DBTT Shift Research Articles

Transition in wear behavior and mechanical properties of novel high nitrogen martensitic steel in cryogenic temperature regimes

In space engines with liquid cryogenic LH2/ LO2 propellants, bearings are used in cryopumps subjected to harsh working conditions. High nitrogen martensitic stainless (HNMS) steel is a newly developed steel modifying currently used AISI 440C steel by partial substitution of C with N and is a potential replacement for AISI 440C as an antifriction bearing material In the present study, HNMS samples are austenitized at 1075 °C followed by cooling to −80 °C (CT) and then double tempering at 500 °C followed by cryogenic treatment at −196 °C for 8 h and 1 h soft tempering at 100 °C (CTC). Hardness increment by 7.9% and 7.6%, wear rate decrement by 11.11% and 7.69% and shift in DBTT from − 60 °C to − 90 °C was observed for CTC samples compared to CT samples at −196 °C and RT respectively. The improved properties for CTC samples might be due to precipitation and uniform distribution of tertiary nano precipitates.

Development of innovative materials and thermal treatments for DEMO water cooled blanket

One of the options currently taken into account for the realization of the first DEMO reactor is the “water-cooled lanket”. This option implies an irradiation temperature for the blanket material in the range of 280–350 °C. Therefore, in light of the under irradiation behaviour of EUROFER, namely of the DBTT shift toward high temperature due to the low irradiation temperature embrittlement, the target of the hereby reported activities is the development of much tougher alloys, to try to tolerate the embrittlement due to the low irradiation temperature. We report in this paper the work done to optimize the toughness of Eurofer 97, increasing the normalizing temperature and maintaining a small grain size using multiple normalizing treatments. We report also the mechanical behaviour of two 9Cr1WTa type alloys, produced and tested with the same aim to find alloys more resistant to embrittlement at low irradiation temperature.

Nanocrystalline-grained tungsten prepared by surface mechanical attrition treatment: Microstructure and mechanical properties

A nanostructured surface layer was fabricated on commercial pure tungsten using the method of surface mechanical attrition treatment (SMAT). The microstructure evolution of the surface layer was characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and its formation mechanism was discussed as well. Both refinement and elongation of the brittle W grains were confirmed. The elongated SMATed W was heavily strained, the maximum value of the strain at the grain boundaries reaches as high as 3–5%. Dislocation density in the SMATed W nanograins was found to be 5 × 1012 cm−2. The formation of the nanograins in the top surface layer of the W was ascribed to the extremely high strain and strain rate, as well as the multidirectional repetitive loading. Bending strength of commercial W could be improved from 825 MPa to 1850 MPa by SMAT process. Microhardness results indicated the strain range in SMATed W can reach up to 220 μm beneath the top surface. The notched Charpy testing results demonstrated that SMATed W possess higher ductility than that of commercial W. The top surface of the W plates with and without SMATe processing possesses residual compressive stress of about −881 MPa and −234 MPa in y direction, and −872 MPa and −879 MPa in x direction respectively. The improvement of toughness (DBTT shift) of SMATed W may be the synergistic effect of residual compressive stress, dislocation density improvement and microstructure refinement induced by SMAT processing. SMAT processing could be a complementary method to further decrease the DBTT value of tungsten based materials.

S083022 RV監視試験片データから見たHAZ遷移温度上昇ラボデータの統計的性質

S083022 RV監視試験片データから見たHAZ遷移温度上昇ラボデータの統計的性質

Impact Properties of Low Activation Vanadium Alloy After Low-Temperature Neutron Irradiation at 450°C and Below

A reference low-activation vanadium alloy NIFS-HEAT-2 was neutron-irradiated at 450 °C and below, in order to estimate the resistance to low temperature irradiation. DBTT of NIFS-HEAT-2 was -85 °C after irradiation up to 8.5 dpa at 450 °C in Na atmosphere, while DBTT was below -196 °C for 3.7 dpa at 430 °C in Li atmosphere. On the other hand, DBTT was lower than about -90 °C for the irradiation up to 0.1˜1 dpa at 60, 290 and 400 °C. The DBTT shift was increased with increasing hardness after neutron irradiation for limited irradiation conditions. The mechanisms of DBTT shift and irradiation hardening at low temperature was discussed.

Reduction method of DBTT shift due to irradiation for reduced-activation ferritic/martensitic steels

The method for reducing irradiation-induced DBTT shift of reduced-activation ferritic/martensitic steels was examined. F82H-LN (low nitrogen, 20 ppm), F82H+60 ppm 11B+200 ppmN and F82H+60 ppm 10B+200 ppmN steels tempered at 780 °C for 0.5 h were irradiated at 250 °C to 2 dpa, and the results for Charpy impact tests were analyzed. The upper shelf energy of F82H+ 11B+N steel was hardly changed by the irradiation, and DBTT shift was very small. From our research, DBTT shift due to irradiation can be reduced by the control of tempered conditions before irradiation, and it is found to be furthermore reduced by impurity doping with 60 ppm 11B and 200 ppmN to F82H steel.

