10.

Task １０ Development of Cryogenic Materials Technology

10.1.
10.1.

R & G Goals

10.1.110.1.1

Goals of Phase II

The goals of phase II are to test material properties under liquid hydrogen environment
and to develop elemental technology related optimized welding method.

The material

characteristic database will also be enhanced.
10.1.210.1.2

Goals for FY 2001

(1) Evaluation test will be implemented on the base metal and weld metal selected as
candidate materials in the temperature area of liquid hydrogen, and basic material
characteristic data will be enhanced. Evaluation on thin walled material will also bestudied.
(2) Properties will be improved by conventional welding method, and elemental technologies
will be developed with respect to new welding methods such as reduced pressure electron
beam welding and friction stir welding.
(3) Additional data of the cryogenic material characteristic database will be input forenlargement of the database.
(4) The material structure, chemical compositions, transformation behavior and mechanism of
low temperature embrittlement and hydrogen embrittlement will be clarified.
10.2
10.2

Results of FY 2001 Research and Development

In Phase II of the Research and Development, evaluation was made on the material
characteristics of the base material and weld material in the low temperature including theliquid hydrogen environment using the selected candidate materials (austenitic stainless steel:
SUS304L, SUS316L, SUS316LN, aluminum alloy: A5083A and A5454). As a result, it was
found out that, while the selected base material maintained sufficient properties in liquidhydrogen environment, improvement of toughness was important for the weld metal.
Accordingly, from the viewpoint of improving toughness, evaluation has been made on theapplicability of new welding methods and welding materials that are emerging in recent years,
and their effects have been verified.
In FY 2001, we have evaluated new welding methods and welding materials by expanding
the scope of metal types to be applied. At the same time, we have analyzed the factors affecting
toughness at a low temperature and clarified the mechanism of embrittlement. Conventional
tests of material characteristics such as fatigue characteristics have been continued to enhancethe database.

-99-The following describes the major achievements obtained from the project of this fiscal
year:
10.2.110.2.1

Evaluation of stainless steel material characteristics

It has been known so far that, stainless steel weld material, the presence of even a small
quantity of δ-ferrite serves as a fracture propagation path at a low temperature, with theresult that toughness is reduced. However, the δ -ferrite is essential to prevent
high-temperature fracture in the welding operation, and the problem is how to ensurecompatibility between the low-temperature toughness and prevention of welding fracture. By
the end of the previous fiscal year, we demonstrated that SUS304 and SUS316L had a highdegree of toughness at a low temperature despite the presence of δ-ferrite according to the
reduced pressure electron beam welding method, and the TIG welding by high manganese
based perfect γ- type welded metal characterized by reduced susceptibility to fracture at ahigh temperature had an excellent toughness at a high temperature and welding fracture
preventive function.
In the program for this fiscal year, reduced pressure electron beam welding method has
been applied to the high-strength SUS316LN effective in reducing the weight of atransportation container, and characteristics at a low temperature and hydrogen susceptibility
have been evaluated. Further, perfect γ -type welding metal and TIG welding was also
examined for hydrogen susceptibility using SUS316L.
Fig. 10.2.1-1 shows the Charpy absorbed energy and fracture toughness value (KIC) of
reduced pressure electron beam (RPEB) of SUS316LN. Although SUS316LN involves theproblem of easier generation of blowhole-shaped welding defects caused by a high nitrogencontent in the base material, the sound portion of SUS316LN ensures much higher toughness
at a low temperature than that in the TIG welding method, similarly to the case of SUS304Land SUS316L. Further, neither reduction in toughness that would be caused by hydrogencharging nor remarkable susceptibility to hydrogen embrittlement were observed.
Fig. 10.2.1-2 shows the toughness of perfect γ-type weld metal and TIG weld metal (JJ1)of the SUS316L at a low temperature. It exhibits higher Charpy absorbed energy and KIC thanthese in the conventional high δ -ferrite TIG welding method. Further, no reduction oftoughness caused hydrogen charging was observed.

-100-800

SUS316LNSUS316LN

RPEB H 2 charged

400√m
K I C (J) / MPaMPa√

Charpy Absorbed Energ y / J500

base metal

300

RPEB

200

100

600

RPEB

RPEB H 2 c harged

400

TIG

base metal
200

TIG
0001002000300

100

Temperatu re / K200

300

Te mpe rature / KFig. 10.2.110.2.1-1 Charpy impact properties and fracture toughness of SUS316LNreduced electron beam welding metal800

SUS316LSUS316L
400

base metal√mK I C (J) / MPaMPa√

Charpy Abs orbed Energ y / J

500300TIG JJ1TIG-JJ1 H2 c harged

200100

TIG0base metal

600

TIG JJ1400

TIG- JJ1 H2 charged
200

TIG

00

100

2003000Te mpe rature / K

Fig. 10.2.110.2.1-2

100

200

300

Temperatu re / K

Charpy impact properties and fracture toughness of SUS316L TIG welding
metal

Following the project in the previous fiscal year, we have conducted a fatigue test ofSUS304L, SUS316L and SUS316LN base metals and weld metals, and have enhanced thedatabase. As a result, it has been found out that all base metals have a fatigue life of 106 to 107
times or more against fatigue under a higher stress equivalent to 0.2% proof stress. However,some of the TIG weld metals have a shorter fatigue life against fatigue, depending on the weld
joint manufactured, and this defect has been estimated to be caused by very small fracture
defects due to solidification.
10.2.210.2.2

