Charpy impact properties, especially in terms of the ductile-to-brittle transition temperature (about 55 °C).     

Tensile and Charpy impact properties of an ODS ferritic/martensitic steel 9Cr–1.8W–0.5Ti–0.35Y2O3

A 9Cr-ODS ferritic/martensitic steel with a composition of 9Cr–1.8W–0.5Ti–0.35Y₂O₃ was fabricated by mechanical alloying and hot isostatic pressing, followed by hot rolling. Tensile properties were measured at room temperature (23 °C) and 700 °C in the rolling direction (LT) and the transverse direction (TL). The ultimate tensile strength (UTS) of the as-rolled samples in both directions reached 990 MPa at 23 °C, and still maintained at 260 MPa at 700 °C. The tensile strength and elongation of the rolling direction was greater than that of the transverse direction. The Charpy impact was tested from −100 to 100 °C in the LT direction. The lower shelf energy (LSE) was more than 65% of the upper shelf energy (USE). The total absorbed energy was separated into the energies for crack initiation and propagation. The propagation energy was always higher than the initiation energy in the range of temperatures tested. The ductile-to-brittle transition temperature (DBTT) of the rolled 9Cr ODS evaluated by an absorbed energy curve was about 0 °C. However, the high LSE and the fracture surface that still contained dimples at lower shelf indicated good toughness of the as-rolled 9Cr ODS steels at temperature of −60 °C.     

Effects of the fabrication process parameters on the precipitates and mechanical properties of a 9Cr-2W-V-Nb steel

The effects of the fabrication process parameters such as a tempering temperature, cold rolling and annealing condition on the precipitates and mechanical properties of a normalized 9Cr-2W-V-Nb steel were evaluated. Nb-rich MX precipitates were found in the specimen tempered at 550 °C while M₂₃C₆, Nb- and V-rich MX ones were observed in the specimen tempered at 750 °C. A cold rolling and an annealing at 750 °C of the specimen tempered at 550 °C induced the formation of large inhomogeneous M₂₃C₆ carbides, causing a reduced tensile strength. However, the cold rolling of the specimen tempered at 750 °C provided fine precipitates due to a fragmentation of some of the M₂₃C₆ carbides, and an annealing at 700 °C for 30 min was found to be suitable to recover the degraded mechanical properties from a cold working.     

Microwave sintering of 8 mol% yttria-zirconia (8YZ): An inert matrix material for nuclear fuel applications

This study focused on reducing overall processing time and temperature for fully stabilized zirconia, an inert matrix material candidate, to minimize the loss of actinides (that will be incorporated into the matrix material), while maintaining at least 90% theoretical density (TD). The effects of different processing routes on bulk density and microstructure were evaluated. The results obtained by adopting microwave sintering for 8 mol% Y₂O₃-ZrO₂ were compared to conventional sintering. A 20 min soak time at 1300 °C resulted in pellets with 90% TD for microwave-processed samples, compared to 77% TD for pellets processed conventionally. A similar density was obtained at lower temperature (1200 °C) by increasing the soak time to 100 min in microwave processing. This time and temperature resulted in 60% TD conventionally processed pellets. Compressive strength values obtained for a 1300 °C (20 min soak time) microwave-processed sample were higher (1600 MPa) as compared to a conventionally processed sample (1300 MPa).     

Long-term properties of reduced activation ferritic/martensitic steels for fusion reactor blanket system

Thermal aging properties of reduced activation ferritic/martensitic steel F82H was researched. The aging was performed at temperature ranging from 400 °C to 650 °C up to 100,000 h. Microstructure, precipitates, tensile properties, and Charpy impact properties were carried out on aged materials. Laves phase was found at temperatures between 550 and 650 °C and M₆C type carbides were found at the temperatures between 500 and 600 °C over 10,000 h. These precipitates caused degradation in toughness, especially at temperatures ranging from 550 °C to 650 °C. Tensile properties do not have serious aging effect, except for 650 °C, which caused large softening even after 10,000 h. Increase of precipitates also causes some degradation in ductility, but it is not critical. Large increase in ductile-to-brittle transition temperature was observed in the 650 °C aging. It was caused by the large Laves phase precipitation at grain boundary. Laves precipitates at grain boundary also degrades the upper-shelf energy of the aged materials. These aging test results indicate F82H can be used up to 30,000 h at 550 °C.     