Study of helium embrittlement in boron doped EUROFER97 steels

To simulate helium effects in Reduced Activation Ferritic/Martensitic steels, experimental heats ADS2, ADS3 and ADS4 with the basic composition of EUROFER97 (9%Cr-WVTa) were doped with different contents of natural boron and separated 10B-isotope (0.008–0.112 wt.%) and irradiated in High Flux Reactor (HFR) Petten up to 16.3 dpa at 250–450 °C and in Bor-60 fast reactor in Dimitrovgrad up to 31.8 dpa at 332–338 °C. The embrittlement and hardening are investigated by instrumented Charpy-V tests with subsize specimens. Complete burn-up of 10B isotope under neutron irradiation in HFR Petten led to generation of 84, 432 and 5580 appm He and partial boron-to-helium transformation in Bor-60 led to generation of 9, 46, 880 appm He in ADS2, ADS3 and ADS4 heats, respectively. At low irradiation temperatures T irr ⩽ 340 °C the boron doped steels show progressive embrittlement with increasing helium amount. Irradiation induced DBTT shift of EUROFER97 based heat doped with 1120 wppm separated 10B isotope could not be quantified due to large embrittlement found in the investigated temperature range. At T irr ⩽ 340 °C helium induced extra embrittlement is attributed to material hardening induced by helium bubbles and described in terms of phenomenological model.

Recent progress in US–Japan collaborative research on ferritic steels R&D

The mechanisms of irradiation embrittlement of two Japanese RAFSs were different from each other. The larger DBTT shift observed in F82H is interpreted by means of both hardening effects and a reduction of cleavage fracture stress by M 23C 6 carbides precipitation along lath block and packet boundaries, while that of JLF-1 is due to only the hardening effect. Dimensional change measurement during in-pile creep tests revealed the creep strain of F82H was limited at 300 °C. Performance of the weld bond under neutron irradiation will be critical to determine the life time of blanket structural components. Application of the ODS steels, which are resistant to corrosion in supercritical pressurized water, to the water-cooled blanket is essential to increase thermal efficiency of the blanket systems beyond DEMO. The coupling of RAFS and ODS steel could be effective to realize a highly efficient fusion blanket.

Radiation embrittlement behavior of fine-grained molybdenum alloy with 0.2 wt%TiC addition

In order to elucidate the effects of pre-irradiation microstructures and irradiation conditions on radiation embrittlement and radiation-induced ductilization (RIDU), fine-grained Mo–0.2 wt%TiC specimens with high and low reduction rates in plastic working, which are designated as MTC-02H and MTC-02L, respectively, were prepared by powder metallurgical methods. The specimens were neutron irradiated to 0.1–0.15 dpa with controlled 1-cycle and 4-cycle heating between 573 and 773 K, and 473 and 673 K, respectively, in JMTR. Vickers microhardness and three-point bending impact tests and TEM microstructural examinations were made. The degree of radiation embrittlement, assessed by DBTT shift due to irradiation, was strongly dependent on the reduction rate and cycle number. The 4-cycle irradiation suppressed the radiation embrittlement compared with the 1-cycle irradiation, and the suppression was much more significant in MTC-02L than in MTC-02H. The observed behavior is discussed in connection with RIDU and microstructural evolution caused by the 4-cycle irradiation.

Irradiation resistance of Eurofer97 at 300 °C up to 10 dpa

Eurofer97 has been irradiated in material test reactors in Mol, Belgium and Petten, The Netherlands at a temperature of 300 °C from 0.2 to 10 dpa. Tensile tests show a continuous logarithmic hardening trend for this dose range of 280 MPa per decade dpa. Impact tests on miniature Charpy-type specimens prove the superiority of Eurofer97 over F82H-mod. when DBTT shift is considered. DBTT shift in degree centigrade is about one fourth the hardening in MPa for the former. The upper shelf energy drops 0.3 J per dpa. Eurofer97 25 mm plate shows worse impact properties than the other product forms of Eurofer97.

