M3LW-18OR0402023 - Austenitic stainless steel samples

ORNL/TM-2018/113792 M3LW-18OR0402023

Preparation and Analysis of Austenitic Stainless Steel Samples Irradiated at Very High Damage Doses

Maxim N. Gussev Scarlett R. Clark James T. Dixon Keith J. Leonard

July 2018

Approved for public release. Distribution is unlimited.

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ORNL/TM-2018/113792 M3LW-18OR0402023

Fusion and Materials for Nuclear Systems Division Materials Science and Technology Division

Preparation and Analysis of Austenitic Steel Samples Irradiated at Very High Damage Doses

Maxim N. Gussev Scarlett R. Clark James T. Dixon Keith J. Leonard

Date Published: July 2018

Prepared under the direction of the U.S. Department of Energy Office of Nuclear Energy

Light Water Reactor Sustainability Program Materials Research Pathway

Prepared by OAK RIDGE NATIONAL LABORATORY

Oak Ridge, TN 37831-6285 managed by

UT-BATTELLE, LLC for the

U.S. DEPARTMENT OF ENERGY under contract DE-AC05-00OR22725

(This page intentionally left blank)

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CONTENTS

LIST OF FIGURES ...................................................................................................................................... v LIST OF TABLES ........................................................................................................................................ v ABBREVIATED TERMS .......................................................................................................................... vii EXECUTIVE SUMMARY ......................................................................................................................... ix 1. DESCRIPTION OF HIGH-DOSE SPECIMENS ................................................................................. 1

1.1 BACKGROUND ........................................................................................................................ 1 1.2 ORIGIN, COMPOSITION, AND DIMENSIONS OF VERY-HIGH-DOSE

SPECIMENS............................................................................................................................... 2 2. SPECIMEN HANDLING AND PREPARATION AT ORNL ............................................................ 3

2.1 STEPS PERFORMED AT THE HOT CELL FACILITY .......................................................... 3 2.2 SPECIMEN PREPARATION AT LAMDA .............................................................................. 5 2.3 SHIPMENT PREPARATION .................................................................................................... 5

3. ANALYSIS OF HIGH-DOSE TENSILE SPECIMENS ...................................................................... 6 3.1 AVAILABILITY OF ARCHIVE MATERIALS ........................................................................ 6 3.2 THE MICROSTRUCTURE OF ARCHIVE MATERIALS ....................................................... 6

3.2.1 304L STEEL .................................................................................................................. 6 3.2.2 316L STEEL .................................................................................................................. 8

3.3 MAGNETIC PHASE IN HIGHLY IRRADIATED SPECIMENS .......................................... 11 3.3.1 POSSIBLE CORRELATION BETWEEN MAGNETIC PHASE(S) AND

IASCC .......................................................................................................................... 11 3.4 EVALUATION OF THE EDM CAPABILITIES FOR PRODUCING MINIATURE

SUBSPECIMENS ..................................................................................................................... 13 4. SUMMARY AND CONCLUSIONS ................................................................................................. 15 5. ACKNOWLEDGMENTS .................................................................................................................. 15 6. REFERENCES ................................................................................................................................... 15


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LIST OF FIGURES

Figure 1. Halden shipment cask (left), holder with specimens (middle), and specimens in individual containers (right). Photo courtesy: M. Delph (ORNL’s IMET facility supervisor) ....................................................................................................................................... 1

Figure 2. Shape and dimensions of the tensile specimen with 2 mm diameter tensile gauge ....................... 2 Figure 3. Dimple at the tensile specimen head. Trial cutting (left) demonstrated that the dimples

are deep enough (more than ~1.5–2 mm). ....................................................................................... 4 Figure 4. 125 dpa tensile specimens received at LAMDA. .......................................................................... 5 Figure 5. Irradiated tensile bar prior (top) and after (bottom) cutting........................................................... 5 Figure 6. General view of the 304L archive material’s grain structure. Black arrows indicate

retained ferrite. RD: assumed rolling direction. ............................................................................... 6 Figure 7. SEM-BSE image of the reference specimen showing elongated bamboo-like ferrite

colonies as well as isolated small ferritic grains. Black arrows indicate retained ferrite. ................ 7 Figure 8. SEM-BSE image of the reference 304L specimen showing ferrite colonies and single

isolated small ferritic grains. Black arrows indicate retained ferrite. One may see weak “halos” near the ferrite particles; these may reflect composition variations or may be just an imaging technique artifact. .......................................................................................................... 7

