24P047-P
Study on Dissimilar Metal Joint of Reduced Activation Ferritic/Martensitic Steeland Austenitic Stainless Steel by Electron Beam Welding
電子ビーム溶接による低放射化フェライト鋼とオーステナイトステンレス鋼の異種金属接合に関する研究
Masahiro Waki, Shuhei Nogami, Takuya Nagasaka, Akira Hasegawa, Takeo Muroga
和氣昌弘 1, 野上修平 1, 長坂琢也 2, 長谷川晃 1, 室賀健夫 2
1

Tohoku University, 6-6-01-2, Aramaki-aza-Aoba, Aoba-ku, Sendai, 980-8578, Japan1
東北大学 〒980-8579 仙台市青葉区荒巻字青葉 6-6-01-2
2
National Institute for Fusion Science, 322-6, Oroshi, Toki, Gifu, 509-5292, Japan2
核融合科学研究所 〒509-5292 岐阜県土岐市下石町 322-6

The dissimilar metal joint of reduced activation ferritic/martensitic (RAFM) steel and austenitic
stainless steel by electron beam welding (EBW) was investigated. The EBW joint was categorized into
five regions: the base metal of F82H (F82H-BM), the heat-affected zone of F82H (F82H-HAZ), theinterlayer at the edge of F82H-HAZ (IL), the weld metal (WM), and the base metal of SUS316L(SUS316L-BM). No hardening due to welding and no significant change in hardness due to thepost-welding heat treatment (PWHT) above 640ºC were observed in the F82H-BM, SUS316L-BM, andthe WM. On the other hand, a significantly higher hardness due to welding and a significant reduction ofhardness due to the PWHT above 640ºC was observed in the F82H-HAZ and IL.

1. Introduction
Reduced activation ferritic/martensitic (RAFM)
steel and austenitic stainless steel are considered tobe applied as a structural material and the peripheral
equipment material of fusion reactor blanket,
respectively [1]. Therefore, it is necessary todevelop the dissimilar metal jointing technique ofthese materials for the fabrication of fusion reactor.
The authors have been studying the dissimilar metal
jointing of RAFM steel, F82H, and austenitic
stainless steel, SUS316L, by the electron beam
welding (EBW), and suggested the necessity of theoptimization of post-weld heat treatment (PWHT) to
improve mechanical property of the joint [2]. The
objective of this study is to investigate the effect ofPWHT
temperatureonmechanicalandmetallographical properties of dissimilar metal EBW
joint of F82H and SUS316L steels.
2. Experimental
Materials used in this study are the F82H
IEA-heat and the SUS316L steels. Electron beam
butt-welding was carried out using flat plates
measuring 50 mm × 50 mm × 7 mm. The
acceleration voltage, beam current, and welding
speed were 150 kV, 20 mA, and 1000 mm/min,respectively. The electron beam was shifted by 0.2

mm on the SUS316L side, which was optimized in
the previous study [2]. PWHT was performed in avacuum at temperatures ranging from 640ºC to750ºC for 1 h.
Vickers hardness measurement of thecross-sectional surface of the weld joint wasconducted using a Vickers hardness tester.
Metallographical analysis was performed by using
an optical microscope, scanning electron microscope(SEM) with an energy-dispersive spectroscopy(EDS) system, and a field-emission electron probe
micro-analysis system (FE-EPMA).
3. Result and Discussion
The EBW joint was categorized into five
regions: the base metal of F82H (F82H-BM), theheat-affected zone of F82H (F82H-HAZ), theinterlayer at the edge of F82H-HAZ (IL), the weld
metal (WM), and the base metal of SUS316L(SUS316L-BM). Fig. 1 shows the cross-sectional
Vickers hardness distribution in the as-welded andpost-welding heat treated EBW joints. PWHT was
not confirmed to have any influence on the hardnessof the F82H-BM and SUS316L-BM at PWHT
temperatures between 640ºC and 720ºC.
Although a much higher hardness (Hv420) wasobserved in the as-welded F82H-HAZ, a significant