Mechanical properties of irradiated ODS-EUROFER and nanocluster strengthened 14YWT

Irradiations to 1.5 dpa at 300-750 °C were conducted to investigate the changes in mechanical properties of an advanced nanocluster strengthened ferritic alloy, designated 14YWT, and an oxide dispersion strengthened ferritic alloy ODS-EUROFER. Two non-dispersion strengthened variants, 14WT and EUROFER 97, were also irradiated and tested. Tensile results show 14YWT has very high tensile strengths and experienced some radiation-induced hardening, with an increase in room temperature yield strength of 125 MPa after irradiation, while results for ODS-EUROFER show a 275 MPa increase following irradiation. Master curve fracture toughness analysis show 14YWT has a cryogenic T_o reference temperatures before and after irradiation of about −188 and −176 °C, respectively, and upper-shelf K_JIc values between 175 and 225 MPa√m. The favorable fracture toughness properties and resistance to radiation-induced changes in mechanical properties observed for 14YWT are attributed to a fine grain structure and high number density of Y-Ti-O nanoclusters.     

Mechanical properties of Y2O3-doped W-Ti alloys

W and W alloys are currently considered promising candidates for plasma facing components in future fusion reactors but most of the information on their mechanical properties at elevated temperature was obtained in the 1960s and 1970s. In this investigation, the strength and toughness of novel Y₂O₃-doped W-Ti alloys manufactured by powder metallurgy were measured from 25 °C up to 1000 °C in laboratory air and the corresponding deformation and failure micromechanisms were ascertained from analyses of the fracture surfaces. Although the materials were fairly brittle at ambient temperature, the strength and toughness increased with temperature and Ti content up to 600 °C. Beyond this temperature, oxidation impaired the mechanical properties but the presence of Y₂O₃ enhanced the strength and toughness retention up to 800 °C.     

Mechanical property and microstructural change by thermal aging of SCS14A cast duplex stainless steel

The aging behavior, especially saturation, of JIS SCS14A cast duplex stainless steels was investigated on the basis of the mechanical properties and microstructural changes during accelerated aging at 350 °C and 400 °C. The aging behavior of the materials mainly proceeds via two stages. During the first stage, the generation and concentration of the iron-rich and chromium-enriched phase in ferrite occurs by phase decomposition. The first stage corresponds to aging times of up to 3000 h at 400 °C. During the first stage, the ferrite hardness achieved is approximately 600 VHN, and the Charpy impact energy is almost saturated. During the second stage, the precipitated chromium-enriched phase aggregates and coarsens, and the G phase precipitation also occurs. The second stage corresponds to the aging times range of 3000-30 000 h at 400 °C. During the second stage, the ferrite hardness achieved is about 800 VHN; however, further hardening exceeding 600 VHN does not influence the Charpy impact energy.     

Comparison of microstructural properties and Charpy impact behaviour between different plates of the Eurofer97 steel and effect of isothermal ageing

The microstructure and fracture properties of the Eurofer97 steel plates of thickness 14 mm and 25 mm were investigated in as-received state and in state after long-term thermal ageing (550 °C/5000 h). Detailed microstructure studies were carried out by means of optical light, electron and quantitative electron microscopy. Mechanical properties were evaluated by means of Charpy impact testing and hardness testing and fracture surfaces were fractographically analysed in macro and microscales. The microstructure of the Eurofer97 consisted of tempered martensite with M₂₃C₆ and MX precipitates. Microstructure of 14 mm plate was more homogenous and fine grained than 25 mm plate. Due to different microstructure the t_DBTT of thicker plate was on +10 °C higher than for 14 mm plate for which reached −60 °C. Slight microstructural changes on the level of subgrain consisting of their partial recrystallization and slight carbide coarsening were observed after applied ageing. The isothermal ageing caused evident shift in t_DBTT about +5 °C, which was most likely caused by recrystallization of subgrains.     

