Materials Transactions, Vol. 45, No. 2 (2004) pp. 407 to 410
#2004 The Japan Institute of Metals
Mechanical Properties and Microstructure of F82H Steel doped with Boron
or Boron and Nitrogen as a Function of Heat Treatment*
Eiichi Wakai, Michitaka Sato, Tomotsugu Sawai, Kiyoyuki Shiba and Shiro Jitsukawa
Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
Effect of heat treatment on mechanical properties and microstructures of Fe-8Cr-2W-0.1C-0.2V-0.04Ta martensitic steel F82H doped with
about 60 mass ppm B or both of 60 mass ppm B and 200 mass ppm N has been examined. The normalization was heated at temperatures from
950 to 1250 C for 1.8 ks, followed by air cooling or water quenching. After tempering treatment at 780 C or 750 C, the distributions of boron,
boron nitride and oxygen were measured by a secondary ion mass spectrometry (SIMS). Optical microstructural observation and tensile and
Charpy impact tests were performed also. In the boron doped F82H the tensile properties were similar to the non-doped F82H, but the ductilebrittle transition temperature (DBTT) shifted from 43 C to 15 C. SIMS images with high intensity of boron were observed in localized
regions of the boron doped F82H. Water quenching reduced the DBTT shift, about 30 C, and the localized boron intensity was slightly
decreased. In the boron and nitrogen doped tempered-F82H heat-treated by the water quenching from the normalizing temperature, the
properties of tensile and Charpy impact were similar to the non doped F82H, and no pronounced localized boron image was observed in the
SIMS image and no intensities of oxides and boron nitride were observed either.
(Received November 10, 2003; Accepted January 16, 2004)
Keywords: mechanical property, ductile-brittle transition temperature (DBTT), boron, nitrogen, martensitic steel, secondary ion mass
spectrometry, heat treatment
1.
Introduction
Low-activation ferritic/martensitic steels are candidate
materials for the first wall and blanket structure of fusion
reactors1) and the target vessel of neutron spallation station.2–5) The high-energy neutrons produced in the D-T
fusion reaction induce displacement damage and generate
gas atoms from (n; p) and (n; ) reactions in the materials. In
order to examine the effect of gas atoms on materials
properties, one is possible to use the reaction of 10 B(n; )7 Li
in steels doped with 10 B and irradiating with thermal neutrons
in a mixed-spectrum fission reactor, such as the high flux
isotope reactor (HFIR) and Japan Materials Testing Reactor
(JMTR).6)
The roles of boron addition were investigated to obtain
higher strength with heavy thickness for boron treatment and
superior toughness of weld bond of large heat-input welded
steel plane. The effect of additions of B, N and Ti elements on
the toughness and microstructures was very effective.7–9) The
toughness of steels is easily affected by the segregation and
precipitation of boron and boron nitride.10–12) Purpose of the
Table 1
present study is to prepare the testing specimens without the
effect of chemical boron on mechanical properties for the
HFIR and JMTR irradiation experiments.
2.
Experimental Procedure
The specimens used in this study are Fe-8Cr-2W-0.1C0.2V-0.04Ta martensitic steel F82H and F82H doped with
about 60 mass ppm B or both of 60 mass ppm B and 200 mass
ppm N. The chemical compositions of the specimens are
given in Table 1. The alloys were made from high-purity
commercial raw materials of a 50 kg or 100 kg using a
vacuum induction furnace. The ingot was heated at 1200 C
for 36 ks and rolled into 15 mm thickness plates. The
normalization was performed at temperatures from 950 to
1250 C for 1.8 ks, following air cooling or water quenching.
After the normalizing, tempering treatment was performed at
750 or 780 C. The plates were also heat-treated under some
conditions as below: N&T&N&T treatment (normalized for
1.8 ks at 1150 C, water-quenching and followed by tempering for 7.2 ks at 700 C, normalizing for 600 s at 950 C (or
The chemical compositions of the specimens used in this study.
(mass %)
Alloy
B
N
Cr
W
C
F82H
0.0002
0.0023
7.92
1.97
0.099
O
0.004
Al
0.004
F82H+B
0.0057
0.0011
7.83
2.09
0.099
<0.01
<0.001
F82H+B+N
0.0059
0.0190
8.09
2.10
0.099
<0.01
<0.001
Alloy
Si
P
S
Ti
V
Mn
Ta
F82H
0.11
0.007
0.001
0.006
0.18
0.10
0.05
F82H+B
0.10
0.006
0.001
<0.002
0.30
0.10
0.042
F82H+B+N
0.099
0.006
0.001
<0.002
0.30
0.10
0.039
*This Paper was
Presented at the Autumn Meeting of the Japan Institute of
Metals, held in Sapporo, on October 12, 2003.
