J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Crack Growth Behavior of F82H Steel in the 288°C water
Yuzuru ITOa, Masahiro SAITOb, Katsunori ABEb and Eiichi WAKAIaab

Japan Atomic Energy Agency, Ibaraki, Japan

Hachinohe Institute of Technology, Aomori, Japan(Received: 17 September 2014 / Accepted: 19 January 2015)

Crack growth is a key mechanical property that must be considered in the design of fusion reactor materials to
be tested at the high flux test module in the International Fusion Materials Irradiation Facility. In this study, crack
growth behavior of F82H steel was investigated to determine its crack growth rate in water at 288qC. The crack
growth tests were conducted for approximately 1000 h using specimens with dimensions similar to the standard
dimensions. A typical intergranular fracture surface was obtained during crack propagation at room temperature in
air. The precipitation of chromium carbide, Cr23C6, along the grain boundaries in F82H steel may influence thefracture behavior in cases such as fatigue crack propagation at room temperature in air. The crack growth rate at 30
MPam1/2 was evaluated to be approximately 7 ×10−11 m/s
Keywords: IFMIF, F82H steel, Crack growth, Chromium carbide, High temperature watersome SSTTs, such as fracture toughness, fatigue, andfatigue crack growth rate, are measured. Because themeasurement of the crack growth rate is one of the tests
planned for the specimens irradiated at the HFTM, asmall-sized specimen for the crack growth has beendeveloped in previous studies [10-11].
Stress corrosion cracking (SCC), i.e., environmentallyassisted cracking (EAC), is an important concern in fission
reactors. Numerous studies have been performed to
examine EAC to understand its mechanisms and thereby
enhance the safety of reactors. In the case of fusion
reactors, EAC is not expected to be a serious concern
because the blanket is designed to be replaced periodicallyto maintain its performance. Actually, no SCC
susceptibility was reported for blankets subjected tohigh-temperature water (between 240qC and 330qC)
containing 10 ppm dissolved oxygen (DO) or for thosesubjected to 300qC water containing 40 ppb DO [12]. In
addition, slow strain rate tensile (SSRT) tests have shownthat F82H, T91, and T92 steels do not exhibit SCC
susceptibility when exposed to supercritical water [13-16].
However, research on the susceptibility of theblanket to EAC should be required because the blanket
materials have been demonstrated to be damaged byheavy irradiation in fusion reactors [12]. For example,
transgranular stress corrosion cracking has been reported
to occur in irradiated F82H specimens during a SSRT test
at 300°C in hydrogenated water or in oxygenated water[17]. Therefore, the objective of this study is to determine
the crack growth rate of F82H steel in 288°C water using
specimens of approximately standard size.

1. Introduction

Structural and functional materials used in the fusion
demonstration reactor (DEMO) will be exposed toneutrons with energies as high as approximately 14 MeV
at approximately 3.5 MW/m2 (5.0 MW/m2 at the peak)
and with a fluence greater than 10 MW·s/m2 during thereactor’s operation. Radiation damage of materials in afusion reactor environment can be characterized bysynergistic effects of displacement damage and nuclear
transmutation products such as hydrogen and helium
atoms [1-6]. These damages result in the degradation of
mechanical properties. For fusion nuclear reactors to besafely operated, the detailed behavior of material
degradation with respect to a 14 MeV neutron dose must
be understood. The International Fusion Materials
Irradiation Facility (IFMIF) generates high-energy
deuterium–lithium neutrons for irradiation experiments ofcandidate fusion reactor materials; researchers are using
this neutron source to construct a database from a seriesof tests of small-sized specimens for the fusion DEMO
reactor’s design and licensing [7]. The irradiation volumeof the IFMIF is approximately 14 L, and the greatest
displacement damage is greater than 20 dpa (displacement
per atom)/year in a volume of 0.5 L. Approximately 1000
specimens need to be irradiated in the IFMIF high flux test
module (HFTM) within a limited volume of approximately
0.5 L, as recommended by [8]. Therefore, small specimensmust be used for this purpose; the development of a small
specimen test technique or technology (SSTT) is therefore
very important [9]. In the IFMIF/EVEDA (Engineering
Validation and Engineering Design Activities) program,
author’s e-mail: ito.yuzuru@jaea.go.jp

73©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Y. Ito et al., Crack Growth Behavior of F82H Steel in the 288˚C Water

Table 1

Chemical composition of F82H-IEA heat (wt.%)

Table 2

2. Material and Methods
2.1 Material

F82H steel, supplied by the Japan Atomic Energy
Agency (JAEA), was used in crack growth tests. The
F82H steel was from F82H-IEA heat No. 9753 [18]. The
chemical composition of the material is summarized inTable 1. The yield stress of the F82H steel wasapproximately 607 MPa at room temperature (RT).