Volume conservation during irradiation growth of Zr-2.5Nb

Dimensional changes are reported in three dimensions for cold-worked Zr-2.5 Nb pressure tube material irradiated to a fast fluence of 174 × 10²⁴ nm⁻², E > 1 MeV at a nominal temperature of 250 °C. The observed dimensional changes in the longitudinal and transverse directions (up to ∼1.2% and ∼−0.5%, respectively) are consistent with earlier data at 280 °C and 310 °C, and the previously reported negative temperature dependence. The observed growth in the radial direction is negative (up to ∼0.7%). Initially, there is a small volume increase (0.05-0.1%) but this gradually decays to < 0.05% and the long term rate of volume change is negligible, within the accuracy of the measurement, demonstrating that the phenomenon observed is, indeed, irradiation growth.     

Deformation and fracture properties in neutron irradiated pure Mo and Mo alloys

The effect of neutron irradiation on the mechanical properties of select molybdenum materials, unalloyed low carbon arc-cast (LCAC) Mo, Mo-0.5% Ti-0.1% Zr (TZM) alloy, and oxide dispersion-strengthened (ODS) Mo alloy, was characterized by analyzing the temperature dependence of mechanical properties. This study assembles the tensile test data obtained through multiple irradiation and post-irradiation experiments, in which tensile specimens were irradiated up to 13.1 dpa at 80-1000 °C and tested at −194 to 1000 °C. Irradiation at 80-609 °C increased yield stress significantly, up to 170%, while the increase of yield stress after irradiation at 784-936 °C was not significant. The plastic instability stress was strongly dependent on test temperature but was nearly independent of irradiation dose and temperature. The true fracture stress showed weak dependences on test temperature, irradiation dose and temperature when ductile failure occurred. Among the test materials the stress-relieved ODS material in the longitudinal direction (ODS-LSR) displayed the highest resistance to irradiation embrittlement due to its relatively high fracture stress. The critical temperature for shear failure (CTSF) was defined and evaluated for the test materials and the CTSF values were compared with the ductile-to-brittle transition temperatures (DBTT) based on ductility data.     

Deep drawing of tungsten plates for structural divertor applications in future fusion devices

The reference design of a helium cooled divertor for future fusion reactors makes use of hundreds of thousands of finger units consisting of a pressurized structural part called a thimble. Due to the high number of parts needed, the thimble has to be fabricated by mass production techniques like deep drawing. As the thimble is a pressurized part exposed to an internal pressure of 100 bar, the demands for the material are high, which means that it requires the best available tungsten material. Former work has shown that pure tungsten material has the best impact properties and has to be preferred over other commercially available tungsten materials, such as that doped with potassium or strengthened with oxides like lanthanum oxide.Furthermore the inherent weakness of the grain boundaries has to be taken into account, which requires the need for grains that are aligned to the contour of the part (grain boundary alignment).This paper describes the successful deep drawing of a 1 mm tungsten plate in high vacuum at 600 °C. In doing this, a thimble can be machined with grains that follow the contour. Furthermore the characterization of a 1 mm tungsten plate is conducted by tensile tests at room temperature and at 600 °C, as well as by Charpy tests taking into account the anisotropic material behaviour.     

Effect of thermo-mechanical processing on microstructure and creep properties of the foils of alloy 617

The effect of rolling and annealing on the microstructure and high temperature creep properties of alloy 617 were investigated. Two types of foil specimens with different thickness reductions were prepared by thermo-mechanical processing. Recrystallization and grain growth were readily observed at specimens annealed at 950 and 1100 °C. The uniform coarse grains increase resistance against creep deformation. The grain size effect in creep deformation was dominant up to 900 °C, while dynamic recrystallization effect became dominant at 1000 °C. Dynamic recrystallization was observed in all the creep deformed foils, even though some specimens had already been (statically) recrystallized during annealing. Steady state creep rates decreased with increasing annealing temperature in the less rolled foils. The apparent activation energy Q_app for the creep deformation increased from 271 to 361 kJ/mol as the annealing temperature increased from 950 to 1100 °C.     

