E-Journal of Advanced Maintenance Vol.9-2 (2017) 91-96
Japan Society of Maintenology

Evaluation of environmental compatibility of EHP (extra high purity)using austenitic stainless steel cladding material
Junji Etoh1, Takaki Ashida1, Takamasa Ochiai1, Kiyoshi Kiuchi1, Masayuki Takizawa1, Junpei
Nakayama2
1

Mitsubishi Research Institute, Inc., 10-3, Nagatacho 2-Chome, Chiyoda-Ku, Tokyo 100-8141,
JAPAN2
Kobe Steel, Ltd., 2-4, Wakinohama-Kaigandori 2-chome, Chuo-ku, Kobe, Hyogo 651-8585, JapanABSTRACT
The resistance to environment-assisted cracking (EAC) like SCC (stress corrosion cracking) and aging
embrittlement are the most important problems on the materials performance of type 316L steel used in theadvanced nuclear power plants. Extra high purity, grade type 25Cr-35Ni EHP, austenitic stainless alloy wasdeveloped by means of minimizing impurity using the special melting technology. It has the excellent
corrosion resistance in LWR environments and high temperature steam under gamma ray irradiation.
In this research, cladding technology of type 25Cr-35Ni EHP alloy on the base metal of type 316L steel isdeveloped for reactor core materials by the diffusion bonding method using hot rolling. The corrosion resistance
was tested by aging up to 600 degrees in Ar under gamma-ray irradiation. The effect of cladding and aging
embrittlement of type 316L steel due to sigma phase precipitation is evaluated by Charpy impact tests.
From these results, it was clarified that the resistance to corrosion and aging embrittlement of type 316L steel
are possible to improve by including cladding of type 25Cr-35Ni EHP alloy, of a thickness of more than 2 mm .
The cladding of EHP alloy is considered to be one of the most widely used materialtechnology for preventing
the aging degradation of austenitic stainless steels like type 316L used in current nuclear power plants.
KEYWORDS
Extra high purity, Austenitic stainless steel, Corrosion, Environmentally- assisted cracking, Cladding material,
Charpy impact test

1.

Introduction

Type 316L austenitic stainless steel was currently performed as the major structure materials usedin the LWRs and the advanced nuclear reactors. However, the aging embrittlement in a region at hightemperature of more than 500 degrees is one of the important license problems on a reactor vessel ofLMFBR and reactor components like SCWR. It is easy to proceed with the precipitation of sigma
phase from the residual ferrite phase and the local segregation of Mo due to grain boundary
segregation during cooling [1]. Extra high purity, grade type 25Cr-35Ni EHP, alloy without residualalpha phase and Mo, was designed for obtaining the sufficient austenitic phase stability by inhibiting
the solidification cracking due to residual impurities in alloys [2-3]. It was achieved with theminimization of the residual impurities by means of the (advanced purification melting technology). It
has also an excellent corrosion resistance to environmenlyassisted cracking by the rapid formation ofpassive oxide film. However, reduction in mechanical strength as an adverse effect and increase in
manufacturing cost by extra high purity were problems.
In this research, in order to overcome
these problems, cladding of type 25Cr-35Ni
EHP alloy, on the base material of type 316L
steel used in current plants was made with thediffusion bonding method by means of hotrolling(Fig.1). Hot rolling is a metal forming
process in which metal stock is passed throughone or more pairs of rolls to reduce thethickness and to make the thickness uniform atabove its recrystallization temperature. TwoFigure. 1. A schematic sketch of hot rollingtypes of cladding with and without of Ni insert
was examined for the bonding compatibility

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F. Lastname, et al./ Title of the Paperand the degradation during thermal aging at high temperature. The mutual diffusion behavior wasminutely examined to the joining interface region between type 25Cr-35Ni EHP and type 316L undergamma ray irradiation. The optimal 25Cr-35Ni EHP condition is evaluated by Charpy impact tests ofspecimens with different cladding thickness and metallographic examination of bonding regions.

2.

Experiment2.1. Corrosion test and Charpy impact test
The 25Cr-35Ni EHP alloy was made by theadvanced melting technology in order to minimize
the total residual impurities (C, O, N, P, S, B, Si,Mn) less than 100ppm and harmful elements of Band alkaline metals etc. This alloy is an immunity
to the grain boundary attack in the overall corrosion
potential, even in the transpassive region. The
chemical composition of specimen materials oftypes 25Cr-35Ni EHP and 316L steels (hereafter,abbreviates as EHP and 316L) used in this study are
shown in Table1. A cladding material is made by
hot rolling method of EHPproduced from the basematerial of type 316L by considering the effect of
Ni insert for inhibiting the mutual diffusion during
the thermal history.
Thermal aging test was performed at around
600 degrees under gamma-ray irradiation (16kGy/h) for about 1500hr based on the thermal aging
Figure. 2. The effect of aging temperature
test results of each single material (EHP and 316L).
(single material) on the Impact value as theThese ageing experimental condition were set taking
function of aging temperature
into consideration of the environment assumed fornuclear equipment. Fig.2 indicates the impact value
by CVT as the function of aging temperature (400 ~800 degrees). Comparing with EHP, the impact
value of CVT of 316L in the temperature region at
higher than 500 degrees is significantly reduced as aresult of the precipitation of sigma phase. The
corrosion behavior after aging of cladding materials
was examined by scanning electron microscopy
after Coriou and Strauss corrosion tests forclarifying grain boundary degradation and sigma
Figure. 3. Charpy V-notch specimen
phase precipitation. In addition to these（10mm×55mm×7.5mm, t：0 , 1, 2, 3 mm）
observations, Charpy impact test (CVT) waspursued for evaluating the cladding thickness of EHP , which is necessary to suppress the reduction inmechanical strength by extra high purity, by changing up to 3 mm in accordance with JIS Z 2242(Fig.3).
Table1. Chemical composition of specimen materials