dimensions of 61_mm ×47 mm ×t16 mm was used in thepresent work to minimize the effect of the specimen size on
crack growth in water at 288°C because the 0.75T-WOL
specimen was the largest specimen obtained from the
supplied F82H steel. The geometry of the 0.75T-WOL
specimen with a thickness of 16 mm is shown in Fig._1.
Given the yield stress of 607 MPa in the F82H steel and thespecimen thickness required by ASTM-E399, the stressintensity factor, K, of 48.6 MPam1/2 was applied to the
0.75T-WOL specimen in this study (Table 2).
The stress intensity factor, K, for the WOL specimen
is calculated from the displacement induced at the mouthof the crack by tightening the loading bolt, as monitoredby the crack-opening displacement (COD) gauge, andfrom the crack length at that moment. The displacement
at the mouth of the crack required to apply the stressintensity factor K0 with crack length a0 is calculated byequation (1) [19].

2.2 Specimens

For mechanical tests of small specimens, highly
accurate control of the applied stress and displacement isrequired [9]. Therefore, the loading mechanism and/or
method for the specimen should be simple and not
involve large loading devices for manipulation of theirradiated specimens in the hot cell. Although compact
tension (CT) specimens are commonly used in fracture
tests, the wedge-opening load (WOL) specimen was usedin this study because only the loading bolt and pin are
required for the fixturing and only tightening of theloading bolt is necessary to apply a load to the specimen.
The tests could be simplified if the WOL specimen was
set up with the loading bolt and pin before irradiation
because mere tightening of the loading bolt could initiate
the crack growth test.
The WOL specimen was designed in accordance with
the Standard Test Method for Stress Corrosion Crackingedited by the 129th Committee on Strength and Fracture ofAdvanced Materials, Society of Materials Science, Japan[19]. Although the standard size of a WOL specimen isregarded as 81.0 mm ×63.0 mm ×t25.4 mm, fabrication ofsuch standard-sized WOL specimens from the supplied F82H
steel was impossible because the size of the as-obtained
specimen was limited. However, stress concentration has beenreported to strongly affect crack growth in simulated boilingwater reactor (BWR) environments [20], and the crack
growth rate is known to increase when the plastic area
becomes large [21]. Thus, a 0.75T-WOL specimen with

Fig. 1

The 0.75T-WOL specimen with 16 mm-thickness.

The required specimen thickness

V0ª C6 a0 W ºª K0 º«¬ E »¼ a0 « C a W »
¬ 3 0
¼

(1)

where V0 is the displacement at the mouth of the crack,
which is identical to the COD, W is the width of thespecimen, K0 is the stress intensity factor, E is the elastic
modulus, a0 is the crack length, and C6 (a0/W) and C3(a0/W) are coefficients. The coefficients C6 (a0/W) and C3(a0/W) are calculated from equations (2) and (3):
2

3

4

§a·
§a·
§a·
§a·
§a·
C3 ¨ ¸ 30.96  195.8¨ ¸  730.6¨ ¸  1186.3¨ ¸  754.6¨ ¸
©W ¹
©W ¹
©W ¹
©W ¹
©W ¹
2
3
4
ª
§a·
§a·
§a·
§a·
§a· º
C6 ¨ ¸ exp «4.495  16.130¨ ¸  63.838¨ ¸  89.125¨ ¸  46.815¨ ¸ »
W
W
W
W
©W ¹
©
¹
©
¹
©
¹
©
¹
¬«
¼»

(2)
(3)

For the WOL specimen, we note that K decreases as theFig. 2
74

The variation of K due to the crack propagation in case of
WOL specimen and CT specimen.

Y. Ito et al., Crack Growth Behavior of F82H Steel in the 288˚C Water

crack propagates because of the constant COD, as shownin Fig._2. Reduction of K due to crack propagation isestimated to be approximately 11% for 1.0 mm of crack
propagation. In comparison, the variation of K caused by
the crack propagation of a CT specimen tested underconstant loading mode is also plotted in Fig. 2. In the caseof the CT specimen, crack growth rate tests are generally
performed under constant loading mode. The value ofK_increases approximately 6% at 1.0 mm of crack
propagation.

environment for materials. However, higher DO levels do
not represent a severe environment for F82H steel, as
indicated by previous reports [12] of the general corrosion
performance of F82H steel under higher DO levels.
Therefore, the crack growth tests were conducted in watercontaining 2 ppm DO by referring to the water conditions[22] under which crack growth was observed in type 304
stainless steel. Notably, zirconium metal was used tofabricate the loading bolt and pin. After the loading bolt
and pin were fabricated, a zirconium oxide film wasformed on the fixtures. This film provides an electricallyinsulating layer between the WOL specimen and theloading bolt and pin, which prevents the formation of a
local cell between the WOL specimen and its fixtures in
the water.

