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

Microstructural Evolution of 9Cr-ODS and JLF-1 in a HIP Joint after
Post-bond Heat Treatment
Haiying FU 1), Takuya NAGASAKA 1, 2), and Takeo MUROGA 1, 2)
1)

SOKENDAI (The Graduate University for Advanced Studies), Toki, Gifu, JAPAN
2)
National Institute for Fusion Science, Toki, Gifu, JAPAN(Received: 23 September 2014 / Accepted: 19 January 2015)

A joint of 9Cr-ODS and JLF-1 (9Cr-2W RAFM) was fabricated by hot isostatic pressing (HIP) at 1000 oC for 3hunder a pressure of 191MPa with a slow cooling rate. JLF-1 was hardened and 9Cr-ODS was softened after the HIP.
By microstructure analysis, JLF-1 showed quenched martensitic phase with MX particles and high density dislocation
tangles in the matrix, while 9Cr-ODS showed ferritic phase with low density of carbides and coarse carbides on grainboundaries. After post-bond heat treatment (PBHT) with normalization and tempering at a fast cooling rate,
microstructure of both 9Cr-ODS and JLF-1 was recovered to the same with that before HIP. Both of them weretempered martensitic phase with laths, carbides, and dislocations.
Keywords: microstructure, 9Cr-ODS, JLF-1, hot isostatic pressing, post-bond heat treatment

on the JLF-1 side at the interface appeared after HIP.
While after PBHT, the few-particle layer disappeared, but
normal tempered martensitic phase of M23C6 carbides andMX particles formed. For the application of the bonded
materials to the fusion components, characterization of theproperty change of the base metals by the heating historyis necessary in addition to that of the bonding interfaces. In
this work, microstructure evolution of base metals of 9CrODS and JLF-1 by the heating history is mainly
investigated.

1. Introduction

Oxide dispersion strengthened (ODS) ferritic steel isa kind of reduced activation ferritic/martensitic steel(RAFM) for fusion application because of its excellent
high temperature mechanical properties [1]. When thematerial is applied to the first wall and structuralcomponent of fusion reactor blankets by joining to theconventional non-ODS RAFM, it would increase theapplication temperature of blanket significantly [2]. Thus,bonding technology of ODS with non-ODS RAFM steels
is very important for the construction of an advanced
fusion reactor in the future. Up to now, lots of techniques
have been utilized for the bonding, such as electron beam
welding [3], friction welding [4,5], diffusion bonding [6]etc. Unlike conventional melting-solidification bonding
techniques, diffusion bonding such as hot isostatic pressing(HIP) is a solid-state process, and does not create moltenzone for the joint. Thus degradation of microstructure andproperties at the interface would be avoided or diminished[6]. In the previous work [7], ODS-RAFM steel, 9Cr-ODS,
and non-ODS RAFM steel, JLF-1, were bonded by HIP at
1000 oC for 3h under 191MPa. Post-bond heat treatment(PBHT) with normalization and tempering at a fast coolingrate was utilized to recover the hardness of both 9Cr-ODS
and JLF-1 base metals to the level similar to that before
HIP. In the previous results, the microstructure of theinterface between 9Cr-ODS and JLF-1 was mainly studied.
The interface was softened compared to the base metals,
because the carbides at the interface are coarser than thosein the base metals. A layer of grains with few MX particles

2. Materials and experiment

Materials used in this study were 9Cr-ODS and JLF-1.
9Cr-ODS was fabricated by mechanical alloying followedby extrusion and forging at 1150 oC [8]. JLF-1 was madeby vacuum induction melting (VIM), forging, and rolling[9]. The normalization was carried out at 1050 oC for 1hwith air cooling (AC), followed by tempering at 800oC for
1h for 9Cr-ODS, and at 780 oC for 1h for JLF-1. The
chemical composition of the materials is shown in Table 1.
Table 1 Chemical composition of 9Cr-ODS and JLF-1 (wt. %).

