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.

References

References
1.
JSME
,
2012
, “
Codes for Nuclear Power Generation Facilities—Rules for Design and Construction for Nuclear Power Plants—Section II: Fast Reactor Standards
,” Japan Society of Mechanical Engineers, Shinjuku-ku, Tokyo, Japan, Standard No. JSME S NC2-2012 (in Japanese).
2.
ASME
,
2013
, “
II—Material
,”
Boiler and Pressure Vessel Code, Section II Part A
,
ASME
,
New York
.
3.
Kimura
,
K.
,
Kushima
,
H.
, and
Abe
,
F.
,
2002
, “
Degradation and Assessment of Long-Term Creep Strength of High Cr Ferritic Creep Resistant Steels
,”
International Conference on Advances in Life Assessment and Optimization of Fossil Power Plants
, Orlando, FL, Mar. 11–13.
4.
Kimura
,
K.
, and
Takahashi
,
Y.
,
2012
, “
Evaluation of Long-Term Creep Strength of ASME Grade 91, 92, and 122 Type Steels
,”
ASME
Paper No. PVP2012-78323.
5.
Tabuchi
,
M.
, and
Takahashi
,
Y.
,
2006
, “
Evaluation of Creep Strength Reduction Factors for Welded Joints of Modified 9Cr-1Mo Steel (P91)
,”
ASME
Paper No. PVP2006-ICPVT11-93350.
6.
Yaguchi
,
M.
,
Matsumura
,
T.
, and
Katsuaki
,
H.
,
2012
, “
Evaluation of Long-Term Creep Strength of Welded Joints of ASME Grade 91, 92, and 122 Type Steels
,”
ASME
Paper No. PVP2012-78393.
7.
Bell.
,
K.
,
1997
, “
Elevated Temperature Midlife Weldment Cracking (Type IV)-A Review
,” The Welding Institute, Cambridge, UK, TWI Report No. 597.
8.
Eggerler
,
G.
,
Ramteke
,
A.
,
Coleman
,
M.
,
Chew
,
B.
,
Peter
,
G.
,
Burbies
,
A.
,
Held
,
J.
,
Jefferey
,
C.
,
Rantala
,
J.
,
Witte
,
M.
, and
Mohrmann
,
R.
,
1994
, “
Analysis of Creep in a Weldes P91 Pressure Vessel
,”
Int. J. Pressure Vessels Piping
,
60
(
3
), pp.
237
257
.
9.
Wakai
,
T.
,
Nagae
,
Y.
,
Onizawa
,
T.
,
Obara
,
S.
,
Xu
,
Y.
,
Otani
,
T.
,
Date
,
S.
, and
Asayama
,
T.
,
2010
, “
Creep Strength Evaluation of Welded Joint Made of Modified 9Cr-1Mo Steel for Japanese Sodium Cooled Fast Reactor (JSFR)
,”
ASME
Paper No. PVP2010-26014.
10.
Komai
,
N.
, and
Masuyama
,
F.
,
2002
, “
Microstructural Degradation of the HAZ in 11Cr-0.4Mo-2W-V-Nb-Cu Steel (P122) During Creep
,”
ISIJ Int.
,
42
(
12
), pp.
1364
1370
.
11.
Albert
,
S. K.
,
Matsui
,
M.
,
Hongo
,
H.
,
Watanabe
,
T.
,
Kubo
,
K.
, and
Tabuchi
,
M.
,
2004
, “
Creep Rupture Properties of HAZs of a High Cr Ferritic Steel Simulated by a Weld Simulator
,”
Int. J. Pressure Vessels Piping
,
81
(
3
), pp.
221
234
.
12.
Li
,
D.
,
Shinozaki
,
K.
, and
Kuroki
,
H.
,
2003
, “
Stress-Strain Analysis of Creep Deterioration in Heat Affected Weld Zone in High Cr Ferritic Heat Resistant Steel
,”
Mater. Sci. Technol.
,
19
(
9
), pp.
1253
1260
.
13.
Watanabe
,
T.
,
Tabuchi
,
M.
,
Yamazaki
,
M.
, and
Hongo
,
M.
,
2006
, “
Creep Damage Evaluation of 9Cr-1Mo-V-Nb Steel Welded Joints Showing Type IV Fracture
,”
Int. J. Pressure Vessels Piping
,
83
(
1
), pp.
819
825
.
14.
Masuyama
,
F.
,
2006
, “
Creep Degradation in Welds of Mod.9Cr-1Mo Steel
,”
Int. J. Pressure Vessels Piping
,
83
(
11–12
), pp.
819
825
.
15.
Ministry of Economy, Trade and Industry
,
2014
, “
Hatstudenyo Karyoku Setsubi ni Okeru Kokuromuko ni Taisuru Jyumyo Hyoukashiki ni Tsuite
,” Ministry of Economy, Trade and Industry, Tokyo, Japan (in Japanese).
16.
Kimura
,
K.
,
Kushima
,
H.
, and
Abe
,
F.
,
2000
, “
Heterogeneous Changes in Microstructure and Degradation Behaviour of 9Cr-1Mo-V-Nb Steel During Long Term Creep
,”
Key Engineering Materials
,
171–174
, pp.
483
490
.
17.
ASME
,
2013
, “
III Rules for Construction of Nuclear Facility Components, Division 1—Subsection NH
,”
Boiler and Pressure Vessel Code
,
ASME
,
New York
.
18.
Takahashi
,
Y.
, and
Tabuchi
,
M.
,
2006
, “
Evaluation of Creep Strength Reduction Factors for Welded Joint of HCM12A (P122)
,”
ASME
Paper No. PVP2006-ICPVT-11-93488.
You do not currently have access to this content.