2.3 Fatigue pre-cracking in air

After the WOL specimen was fabricated, a fatigue
pre-crack of approximately 1.0 mm was introduced at RT
in air. Because we intended to conduct the crack growth
tests at K = 25 MPam1/2, the maximum K of the fatigue
pre-cracking was 20 MPam1/2, which is approximately
20% less than that of the crack growth test. The stressintensity factor of the WOL specimen was calculatedusing the following equation (4) [19]:
K

2
3
4
P a ª
º
30.96  195.8 a
 730.6 a
 1186.3 a
 754.6 a
W
W
W
W »¼
BW «¬

3. Results and Discussion
3.1 Fracture surface observations

The surface of the specimen was fractured via fatigue
post-cracking at RT in air after the crack growth test was
terminated. Optical microscopy (OM) and scanningelectron microscopy (SEM) were used to observe thefracture surface. The boundary between the pre-cracked
area and post-cracked area was sharply defined (Fig. 4). A
typical intergranular fracture surface and some oxides wereobserved, as shown in Fig. 5. Figures 6 (a) and (b) show
images of typical intergranular fracture surfaces at the
pre-cracked area and the post-cracked area, respectively.
Because the pre-cracked area was exposed to 288°C water
during the test period, some oxides were observed on thesurface. The features of the surfaces observed in Figs. 6 (a)and (b), however, were similar, with the exception of theoxides on the surface in Fig. 6 (a). Therefore, the typicalintergranular fracture surface was obtained during thecrack propagation at RT in air. Therefore, the area tested
for crack growth and the pre-cracked area were difficult
to distinguish by merely observing the features of thefracture surface. The possible mechanism of theintergranular fracture of F82H steel at RT in air isdiscussed in section 3.3.

(4)where, P is the applied load, B is the specimen thickness(=16 mm), and W is the specimen width (=48.38 mm).

2.4 Crack growth tests in 288qC water

The crack growth tests in water at 288°C with anelectrical conductivity less than 2 u10−4 mS/cm under2_ppm DO were conducted using an autoclave
manufactured by TOSHIN KOGYO CO., LTD. The WOL
specimen was placed in the autoclave after the test load
was applied at RT in air. Then, the water was heated to
288°C and maintained at this temperature in the autoclave
during the test period to immerse the WOL specimen. The
test period in this study was approximately 1027 h. The
loading procedure is shown in Fig. 3. In this study, a 5%
overload, K = 31.5 MPam1/2, compared to the test load
was applied approximately 1 minute at RT in air before thetest load was applied, which was expected to promote EAC
under the experimental conditions because of further
hardening of the plastic zone at the specimen.
Water with 8 ppm DO, where the water is almost
saturated with oxygen, is normally regarded as a severe

Fig. 3

3.2 Crack growth rate in 288°C water

As previously mentioned, assessing whether the crack
growth occurred during the test period only throughobservation of the features of fracture surface is difficult.
Figure 7 shows typical intergranular fracture surfaces
following the boundary between the pre-cracked area andpost-cracked area. Although the intergranular fracturesurfaces are identified in both Figs. 7 (a) and (b), oxides
were not always observed on the surfaces, as shown in Fig. 7(b). Hence we hypothesized that the intergranular fracturesurface with some oxides along the boundaries between the
pre-cracked and post-cracked areas was evidence of crack
growth during the test period, as shown in Fig. 7 (a).

Loading procedure for 0.75T-WOL specimen.
75

Y. Ito et al., Crack Growth Behavior of F82H Steel in the 288˚C Water

Fig. 4Fracture surface observed by (a) optical microscope, and (b) scanning electron microscope.

(a)Fig. 5

Fig. 6(b)
Typical intergranular fracture surfaces, (a) “I” zone, and (b) “II” zone.

(a)(b)
Typical intergranular fracture surfaces, (a) with some oxides on the surface at “A” area in “I” zone, (b) without oxides on thesurface in the post-cracked area.