E. Wakai, M. Sato, T. Sawai, K. Shiba and S. Jitsukawa
1000 C and 1040 C), water-quenching and followed by
tempering for 1.8 ks at 780 C. The objective of the first
normalizing at 1150 C was to solve BN clusters somewhat in
matrix and to reduce the size of BN clusters and that of the
first tempering was to reduce boron segregation at grain
boundaries and to collect boron in high-number density
carbides. The 2nd normalizing was performed to reduce grain
sizes and to transfer boron atoms from carbides to BN, and
the 2nd tempering was also performed to trap free boron
atoms in matrix into carbides. The distributions of boron,
boron nitride and oxygen were measured by a secondary ion
mass spectrometry (SIMS) of CAMECA IMS-3F type
operated at 8 kV for positive ions and 17 kV for negative
57
þ
ions, and 16 O , 25 BN , 42 BO
2 , Fe ions were measured in
a region with diameter of 0.15 mm. The surface metallurgical
observation was examined. In tensile testing, JIS 14 A tensile
specimen (6 mm diameter and 33 mm length in the gauge
region) were measured at room temperature under a strain
rate of 2:5 103 s1 . The ductile-brittle transition temperature of the specimens was measured by using half-size
Charpy impact specimens (5 mm width, 10 mm height,
50 mm length) with V-notch.
3.
150
Absorbed Energy, E/J
408
F82H
F82H+B
100
50
0
−150 −100
−50
0
50
100
Temperature, T / °C
Fig. 2 Ductile-brittle transition temperatures of F82H and F82H doped
with 60 mass ppm B.
Ion Counts
42BO 2
Results and Discussion
Effect of air-cooling from normalizing on mechanical properties in F82H doped with boron
Yield and ultimate tensile stresses (YS and UTS) of F82H
and F82H+B steels are given in Fig. 1(a), and the values
were very similar to each other. Uniform and total elongations (UE and TE) of F82H and F82H+B steels are given in
Fig. 1(b), and the values were also very similar to each other.
However, the ductile-brittle transition temperature (DBTT)
of F82H+B steel was higher than that of F82H steel as shown
in Fig. 2. The DBTT was shifted to higher temperature about
70 C. The distribution of boron element in F82H+B steel
Stress, σ /MPa
3.1
800
700
600
500
400
300
Heat condition: 1040°C 1.8 ks AQ, 750°C 3.6 ks
Fig. 3 The SIMS image of 42 BO
2 ions of F82H+B steel heat-treated by
air-cooling after the normalizing. The localization of boron with high
intensity was observed.
was measured by the SIMS under a high-sensitivity range in
order to detect a small amount of boron segregation to sinks
such as grain boundaries. As shown in Fig. 3, the localization
of boron with high intensity was observed in the SIMS image
of 42 BO
2 and was not corresponds to precipitates such as BN
and oxides Therefore, the embrittlement of F82H+B steel
might be caused by the boron localization.
200
100
0
Elongation (%)
(a)
F82H
F82H+B
0.1 mm
20
3.2
YS
UTS
(b)
F82H
F82H+B
15
10
5
0
UE
TE
Fig. 1 Tensile properties of F82H and F82H doped with 60 mass ppm B at
room temperature.
Effect of water quenching from normalizing on
mechanical properties in F82H+B and F82H+B+N
steels
The grain size of F82H+B steel normalized at 950 C was
comparable to that of F82H+B+N steel normalized at
950 C. The grain size of F82H+B+N steel was increased
with elevating normalizing temperature from 950 to 1040 C.
The microstructures of F82H+B and F82H+B+N steels
were full martensite structure. The tensile properties of these
steels are given in Table 2.
The Charpy impact testing was performed for the F82H+B
and F82H+B+N steels heat-treated by water quenching from
normalizing at temperatures from 950 C to 1250 C as given
in Fig. 4. The DBTT of F82H+B steel was decreased by the
treatment of water quenching from normalizing temperature,
Mechanical properties and microstructure of F82H steel doped with boron or boron and nitrogen as a Function of Heat Treatment
409
Table 2 Tensile properties of F82H, F82H+B and F82H+B+N steel.