Figure 9. Reference 304L specimen surface at high magnification. One may see minor surface damage (indicated by black arrows). ............................................................................................... 8

Figure 10. Archive 316L steel specimen. General view of the grain structure. SEM-BSE images. RD: assumed rolling direction. ........................................................................................................ 9

Figure 11. Signs of plastic deformation (multiple slip lines) in the structure of 316L reference materials. Black lines show active slip planes in the selected austenitic grain. Some areas with pronounced gray color intensity changes (lattice misorientation gradients) are indicated by white arrows. ............................................................................................................... 9

Figure 12. Slip lines and deformation twins in the structure of the 316L specimen. .................................. 10 Figure 13. The appearance of inclusion in BSE (left) and SE (right) SEM modes..................................... 11 Figure 14. The relationship between IG% and magnetic flux density for model alloys irradiated in

JRR-3 (after [8]). ............................................................................................................................ 12 Figure 15. Correlation between intergranular fracture fraction (IG%) and ferrite (magnetic phase)

amount in the same specimens (IG%: [4], ferrite amount: [5]). ................................................... 12 Figure 16. Estimated magnetic phase amount in the high-dose BOR-60 specimens. Data for

duplex 308 steel are not shown. ..................................................................................................... 13 Figure 17. Typical appearance of the in-house–produced miniature tensile specimens after the

rough cut. The tilted light source was used to highlight the surface roughness. ............................ 14 Figure 18. The appearance of the surface and specimen edge after EDM cutting. An optic

microscope, same magnification, and light conditions were used to examine both images. The sample at left appeared to have a rougher surface, compared with the sample on the right. Surface appearance and colors are slightly different. The top object is yellow (or light yellow) in color, most likely, due to the copper presence; the bottom object is gray or light gray in color, typical to an iron or steel surface and indicating a much smaller copper fraction. .............................................................................................................................. 14

LIST OF TABLES

Table 1. High-dose tensile specimens delivered at ORNL for post-irradiation evaluation and IASCC testing .................................................................................................................................. 3

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Table 2. Composition (wt. %) of the materials of interest ............................................................................ 3

vii

ABBREVIATED TERMS

bcc body-centered cubic BSE backscatter electrons dpa displacement per atom EBSD electron backscatter diffraction EDM electric discharge machine fcc face-centered cubic IASCC irradiation-assisted stress corrosion cracking IMET Irradiated Materials Evaluation and Testing LAMDA Low Activation Materials Development and Analysis LWR light-water reactor LWRS Light-Water Reactor Sustainability NWC normal water chemistry ORNL Oak Ridge National Laboratory PW primary water SEM scanning electron microscopy SEM-EBSD scanning electron microscopy coupled with electron backscatter diffraction TEM transmission electron microscope/microscopy


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EXECUTIVE SUMMARY

This report describes the preliminary results of the visual inspection, cutting, preparation for IASCC (irradiation-assisted stress corrosion cracking) testing, and property evaluation of austenitic stainless steel specimens irradiated at very-high-damage doses (up to 125 dpa) in the BOR-60 fast reactor. Work was performed at Oak Ridge National Laboratory using the Irradiated Materials Examination and Testing (hot cell) Facility and the Low Activation Materials Development and Analysis (LAMDA) laboratory. Report sections describe specific aspects of the research activity.

Section 1 reviews the general background, material, and specimen origin, and provides detailed information on material composition and damage dose level. Section 2 describes operations performed with irradiated specimens at the hot cell facility and LAMDA laboratory. Tensile specimen preparation is completed, and the tensile bars are ready for shipment to the University of Michigan for stress corrosion cracking testing. Section 3 discusses the microstructure of archive alloys and post-irradiation material properties; because of the very limited timeframe, only selected results are given on the magnetic properties of the irradiated specimens. Section 4 summarizes the work performed, presents conclusions, and discusses future activities within the line of research.