Material
9Cr-ODS
JLF-1

C
0.14
0.09

Si
0.06
0.05

Mn
0.09
0.49

P
<0.005<0.003S
0.004
0.0005

Cr
9.08
9.00

Material
9Cr-ODS
JLF-1

W
1.97
1.98

V

Ta

0.20

0.083

Ti
0.23

Y
0.29

N
0.013
0.015

O
0.16
0.002

The 9Cr-ODS disk used in this study was 5 mm thickand 24 mm in diameter. The JLF-1 blocks were 20 mm

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61©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

H. Fu et al., Microstructural Evolution of 9Cr-ODS and JLF-1 in a HIP Joint after Post-Bond Heat Treatment

thick and 24 mm in diameter. The blocks were electropolished to remove the contamination of the surfaces withetching solution of 13% HClO4, 13% ethylene glycol
monobutyl ether, and 74% acetic acid, at 15V below 5oC.
After cleaned and dried, the 9Cr-ODS disk was
sandwiched between two JLF-1 blocks and sealed into asoft steel capsule using electron-beam welding, and thenhot isostatically pressed at 1000 oC for 3h at 191MPa. The
cooling rate of the HIP was 5oC per minute. Coupon
specimens provided with the bonding interface in thecenter were machined for microstructure analysis andhardness test. PBHT was carried out for the joint by
normalization at 1050 oC for 1h and tempering at 720820oC for 1h. The cooling rate after normalization andtempering was 34~36oC per minute. Fig. 1 depicts the HIP
and the following PBHT procedure for the joint. Hardness
tests were carried out at 300g for 30s for the base metals
as-HIPed and after PBHT. Microstructural analysis bytransmission electron microscopy (TEM) and scanningelectron microscopy (SEM) was executed for the joint
after electro-etching. The etching solution is also 13%
HClO4, 13% ethylene glycol monobutyl ether, and 74%
acetic acid. More details about the experimental procedure
are shown in the previous work [7].

9Cr-ODS base metal
JLF-1 base metal

Hardness (HV)

400
300
200
100

1050 oC×1h 1050 oC×1h 1050 oC×1h
as-HIPed+720 oC×1h +760 oC×1h +800 oC×1h01050 oC×1h 1050 oC×1h 1050 oC×1h
o
Before HIP 1050 C×1h +740 oC×1h +780 oC×1h +820 oC×1h

Fig. 2. Hardness of the base metals of 9Cr-ODS and JLF-1 before
HIP, as-HIPed and after PBHT, reproduced from ref.[7].

Fig. 3 depicts the grain size of 9Cr-ODS and JLF-1
base metals. For 9Cr-ODS base metal, the grain size isabout 1μm with no significant change after HIP and PBHT,
because the high density of nano-oxide particles in thematrix could retard grain growth [10]. For JLF-1 basemetal, the grain size increased after HIP, and further
increased after PBHT. There are sub-grains in a prioraustenite grain of RAFM steel. While tempering at hightemperature, especially near the recrystallization
temperature, dislocations rearranged, and the sub-grains
would grow up. Thus the grain size may increase whentempering at 780oC and 800oC in this study.
50

Grain size ( μm)

5oC/min500

o34~36 C/min

40

9Cr-ODS base metal
JLF-1 base metal

30
20
10

1050oC×1h+740o C×1h

as-HIPed0
before HIP 1050oC×1h

1050oC×1h+780oC×1h

1050oC×1h+760o C×1h

1050oC×1h+800oC×1h

Fig. 3. Grain size evolution of 9Cr-ODS and JLF-1 base metals,
reproduced from ref.[7].

Fig. 1. HIP and PBHT procedure.

3. Results and discussion
3.1 Hardness and grain size after PBHT

3.2 Microstructure evolution of 9Cr-ODS base
metal

Fig. 2 shows the hardness of 9Cr-ODS and JLF-1
base metals before HIP (as-received), after HIP (as-HIPed),and after PBHT. After HIP, because of the cooling rate of
5oC per minute was too slow to quench, 9Cr-ODS basemetal was softened. For JLF-1 base metal, hardening
occurred, on the other hand. HIP at 1000oC induced
normalization and quenching for JLF-1, and produced asquenched martensitic phase. After PBHT withnormalization at 1050oC at a cooling rate of 34~36 oC per
minute, both 9Cr-ODS and JLF-1 base metals werequenched. After the following tempering at 720-820oC, astemperature increased, the hardness of the both decreased.
Tempering at 740oC and 760oC seems to be appropriate torecover the hardness to the level similar to that before HIP.