reduction of hardness occurred due to the PWHT
above 640ºC. Similar to the F82H-BM, a martensitic
phase was observed in the F82H-HAZ, and a more
complex metallographical appearance was observedin the as-welded F82H-HAZ than in the as-welded
F82H-BM. Thus, the higher hardness of theas-welded F82H-HAZ was considered to be a resultof the higher density of the lath in the martensitic
structure formed by cooling after the welding.
A much higher hardness under the as-welded
condition and a significant reduction of hardness dueto the PWHT were also observed in the IL. A further
increase in the PWHT temperature caused a lower
hardness of the IL than that of the BMs. Nomartensitic phase was observed in the IL, and EDS
and EF-EPMA analyses showed no significant
diffusion of major elements such as Fe, Cr, and Ni,
and minor elements such as C. Based upon the phase
diagram of the Fe-Cr binary system, the formationof δ-ferrite occurs above approximately 1300°C orso. Therefore, δ-ferrite was considered to be one ofthe possible phases of the IL. During welding, the IL
region might be transformed into the δ-ferrite phase,and then remain as the δ-ferrite phase as a result ofrapid cooling.
No hardening due to welding was observed in
the WM of the as-welded joint. An austenitic phase
was observed in the WM, which was attributed tothe electron beam being shifted by 0.2 mm on theSUS316L side [2]. Therefore, the hardness of theWM was considered to be relatively stable, similar
to SUS316L-BM under PWHT temperatures
between 640ºC and 750ºC.

Vickers hardness, Hv

F82H-BM F82H-HAZ IL
450
400
350
300
250200150
100
500

Vickers hardness, Hv-0.51)

2)00.5

11-0.5450
Distance[mm]400350300250200150100
500[1] Y. Nomoto, S. Suzuki, K. Ezato, T. Hirose, D. Tsuru,H. Tanigawa, T. Hatano, M. Enoeda, M. Akiba: Fus.
Eng. Des. 81 (2006) 719.
[2] N. Hara, S. Nogami, T. Nagasaka, A. Hasegawa,H. Tanigawa and T. Muroga: Fus. Sci. Tech. 56-1
(2009) 318.
SUS316L-BM660ºC x 1 h
00.5

11-0.5-0.5680ºC x 1 h
00.5

Distance [mm]1-0.5

11-0.500.51Distance [mm]750ºC x 1 h720ºC x 1 h
00.5

Distance [mm]Distance [mm]700ºC x 1 h-1

The EBW joint was categorized into fiveregions: the base metal of F82H (F82H-BM),the heat-affected zone of F82H (F82H-HAZ),
the interlayer at the edge of F82H-HAZ (IL),the weld metal (WM), and the base metal ofSUS316L (SUS316L-BM).
No hardening resulting from welding and no
significant change in hardness resulting fromPWHT above 640ºC were observed in theF82H-BM, SUS316L-BM, and WM. On theother hand, a significantly higher hardness wasobserved in the as-welded F82H-HAZ and IL
than in the other region, and a significant
reduction of hardness occurred in theF82H-HAZ and IL caused by PWHT above
640ºC.

References

640ºC x 1 hAs-welded-1

WM

4. Conclusions
The effects of PWHT on the mechanical andmetallographical properties of a dissimilar metal
EBW joint of F82H IEA-heat and SUS316L steels
were investigated to support in the development ofdissimilar metal welding techniques for RAFM andaustenitic stainless steel. The results of this study aresummarized as follows;

00.5

11Distance[mm]
Distance fromthe centerof WM [mm]

-0.5

00.5

1Distance [mm]Fig. 1. Cross-sectional Vickers hardness distribution in the as-welded and post-welding heat treated F82H/SUS316L
EBW joints (PWHT temperatures: 640, 660, 680, 700, 720, 750ºC)