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1. DESCRIPTION OF HIGH-DOSE SPECIMENS

1.1 BACKGROUND

Materials in nuclear power plants are exposed to a harsh, highly aggressive environment that combines mechanical stresses, elevated temperature, radiation fields, transmutation effects, and other degradation modes. Because of intensive research in the area of radiation materials science, many radiation-induced degradation modes have been successfully mitigated or, at least, understood well enough for the low-dose area (<20 dpa). However, there is limited work for moderate- and high-dose regimes (>20 and 50–100 dpa, respectively), and only a few research teams have worked with very high neutron-induced damage doses (>100 dpa). As nuclear reactors age, many in-core and even peripheral components will reach high and very high damage doses. To ensure safe operations, it is important to explore these dose ranges, ensuring all processes are understood, or, at least, known. During FY2016–2017, a collaborative effort was launched to transport several high- and very-high-dose specimens (up to 125 dpa) from Dimitrovgrad, Russia, to Halden, Norway, and after that to Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee. Much of the effort involved acquiring the proper license(s) and documenting that irradiated materials were appropriately prepared for shipment. The irradiated materials arrived in Halden, Norway, on August 23, 2016. After that, new activity calculations, inventory, additional packaging, documentation, and licensing efforts were pursued. The materials were shipped from the Halden reactor to ORNL in early 2018. In March 2018, the shipment cask (Figure 1) was delivered to ORNL’s Irradiated Materials Examination and Testing (IMET) hot cell facility. The internal holder with specimens was extracted and examined, revealing no transportation-related issues. Finally, specimens were unloaded and underwent individual examination and inventory. Historically, this is one of the first, if not the first, shipment of tensile specimens to arrive at ORNL after neutron irradiation at doses over 100 dpa.

Figure 1. Halden shipment cask (left), holder with specimens (middle), and specimens in individual

containers (right). Photo courtesy: M. Delph (ORNL’s IMET facility supervisor) Several shipments of specimens prepared at ORNL will be made to the University of Michigan for continuous extension rate testing under different light-water reactor (LWR) environments (i.e., normal water chemistry [NWC] or primary water [PW]). The University of Michigan is limited on the amount of

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activity allowed at their facility at a given time; therefore, several shipments of material will be made between the university and ORNL. Upon their return to ORNL, the high-fluence samples will undergo extensive post-test characterization and will be subjected to further sectioning to create samples for different purposes (e.g., TEM analysis, EBSD, APT). As expected, these materials will provide a unique object set for examining (1) the irradiation-assisted stress corrosion cracking (IASCC) initiation and crack growth phenomenon, (2) changes in the defect microstructure, (3) swelling, and (4) precipitate phase formation. Microstructural information from these very-high-dose samples will provide additional data for examining void development and evolution at high doses, radiation-induced solute segregation at grain boundaries, and radiation-induced changes to the precipitate structures within the material. Note that current models of segregation suggest saturation by 10 dpa but may be further influenced by other high-fluence phenomena. Thus, the scientific and practical value of materials irradiated to high and very high doses cannot be overestimated.

1.2 ORIGIN, COMPOSITION, AND DIMENSIONS OF VERY-HIGH-DOSE SPECIMENS

Between 2000 and 2010, tensile and compact tension specimens of multiple materials, as well as transmission electron microscope (TEM) discs, were irradiated in the Russian BOR-60 fast reactor in the framework of BORIS (BOR-60 Internal Study) irradiation experiments [1,2]. Typical dose range was 5–20 dpa, but several capsules reached or exceeded 100 dpa. Irradiated specimens were involved in multiple programs including the Cooperative IASCC Research program [1]. Currently, under certain conditions, irradiated materials and untested specimens are available for research teams working in the radiation materials field.

2 mm2 mm

5 mm

8,5 mm16 mm

12 mm5 mm

8,5 mm

33 mm

Diameter 3,1 mm

3,5 mm

Figure 2. Shape and dimensions of the tensile specimen with 2 mm diameter tensile gauge (CEA type GI-116).

The pool of irradiated but untested specimens was examined, and 13 tensile specimens were selected for IASCC testing that included pre- and post-test analysis in the framework of the Light-Water Reactor Sustainability (LWRS) program (Table 1). The specimens were tensile bars of type GI-116 geometry [1,2] (Figure 2). In the table, the suffix E stands for cold-worked material, and the suffix H represents the traditional solution annealed condition. Specimens of this geometry have a round gauge section and utilize a pin-loading scheme, which may be replaced by the shoulder-loading scheme. Routine post-irradiation testing may be performed using as-received specimens; however, IASCC testing requires additional preparation. Table 2 shows the composition of the selected high-dose materials. The first one, AISI 308 steel, is expected to have a duplex structure—a mix of austenite (fcc) and ferrite (bcc) phases. The second, AISI

3

316L steel, is expected to be fully austenitic prior to irradiation. The third, AISI 304L steel, may have some amount of retained ferrite, depending on the final thermo-treatment of the particular heat.