Core materials development for the fuel cycle R&D program

The Fuel Cycle Research and Development program is investigating methods of burning minor actinides in a transmutation fuel. One of the challenges of achieving this goal is to develop fuels capable of reaching extreme burnup levels (e.g. 40%). To achieve such high burnup levels’ fast reactor core materials (cladding and duct) must be able to withstand very high doses (>300 dpa design goal) while in contact with the coolant and the fuel. Thus, these materials must withstand radiation effects that promote low temperature embrittlement, radiation induced segregation, high temperature helium embrittlement, swelling, accelerated creep, corrosion with the coolant, and chemical interaction with the fuel (FCCI).To develop and qualify materials to a total fluence greater than 200 dpa requires development of advanced alloys and irradiations in fast reactors to test these alloys. Test specimens of ferritic/martensitic alloys (T91/HT-9) previously irradiated in the FFTF reactor up to 210 dpa at a temperature range of 350-750 °C are presently being tested. This includes analysis of a duct made of HT-9 after irradiation to a total dose of 155 dpa at temperatures from 370 to 510 °C. Compact tension, charpy and tensile specimens have been machined from this duct and mechanical testing as well as SANS and Mossbauer spectroscopy are currently being performed. Initial results from compression testing and Charpy testing reveal a strong increase in yield stress (∼400 MPa) and a large increase in DBTT (up to 230 °C) for specimens irradiated at 383 °C to a dose of 28 dpa. Less hardening and a smaller increase in DBTT was observed for specimens irradiated at higher temperatures up to 500 °C. Advanced radiation tolerant materials are also being developed to enable the desired extreme fuel burnup levels. Specifically, coatings are being developed to minimize FCCI, and research is underway to fabricate large heats of radiation tolerant oxide dispersion steels with homogeneous oxide dispersions.     

Heat treatment effect of T91 martensitic steel on liquid metal embrittlement

The sensitivity of liquid metal embrittlement of the T91 martensitic steel is investigated with small punch tests at 300 °C in air and in lead bismuth eutectic (LBE). The material was studied in six tempering conditions corresponding to different values of hardness. An effect of LBE has been observed for all the materials excepted for T91 steel tempered at 750 °C, the more ductile material. In high strength materials (T91 steel as quenched, tempered at 600 °C or 500 °C), a ductile to brittle transition is induced by liquid metal, confirmed by the observation of brittle fracture. In relative high strength materials (tempered at 650 °C and 700 °C), LBE promotes a decrease in mechanical properties and a reduction of the ductility of materials, with a mixed ductile and brittle fracture.     

Influence of tempering temperature upon precipitate phases in a 11%Cr ferritic/martensitic steel

The effect of tempering temperature on the precipitate phases in a 11%Cr ferritic/martensitic steel normalized at 1050 °C for 1 h and tempered for 2 h at temperatures ranging from 600 to 780 °C has been investigated using transmission electron microscope and energy-dispersive X-ray spectroscopy. The results show that tempering temperature does not affect the existences of niobium-rich carbonitrides, (Nb_0.7V_0.2Cr_0.1)(C,N) and (Nb_0.55V_0.35Cr_0.1)(C,N), vanadium-niobium-rich carbonitride (V_0.45Nb_0.45Cr_0.1)(C,N), chromium-rich carbonitride (Cr_0.83V_0.12W_0.05)₂(C,N) and chromium-rich carbide (Cr_0.7Fe_0.25W_0.05)₂₃-C₆, whilst the precipitations of vanadium-rich carbonitrides, (V_0.65Nb_0.2Cr_0.15)(C,N) and (V_0.55Nb_0.25Cr_0.2)(C,N) are dependent on tempering temperature, which were detected only at the higher tempering temperatures of 750 and 780 °C. No coarsening was occurred during the temperings for the niobium-rich and spherical vanadium-rich carbonitrides. There was a low coarsening rate for the chromium-rich carbonitrides and chromium-rich carbides with increasing the tempering temperature from 600 to 700 °C and 650 to 780 °C, respectively, and a high coarsening rate for the chromium-rich carbonitrides and chromium-rich carbides at the tempering temperatures 750 through 780 °C and 650 °C, respectively. The compositions show an increase in vanadium and a decrease in niobium and chromium contents for the niobium-rich carbonitrides, and a decrease in niobium and an increase in vanadium and chromium contents for the vanadium-niobium-rich carbonitrides, and an increase in vanadium and a decrease in tungsten contents for the chromium-rich carbonitrides. The chromium-rich carbides show an increase and a decrease in their iron and chromium contents, respectively, with increasing the tempering temperature from 650 to 780 °C.     

