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

Effects of Heat Treatments and Addition of Minor Elements of Boronand Nitrogen on Mechanical Properties and Microstructures of
Reduced-Activation Ferritic/Martensitic Steel
Eiichi WAKAI, Masami ANDO, Nariaki OKUBO
Fusion R&D Directorate, Japan Atomic Energy Agency, Tokai-mura, Ibaraki, Japan(Received: 30 September 2014 / Accepted: 19 January 2015)

Effects of heat treatments on the microstructures and mechanical properties of RAFM steels, including irradiation
damage, were examined using the F82H steel and the F82H steel doped with boron and nitrogen. From a time of flightsecondary ion mass spectroscope (TOP-SIMS) analysis, it was found that a reheat treatment of the 2nd normalizing andtempering reduced the inhomogeneity of the material, and decreased the DBTT. Irradiation hardening andembrittlement of the F82H steel were changed by the initial tempering conditions before irradiation. It was also foundthat the DBTT shift can be reduced by the initial heat treatment and that the additions of miner elements of 60 wtppm
11
B and 200 wtppm nitrogen result in an effective reduction of irradiation embrittlement. The cause of reduction ofirradiation embrittlement may be due to the formation of small BN clusters.

Keywords: F82H, RAFM, Heat treatment, Irradiation hardening, Irradiation embrittlement, DBTT, Tempering,
Boron.

2. Effect of heat treatment on the
microstructures
andmechanicalproperties on RAFM steels

1. Introduction

The materials development of reduced-activation
ferritic/martensitic (RAFM) steels for the fusion DEMO
reactor was started around 1980s. Some important studies
and evaluations for fusion reactor materials developments
were performed [1-4], and the properties of Eurofer97
and IEA F82H are well known in general. RAFM steels
are the first candidate materials for the first wall andblanket structure of fusion DEMO reactors, and the target
back-plate and the target assembly of Irradiation Fusion
Materials Irradiation facility (IFMIF). Recently, a new 5
tons F82H was produced in 2007 and 20 tons F82H wasproduced in 2011 under BA program [3-4]. In this study,
several basic studies of radiation effects such as He and H
on properties of materials are mainly evaluated, and therequired materials properties for DEMO and IFMIF arealso briefly introduced. In this study, there are two
subjects as follows.
(1) Effect of initial heat treatment on the
microstructures and mechanical properties of RAFM
steels, including irradiation damage.
(2) Effects of additional minor elements of boron andnitrogen on the microstructures and mechanicalproperties of RAFM steels, including irradiation damage.

The irradiation hardening of RAFM steel changes
depending on the irradiation temperature. The
degradation of mechanical properties of RAFM steels
induced by irradiation at around 300oC is thought to bemainly induced by the irradiation-hardening and-embrittlement. It is known that the irradiation hardening
increases with irradiation dose, gradually saturating at
about 50 dpa [5]. The increment of yield strength inEurofer97 and F82H steels irradiated at 330 oC reaches
about 400 – 500 MPa, and the DBTT shift increases to200 – 250oC by the neutron irradiation due todisplacement damages [5]. In the temperature region
above 400oC, the irradiation swelling of RAFM steels is
induced [6-10].
Here, the factors of the irradiation hardening and theembrittlement are given as below.
(a) Factors of irradiation hardening
Formation of dislocation loops, cavities, black dot
clusters and precipitates (M6C, D’-precipitate, MX, etc.),
the increment of dislocation density.

author’s e-mail: wakai.eiichi@jaea.go.jp

104©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

E. Wakai et al., Effects of Heat Treatments and Addition of Minor Elements of Boron and Nitrogen on Mechanical Properties and Microstructures of Reduced-Activation Ferritic/Martensitic Steel

Alloy

B

F82H

0.0002

Table 1 The chemical composition of the IEA heat F82H steel used in this study (wt%).
N
Cr
W
C
O Sol.
Si
P
S
Ti
V
Ni
Cu
Al
0.007 7.82 1.98 0.09 - 0.001 0.07 0.003 0.001 0.004 0.19 0.02 0.01

Mn

Nb

Ta

0.1

0.0002

0.04

Table 2 The chemical compositions of the other heat F82H steel and F82H+B and F82H+B+N steels used in this study (wt%).
B
N
Cr
W
C
O
Al
Si
P
S
Ti
V
Mn
Ta
F82H
0.0002 0.0023 7.92 1.97 0.099 0.004
0.004 0.11 0.007 0.001 0.006 0.18 0.12 0.05
F82H+B
0.0059 0.0021 7.9
2.1
0.10 0.007
0.001 0.10 0.006 0.001 0.002 0.30 0.10 0.04
F82H+11B+N 0.0057 0.0200 7.83 2.09 0.099 <0.01 <0.001 0.10 0.006 0.001 0.002 0.30 0.10 0.042