Table 1. High-dose tensile specimens delivered at ORNL for post-irradiation evaluation and IASCC testing

Irradiation experiment

Specimen ID Alloy Dose

(dpa)

BORIS 2 SA5 308 19.7 BORIS 2 SA6 308 19.7 BORIS 4 B98 316 E 42.0

BORIS 6R 1-3 A84 304 H 5.4 BORIS 5 B101 316 E 46.9 BORIS 5 B102 316 E 46.9 BORIS 5 B103 316 E 46.9 BORIS 5 B89 316 E 67.4 BORIS 5 B95 316 E 67.4 BORIS 5 A98 304 H 69.0

BORIS 6R 2-1 A96 304 H 95.0 BORIS 6R 2-1 A32 304 H 125.4

BORIS 5 B35 316 E 125.4

Table 2. Composition (wt. %) of the materials of interest

Material ID

Element composition (wt. %) N (ppm)

Other (wt. %) C S P Si Mn Ni Cr Mo Cu Co

308-1 SA 0.006 0.012 0.018 0.38 1.84 9.65 20.30 0.16 – 0.05 479 Nb/Ta:

<0.005

316-1E 0.054 0.022 0.027 0.68 1.12 10.60 16.60 2.25 0.24 0.12 230

Ti: <0.01; O:

41 ppm; B: 5 ppm;

Nb/Ta: 0.01

304-1H 0.022 0.0007 0.032 0.36 1.79 9.88 18.61 – 0.25 0.064 610 B: 9 ppm

2. SPECIMEN HANDLING AND PREPARATION AT ORNL

2.1 STEPS PERFORMED AT THE HOT CELL FACILITY

Low- and moderate-dose (<50 dpa) specimens of the same geometry were successfully prepared at ORNL earlier to support multiple projects [3,4]. Based on these projects [3,4], a plan was developed to perform visual specimen inspection; then after that to cut slices from the specimen heads to reduce tensile bar activity; and finally perform light grinding and electropolishing of the gauge portion to provide a mirror-

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quality, cold-work, and damage-free surface. High surface quality is a mandatory condition for observing and studying the IASCC phenomenon and deformation localization. The first step—evaluation of specimen geometry and appearance—was performed inside the hot cell using a remote-controlled digital camera. No severe scratches, cracks, or bending were observed; all specimens appeared to be damage free, with light yellow, slightly oxidized surfaces. At the same time, it was established that all received specimens have dimples at the specimen ends (Figure 3). The dimples were not expected, as the specimen drawings (Figure 2) did not show such features. The presence of the dimples severely limited the use of the irradiated material as well as the number and geometry of any possible sub-specimens. For instance, in earlier testing, four bend samples (thin slices with dimensions of 5.5 × 3 × 0.8 mm) were produced from each irradiated tensile bar of the same geometry; these subsamples were successfully used to support multiple research projects in the framework of the LWRS program [3,5–7] including deformation experiments and advanced four-point-bend tests (e.g., instrumented deformation experiments [6] and controlled corrosion tests [3]). The trial cut was performed at the hot cell facility to estimate the dimple depth (Figure 3, right). The depth was found to be significantly higher than ~1.5 mm. Larger damage doses—up to 125 dpa compared with ~47 dpa in [4]—will attract much more attention from researchers, and material demand will be higher. In this light, the presence of the dimples is a negative circumstance. Large dimple depth allows for not more than two bend slices or TEM samples per tensile object (i.e., one per each head) if the low-speed saw is used for cutting.

Figure 3. Dimple at the tensile specimen head. Trial cutting (left) demonstrated that the dimples are deep

enough (more than ~1.5–2 mm).

According to the initial plan, specimen cutting, grinding, and electropolishing procedures were to be performed at IMET due to expected high dose rates (>200–300 mR/h/1 ft per tensile object). Specimens with such dose rate values could not be transferred to the Low Activation Materials Development and Analysis (LAMDA) laboratory. However, a specimen survey performed by radiation control technicians revealed much smaller dose rates, by a factor of four minimum, putting the specimens under the allowable LAMDA limit (100 mR/h/1 ft). Direct access to the specimens allows for faster, more accurate, and cheaper work; thus it was decided to transfer the samples to LAMDA.