Fig. 4 shows microstructural evolution of 9Cr-ODS
before HIP, as-HIPed, and after PBHT. During the HIP
and PBHT procedure, the microstructure of 9Cr-ODS basemetal has changed. Before HIP, 9Cr-ODS is tempered
martensitic phase, with M23C6 type of carbides [11]distributed on the grain boundaries. TEM image shows
tempered martensitic laths and dislocations. There may beerrors for the statistics on the size and density for the nanooxide particles originated from the thickness estimationand imaging conditions during the TEM observation.
Moreover, the nano-oxide particles may be different indifferent sampling area, thus further investigation isnecessary to extract statistically significant changes.

62

H. Fu et al., Microstructural Evolution of 9Cr-ODS and JLF-1 in a HIP Joint after Post-Bond Heat Treatment

However, from the preliminary rough estimate in thisstudy, the number density of nano-oxide particles in asreceived is about 1.6×10 23m-3 with mean diameter of
7.4nm, as shown in Table 2. After HIP, the carbides, with
150-400nm in length, and 60-180nm in width, clustered on
the grain boundaries. The clusters are about 150-1800nm
in length. The carbides are expected to be M3C (M=Fe, Cr,W) according to Fe-Fe3C phase diagram [12]. The
microstructure is ferritic phase according to CCT diagramof 9Cr-ODS [13] and the TEM image in Fig. 4(d) whichshows low density of dislocations and carbide precipitates.
The density of the nano-oxide particles decreased to about
1.2×10 23m-3, with the mean diameter increased to 9.4nm.
While after PBHT with normalization at 1050oC for 1hfollowed by tempering at 760oC for 1h, microstructure of
9Cr-ODS was recovered to tempered martensitic phasewith high density of carbides, dislocations, laths, andnano-oxide particles with number density of 1.5×1023m-3,
similar to that before HIP, but with a smaller particle sizeof 5.2nm.

60

(a)

(b)

Asreceived
50nm

500nm

AsHIPed
2μm

After
PBHT
50nm

As-received
2
23.2
7.4
1.6

As-HIPed
3.5
39.7
9.4
1.2

(b)

40
30

As-HIPed

20
10

(c)

40
30

500nm

After PBHT

20
10
0

0

5 10 15 20 25 30 35 40
Particle diameter (nm)Fig. 5. Size distribution of nano-oxide particles in 9Cr-ODS basemetal, (a) as-received; (b) as-HIPed; (c) after PBHT at
1050oC×1h+760oC×1h.

Fig. 4. Microstructure of 9Cr-ODS before HIP (a-b), as-HIPed(c-d), and after PBHT at 1050oC×1h+760 oC×1h (e-f);(a, c, e)— SEM images, (b, d, f) —TEM images.
Table 2 Data of nano-oxide particles in 9Cr-ODS as-received, asHIPed, and after PBHT at 1050oC×1h+760oC×1h.
Min. diameter (nm)Max. diameter (nm)Mean diameter (nm)Density N (1023 m-3)

10

50

(f)

2μm

As-received

20

0
60

500nm
Fraction (%)

(e)

50nm

30

50

(d)

(c)

40

0
60

Fraction (%)

2μm

(a)

50

TEM

Fraction (%)

SEM

Fig. 5 shows the size distribution of nano-oxide
particles in 9Cr-ODS base metal. Most of the nano-oxide
particles in the as-received (before HIP) are in the range of
2-8nm in diameter; the maximum is about 20nm. But after
HIP, the size of nano-oxide particles was about 3.5-15nm
with some large particles of 40nm in diameter. Table 2
summarizes the data of the nano-oxide particles in 9CrODS. It can be seen that, after HIP with a slow coolingrate, the nano-oxide particles slightly grow up with thenumber density decreased. After PBHT with normalization
at 1050oC for 1h and tempering at 760oC for 1h, themicrostructure has recovered to tempered martensitic
phase. The nano-oxide particles were 1.9-7nm in diameter,
the maximum is around 20nm, with size and density a little
smaller than that before HIP. However, as shown in Fig.
4(c, d) and Table 3, low density dislocations in ferrite
matrix and uneven distribution of coarse carbide clusters
on grain boundaries may mainly contribute to the softeningof the 9Cr-ODS base metal after HIP.