Charpy V-notched (CVN) impact (3.3 mm in width, 3.3
mm in height, 20 mm in length) specimens and 3mm-disk
specimens for a TEM (a transmission electron
microscope) observation were also fabricated. The
Vickers hardness was measured at 10 kg by using a test
machine of SHIMADZU co. Ltd., and Charpy impact
tests were carried out using a test machine of JTT
CIEM30D-01. Elemental distributions of Fe, B, C, N, Al,
and P in the IEA F82H and the reheated F82H steel were
measured by a time of flight-secondary ion mass
spectroscope (TOF-SIMS) with 15 keV Ga+ ions over thescanned area of 100 × 100 μm after the pre-sputtering.
The pre-sputtering was performed by 13 keV O+ ions and25 keV Ga+ ions, for imaging the surface morphology
and removing surface contamination, respectively. The
microstructures were observed by a Hitachi HF-2000 of
TEM operated at 200 kV.
The neutron irradiation was performed in the Japan
Materials Test Reactor (JMTR) at 250oC to about 2 dpa.

(b) Factors of irradiation embrittlement
Irradiation hardening, the formation and growth of
precipitates, and the degradation of the binding ofboundaries due to irradiation-induced segregation ofsolute atoms and/or trapping of helium and hydrogenatoms on the boundaries, etc.
It was reported that the irradiation hardening andembrittlement of RAFM steels were less than those of theHT9 steel which was developed as a structural material
for fast reactors [11]. Several types of RAFM steels weredeveloped by several institutes, and the chemicalelements are almost identical but the concentrations of
the minor elements are also different. The heat treatmentsof RAFM steels for the tempering and normalizing areslightly different in each of RAFM steel. The differencein the response of mechanical properties such as
embrittlement behaviors to neutron irradiation were
estimated to be caused by the combined effects due tothese differences [12]. The effects of tempering time andtemperature on microstructures and mechanical properties
in F82H steel were previously reported [13-19], and themethod is very useful to control the materials properties
and radiation resistance.

4. Results of mechanical properties andmicrostructures in the reheated F82H
Fig. 1 shows Charpy impact energies as a functionof test temperature in the IEA heat F82H steel before andafter the reheat treatment (the second normalization:
1040oC for 0.5 h, the second tempering: 750oC for 1 h).
The DBTT after the reheat treatment was decreased from

3. Experimental procedures
F82H steel used in this study is IEA heat and theother heat F82H, F82H+B, F82H+B+N steels, and thechemical compositions are given in Tables 1 and 2. In the
IEA heat F82H, the heat treatments were normalized at
1040oC for 0.5 h and tempered at 750oC for 1 h, and thedetails were described in a reference [20]. In this study,
additional heat treatments of re-normalizing andtempering were performed on the F82H steel. It was
firstly re-normalized at 1040oC for 0.5 h in a vacuum, andit was cooled to room temperature. Then it was tempered
at temperatures of 750, 780 and 800oC for 0.5 h in a
vacuum. The tempering time at 750oC was varied
between 0.5 and 10 h. The tensile test specimens of SS-3
type (gage size: 7.62 mm in length, 1.52 mm in width,
0.76 mm in thickness) were fabricated. Miniaturized

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㻙㻝㻜㻜

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㻜

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Testenergies
Temperature,
T/Υ
Fig. 1. Charpy impact
of F82H steel
before andafter the reheat treatment (second normalization: 1040oC
for 0.5 h, second tempering: 750oC for 1 h).

105

E. Wakai et al., Effects of Heat Treatments and Addition of Minor Elements of Boron and Nitrogen on Mechanical Properties and Microstructures of Reduced-Activation Ferritic/Martensitic Steel(a)Total ion image(b) Fe image(c) FeN image(e) B image(f) PO2 image(g) Al image(d) CN image

Fig. 2 SIMS images of the IEA heat F82H steel before the re-heat treatment.

