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SPECIAL SECTION PAPERS: Materials and Fabrication

The Welded Joint Strength Reduction Factors of Modified 9Cr–1Mo Steel for the Advanced Loop-Type Sodium Cooled Fast Reactor

[+] Author and Article Information
Takuya Yamashita

Japan Atomic Energy Agency,
Ibaraki 311-1393, Japan
e-mail: yamashita.takuya38@jaea.go.jp

Takashi Wakai

Japan Atomic Energy Agency,
Ibaraki 311-1393, Japan
e-mail: wakai.takashi@jaea.go.jp

Takashi Onizawa

Japan Atomic Energy Agency,
Ibaraki 311-1393, Japan
e-mail: onizawa.takashi@jaea.go.jp

Kenichiro Satoh

Mitsubishi FBR Systems,
Tokyo 150-0001, Japan
e-mail: kenichiro_satoh@mfbr.mhi.co.jp

Kenji Yamamoto

Mitsubishi Heavy Industries,
Takasago 676-8686, Japan
e-mail: kenji4_yamamoto@mhi.co.jp

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received November 30, 2015; final manuscript received June 16, 2016; published online September 2, 2016. Assoc. Editor: Xian-Kui Zhu.

J. Pressure Vessel Technol 138(6), 061403 (Sep 02, 2016) (6 pages) Paper No: PVT-15-1260; doi: 10.1115/1.4034017 History: Received November 30, 2015; Revised June 16, 2016

Creep strength enhanced ferritic (CSEF) steels including ASME Gr.91 are widely used in fossil power plants. In the advanced loop-type sodium-cooled fast reactor (SFR), modified 9Cr–1Mo steel (ASME Gr.91) is going to be adopted as a structural material. Modified 9Cr–1Mo steel was registered in the Japan Society of Mechanical Engineers (JSME) code as a new structural material for SFRs in the year 2012. The creep-rupture curve of the base metal of this steel was standardized using region splitting analysis method. According to this method, creep-rupture data were divided into two regions, high-stress and low-stress regimes, and those regions were individually evaluated by regression analyses with the Larson–Miller parameter (LMP). The difference in the creep failure mechanisms between the high-stress and low-stress regions was considered in this method. The boundary between these regions was half of the 0.2% proof stress of the base metal at the corresponding temperature. In the modified 9Cr–1Mo steel welded joint, creep strength may markedly degrade, especially in the long-term region. This phenomenon is known as “type-IV” damage due to creep voids and cracks in the fine-grained heat-affected zone (HAZ). There is no precedent for indicating the obvious creep strength degradation of welded joints under SFR temperatures (550 °C or less). Although obvious strength degradation of the welded joints has not yet been observed at 550 °C, it is fair to assume that the strength degradation will occur due to very long-term creep. Therefore, considering strength degradation due to “type-IV” damage is necessary. This paper proposes the creep-rupture curve and the welded joint strength-reduction factor (WJSRF). The creep-rupture curve of the welded joint was proposed by employing a second-order polynomial equation with LMP using region splitting analysis method, which is used for the base metal as well. The WJSRFs were proposed on the basis of design creep-rupture stress strength. The resulting allowable stress was conservative compared with that prescribed in ASME code and the Japan domestic regulation for thermal plants. In addition, the design of the hot-leg pipe in SFR was reviewed considering the WJSRFs.

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Figures

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Fig. 1

Compact plant design achieved by employing some innovative technologies

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Fig. 2

Available creep test data of welded joints

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Fig. 3

Ultimate tensile strength versus temperature for the welded joint

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Fig. 4

Relation of standard deviation to order of regression analysis

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Fig. 5

Several creep-rupture curves by using from first- to fourth-order regression analysis

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Fig. 6

Obtained creep-rupture curve and analyzed data

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Fig. 7

Comparison between nominal creep-rupture curves of the base metal and welded joint

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Fig. 8

Comparison of design creep-rupture stress strength proposed in this study with ASME B&PV code section 3, subsection NH

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Fig. 9

Comparison of design creep-rupture stress strength proposed in this study with code for thermal power plants

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Fig. 10

Comparison of design creep-rupture stress strength proposed in this study with those of the previous study

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