Tensile bar head

The cut slice (thickness =

1 mm)

Blade

Residual material after

cutting (0.4 mm)

Dimple

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2.2 SPECIMEN PREPARATION AT LAMDA

Figure 4. 125 dpa tensile specimens received at LAMDA.

The specimen preparation procedure was kept as close as possible to the steps performed for the specimen batches prepared in FY2010-FY2016 [4]. Upon receipt, the tensile specimens (Figure 4) were surveyed, cleaned, and inventoried. A low-speed saw was employed to cut the heads (Figure 5), further reducing the radiation impact on LAMDA staff. After that, each sample was both mechanically and electrochemically polished. Mechanical polishing was performed using a rotating tool with custom specimen holder and 1200 grit SiC sandpaper to remove sodium and surface contaminants left by the reactor coolant during irradiation.

Figure 5. Irradiated tensile bar prior (top) and after (bottom) cutting.

Each sample was electropolished using the A2 solution recommended by Struers for austenitic stainless steels (composition by volume: 73% ethanol, 10% ethylene glycol monobutyl ether, 9% distilled water, and 8% perchloric acid). An ice-bath was then employed to cool the solution to below 20°C. A 30 V DC was applied for four 15 s periods per sample to achieve a mirror-quality surface finish. Temperature varied ±3.5°C during one electropolishing cycle. The electropolishing duration was adjusted as necessary for some specimens after surface evaluation with a light microscope.

2.3 SHIPMENT PREPARATION

Tensile specimen preparation is complete, and 13 tensile bars are ready for shipment to the University of Michigan.

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3. ANALYSIS OF HIGH-DOSE TENSILE SPECIMENS

3.1 AVAILABILITY OF ARCHIVE MATERIALS

Archive materials were provided by French Alternative Energies and Atomic Energy Commission (CEA) and Électricité de France S.A. (EDF) as three pieces of 316-E, 304-H, and 308-1 heats (see Table 2). These materials correspond to steels irradiated under EDF/CEA irradiation programs at the BOR-60 fast reactor [1,2]. As expected, the archive materials will provide the baseline for analysis of radiation-induced changes in the high-dose specimens.

3.2 THE MICROSTRUCTURE OF ARCHIVE MATERIALS

The pieces of archive materials were sectioned as necessary and epoxy mounted and polished using standard metallography procedures to provide suitable surface quality for the electron backscatter diffraction analysis (EBSD) and focused ion beam lift-outs. Special care was taken to minimize sample size to retain as much archive material as possible for future use. Colloidal silica polishing was used as a final preparation step to keep the sample surface flat and preserve inclusions for further energy dispersive x-ray spectroscopy analysis. Scanning electron microscope (SEM) equipped with a backscatter electron (BSE) detector was employed to evaluate grain structure, possible texturing, cold-work effects, and the presence of inclusions and their density and morphology. The BSE detector provides information on grain size and shape and allows for scanning and evaluating much larger areas, compared with EBSD.

3.2.1 304L STEEL

Figure 6 shows a general grain structure of the archive 304L steel. Scanning electron microscopy-backscatter electron (SEM-BSE) analysis revealed round, equiaxial austenitic grains with numerous annealing twins. The density of nonmetallic inclusions was found to be small enough compared with the ordinary commercial 304L heats. Additionally, retained ferrite was observed in the structure. Ferrite grains formed specific line-like colonies (indicated by black arrows in Figure 6); their orientation (almost vertical in Figure 6 and Figure 7) indicates the former rolling direction. Each ferrite colony consisted, as a rule, of several small ferrite grains forming a bamboo-like structure (Figure 7 and Figure 8).

Figure 6. General view of the 304L archive material’s grain structure. Black arrows indicate retained ferrite. RD: assumed rolling direction.

RD

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Such ferrite morphology (i.e., bands of ferritic grains) may be a detrimental factor, affecting material deformation behavior and performance in a corrosive environment. Since relative orientation of the archive material plate and irradiated specimens is not known, some additional evaluation may be needed prior to making a final decision on the irradiated material cutting scheme and sub-specimen preparation.

Figure 7. SEM-BSE image of the reference specimen showing elongated bamboo-like ferrite colonies as well

as isolated small ferritic grains. Black arrows indicate retained ferrite.

