Irradiation Effects on Precipitation in Reduced-Activation Ferritic/Martensitic Steels

It was previously reported that reduced-activation ferritic/martensitic steels (RAFs), such as F82H-IEA and its heat treatment variant, ORNL9Cr-2WVTa, JLF-1 and 2%Ni-doped F82H, show a variety of changes in ductile-brittle transition temperature (DBTT) and yield stress after irradiation at 573 K up to 5 dpa. These differences could not be interpreted as an effect of irradiation hardening caused by dislocation loop formation. In this paper, the effects of irradiation on precipitation of RAFs were investigated to determine how these effects might affect the mechanical properties. The precipitation behavior of the irradiated steels was examined by weight analysis, X-ray diffraction analysis and chemical analysis on extraction residues. These analyses suggested that irradiation caused (1) an increase of the amount of precipitates (mainly M 23 C 6 ), (2) an increase of Cr and decrease of W contained in precipitates, and (3) the disappearance of MX (TaC) in ORNL9Cr and JLF-1

Material strength standard of F82H for RCC-MRx

Non-irradiated and irradiated material properties of a reduced activation ferritic/martensitic steel, F82H, have been extensively accumulated through international collaborative researches. F82H then demonstrates material properties applicable to the structural material of breeding blanket module. At present, material strength standard needs to be developed for the design activity of blanket module. Material strength standard generally gives average, minimum, and design values of mechanical property and their equations complying with the definition in construction code. In this work, we summarized and analyzed the experimental data of tensile, creep, and fatigue properties, and developed the material strength standard for RCC-MRx. Obtained values were then compared to those of Eurofer. Though some minor differences were observed in minimum and average values of tensile and creep properties, no significant differences were observed in their design values. As for fatigue property, F82H showed conservative design curves

Evaluation of fatigue properties of reduced activation ferritic/martensitic steel, F82H for development of design criteria

Reduced activation ferritic/martensitic (RAFM) steels are promising candidate for structural material of tritium breeding blanket in a fusion reactor. Database accumulation and definition of design criteria for RAFM have been intensively studied together with the progress of blanket design activities. As a part of database accumulation for fusion blanket, a RAFM steel, F82H was fatigue-tested at 573 and 673 K in the air. Axial strain-controlled fatigue tests were carried out with a cylindrical specimen with 8 mm of diameter with -1 of strain ratio condition in accordance with Japanese Industrial Standard, JIS Z 2279, “Method of high-temperature low cycle fatigue testing for metallic materials.” For high cycle tests, the maximum test cycles exceeded 10^6 cycles. Fatigue lifetime of F82H at temperatures ranging from room temperature to 673 K fell into the factor of 2 of the empirical fitting curve for 673 K. It was studied using experimental results that fatigue-related design limit based on RCC-MRx, such as fatigue design curves, half-life cyclic curves, and related coefficients

Non-contact strain evaluation for miniature tensile specimens of neutronirradiated F82H by digital image correlation

The small specimen tensile test is commonly used as the basic test to evaluate the irradiation effect on the strength of fusion structural materials after irradiation, and a lot of attempts were made to qualify the data obtained by small size specimen, but the specific attention on the evaluation of strain were not satisfactory given. This study therefore aims to propose a novel non-contact strain measurement technique, based on digital image correlation (DIC) method, for reduced-activation ferritic/martensitic (RAFM) steel, e.g., F82H, as the leading candidate structural material of fusion in-vessel components in Japan. A flat rectangular miniature specimen, e.g., SS-J3 type, was adopted to evaluate non-irradiated and neutron-irradiated tensile properties. In the proposed DIC, very tiny surface machining flaws were recognized by the DIC computation program and utilized to set several pairs of the gauge end points with a fixed gauge length. In case of the room-temperature test, it was clearly demonstrated that this procedure could provide more precise and reliable data compared with the previous approach of the single-scan measurement under floated gauge length. Superior linearity was achieved and recognized DIC points of concern were traceable during the entire period of the test. By applying this technique, superior high-dose stability of elastic properties of F82H was first demonstrated

An F82H steel pressurized tube creep capsule for irradiation in HFIR

A novel capsule design has been developed for measurement of irradiation creep in pressurized tubes and is being used to irradiate reduced activation ferritic/martensitic F82H steel creep test specimens. These tests are being conducted in the flux trap of the High Flux Isotope Reactor at the Oak Ridge National Laboratory. The capsule design uses a tight-fitting corrugated aluminum foil placed in the center of a vanadium alloy holder to conduct heat from the centrally located test specimen. The foil acts as a compressible thermal interface between the pressurized tube and holder, maintaining a constant thermal resistance (and thus a constant tube temperature gradient) during irradiation, regardless of differential thermal expansion, creep, and swelling in the test specimen. Mechanical interference with creep deformation of the specimen tube is minimized by using a thin (0.05 mm) foil with sufficient room to crush. Finite element analysis of the contact pressure between the specimen and foil, combined with thermal creep in the foil, showed little interference with specimen stress conditions. Specimens were designed to experience hoop stresses of 380, 300, 150, and 0 MPa at a temperature of 300 °C while being irradiated to a dose of 3.7 dpa. Passive SiC thermometry is located within the pressurized tube and the holder material for confirmation of experiment irradiation target temperatures. This work discusses aspects of the capsule's fabrication and design, including thermal models of the capsule during irradiation. Keywords: Pressurized tube, Irradiation creep, F82H IEA heat, HFI

Strain evaluation using a non-contact deformation measurement system in tensile tests of irradiated F82H and 9cr ODS steels

We developed a non-contact deformation measurement system to accurately evaluate strain for post irradiation tensile testing, since conventional strain gages cannot be used for small size specimens. The strain calculated from cross-head displacement generally includes deformation from specimen shoulders, fixtures, and the test frame in addition to the deformation from the specimen gauge section. In our system, the distance between painted marks within the specimen gauge section was measured using a high resolution video camera to evaluate the specimen deformation during room temperature tensile testing. The test materials were F82H and 9Cr ODS steels irradiated up to ≈71 displacements per atom (dpa) at about 573 K in High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL). Our system yielded accurate stress strain curves without deformations other than the specimen gage section, and the elongation was less than that calculated from cross-head displacement. This system can contribute to expanding the technically reliable database for the design activity of fusion reactor blanket, including the effects of irradiation on tensile properties. Keyword: Reduced activation ferritic/martensitic steel, Oxide dispersion strengthened steel, Post irradiation experiment, Small specimen technology, Tensile test, Strai