T. Sawai

Microstructural evolution of welded austenitic stainless steels irradiated in the spectrally-tailored ORR experiment at 400°C

Abstract Microstructural evolution of austenitic stainless steels and their welds has been examined after spectrally-tailored neutron irradiation. JPCA and 316W, containing 0.24 and 0.08 wt% of titanium, respectively, were electron-beam welded. TEM disks taken from these weld joints were irradiated in the ORR (Oak Ridge Research Reactor), to 7.4 dpa and almost 100 appm He. Base metal specimens of 316R with very low titanium content (0.005 wt%) were also irradiated. Specimens were examined by precision immersion densitometry before TEM observation. Only the 316R base metal showed measurable swelling by density change. Cavity swelling, determined by TEM observations in the base metals, was 0.29% for 316R, 0.06% for 316W and 0.03% for JPCA. Titanium effectively suppressed the cavity swelling of the base metals. The cellular microstructure of fusion zone remained after this irradiation both in JPCA and 316W with uniform distribution of cavities. Welding did not degrade the swelling resistance as measured either by immersion densitometry or TEM.

Microstructural evolution of austenitic stainless steels irradiated in spectrally tailored experiment in ORR at 400°C

Abstract Several different heats of austenitic stainless steel, including Japanese-PCA(JPCA), were irradiated in the spectrally tailored ORR experiment at 400°C to 7.4 dpa. The levels of helium generated were 155 appm for JPCA (16Ni, 30 wppm B) and 102 appm for standard type 316 steel (13Ni). The mean He: dpa ratio throughout the irradiation falls between 15 and 20 appm He/dpa, which is close to the He/dpa values expected for fusion. Swelling was measured by transmission electron microscopy and by precision immersion densitometry. All the CW alloys showed swelling that was at or below the detection limit of the densitometer (0.1%). No measurable swelling was detected in the SA JPCA alloy, while the highest value of 0.8% was observed in the SA high-purity alloy. One Ti-modified steel with low C also showed a relatively high swelling value of 0.5%, while standard type 316 steel showed only 0.15% swelling. TEM observation gave consistent but slightly larger values of swelling.

Comparison of immersion density and improved microstructural characterization methods for measuring swelling in small irradiated disk specimens

Abstract The procedure of obtaining microstructural data from reactor-irradiated specimens has been carefully checked for accuracy by comparison of swelling data obtained from transmission electron microscopy (TEM) observations of cavities with density-change data measured using the Precision Densitometer at Oak Ridge National Laboratory (ORNL). Comparison of data measured by both methods on duplicate or, in some cases, on the same specimen has shown some appreciable discrepancies for US/Japan collaborative experiments irradiated in the High Flux Isotope Reactor (HFIR). The contamination spot separation (CSS) method was used in the past to determine the thickness of a TEM foil. Recent work has revealed an appreciable error in this method that can result in an overestimation of the foil thickness. This error causes lower swelling values to be measured by TEM microstructural observation relative to the Precision Densitometer. An improved method is proposed for determining the foil thickness by the CSS method, which includes a correction for the systematic overestimation of foil thickness.

Swelling behavior of welded type 316 stainless steel and its improvement

Abstract Type 316 stainless steel was electron beam welded with titanium foil insertion. The concentration of introduced titanium in the weld metal was 0.1, 0.3 and 0.6 wt% corresponding to the inserted foil thickness of 10, 30 and 60 μm, respectively. All the weld joint showed good mechanical performance. The swelling resistance of the weld metal is effectively improved by the introduced titanium. Although inhomogeneous distribution of titanium makes it difficult to estimate the extent of the improvement quantitatively, the results suggest the applicability of this method to Ti-modified 316 stainless steel, where weld metal is already reported to show reduced swelling resistance.

Microstructural development of 10Cr-2Mo ferritic steel irradiated to 57 dpa in HFIR

Abstract Microstructural development under irradiation of the 10Cr-2MoVNb Japanese ferritic/martensitic steel (JFMS) was investigated. Irradiation was performed in the High Flux Isotope Reactor (HFIR) at temperatures that ranged from 400 to 600° C to doses of up to 57 dpa. Voids were observed only after irradiation at 500°C to 57 dpa. x-Phase was formed during the irradiation. The microstructural stability, or chemical composition of thermal precipitates which were produced by tempering (M23C6, M6C) or by thermal aging (Laves phase) were also greatly affected by irradiation. Microcompositional analysis was also able to detect nickel and silicon segregation around helium bubbles.