(a)Total ion image(e) B image(b) Fe image(f) PO2 image(c) FeN image(d) CN image(g) Al image

Fig. 3 SIMS images of the IEA heat F82H steel after the re-heat treatment.

region of aluminum. The precipitates including aluminum
element might be formed by the processing in theremoval of oxygen using Al during the melting, and they
are thought to be remained as aluminum oxide during theelimination process of oxides in the melting. By thereheat treatment, the isolated B distribution was reduced
and the segregation of P disappeared, and the precipitates
including Al were decreased.
The impact energies of 1/3 CVN specimens of F82H

about -100oC to about -120oC, and the DBTT of the IEA
heat F82H steel was improved by the reheat treatment. To
examine the cause of the improvement of DBTT, thedistributions of some elements were measured by the
TOP-SIMS. The SIMS images of the F82H steel before
and after the re-heat treatment are shown in Figs. 2 and 3,
respectively. The elements of B and P were segregated at
the boundaries as seen in Fig. 2, and the precipitates of Al
were observed in Fig. 2(g), which was in the localized

106

E. Wakai et al., Effects of Heat Treatments and Addition of Minor Elements of Boron and Nitrogen on Mechanical Properties and Microstructures of Reduced-Activation Ferritic/Martensitic Steel

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Test Temperature, T/Υ

Fig. 4 Charpy impact energies of the F82H tempered at different conditions.

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Yield Stress ,σ/MPa

ЋYS

UEЍ

Uniform Elongation(%)

Yield Stress ,σ/MPa

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Uniform Elongation(%)

(b)

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Fig. 5. Tensile properties of the re-heated F82H steel tempered at different times and temperatures.

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(a)

Vickers Hardness, Hv

Vickers Hardness, Hv

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(b)
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Fig. 6. Vickers hardness of the re-heated F82H steel tempered at different times and temperatures.

steel as a function of temperature are shown in Figs. 4(a)and (b). The highest DBTT was –94oC in the F82H steel
tempered at 750oC for 10 h. DBTT of the specimens
tempered at 750oC for 0.5 h, 1 h and 2 h was about-120oC as seen Fig. 4(a). DBTT of the specimens
tempered at 780 and 800oC for 0.5 h was -104oC and-113oC, respectively, as given in Fig. 4(b).

Fig. 5 shows yield strengths and uniform elongationsof F82H steels tempered at different conditions. The yieldstrength of the specimens decreased with increasing
tempering temperature and time [9]. The Vickers
hardness of the specimens was also given in Fig. 6. The
Vickers hardness also decreased with increasing
tempering temperature and time.

107

E. Wakai et al., Effects of Heat Treatments and Addition of Minor Elements of Boron and Nitrogen on Mechanical Properties and Microstructures of Reduced-Activation Ferritic/Martensitic Steel

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(a)

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750Υ㽢0.5h780Υ㽢0.5h
750Υ㽢0.5h

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750Υ㽢2.0h

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Vickers Hardness, Hv

Yield Stress,σ/MPa

Fig. 7. The relations between DBTT [21 ] and yield stress or Vickers hardness in the reheated F82H steel tempered at different times
and temperatures.

Fig. 7 shows the relations between DBTT and yieldstress [21] and, DBTT and Vickers hardness in thereheated F82H steel tempered at different times andtemperatures. In this heat treatment, the DBTT was notincreased with the increment of the yield strength andVickers hardness, but was roughly constant except for the
lower region of the yield strength and Vickers hardness.
The specimens with higher DBTT had relatively larger
size carbides as shown in Figs. 9 and 10. Fig. 8 shows therelations between DBTT and yield stress in the otherferritic/martensitic
steels,
and
the
other
ferritic/martensitic steels as shown in Fig. 8 had different
concentrations in some elements and were tempered at
different temperatures [22-29]. There was no correlation
between the DBTT and the yield strength. The DBTT of
the reheated F82H steel was lower than those of the othersteels.
Figs. 9 and 10 show the microstructures of thereheated F82H steels. Lath martensitic structures withdislocations and carbides were observed in all specimens.
Larger size carbides were observed in the specimen
tempered at 750oC for 10 h as shown in Fig. 10(d). The
slight growth of carbides of the F82H steel tempered at
780 and 800oC for 0.5 h were seen in Fig. 9.
It was reported that the IEA heat F82H wasevaluated to include inhomogeneity, which was analyzed
from the fracture behavior in the brittle-ductile transition
temperature using the master curve analysis by M.
Sokolov [29], and the large scattering of fracture
toughness data in the brittle-ductile transition temperature
region was shown in the study. The large scattering isthought to be induced by the inhomogeneity of the IEA
heat F82H, and the inhomogeneity can be expected to be
reduced effectively by the re-heat treatment shown in thisstudy.

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Eurofer 97

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Yield Stress,

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MPa

Fig. 8. The relations between DBTT and yield stress in theother ferritic/martensitic steels.

