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E-Journal of Advanced Maintenance Vol.9-2 (2017) 91-96Japan 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, JAPAN 2 Kobe Steel, Ltd., 2-4, Wakinohama-Kaigandori 2-chome, Chuo-ku, Kobe, Hyogo 651-8585, Japan ABSTRACT

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 the advanced nuclear power plants. Extra high purity, grade type 25Cr-35Ni EHP, austenitic stainless alloy was developed 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 is

developed 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 used

in the LWRs and the advanced nuclear reactors. However, the aging embrittlement in a region at high temperature of more than 500 degrees is one of the important license problems on a reactor vessel of LMFBR 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 residual alpha 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 the minimization 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 of passive 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 the diffusion bonding method by means of hot rolling(Fig.1). Hot rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform at above its recrystallization temperature. Two types of cladding with and without of Ni insert was examined for the bonding compatibility

Figure. 1. A schematic sketch of hot rolling

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and the degradation during thermal aging at high temperature. The mutual diffusion behavior was minutely examined to the joining interface region between type 25Cr-35Ni EHP and type 316L under gamma ray irradiation. The optimal 25Cr-35Ni EHP condition is evaluated by Charpy impact tests of specimens with different cladding thickness and metallographic examination of bonding regions. 2. Experiment 2.1. Corrosion test and Charpy impact test

The 25Cr-35Ni EHP alloy was made by the advanced 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 B and 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 of types 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 base material 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 (16 kGy/h) for about 1500hr based on the thermal aging test results of each single material (EHP and 316L). These ageing experimental condition were set taking into consideration of the environment assumed for nuclear 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 a result 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 for clarifying grain boundary degradation and sigma phase precipitation. In addition to these observations, Charpy impact test (CVT) was pursued for evaluating the cladding thickness of EHP , which is necessary to suppress the reduction in mechanical 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

Figure. 2. The effect of aging temperature (single material) on the Impact value as the

function of aging temperature

Figure. 3. Charpy V-notch specimen （10mm×55mm×7.5mm, t：0 , 1, 2, 3 mm）

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2.2. Results Fig.4 shows the effect of EHP thickness in the

cladding material with Ni insert on the impact value by CVT, as a function of EHP thickness in the cladding material with Ni insert. The impact value of cladding material is larger than the sum of each allocation thickness of EHP and SUS316L in each thickness, especially in more than 2 mm thickness of 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 bonding interfaces. This leads to a degradation assumption , due to the precipitation of sigma phase in the bonding interface by mutual diffusion under thermal aging. In order to confirm the precipitation of sigma phase in the bonding interface, intergranular corrosion test is conducted. The behavior is clarified as the precipitation of sigma phase in the SEM image of the interface region after Coriou corrosion test of cladding material, as shown in Fig.5. At the bonding interface region, the concentration of major elements and the corrosion resistance change are due to 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 after thermal 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 to the accelerated diffusion of interstitial non-metallic impurities. Ni insert layer make it possible to effectively suppress it playing an important role on reducing the intergranular corrosion. From these results, it was experimentally clarified that the presence or absence of the Ni insert has a large influence on the formation of the precipitation of sigma phase on the EHP steel side in the bonding interface because the diffusion 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)

Figure. 4. The effect of EHP thickness on the Impact value as the function of EHP

thickness in the cladding material

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Figure. 6. The optical microscope observation image of Strauss corrosion test in the vicinity of the interface 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 evaluated with finite element method (FEM) analysis using LS-DYNA code. Fig.7 shows the analytical model for simulating Charpy impact tests. Cradle and impact blade are modeled by a rigid element. The initial velocity of the shock blade is defined as a boundary condition and the cradle has been completely fixed. The interference portion of the cradle and the test piece in the contact condition is not sticking. This leads to the assumption of simulated fracture i in the pre-fracture surface at the center vertical direction of the test piece toward the vertical direction from the notch bottom to define a contact surface with the dual node. (So that,) The Mises stress exceeds the breaking stress due to the absence of the node. The model of test specimens is divided into four parts. It gives different material properties in consideration of stress concentration and fracture toughness for 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 the analytical 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 for modifying the impact resistance is estimated to be more than 1mm.

Figure. 7. Model Analysis of Charpy impact test

(Number of node and element : 5197, 4792)

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3. 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 for improving 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 the diffusion bonding method, because of the advanced melting method of EHPwhich minimizes the harmful impurities affected to the solidification cracking. The effect of Ni insert is also examined for 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 the mutual diffusion during the thermal aging.

(4) On the other hand, Ni insert cladding materials are minimizing the metallurgical change at the bonding 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 is clarified 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 to EAC 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 of Education, Culture, Sports, Science and Technology Ministry (MEXT).

References

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[2] Junpei Nakayama, Kiyoshi Kiuchi, “Development of Extra-High Purity Stainless Steels for Nuclear Corrosive Environments”, MRS Proceedings, vol.1298 (2011).

Figure. 8. Divided model of test piece and material properties for each part

Figure. 9. Comparison of analytical and experimental results in Charpy Absorbed energy

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[3] Junpei Nakayama, Tsuyoshi Noura, Hitoshi Yamada, Kazumitsu Yamamoto, “Extra High Purity

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[4] Ikuo Ioka, Chiaki Kato, Kiyoshi Kiuchi, Junpei Nakayama, “Intergranular Corrosion for Extra High Purity Austenitic Stainless steel in Boiling Nitric Acid with Cr (VI)”, Journal of Power and Energy Systems, vol.3(2009)

[5] Y. Sawamoto, N. Tanaka, “Stress-Strain Relationship of Steel for Assessment of the Deformation Capacity under Brittle Fracture”, KAJIMA Techinical Research Institute Annual Report, vol.50 (2009).