Figure 8. SEM-BSE image of the reference 304L specimen showing ferrite colonies and single isolated small ferritic grains. Black arrows indicate retained ferrite. One may see weak “halos” near the ferrite particles;

these may reflect composition variations or may be just an imaging technique artifact.

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Grain boundaries of the austenitic phase appeared to be clean, without any visible particles of carbide or another secondary phase. Additionally, the SEM-BSE images did not reveal any signs of cold work in the archive 304L steel. Austenitic grains appeared in uniform color, without any visible gradients (Figure 8). Images taken at high magnification (Figure 9), showed minor surface defects, most likely scratches arising from specimen preparation, but no evidence of slip lines or other strain-induced features were observed.

Figure 9. Reference 304L specimen surface at high magnification. One may see minor surface damage

(indicated by black arrows).

3.2.2 316L STEEL

Figure 10 shows the general grain structure of the archive 316L steel. SEM-BSE analysis revealed a complex structure with slightly elongated austenitic grains; color gradients inside grains suggest significant cold deformation level. Many inclusions observed in the structure have an elongated shape, also suggesting significant plastic deformation. The assumed rolling direction, deducted from grain shapes, is horizontal in Figure 10; however, EBSD scanning is needed to evaluate the actual texture in the archive and irradiated specimens and to define the right coordinate system.

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Figure 10. Archive 316L steel specimen. General view of the grain structure. SEM-BSE images. RD: assumed

rolling direction.

Figure 11. Signs of plastic deformation (multiple slip lines) in the structure of 316L reference materials. Black lines show active slip planes in the selected austenitic grain. Some areas with pronounced gray color intensity

changes (lattice misorientation gradients) are indicated by white arrows.

Images taken at high magnification (Figure 11, Figure 12) show pronounced slip lines in the austenitic grains; most grains contain slip lines belonging to at least three active slip planes (indicated by black lines

RD

10

in Figure 11). Significant variations in the BSE signal intensity present near grain boundaries, suggesting strong local lattice misorientation gradients and confirming material conditions (i.e., significant plastic deformation). In many grains, the observed slip lines have some width that are most likely strain-induced twins (Figure 12). Their origin will need to be confirmed by EBSD, but the line dimensions and appearance strongly suggest deformation twinning.

Figure 12. Slip lines and deformation twins in the structure of the 316L specimen.

Many inclusions appeared as black elongated voids; however, the SEM image in conventional secondary electrons shows some particles inside the void. The weak BSE signal suggests that particles consist of light elements, most likely carbon.

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Figure 13. The appearance of inclusion in BSE (left) and SE (right) SEM modes.

3.3 MAGNETIC PHASE IN HIGHLY IRRADIATED SPECIMENS

3.3.1 POSSIBLE CORRELATION BETWEEN MAGNETIC PHASE(S) AND IASCC

In contrast to the laborious TEM or mechanical testing, magnetic parameters may be evaluated using relatively simple tools. Magnetic measurements are nondestructive and may be conducted in the field (e.g., inside a nuclear reactor vessel during refueling). This circumstance makes them an attractive option for evaluating in-service material degradation. Limited available literature suggests there may be a connection between IASCC susceptibility and magnetic phase amount (Mf). Thus, Takaya et al. [8] established a correlation between the fraction of intergranular fracture (IG%) and magnetic flux for specimens irradiated by neutrons up to 5 dpa (Figure 14). Later, the same team offered a special magnetic sensor for predictive IASCC diagnostics in the nuclear reactor [9]. Recently, this correlation—an increase in IASCC susceptibility when magnetic flux increases—was extended to the austenitic steel with ferrite, which existed prior to irradiation [10]. One may speculate that magnetic measurement results may serve as an indication of critical material conditions (e.g., IASCC susceptibility).

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Figure 14. The relationship between IG% and magnetic flux density for model alloys irradiated in JRR-3

(after [8]). Without any doubt, many open questions remain such as the exact nature of magnetic phase (most authors assume bcc-ferrite, but some report the bcc-phase as martensite [11]) and the role of corrosion environment. Limited statistics present a severe obstacle; very sparse IASCC data exists for materials with known magnetic properties. A rough, qualitative assessment can be made using previous BOR-60 specimen batches: IASCC tests were conducted in different environments [3,4], whereas magnetic phase amount was evaluated at the specimen preparation stage [5]. Examining only data for 304 and 316 steel compositions, one may see (Figure 15) a close-to-linear correlation for the NWC environment; in other words, ferrite presence is detrimental. At the same time, a very weak correlation between IG% and Mf exists for the PW environment. As a first iteration, one may assume that ferrite influences IASCC processes in the NWC environment but not in the PW environment.