0.2%YS
UTS
TE
RA
(MPa)
(MPa)
(%)
(%)
Non
554
673
22.5
79
N: 950 C, 1.8 ks,
468
622
26
80
437
596
28
81
451
616
27
81
Alloy
1st N&T
2nd N&T
F82H
N: 1040 C, 1.8 ks,
T: 750 C, 3.6 ks
N: 1150 C, 1.8 ks,
F82H
F82H+B
F82H+B+N
F82H+B+N
F82H+B+N
T: 700 C, 7.2 ks
T: 780 C, 1.8 ks
N: 1150 C, 1.8 ks,
N: 950 C, 1.8 ks,
T: 700 C, 7.2 ks
T: 780 C, 1.8 ks
N: 1150 C, 1.8 ks,
N: 950 C, 1.8 ks,
T: 700 C, 7.2 ks
T: 780 C, 1.8 ks
N: 1150 C, 1.8 ks,
T: 700 C, 7.2 ks
N: 1000 C, 1.8 ks,
T: 780 C, 1.8 ks
498
647
25
79
N: 1150 C, 1.8 ks,
N: 1040 C, 1.8 ks,
512
660
24
78
T: 700 C, 7.2 ks
T: 780 C, 1.8 ks
DBTT, T/°C
50
0
F82H
F82H+B(A.C.)
−50
F82H+B(WQ)
F82H+B+N(WQ)
−100
900
1000
1100
1200
1300
Normalizing Temperature, T/°C
Fig. 4 The relation of DBTT and normalizing temperature in F82H,
F82H+B and F82H+B+N steels.
42BO 2
16O-
however the DBTT of F82H+B steel was higher about 30 C
than that of F82H steel. The localized boron intensity was
somewhat decreased and no intensities of oxides and boron
nitride were also observed.
In the F82H+B+N steel, the localized boron image was
hardly observed in the SIMS image and no intensities of
oxides and boron nitride were also observed as seen in Fig. 5.
The boron localization of F82H+B+N steel was decreased
with normalizing temperature from 950 to 1040 C, but the
DBTT was inversely increased with it. Therefore, F82H
doped with boron and nitrogen, heat-treated under water
quenching from second normalizing at 1000 C and tempered
at 780 C for 1.8 ks, had an excellent mechanical properties
and it was selected for the irradiation specimens of HFIR and
25BN -
57Fe+
25
42
57
þ
Fig. 5 The SIMS image of 42 BO
2 , BN , BO2 , Fe ions of the F82H+B+N steel. The normalizing temperature was 1000 C. The
boron intensity in the localized region was hardly observed.
410
E. Wakai, M. Sato, T. Sawai, K. Shiba and S. Jitsukawa
JMTR experiments. In the F82H steel doped with boron, the
boron segregation occurred during the process of heat
treatments and the DBTT of F82H+B steel shifted to higher
temperature. In the F82H steel doped with boron and
nitrogen, the DBTT shift to higher temperature in the
F82H+B+N steel was not observed because the boron
segregation was suppressed.
Acknowledgements
The authors would like to thank Dr. H. Tanigawa of JAERI
for valuable discussions. The authors would like to express
Y. Kuriki of Kokan Keisoku Co., Ltd. for the preparation of
F82H, F82H+B and F82H+B+N steels.
REFERENCES
4.
Conclusion
Effect of heat treatment on mechanical properties of Fe8Cr-2W-0.1C-0.2V-0.04Ta martensitic steel F82H doped
with about 60 mass ppm B or both of 60 mass ppm B and 200
mass ppm N has been examined. Purpose of the present study
is to prepare the testing specimens without the effect of
chemical boron on mechanical properties for the HFIR and
JMTR irradiation experiments. The F82H doped with boron
and nitrogen, heat-treated by N&T&N&T (1st N&T: normalized for 1.8 ks at 1150 C and water-quenching, and
followed by tempering for 7.2 ks at 700 C; 2nd N&T:
normalizing for 600 s at 950 C (or 1000 C and 1040 C) and
water-quenching, and followed by tempering for 1.8 ks at
780 C), has an excellent mechanical properties and it was
selected for the irradiation specimens.
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