(a)

(b)

750Υg0.5h

400nm

800Υg0.5h

1000nm

780Υg0.5h

1000nm(c)Fig. 9. Microstructures of F82H steel depending on thetempering temperatures.
(a)

(b)

750Υg0.5h

750Υg1 h

400nm

(c)

1000nm(d)Large size
carbideIrradiation experiments in the JMTR reactor atabout 250oC to about 2 dpa was conducted for thereheated F82H steels as functions of tempering

750Υg2 h

1000nm

750Υg10 h

1000nmFig. 10. Microstructures of F82H steel depending on thetempering time.

108

E. Wakai et al., Effects of Heat Treatments and Addition of Minor Elements of Boron and Nitrogen on Mechanical Properties and Microstructures of Reduced-Activation Ferritic/Martensitic Steeltemperatures and times using the F82H steel of tensile
specimens and Charpy impact specimens [13-17]. The
DBTT shift due to the irradiation depended on thetempering conditions of time and temperature [15] asshown in Fig. 11. Thus, it was revealed that theirradiation hardening and embrittlement of RAF/M steel
was strongly dependent on the heat treatment conditions
before the irradiation. These results indicate that thechange of carbon contents in the matrix and dislocation
densities influenced by the heat treatments before theirradiation would affect the mobility of the point defects
and the growth of defect clusters.
In order to reduce the DBTT shift by the neutron
irradiation, further examination was conducted in adifferent approach using F82H steel doped with minor
elements of boron (11B) and nitrogen atoms. 11B element
has no nuclear reaction with thermal neutrons. The
chemical compositions used in this experiment are given
in Table 2. Figs. 12 (a) and (b) are phase diagrams ofboron and nitrogen atoms in F82H steel calculated by the
Thermo-Calc software. The phases in the boron andnitrogen elements as a function of temperature are shownin the figure. The heat treatments were conducted in two
steps, and the first step normalizing was performed toprevent the formation of delta-ferrite and the following
tempering was performed to suppress the formation of
BN clusters in the specimen. The second normalizing was
performed to adjust the grain size and also to form the
BN clusters, and the second tempering was performed toform the carbides in the specimen. In the F82H+B+N
steel, the procedures of the heat treatment are shown in
Fig. 13. The previous research [30] showed that the boron
examined by the SIMS using 42 BO ions was observed to
be isolated in the F82H+60 wtppm-B steel under thesame heat treatment with the IEA F82H steel as given in
Fig. 14. The isolated size of boron was from a few Pm to20-30 Pm.
In the present study, the distributions of boron,
carbon, nitrogen and aluminum elements were measured
by the TOP-SIMS. The TOP-SIMS images of the F82H
steel, which was heat-treated with a condition of theminimum DBTT shift, and F82H+60 wtppm 11B andF82H+60 wtppm 11B+ 200 wtppmN are given in Fig. 15.
As shown in Fig. 15, slight isolations of boron wereobserved in the F82H steel, and the size was less thanabout 2 Pm. The isolated position of boron corresponds tothe position of aluminum. The position of aluminum islikely to correspond to oxides, not the carbides as seen in
the 26CN ion image. The behavior of the boron isolation
was also observed in the F82H+60 wtppm-B andF82H+60 wtppm-B+200 wtppm-N. The intensity of theisolation boron increased in the F82H+60 wtppm-B, and

200

750oC
1h

DBTT Shift(oC)

150
100

750oC
0.5 h 750oC
2h

750oC1
0h
780oC
0.5 h

50
0

F82H

0

50

100

150

200

250

300

350

400

Irradiation Hardening (MPa)Fig. 11 Relation between the DBTT shift and irradiation
hardening in the F82H depending on tempering conditions
[15].

(a)
M23C6㻌

BN㻌
G㻌

LiquidJ㻌
D㻌(b)
M(C,N)GJ

LiquidBN

D
D

Fig. 12 The calculated positions for (a) boron and (b) nitrogen

Temperature ( oC)

in F82H steel in the phase diagram.

1000

1150 oC, 30 min.

950/1000/1040 oC, 10 min.
2nd. Normalizing

1st. Normalizing

600
200

700 oC, 120 min.
1st. Tempering

780 oC, 30 min.
2nd. TemperingTime

Fig. 13 Procedure of heat treatment of F82H+B, F82H+B+N
and F82H .

42BO 2

100Pm
Fig. 14 SIMS image of F82H+60 wtppm B steel [21].

