Materials Science Forum Vols. 654-656 (2010) pp 528-531 © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.654-656.528
Evaluation of Creep-Fatigue Crack Growth for Grade 91 Steel Wide Plate Hyeong-Yeon Lee1,a, Jong-Bum kim1,b and Jae-Han Lee1,c 1
1045 Daeduk-daero, Yusong-gu, Daejeon 305-353, Korea Atomic Energy Research Institute, Republic of Korea a
[email protected],
[email protected],
[email protected]
Keywords: Creep-Fatigue, crack growth, grade 91 steel, wide plate
Abstract. An assessment of a creep-fatigue crack growth for Mod. 9Cr-1Mo steel wide plates have been carried out based on an extended French high temperature design code, RCC-MR A16 guide. The defect assessment guide of the A16 provides assessment procedures on creep-fatigue crack growth for an austenitic stainless steel, but no guidelines are available yet for a Mod. 9Cr-1Mo steel. In this study, assessments of a creep-fatigue crack growth at defects of Grade 91 steel wide plates have been carried out based on the extended A16 method for austenitic stainless steel. Introduction The integrity of a high temperature structure such as a sodium cooled fast breeder reactor [1] operating at a creep regime is usually limited by the accumulation of creep-fatigue damage or the creep-fatigue crack growth behavior. The A16 guide[2] provides assessment procedures for a creep-fatigue crack initiation and growth for an austenitic stainless steel but it does not provide procedures for a Mod. 9Cr-1Mo (ASME Grade 91) steel. In this study, an assessment of a creep-fatigue crack growth has also been carried out for a Grade 91 steel with an extended A16 procedure. Since the mathematical models for a fatigue crack growth (FCG) and creep crack growth (CCG) for a Grade 91 steel are not available in the A16 guide, the FCG model of CEA[3] was used for the FCG evaluation while CCG model of KAERI at 600°C, and CCG models of CEA[3] at 550°C and 600°C have been used for the CCG evaluation[4]. In previous studies, the creep-fatigue crack initiation and crack growth behavior of austenitic stainless steel and Grade 91steel structure were evaluated and compared with the results by structural tests [4-7]. In this study, the creep-fatigue crack growth has been assessed for Grade 91 wide plates with surface defects and they will be compared with the structural test results currently ongoing to quantify the conservatism of the extended assessment method for Grade 91 steel. Assessment of Creep-Fatigue Crack Growth Selection of Material. Modified 9Cr-1Mo steel has been widely used for a high temperature structure such as a steam generator, intermediate heat exchanger and secondary piping of a liquid metal reactor [1] operating at about 550°C, and boiler components in ultra supercritical thermal power plants operating at about 600°C. Table 1 shows the chemical compositions of Mod.9Cr-1Mo steel. Table 1. Chemical compositions of Grade 91 steel C
Mn
0.116 0.35
P 0.01
S
Si
0.001 0.224
Cu
Ni
Cr
Mo
Al
Fe
0.11
0.15
8.87
0.92
0.21
Bal
V
N
0.18 0.05
Grade 91 steel has excellent thermal properties with low thermal expansion coefficient and high thermal conductivity while it has high design stress intensity as shown in Fig. 1[7]. However, concerns of a cracking in a heat-affected zone and a subsequent failure, commonly known as a Type IV cracking, may exist at the welded joints for long term services. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 147.43.133.47-25/05/10,11:44:18)
Materials Science Forum Vols. 654-656
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500 20
Su
Thermal exp. coeff.(1e-6) Thermal conductivity(Btu/hr ft F)
Sy
Sm
400
Stress (MPa)
15
10
300
200
100
5
0 0
Grade 91
Alloy 800H
316SS
304SS
2.25Cr-1Mo
Grade 91
12Cr
Alloy 800H
316SS
304SS
2.25Cr1Mo
12Cr
Fig. 1 Thermal and mechanical properties of Grade 91 steel
700mm
Finite Element Modeling. A wide plate model shown in Fig. 2(a) was used in this study. Artificial defects with the dimension of 90mm width and 2.5mm crack depth exist for each specimen as shown in Fig. 2(b). The wide plate specimen has the dimensions of 450×700 mm and a thickness of 25 mm. A defect has been machined by EDM at fine grain heat affected zone(FGHAZ) where Type IV cracking may occur and another specimen without welding has defect at the center of the plate specimen as shown in Fig. 2(c).
450mm
(a) Schematic of dual specimens (b) Crack geometry (c) Geometries of wide plate specimens Fig. 2 Grade 91 wide plate specimen and structural test facility.
The specimens are subject to the creep-fatigue load cycles as shown in Fig. 3. The specimen is heated up to 550°C, held at 550°C for one hour and beach-marking tests will be carried out between creep-fatigue load cycles. Since Grade 91 is known to be more damaging in compression hold[3,7], compressive hold loading was applied to the specimen as shown in Fig. 3. P Load (KN)
time (min)
p1 p2 Pre-cracking C-F loading (I) Hold time (1hr)
C-F loading (II) Beach-marking
P
Fig. 3 Mechanical loading conditions and specimens under mechanical loading
A 3D quarter symmetric finite element model was used and the stress contours for the uncracked body and cracked body under mechanical loading of 10 tons and thermal loading are shown in Fig. 4.