Evaluation of aluminum alloy material characteristics

By the end of the previous fiscal year, we demonstrated that friction stir welding (FSW)-101-improves the low temperature toughness of the aluminum alloy through the formation of a finestructure on the weld metal. It has also been made clear that the toughness can be improved by
reducing the magnesium content of the base material. In the program for this fiscal year,friction stir welding (FSW) was applied to low-magnesium (3%) A5454 alloy, and the result
were compared with that of A5083 with magnesium content of 5% obtained in the previousfiscal year and before. This is given in Fig. 10.2.2-1 In both the Charpy absorbed energy andKIC, A5454 exhibits higher low-temperature toughness values than those of the A5083. It
should be noted that, for the Charpy absorbed energy of A5454 according to FSW method, nofracture was caused by the impact of the test equipment used in the present experiment at anytemperature. The actual value is estimated to be greater than this value.
In the meantime, whereas reduction of the magnesium content improves thelow-temperature toughness, it decreases the tensile strength. To keep tack of the balance
between them, we used a new A5086 alloy of 4% magnesium content to examine thecharacteristics of the large current MIG weld metal this year. The result is given in Fig.
10.2.2-2 It indicates that the strength and tensile strength are reduced, and KIC value isincreased with the decrease of the magnesium content. Observation of the fracture surface
suggests that improvement of the toughness is attributable to reduction in the grain boundary
segregation due to decreased magnesium content and improvement of the transgranulardeformation. Further, the reduction of tensile strength in the A5083 can be explained by earlier
fracture having occurred for the same reason due to high magnesium content.

200A545 4 FSW

A5454 FSW

5040√mK I C (J) / MPaMPa√

Char py Abso r be d Ene r gy / J60A508 3 FSW
A5454 base metal

3020A5083 base metal

A5083 MIG

150A5454 base metal

100A5083 FSW

50A5083 MIG

10A5083 base metal

000100200

3000Temperature / K100200Temperature / KFig. 10.2.210.2.2-1 Charpy impact properties and fracture toughness ofA5454 friction stir weldingwelding metal-102-300

600120100

A5 086 MIG
400√m
K IC (J) / MPa
MPa√

TS, Y S / Mpa

A545 4 MIG

A5 083 MIG

TS500

A5 454 MIG

300

YS

200

100

A508 6 MIG
8060

A508 3 MIG
4020000100

2003000Te mpe rature / K

100200300

Te mpe rature / K

Fig. 10.2.210.2.2-2 Influence of magnesium content in aluminum alloy
upon tensile strength and fracture toughness

10.2.3 Study of low temperature and hydrogen embrittlement in stainless steel weld metal
In TIG welding method, the thick-walled material is welded by multi-path welding process.
Thermal impact in this case may affect the low-temperature toughness. Fig. 10.2.3-1 shows
the result of examining the Charpy absorbed energy by heat treatment of the PREB material
based on high-energy 1-path welding method. As the temperature is higher and keeping time islonger under the heat treatment conditions, the TIG weld metal and Charpy absorbed energy
are reduced; thus, the reduction in toughness has been verified. In the subsequent program, weare planning to observe the fracture surface and examine the structure, thereby clarifying thecauses of these phenomena. It should be added that, in the program for this fiscal year, we also
conducted a precision fracture toughness test to clarify the difference in fracture toughness
depending on the position of the weld metal, and checked the low-temperature embrittlement
behavior.

-103-

40 030 025 0at 77K

RPE B

Temperature / K

Charpy Absorbed Energy / J

SUS316L

2 0 mi n . He at i n g

20 015 02 h r s. He at i n g

10 035 0Ａ ｂｓ ｏｒ ｂｅｄ

N o n Ａ ｂｓ ｏｒ ｂｅｄ

30 0SUS304L & SUS316L

TI G50Base m et al & W el d m et al
H oldin g t i m e ： 1004 h r s025 06 00w i t ho ut
H e at i ng

70 080 09 00

H e at i ng T e mp e r at ur e

1 000

11 000/ K

510P r e s s ur e o f hy d r o ge n g asFig. 10.2.310.2.3-1 Influence of thermal treatment
upon Charpy absorbed energy of stainless

15/ MPa

Fig. 10.2.310.2.3-2 Conditions on the intrusion ofhydrogen into stainless ststeel
eelsteel
Upon intrusion of hydrogen into stainless steel, characteristics may be deteriorated in somecases. Fig. 10.2.3-2 shows the result of exposing the stainless steel for a long time in a hydrogengas atmosphere at various temperatures and pressures to examine if hydrogen enters or not.
The critical pressure for hydrogen intrusion at 100 ℃ is estimated at 1 to 5 MPa. The higher
the pressure, the smaller the critical temperature for hydrogen intrusion. Our further task is to
pay attention to the storage of over-20MPa high-pressure hydrogen gas and to examine thepossibility of hydrogen intrusion and its influence upon the material. It should be noted thatthe relationship between the embrittlement in the hydrogen gas environment and stainlesssteel components has also been studied, following the study project for the previous fiscal year.
10.3.
10.3.

Summary and future develo
development
pment issues

We have evaluated new welding methods effective in improving toughness at a lowtemperature by expanding the scope of the metal types to be applied. Reduced pressure
electron beam welding of stainless steel has been shown to provide the same high toughness at
a low temperature in high-strength SUS316NL as in other types of steel. No influence ofhydrogen charging was observed. Perfect γ-weld metal TIG welding method has also shown toprovide a high degree of toughness. No influence upon hydrogen susceptibility was observed.
Further, application of the friction stir welding to the A5454 aluminum alloy of low-magnesium
content has been found to ensure a drastic improvement in the low-temperature toughness. In
this project, we could also extend our knowledge on the strength/toughness balance bymagnesium content.

-104-Since development of dispersed use at the hydrogen supply station and others is expected
to be accelerated and promoted, our subsequent task will be to evaluate the characteristics ofthe thin-walled material for small- to medium-sized containers and to evaluate and study theembrittlement characteristics under high-pressure hydrogen gas environment.

-105-