Monitoring the embrittlement of reactor pressure vessel steels by using the Seebeck coefficient

The degree of embrittlement of the reactor pressure vessel (RPV) limits the lifetime of nuclear power plants. Therefore, neutron irradiation-induced embrittlement of RPV steels demands accurate monitoring. Current federal legislation requires a surveillance program in which specimens are placed inside the RPV for several years before their fracture toughness is determined by destructive Charpy impact testing. Measuring the changes in the thermoelectric properties of the material due to irradiation, is an alternative and non-destructive method for the diagnostics of material embrittlement. In this paper, the measurement of the Seebeck coefficient () of several Charpy specimens, made from two different grades of 22 NiMoCr 37 low-alloy steels, irradiated by neutrons with energies greater than 1 MeV, and fluencies ranging from 0 up to 4.5 × 10¹⁹ neutrons per cm², are presented. Within this range, it was observed that increased by ≈500 nV/°C and a linear dependency was noted between and the temperature shift ΔT_41 J of the Charpy energy vs. temperature curve, which is a measure for the embrittlement. We conclude that the change of the Seebeck coefficient has the potential for non-destructive monitoring of the neutron embrittlement of RPV steels if very precise measurements of the Seebeck coefficient are possible.     

Study of electron beam curing process using epoxy resin system

Polymeric matrix composite (PMC) has been used in engineering applications instead of metal in the last few years, due to its corrosion resistance and excellent relation between tensile strength/density and elastic modulus/density. However, PMC materials cured by thermal process require high temperature and are time-consuming. The electron beam (EB) curing technology allows its use at room temperature and reduced curing times, and this is one of the main advantages over thermal technology. The aim of this work is to investigate electron beam curable epoxy formulations to use in filament winding processes to produce composite material with similar or better properties than thermal curable composites. The study has been made with commercial epoxy resins and cationic initiators. The epoxy resin samples were irradiated for few minutes with total dose of 150 kGy. The glass transition temperatures (T_g) were determined by dynamic mechanical analyzer (DMA) and the result was 137 °C. The thermal process was carried out in a furnace following three steps: 4 h at 90 °C, increasing temperature from 90 °C to 130 °C during 4 h and 12 h at 130 °C. The total process time was 20 h. The T_g of this sample was 102 °C.     

Embrittlement of irradiated ferritic/martensitic steels in the absence of irradiation hardening

Irradiation damage caused by neutron irradiation below 425-450 °C of 9-12% Cr ferritic/martensitic steels produces microstructural defects that cause an increase in yield stress. This irradiation hardening causes embrittlement observed in a Charpy impact test as an increase in the ductile-brittle transition temperature. Little or no change in strength is observed in steels irradiated above 425-450 °C. Therefore, the general conclusion has been that no embrittlement occurs above these temperatures. In a recent study, significant embrittlement was observed in F82H steel irradiated at 500 °C to 5 and 20 dpa without any change in strength. Earlier studies on several conventional steels also showed embrittlement effects above the irradiation-hardening temperature regime. Indications are that this embrittlement is caused by irradiation-accelerated or irradiation-induced precipitation. Observations of embrittlement in the absence of irradiation hardening that were previously reported in the literature have been examined and analyzed with computational thermodynamics calculations to illuminate and understand the effect.