109E. Wakai et al., Effects of Heat Treatments and Addition of Minor Elements of Boron and Nitrogen on Mechanical Properties and Microstructures of Reduced-Activation Ferritic/Martensitic Steel

F82H

F82H+60ppmB+190ppmN

F82H+60 ppmB

Boron

Boron

Boron

10 Pm

10 Pm
26CN

26CN

26CN

70FeN

70FeN

10 Pm
Aluminum

10 Pm

10 Pm
Aluminum

Aluminum

10 Pm
(a)

10 Pm

10 Pm

10 Pm
70FeN

10 Pm

10 Pm
(b)

10 Pm
(c)

Fig. 15 TOP-SIMS images of boron and carbon elements (a) F82H, (b) F82H+60 ppmB, (c)F82H+60 ppmB+200 ppmN.

the boron was very small in the F82H+60 wtppm 11B+
200 wtppmN as seen in Fig. 15(c). Microstructures of the
F82H+60 wtppm11B+ 200 wtppmN steel and F82H steel
tempered at 780oC for 0.5 h taken by a transmission
electron microscope are shown in Fig. 16. The steel has atypical lath martensitic structure, and dislocations andcarbides are seen. The size of carbides of the F82H steel
were slightly larger than those of F82H+60 wtppm 11B+
200 wtppmN. Fig. 17 shows the reduction of DBTT shift
in the F82H and F82H+60 wtppm11B+ 200 wtppmN steel
tempered at 780oC for 0.5 h. The DBTT shift can be

the numbers of the isolation boron was reduced in the
F82H+60 wtppm-B+200 wtppm-N. The size of carbides
in the F82H+60 wtppm-B increased. The larger size
carbides may be induced by the absorption of boron
element to M23C6 carbides due to the low content
nitrogen in the F82H+60 wtppm-B steel as judged from
Fig. 12(a).
However, in the improved heat treatment condition inthis study, the isolated level in the F82H+60 wtppm 11B
was reduced as shown in Fig. 15(b), and the isolations of110

E. Wakai et al., Effects of Heat Treatments and Addition of Minor Elements of Boron and Nitrogen on Mechanical Properties and Microstructures of Reduced-Activation Ferritic/Martensitic Steel

5. Summary and Conclusion
In this study, the effects of initial heat treatments on
the microstructures and mechanical properties of RAFM
steels, including irradiation damage, were examinedusing the F82H steel and the F82H steel doped with 60
wtppm boron and 200 wtppm nitrogen.
From a time of flight secondary ion mass
spectroscope (TOP-SIMS) analysis, the inhomogeneity ofelements such as P, B, C, and N was measured, and thereheat treatment of the 2nd normalizing and tempering in
the F82H steel reduced the inhomogeneity of material,and decreased the DBTT of the F82H. Irradiation
hardening and embrittlement of the F82H steel changedby the initial tempering conditions before irradiation. It
was found that the DBTT shift can be reduced by the
initial heat treatment, and the additions of miner elementsof 60 wtppm 11B and 200 wtppm nitrogen resulted in an
effective reduction of irradiation embrittlement. The BN
clusters can be reduce the average of mobilities of point
defects and may also act as sink sites for point defects. As
the results, the irradiation hardening and embrittlement
would be reduced.

(b)F82H(a)F82H+B+N

1Pm

1Pm

Fig. 16 Microstructures of (a) F82H+B+N and (b) F82H.

DBTT Shift by Irradiation (oC)

200
150
100

6. References

50

A.-A.F. Tavassoli et al., Fusion Eng. Des. 61-62, 617(2002).
[2] A.-A.F. Tavassoli et al., J. Nucl. Mater. 329–333, 257(2004).
[3] H. Tanigawa et al., Fusion Eng. and Des. 86, 2549
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F82H+B+N
F82H
with the improved reheated by thecondition
improved heatcondition

Fig. 17 DBTT shift due to irradiation in the IEA heat F82H,
F82H with the reheated with the improved condition andF82H+B+N with the improved heat condition [15, 19].

reduced by the optimized heat conditions and the
additions of minor elements of boron and nitrogen. The
additional minor elements of 11B and N to RAFM steels
are very attractive for the radiation resistance. The BN
clusters formed in the F82H steel can reduce the averageof mobilities of point defects and may also act as sinksites for point defects. As the results, the irradiation
hardening and embrittlement would be reduced.
The high dose irradiation experiment including the
F82H+11B+N steel is under conducting in the HFIR up to
80 dpa, and the results will be obtained within a few
years later under a collaboration work of JAEA andDOE/ORNL.

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