Figure 15. Correlation between intergranular fracture fraction (IG%) and ferrite (magnetic phase) amount

in the same specimens (IG%: [4], ferrite amount: [5]). The results for high-dose specimens are shown in Figure 16; the magnetic phase amount was measured at the heads of the electropolished tensile bars using industrial MP30 ferroprobe. Figure 16 shows that there is close-to-zero magnetic phase amount in 316-1 heat and the Mf value obviously increased with dose. However, the increase is very slow, and Mf reached only ~0.5% at 125 dpa.

NNWC

PPW

13

An unexpected result was observed for the 304-1 heat. In contrast to the 316-1 heat, this material revealed a decrease in the Mf value. This observation should be confirmed via microstructure analysis; at the moment, the available literature suggests ferrite formation under irradiation, ferrite disappearance was not observed at least at small and moderate doses at the LWR-relevant temperatures.

Figure 16. Estimated magnetic phase amount in the high-dose BOR-60 specimens. Data for duplex 308 steel

are not shown.

Further work is being done to evaluate the structure and properties of the materials irradiated at the high dose.

3.4 EVALUATION OF THE EDM CAPABILITIES FOR PRODUCING MINIATURE SUBSPECIMENS

As mentioned above, subspecimen manufacturing would be beneficial for multiple research activities like TEM or in situ testing. Electric discharge machining (EDM) is a perfect method for cutting miniature objects from a metallic material. In early FY2018, the AgieCharmilles CUT 200 Wire EDM machine was purchased and installed. The EDM purchase was sponsored by ORNL’s Materials Science and Technology Division (J. T. Busby). The EDM system is designed for precise cutting of metals and conductive materials and has maximum work area dimensions (X,Y,Z-travel) of 350 × 220 × 220 mm, which is sufficient for present project needs. Wire diameter may vary from 0.33 to 0.070 mm. The precision of the cut part contour achieves ±2 μm. As expected, the accuracy and performance of this EDM machine will allow for producing a wide range of specimens for different purposes (e.g., tensile and fracture mechanic testing, bend bars, microstructure analysis samples) and will offer great flexibility in using the available irradiated material. Currently, the EDM is installed in a clean room, but the necessary paperwork is being pursued to allow for cutting and preparation of irradiated materials. Final approval is expected to be granted in July or August 2018.

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Whereas the EDM offers better flexibility with regard to specimen design, compared with computer numeric control machining, the appearance of the remelted layer at the specimen surface is a significant drawback. To evaluate the surface quality and dimensional accuracy, several trial cuts were performed with nonirradiated 304L steel. Even the rough EDM cut mode allowed for manufacturing miniature tensile specimens (e.g., specimen in Figure 17) quickly and accurately enough.

Figure 17. Typical appearance of the in-house–produced miniature tensile specimens after the rough cut. The

tilted light source was used to highlight the surface roughness.

The first EDM trial cut produced a slightly rough, yellowish surface with rough edges (Figure 18, left). Surface coloring appeared, as believed, because of copper transfer from the cutting wire to the specimen surface; potentially it may lead to the nonuniform material removal during electropolishing. Additionally, the surface roughness was of concern due to potential increases in specimen preparation time. Several more trials were performed with different cutting parameters. Finally, an improved cutting mode was established, producing improved surface quality (Figure 18, right) with strongly reduced surface roughness and sharp specimen edges. Work is being done to establish the best possible cutting procedure.

Figure 18. The appearance of the surface and specimen edge after EDM cutting. An optic microscope, same magnification, and light conditions were used to examine both images. The sample at left appeared to have a

rougher surface, compared with the sample on the right. Surface appearance and colors are slightly different. The top object is yellow (or light yellow) in color, most likely, due to the copper presence; the bottom object is gray or light gray in color, typical to an iron or steel surface and indicating a much smaller copper fraction.