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F. Lastname, et al./ Title of the Paper

2.2. Results
Fig.4 shows the effect of EHP thickness in thecladding material with Ni insert on the impact value
by CVT, as a function of EHP thickness in thecladding material with Ni insert. The impact valueof cladding material is larger than the sum of each
allocation thickness of EHP and SUS316L in eachthickness, especially in more than 2 mm thicknessof EHP. On the other hand, the cladding specimens
without Ni insert are difficult to evaluate the real
impact value due to the cracking along the bondinginterfaces. This leads to a degradation assumption ,due to the precipitation of sigma phase in thebonding interface by mutual diffusion under thermalaging. In order to confirm the precipitation of sigma
phase in the bonding interface, intergranularcorrosion test is conducted. The behavior is clarified
as the precipitation of sigma phase in the SEM
Figure. 4. The effect of EHP thickness on theimage of the interface region after Coriou corrosion
Impact value as the function of EHP
test of cladding material, as shown in Fig.5. At thethickness in the cladding material
bonding interface region, the concentration of major
elements and the corrosion resistance change are dueto the mutual diffusion. On the other hand, the excellent corrosion resistance of EHP matrix materials
is maintained during the cladding and the additional thermal aging. The differences in optical
microscope photographs of the cladding interface region after Strauss corrosion tests before and afterthermal aged specimens with or without Ni insert under gamma ray irradiation are shown in Fig.6.
The intergranular corrosion at the interface vicinity of EHP of claddings without Ni insert is clearly
accelerated with the diffusion of impurities from the 316L during thermal aging. After thermal aging
under gamma ray irradiation, the intergranular corrosion detected region is widely expanded due tothe accelerated diffusion of interstitial non-metallic impurities. Ni insert layer make it possible toeffectively suppress it playing an important role on reducing the intergranular corrosion. From theseresults, it was experimentally clarified that the presence or absence of the Ni insert has a large influence on theformation of the precipitation of sigma phase on the EHP steel side in the bonding interface because thediffusion of carbon is suppressed by the presence of Ni insert material.

Figure. 5. The SEM observation image of Corio corrosion test in the vicinity of the interface of cladding
material before and after heat aging (750 degrees, 1500hr)

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F. Lastname, et al./ Title of the PaperFigure. 6. The optical microscope observation image of Strauss corrosion test in the vicinity of theinterface of cladding material before and after thermal aging with or without Ni insert(750 degrees, 1500hr, γ-ray irradiation)

2.3. FEM analysis
The improvement mechanism of impact
values created by cladding is minutely evaluatedwith finite element method (FEM) analysis using
LS-DYNA code. Fig.7 shows the analytical model
for simulating Charpy impact tests. Cradle andimpact blade are modeled by a rigid element. The
initial velocity of the shock blade is defined as aboundary condition and the cradle has been
completely fixed. The interference portion of thecradle and the test piece in the contact condition isnot sticking. This leads to the assumption ofsimulated fracture i in the pre-fracture surface at
the center vertical direction of the test piece toward
the vertical direction from the notch bottom todefine a contact surface with the dual node. (Sothat,) The Mises stress exceeds the breaking stressdue to the absence of the node. The model of test
specimens is divided into four parts. It gives
Figure. 7. Model Analysis of Charpy impact
different material properties in consideration oftest
stress concentration and fracture toughness for(Number of node and element : 5197, 4792)
each part based on reference data which is obtained
from the verification of several Charpy impact test analyses by FEM simulation[5]. The divided
model of test piece and material properties for each part is shown in Fig.8. The results of FEM
analysis including the experimental results is shown in Fig.9 for comparing with the validity of theanalytical method. The analytical data are able to reproduce the experimental results by Charpy
impact test, results. From the obtained evaluation results, the cladding thickness of EHP required formodifying the impact resistance is estimated to be more than 1mm.

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F. Lastname, et al./ Title of the Paper

Figure. 9. Comparison of analytical andexperimental results in Charpy Absorbed energy

Figure. 8. Divided model of test piece andmaterial properties for each part3. Conclusion
The following results were obtained from the present research.
(1) For the purpose of improvement of the environmental applicability of type 316L as the major
structural material of current reactors, the application of cladding using EHP alloy is assessed forimproving the resistance to EAC and the aging degradation of several reactor components.
(2) The cladding of EHP alloy with stable austenitic to the type 316L steel was easily made by thediffusion bonding method, because of the advanced melting method of EHPwhich minimizes theharmful impurities affected to the solidification cracking. The effect of Ni insert is also examinedfor maintenance of sufficient strength of bonding interface during the practical aging.
(3) The embrittlement layer at the bonding interface of cladding without Ni insert is formed by themutual diffusion during the thermal aging.
(4) On the other hand, Ni insert cladding materials are minimizing the metallurgical change at thebonding interface during thermal aging and show the excellent impact resistance.
(5) As results of numerical analyses of impact fracture data for simulating the experiment results, it isclarified that the V-notch bottom thickness of EHP alloy of more than 1mm is possible to improve
the type 316L with the low impact resistance due to the aging degradation.
(6) As the practical applicability of EHP /Ni/316L cladding material as reactor materials, the effective
thickness of EHP alloy is estimated to be nearly 2 mm for maintaining the excellent resistance toEAC and the impact resistance, by considering the aging degradation during the long term
operation of reactor plants.
(7) As a future research topic, it is desirable to conduct further soundness assessment tests such as
cladding shear test or tensile test in addition to the Charpy test.

Acknowledgement
This research is a part of the results that has been carried out in the budget of the Ministry ofEducation, Culture, Sports, Science and Technology Ministry (MEXT).

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