Effect of neutron irradiation at low temperature on the embrittlement of the reduced-activation ferritic steels

Effects of neutron irradiation to fluence of 2.0 × 10 24 n/m 2 ( E > 0.5 MeV) in temperature range 70–300°C on mechanical properties and structure of the experimental reduced-activation ferritic 0.1%C–(2.5–12)%Cr–(1–2)%W–(0.2–0.7)%V alloys were investigated. The steels were studied in different initial structural conditions obtained by changing the modes of heat treatments. Effect of neutron irradiation estimated by a shift in ductile-brittle transition temperature (ΔDBTT) and reduction of upper shelf energy (ΔUSE) highly depends on both irradiation condition and steel chemical composition and structure. For the steel with optimum chemical composition (9Cr–1.5WV) after irradiation to 2 × 10 24 n/m 2 ( E ⩾ 0.5 MeV) at 280°C the ΔDBTT does not exceed 25°C. The shift in DBTT increased from 35°C to 110°C for the 8Cr–1.5WV steel at a decrease in irradiation temperature from 300°C to 70°C. The CCT diagrams are presented for several reduced-activated steels.

Status of vanadium alloys for fusion reactors

Advantages of vanadium alloys for fusion reactor structural applications are: low induced activation, excellent thermal stress factor, high strength at elevated temperatures, and superior ductility at low temperatures. Resistance to irradiation damage is also very impressive, i.e. very small DBTT shift by irradiation and low swelling. Research and development of vanadium alloys have made a remarkable progress in recent years partly supported by ITER-related activities. Composition range centered about V4Cr4Ti is the target of many of the studies conducted in the US, Japan and RF. Most of the studies have demonstrated the superior performance of this alloy family, while some alloys containing large amount of impurities have shown relatively poor properties. In addition to the baseline mechanical properties, neutron irradiation effects, e.g., swelling and radiation embrittlement are covered in this paper. Of particular importance is the study of the effects of dynamically-charged helium on mechanical properties and swelling. Recent developments of insulator coatings and welding are also covered. The importance of mechanistic studies of the damage behavior is emphasized for efficient alloy development and prediction of materials life in service.

Charpy impact properties of martensitic 10.6% Cr steel (MANET-I) before and after neutron exposure

An extensive irradiation programme (FRUST/SIENA) was elaborated to study the influence of radiation upon the Charpy impact characteristics of the MANET-I martensitic 10.6% Cr steel. In addition to unirradiated reference specimens, 87 irradiated subsize Charpy specimens (3 x 4 x 27 mm 3) were examined under eight different heat treatments at irradiation temperatures between 290 and 475 °C and exposure doses of 5–15 dpa. A newly developed instrumented Charpy impact bending test and evaluation system was used in the tests. The impact properties characterized by the upper shelf energy (USE) and the transition temperature from brittle to ductile material behaviour (DBTT) were determined before and after irradiation. Clear empirical correlations could be established according to which irradiation at lower temperatures provokes a shift in DBTT and USE independent of the initial material state with dose saturation at 10–15 dpa. An elevated irradiation temperature alleviates the irradiation-induced material deterioration.

Effects of neutron irradiation on microstructural evolution in candidate low activation ferritic steels

Fe-(2.25-12)Cr-2W-V, Ta low activation ferritic steels (JLF series steels) were developed in the fusion materials development program of Japanese universities. Microstructural observations, including precipitation response, were performed after neutron irradiation in the FFTF/MOTA. The preirradiation microstructure was stable after irradiation at low temperature (< 683 K). Recovery of martensitic lath structure and coarsening of precipitates took place above 733 K. Precipitates observed after irradiation were the same as those in unirradiated materials in 7–9Cr steels, and no irradiation induced phase was identified. The irradiation induced shift in DBTT in the 9Cr-2W steel proved to be very small which is a reflection of stable precipitation response in these steels. A high density of fine α' precipitates was observed in the 12Cr steel which might be responsible for the large irradiation hardening found in the 12Cr steel. Void formation was observed in 7–9Cr steels irradiated at 683 K, but the amount of void swelling was very small.

Influence of neutron irradiation on the charpy impact properties of V-15Cr-5Ti

Vanadium alloys are being evaluated for fusion reactor first wall and blanket applications to achieve both low neutron activation and operating temperatures above those suitable for ferritic steels. Both pre- and postirradiation impact tests have been completed on miniature Charpy specimens of the alloy V-15Cr-5Ti in the annealed condition (1200/sup 0/C/1 hr/air cool). Irradiation was conducted in lithium filled TZM capsules at 365, 420, 520 and 600/sup 0/C to doses ranging from 10 to 30 dpa. The ductile-to-brittle transition temperature was unexpectedly high in the unirradiated condition, on the order of 130/sup 0/C. Impact tests demonstrate that irradiation induces a large upward shift in DBTT in this alloy. Fracture occurred primarily by transgranular cleavage in irradiated specimens at temperatures as high as 240/sup 0/C. 10 refs., 5 figs., 1 tab.