0.26 mm, divided by the test period, 1027 h. For comparisonof the calculated crack growth rate in F82H steel, a crack
growth rate of approximately 1 ×10−10 m/s at 30 MPam1/2
has been reported for type-304 stainless steel under BWR
environment [23], which is similar to the environment usedin this study. However, F82H, T91, and T92 steels have notbeen reported to exhibit SCC during SSRT tests under
supercritical water conditions. Because small-sized
specimens for crack growth investigations were developedin previous studies [10-11], further crack growth tests

In this case, the crack growth rate was estimated from thelength of the intergranular fracture surface and the durationof the test period. Because the crack-growth-tested area wasdifficult to distinguish from the pre-cracked area throughmere observation of the fracture surface, a length of 0.26
mm was conservatively regarded as the crack growthdistance during the test period of approximately 1000 h,
although the crack crossed the boundary (Fig. 8). As a result,the crack growth rate at 30 MPam1/2 was approximately 7
×10−11 m/s, as calculated on the basis of the crack length,76

Y. Ito et al., Crack Growth Behavior of F82H Steel in the 288˚C Water

Fig. 7

Fig. 8(a)(b)
Typical intergranular fracture surfaces, (a) with some oxides on the surface at “B” area in “I” zone, (b) without oxides on thesurface at “C” area in “II” zone.

F82H steel has the potential to trap hydrogen in thematerial. Therefore, intergranular fracture of F82H steel at
RT in air has been attributed to hydrogen embrittlement(HE), where the hydrogen is supplied by the Cr23C6
precipitates [11]; that is, some Cr23C6 in the plastic zone,
where cracks are formed, might release captured hydrogen
into the grain boundaries and/or into the matrix because of
the effect of the stress field of the crack on Cr23C6. The
released hydrogen would diffuse along the grain boundary
and/or into the matrix; eventually, some hydrogen would
concentrate in the high tri-axial stress state region wherehigh stress, high strain, and a high dislocation density
coexist. The hydrogen would not only induce HE but also
affect the intergranular fracture at RT in air. According to
the results of tensile low-strain-rate tests of F82H steel
with a 3 wppm hydrogen content, F82H steels exhibit
typical brittle fracture consequences of HE; the F82H
steels even exhibited HE susceptibility at low hydrogenconcentrations of 1–2 wppm [30]. Although Cr23C6
contains less hydrogen initially, the intergranular fracture
in F82H steel containing added P has been reported [31] to
be induced by grain boundary segregation of P. The grainboundary segregation of P as well as that of Cr23C6 wasalso identified on the intergranular fracture surface inAlloy690TT [32]. This result supports our explanation of
why the intergranular fracture was observed at RT in air
in F82H steel as well as in Alloy 690TT.

The measured crack length under the test period, thepicture is magnified from Fig. 7 (a).

under high-temperature water conditions, including
supercritical water conditions, using the same specimens
would be necessary to confirm the crack growth behavior
obtained in the 288qC water used in this study.

3.3 Intergranular fracture in F82H steel

We observed that intergranular fracture occurred evenin the fatigue pre-crack at RT in air. The fraction ofintergranular fracture surfaces at the pre-cracked area in
Fig. 5 (a) was measured to be approximately 58%. An
intergranular fracture at RT in air has also been reported[24] in thermally treated Alloy 690 (Alloy 690TT). Alloy
690TT [25-27] is a nickel-based, high-chromium,heat-treated alloy; it was developed to improve resistanceto intergranular SCC in high-temperature water resulting
from the precipitation of chromium carbide, Cr23C6, along
the grain boundaries. F82H steel also contains precipitatesalong the grain boundaries to enhance the resistance toradiation and to maintain toughness under hightemperatures. As in the case of Alloy 690TT, the major
precipitates in F82H steel are chromium carbide, Cr23C6[28]. Recently, the Cr23C6 in F82H steel was reported tocapture hydrogen isotopes with relatively high stability
because the binding energy for hydrogen in Cr23C6 isapproximately 0.58 eV [29]. This behavior implies that

4. Conclusions

We investigated the crack growth in F82H steel toobtain crack growth data for this steel in water at 288qC. The
results obtained in this study are summarized as follows:
1. A typical intergranular fracture surface in the F82H
steel was obtained during the crack propagation at RT
in air. Therefore, the crack-growth-tested area and thepre-cracked area were difficult to distinguish by mere
observation of the features of the fracture surface.
2. The intergranular fracture surface with some oxides

77

Y. Ito et al., Crack Growth Behavior of F82H Steel in the 288˚C Water

along the boundaries between the pre-cracked andpost-cracked areas provides tentative evidence of crack
growth during the test period. The crack growth rate at
30 MPam1/2 was calculated to be approximately 7
×10−11 m/s because the crack length was 0.26 mm
during the test period of 1027 h.
3. A possible mechanism of the formation of theintergranular fracture surface in F82H steel in thisstudy was proposed: the precipitation of chromium
carbide, Cr23C6, along the grain boundaries in F82H
steel may influence the fracture behavior during casessuch as fatigue crack propagation at RT in air.
4. Further crack growth tests under high-temperature waterconditions, including a supercritical condition, using
small-sized specimens would be necessary to confirm
the crack growth behavior observed in water at 288qC.

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Acknowledgement

This study was performed under the IFMIF/EVEDA
project in the Broader Approach (BA) framework.

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