Table 3 Mean size of carbides in 9Cr-ODS and JLF-1base metals

After PBHT
1.9
20.4
5.2
1.5

Asreceived
As-HIPed
After
PBHT

63

9Cr-ODS
Conditiono1050 C×1h+
800oC×1h, AC
1000oC×3h×191M
Pa, 5oC/min
1050oC×1h+760o
C×1h, 36oC/min

Size(nm)
150
650
120

JLF-1
Conditiono1050 C×1h+
780oC×1h, AC
1000oC×3h×191M
Pa, 5oC/min
1050oC×1h+760o
C×1h, 36oC/min

Size(nm)160—120

H. Fu et al., Microstructural Evolution of 9Cr-ODS and JLF-1 in a HIP Joint after Post-Bond Heat Treatment

3.3 Microstructure evolution of JLF-1 base metal

structure with normal M23C6 carbides formed in the whole
joint.

Fig. 6 depicts the microstructure of JLF-1 base metal.
Before HIP, JLF-1 is tempered martensitic phase withlaths, dislocations, and M23C6 carbides on lath and grainboundaries. The carbide size is shown in Table 3. Whileafter HIP, the microstructure is quenched martensitic phase.
No carbides on the lath and grain boundaries are observed,
instead, MX particles and dislocation tangles form in thematrix. When PBHT with normalization and tempering
was carried out, the microstructure has been recovered totempered martensitic phase with laths, dislocations, andcarbides on lath and grain boundaries. The carbide size issmaller. This may be the reason why the hardening is alittle larger than that before HIP.
SEM

(a)

9Cr-ODS
2μm

500nm
(d)

AsHIPed

2μm
(e)

500nm

JLF-1

2μm

Acknowledgements(f)This work was supported by NIFS budget codes
NIFSUFFF024 and NIFSKOBF020.

After
PBHT

References

2μm

Fewparticle
layer

9Cr-ODS

A diffusion-bonded joint of 9Cr-ODS and JLF-1 was
fabricated by HIP. PBHT with normalization andtempering was utilized to recover the hardness andmicrostructure of the joint. After HIP, the 9Cr-ODS basemetal was softened because of the slow cooling rate of HIP.
The microstructure was ferritic phase with low density
dislocations and coarse carbides M 3C on the grainboundaries. JLF-1 base metal was hardened by the HIPtreatment because quenched martensitic phase formed after
HIP. While after PBHT with normalization and tempering
at a fast cooling rate, hardness of both 9Cr-ODS and JLF-1
base metals was recovered to the level similar to that
before HIP. The microstructure for the whole joint
including the interface and the base metals was tempered
martensitic structure with M 23C 6 carbides.

Asreceived

(c)

JLF-1

(b)

4. Conclusions

(b)

2μm

InterfaceFig.7. Microstructure of the interface in as-HIPed (a), and after
PBHT at 1050oC×1h+760 oC×1h (b).

TEM

(a)

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[6] S. Noh, et al., J. Nucl. Mater. 426, 208 (2012).
[7] H.Y. Fu, et al., Fusion Eng. Des. 89, 1658 (2014).
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[9] A. Kohyama, et al., J. Nucl. Mater. 258–263, 1319 (1998).
[10] J. Gan, et al., Report of Idaho National Laboratory, 2007.
[11] J.K. Goddard, Characterization of two ODS alloys: 18Cr
ODS and 9Cr ODS (Theses and Dissertations for Universityof South Carolina, 2013) P.29.
[12] W.D. Callister, Fundamentals of Materials Science andEngineering (An Integrated Approach, 4 edition, Wiley,
Hoboken, N.J, 2012) P.303.
[13] S. Ukai, Oxide Dispersion Strengthened Steels-Compr.
Nucl. Mater., (Elsevier, Oxford, 2012) P. 241.

500nm

Fig. 6. Microstructure of JLF-1 before HIP (a-b), as-HIPed (c-d),
and after PBHT at 1050 oC×1h+760oC×1h (e-f). (a, c, e )—SEM images, (b, d, f ) —TEM images.

3.4 Microstructural evolution of the interface
Fig.7 shows the interface between 9Cr-ODS and JLF1 in the conditions of as-HIPed and after PBHT. After HIP,
as depicted before in [7], coarse carbides M3C emerge on
the 9Cr-ODS side, while quenched martensitic structureappears on the JLF-1 side. However, there is a layer ofgrains with few MX nano-particles and no lath structure at
the interface on the JLF-1 side. It seems to be ferritic
structure formed during the HIP process. After PBHT
with normalization and tempering, both the coarse carbides
on the 9Cr-ODS side and the few-particle layer on the
JLF-1 side disappear. Instead, tempered martensitic

64