530
PRICM7
(a) vertical normal stress (defect free)
(b) normal stress (cracked body)
Fig. 4 Stress contours for 1/4 symmetric model of the wide plate model
Assessment of a Creep-Fatigue Crack Growth Mathematical Models of Crack Growth for Grade 91 Steel. The amount of creep-fatigue crack growth in the A16 guide is determined by a linear summation of FCG and CCG based on a linear elastic fracture mechanics analysis. As for the FCG for the Grade 91 steel base metal of the CT (1 inch thickness) specimen, the mathematical model for base metal of Eq. 1 at 550°C [3] was used. 1.83 da = 9.3 ×10−7 ∆K eff d where ∆K eff is an effective stress intensity factor range.
(1)
As for the CCG assessment for Grade 91 steel base metal, the CCG model of Eq. 2 at 600°C [4] was used. 0.78 da = 2.11×10−2 C * (2) dt and another CCG models for base and weld metal at 550°C [3] of Eq. 3 were used. 0.642 0.678 da da = 4.8 ×10−3 C * = 1.858 ×10−2 C * , (3) dt dt Then the amount of an FCG is determined from Eq. 1.
( )
( )
δ ai = C ∆K eff
( )
n
(4) *
For the calculation of a CCG, the fracture parameter of C is determined during the hold time. The amount of a CCG during the given hold time tmi is calculated from the following Eqs. 5 and 6. C *s =
(
Csme + κ C* ⋅ kC*th ⋅ J elth ti + tmi
(δ ac )i = ∫t
i
)
2
(5)
q
A Ci* (t ) dt
(6)
where Csme is the C* integral for the mechanical loads, J elth is the J-integral under the thermal loads and Ci* (t ) is the C* integral at time t. Evaluation of Creep-Fatigue Crack Growth. Assessment of creep-fatigue crack growth has been carried out for Grade 91 wide plate specimens with defects. Creep-fatigue loading with one hour hold time at 550°C is applied and observed images are to be compared with the assessment results when the test results are to be available. Assessments of creep-fatigue crack growth have been carried out for the creep strain rules of primary creep law in Eq. 7 and secondary creep law in Eq. 8, which are used in the calculation of the C* integrals. .
ε p = 1.5803 × 10−29 × ε −1.9268 × σ 11.133
(7)
.
ε s = 1.0745 ×10−20 ⋅ σ 8.2497
(8)
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10
KAERI-600C-Prim.creep
KAERI-600C-Prim.creep
KAERI-600C-Second.creep
KAERI-600C-Second.creep
8
8
CEA-550C-Prim. creep
C-F crack growth (mm)
C-F crack growth (mm)
531
CEA-550C-Second. creep
6
4
CEA-550C-Prim. creep CEA-550C-Second. creep
6
4
2
2
0
0 0
100
200
300
400
500
0
100
C-F load cycles (N)
(a) Base metal Fig. 5 Creep fatigue crack growth
200
300
400
500
C-F load cycles (N)
(b) Weld metal
Since present hold time is one hour, primary creep should be valid. The amount of creep-fatigue crack growth after 500 load cycles was maximum 5.72 mm for base metal while it is 9.72 mm for weld metal when KAERI model is used as shown in Fig. 5.
Summary Mod. 9Cr-1Mo(Grade 91) steel is widely used in fast reactor but currently no assessment guideline on creep-fatigue crack growth and leak before break is available for the Grade 91 steel. In this study, a French A16 method was extended to Grade 91 steel wide plate and the crack growth under creep-fatigue loading was evaluated. When the mathematical models of KAERI and CEA on fatigue crack growth and creep crack growth were used, KAERI model for 600°Cgave 4.5 times higher for base metal and twice higher for weld metal in creep-fatigue crack growth than the CEA model. These assessment results will be compared with those of the ongoing structural tests.
Acknowledgements This study was supported by the Korean Ministry of Education, Science & Technology through its National Nuclear Technology Program.
References [1] [2] [3] [4] [5] [6] [7] [8]
D.H Hahn, et. al., KALIMER-600 Conceptual Design Report, KAERI/TR-3381, Korea Atomic Energy Research Institute, Daejeon (2007). RCC-MR, Section I Subsection Z, Technical Appendix 16, 2007 Edition, AFCEN (2007). O. Ancelet, S. Chapuliot, Proceedings of ICAPP 2007, Nice, France, May 13-18, Paper 7182 (2007). H.Y. Lee, J.B. Kim, W.G. Kim, J.H. Lee, The Transactions of the Indian Institute of Metals, in press (2010). H.Y. Lee, J.B. Kim, S.H. Lee, J-H Lee, Int. J. of Pressure Vessel and Piping, Vol. 83 (2006), p.826. H.Y. Lee , J.H. Lee, B.H. Kim, J. Mechanical Sci. and Technology, Vol. 20(12) (2006), p.2076. H.Y, Lee, S.H. Lee, J.B. Kim, J.H.Lee, Int. J. of Fatigue, Vol. 29 (2007), p. 1868. ABAQUS Users manual, Version 6.9, H.K.S, USA; DS Simulia (2009).