1 mm

First trial, rough cut (high current) Second trial, fine cut (low current)

1 mm

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4. SUMMARY AND CONCLUSIONS

This report documents activity and operations performed with stainless steel specimens irradiated at very-high-damage doses (up to 125 dpa) in the BOR-60 fast reactor. Thirteen tensile bars (three different steels) were delivered at ORNL early in 2018 and then surveyed and inventoried at the IMET (hot cell) facility. Upon transfer to LAMDA in April 2018, the specimens were cut to provide extra material for additional research activities; the specimen gauges were ground and electropolished to provide suitable surface quality for IASCC tests. Prepared tensile objects are ready for shipping to the University of Michigan.

Future research activity will include microhardness measurements, density measurements, and microstructure analysis. Subspecimens for bend tensing and microstructure analysis will be produced using unique wire EDM capability at the LAMDA laboratory.

5. ACKNOWLEDGMENTS

This document has been authored by the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the US Department of Energy. The US Government retains and the publisher, by accepting this document for publication, acknowledges that the US Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript or allow others to do so, for US Government purposes.

Invaluable help and support from Faiza Sefta (Électricité de France S.A., EDF) and from Torill Karlsen and Helge Valseth (Institute for Energy, Norway) in preparing the sample shipment are gratefully acknowledged. The authors also would like to thank French Alternative Energies and Atomic Energy Commission (CEA) and Électricité de France S.A. (EDF), and personally Dr. B. Tanguy (CEA), for providing archive materials.

Also, authors would like to thank M. Delph (ORNL, IMET supervisor) for help with shipment receiving and handling, as well as J. Schmidlin (LAMDA facility supervisor), P. Tedder, S. Curlin, B. Eckhart, and W. Commings (ORNL) for their valuable help with irradiated material handling and preparation. Finally, the authors would like to thank C. Massey (ORNL) for reviewing the report and providing valuable comments and suggestions and L. Varma (ORNL) for help with document preparation.

6. REFERENCES

[1] J. Massoud, P. Dubuisson, P. Scott, and V. Chamardine, “CIR II program: Description of the Boris 6 and 7 experiments in the BOR-60 fast breeder reactor,” EPRI Report, vol. 1011787, 2005.

[2] P. Scott, “Materials reliability program: A review of the cooperative irradiation assisted stress corrosion cracking research program (MRP-98),” EPRI Report, vol. 1002807, 2003.

[3] K. J. Stephenson and G. S. Was, “The role of dislocation channeling in IASCC initiation of neutron irradiated stainless steel,” Journal of Nuclear Materials, vol. 481, 2016, pp. 214–225.

[4] K. J. Stephenson and G. S. Was, “Crack initiation behavior of neutron irradiated model and commercial stainless steels in high temperature water,” Journal of Nuclear Materials, vol. 444, 2014, pp. 331–341.

[5] M. N. Gussev, J. T. Busby, L. Tan, and F. A. Garner, “Magnetic phase formation in irradiated austenitic alloys,” Journal of Nuclear Materials, vol. 448, 2014, pp. 294–300.

[6] M. N. Gussev, K. G. Field, and J. T. Busby, “Deformation localization and dislocation channel dynamics in neutron-irradiated austenitic stainless steels,” Journal of Nuclear Materials, vol. 460, 2015, pp. 139–152.

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[7] M. N. Gussev, K. G. Field, and J. T. Busby, “Strain-induced phase transformation at the surface of an AISI-304 stainless steel irradiated to 4.4 dpa and deformed to 0.8% strain,” Journal of Nuclear Materials, vol. 446, 2014, pp. 187–192.

[8] S. Takaya, Y. Nagae, T. Yoshitake, Y. Nemoto, J. Nakano, F. Ueno, K. Aoto, and T. Tsukada, “Examination of relation between IASCC susceptibility and magnetic property,” E-Journal of Advanced Maintenance, vol. 1, 2009, pp. 44–51.

[9] Y. Nemoto, S. Keyakida, T. Uchimoto, S. Takaya, and T. Tsukada, “Development of a magnetic sensor system for predictive IASCC diagnosis on stainless steels in a nuclear reactor,” International Journal of Applied Electromagnetics and Mechanics, vol. 35, 2011, pp. 123–139.

[10] (in Japanese) , , , , and , , , vol. 14, 2016, pp. 83–90.

[11] H.-H. Jin, S. Lim, and J. Kwon, “Characterization of the martensite phase formed during hydrogen ion irradiation in austenitic stainless steel,” Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, vol. 409, 2017, pp. 318–322.

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M3LW-18OR0402023 - Austenitic stainless steel samples