Void formation in 10Cr-2Mo ferritic steel irradiated in HFIR

Abstract Microstructural evolution of a 10Cr-2Mo ferritic steel was investigated after irradiation in the High Flux Isotope Reactor (HFIR) at 500°C and doses up to 57 dpa. About 300 appm of helium was also produced after 57 dpa because the steel contained 1 wt% Ni. Many tiny helium bubbles were observed in a specimen irradiated to 34 dpa, either inside of or adjacent to particles of radiation-produced x- and Laves-phase precipitates. By contrast, after 57 dpa many of the tiny bubbles had developed into larger voids. Most of the voids were associated with radiation-induced x-phase particles.

Swelling susceptibility of electron-beam welded austenitic stainless steel

The void swelling susceptibility of austenitic stainless steel weldments has been examined. Materials used were Type 316 stainless steel containing 0.08% Ti and JPCA. Plates of these steels were electron-beam welded in a vacuum, and disks for irradiation experiments were obtained from the transverse sections, corresponding to the base metal, heat affected zone and weld metal. Irradiation in a High Voltage Electron Microscope (HVEM) was carried out at 773 K up to 15 dpa. In both steels this produced more void swelling in the weld metal than in the base metal, and more void swelling in the heat affected zone than in the base metal for Type 316 stainless steel. Segregation during solidification was detected in the weld metal and this may affect the void swelling susceptibility through the compositional change along with precipitation.

Microstructure and mechanical properties of α-particle irradiated stainless steels

The effect of helium injected in stainless steels on the mechanical properties are investigated. The materials used are 100 μm thick foils of type 316 austenitic stainless steel, dual phase (9Cr-2Mo, ferritic and martensitic phase) stainless steel and duplex (22Cr-5Ni, ferritic and austenitic phase) stainless steel. The tensile strength (0.2% proof strength and ultimate strength in room temperature tensile test) of these materials increased slightly with the amount of injected alpha particles, but the elongation of these steels decreased with the alpha particle dose. Intergranular type fracture was observed only on the 316 stainless steel in which alpha particles had been injected in amounts up to 1.3 × 10 17 cm −2 at the depth of range. Transgranular fracture occurred on a post-injection annealed 316 stainless steel and on the other materials. The transmission electron microscope observation of helium bubble distribution established that the intergranular fracture is caused by a highly dense distribution of small helium bubbles at grain boundaries.

Microstructural evolution of He-preinjected austenitic stainless steels under HVEM irradiation

Abstract Two kinds of austenitic stainless steels, 316R (standard type 316 stainless steel) and alloy K (low-C and 0.29 wt% Ti) were irradiated in a high voltage electron microscope (HVEM) after He injection to 2.5 and 10 appm. 45 keV He was injected into the prepared transmission electron microscope (TEM) foils and HVEM irradiation was performed at 773 K to 14 dpa. Without helium preinjection, alloy K shows very few void production and thus the void swelling of alloy K was lower than that of 316R. Injected helium of 2.5 and 10 appm led to the void number densities of 10 22 /m 3 level or more in both steels. Void swelling was largest for 2.5 appm He specimens for both steels and the void growth rate was higher in alloy K than in 316R, and void swelling was higher in alloy K. The order of swelling resistance of these two steels as thus changed after 2.5 appm He injection.

Measurement of mean projected ranges for carbon-ions irradiated into stainless steel

Abstract Type 316 stainless steel was irradiated at ambient temperature with C-ions of 40 MeV to 1.4 dpa at the peak. A band was revealed by chemical etching on the cross-sectional plane normal to the irradiated surface at the distance of 18–19 μm from the ion-incident surface. In the region of the band, the radiation-produced damage structure was observed with electron microscopy. The center of the band at 18.5 μm approximates the mean projected range and agrees well with the value of 18.6 μm calculated by the extended E-DEP-1 computer code with Zieglers electronic stopping powers. The mean projected ranges for 60. 80 and 90 MeV C-ions irradiated into Type 316 stainless steel are measured to be 31. 47 and 57 μm, respectively, which are also in good agreement with calculated values of 32.7, 49.5 and 59.2 μm.