Research method

Research and Development of Stainless Steel with
Hydrogen Embrittlement Resistance

X-ray diffraction/
scattering

Analysis of crystalline phase causing hydrogen embrittlement

Role of SPring-8

Achievements

Demonstration of the existence of a phase causing hydrogen embrittlement**
by observing the crystalline microstructure of stainless steel* at room
temperature
Contribution to the research and development of a stainless steel resistant tohydrogen embrittlement and its applications
R&D facility: NIPPON STEEL Stainless Steel Corporation and Osaka Prefecture University*Stainless steel: Stainless steel is an iron alloy containing a minimum of 11% Cr. About ~ 50 million tons of stainless steel is produced worldwide
per year. SUS304 (18-8 stainless steel) is a typical stainless steel that contains 18% Cr and 8% Ni and has high strength, ductility, and corrosion
resistance. Therefore, it has been widely used as a material in built-in kitchens, elevators, and chemical plants and tanks.
**Hydrogen embrittlement: Hydrogen embrittlement is a phenomenon that occurs in metals. When an external force is applied to metals into
which hydrogen penetrates, their ductility and toughness decrease and they become brittle. Metals suffering from hydrogen embrittlement may
break upon the application of force.
***Lorentz transmission electron microscope (TEM): A Lorentz transmission electron microscope is used to observe the magnetic domains ofmagnetic materials. Electrons that enter a ferromagnetic substance change their direction of travel upon the application of a Lorentz force via amagnetic field (deflection). Electrons are subjected to different deflections from neighboring magnetic domains. This enables a contrast to beobserved at the neighboring magnetic domains in images, revealing the boundary of the phases.

(b) α’ phase (bcc):
ferromagnetic

In addition to the use of renewableenergy as an alternative to fossil fuels,hydrogen has been attracting muchattention as a source of energy. Stainlesssteel is a promising material for containers
used for the storage and transport ofhydrogen because of its excellent mechanicalproperties such as high strength and ductility
as well as corrosion resistance. However, it
may undergo hydrogen embrittlement.
To develop stainless steel with hydrogen
embrittlement resistance, the mechanism
underlying hydrogen embrittlement should
be clarified.
However, the theory ofhydrogen embrittlement has not been
explained, and it has been difficult to observe
the change in the crystalline microstructureduring the processing of stainless steel atroom temperature at an atomic scale.

In situ X-ray diffraction observation of SUS304 specimens
with and without prestrain was carried out at SPring-8
while the specimens were strained using a tensile testingmachine. The results indicate that the ε phase, which isconsidered to be the cause of hydrogen embrittlement,
was observed when the crystalline microstructure of thespecimen transformed from the nonmagnetic γ phaseto the ferromagnetic α’ phase. In addition, the sitewhere the crystalline phase was formed was observed
using a Lorentz transmission electron microscope***.
The ε phase was observed at the twin boundary between
the nonmagnetic γ phase or ε phase and the ferromagnetic
α’ phase.
The above results indicate the existence of the ε phase,
which is considered to cause the hydrogen embrittlementof stainless steel at room temperature, but it has not been
observed thus far. It is expected that a new stainless steel
with hydrogen embrittlement resistance will be developed
and that new applications of this stainless steel will beproposed in the future.

Observation of X-ray diffraction profiles using SUS304 specimens with and without prestrain
X-ray diffraction experiment at SPring-8

10Intensity (arb. units)(a) γ phase (fcc):
nonmagnetic

Results

1050 %
40 %
30 %
20 %
10 %

5With increasing amount oftensile strain (10%, 20%,…),
the area of the peak of the εphase in the profile increases
up to a tensile strain of 40%,
after which it decreases. From
this result, it is found that theε phase starts to transform tothe ferromagnetic α’ phase ata tensile strain of ~ 30%.

γ111
γ200α’110

4ε110

10 3γ111
γ200α’110

ε110
10 310 2To observe the transformation
process from the ε phase to theα’ phase, in situ observation ofstrained SUS304 specimens
without tensile strain wascarried out. With increasing
amount of strain, the amountof the ε phase increases, which
is followed by the appearanceof the α’ phase.

2θ(degree)

Observation of magnetic phase: Lorentz TEM image of fracture specimen(c) ε phase (hcp): nonmagnetic

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10480 %40 %
30 %
25 %
17 %
10 %5%

10.5 11.0 11.5 12.0 12.5 13.0 13.5

10.5 11.0 11.5 12.0 12.5 13.0 13.5
2θ(degree): Fe, Cr, Ni

SUS304 crystals are nonmagnetic and have theface-centered-cubic crystalline structure (γ phase). Whena SUS304 specimen is strained at room temperature,the γ phase is transformed to the body-centered-cubic
crystalline microstructure (α’ phase) and the specimen
becomes ferromagnetic (deformation-induced martensite
transformation). This transformation improves thestrength and ductility of SUS304. In this study, thegeneration of the ε phase, which is considered to causehydrogen embrittlement, was confirmed during thetransformation process from the γ phase to the α’ phase.

10 5Intensity (arb. units)

Change in crystalline microstructure of SUS304

BackgroundA multiaxis diffractometer, which enables easy control of the incident
angle of X-rays and the detection angle of diffracted X-rays, and a tensile
testing machine were installed in the station of BL02B1. The in situ
observation of stretched SUS304 specimens with and without prestrain
was carried out.

γα’γ+α’A BAγεγ+α’α’

2 µm

The boundary between
black and white magnetic
phases was observed inthe area of the α’ phase ina Lorentz TEM image of afractured specimen.

1 nmTBAn array of crystals uniqueto the ε phase (“ABAB”)
was observed in thehigh-resolution TEM imageof a SUS304 specimen
subjected to 20% tensile
strain (compared with“ABC” in the general
γ phase).

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