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International Journal of Pressure Vessels and Piping 90-91 (2012) 61e68
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International Journal of Pressure Vessels and Piping
journal homepage: www.elsevier .com/locate/ i jpvp
Fracture assessment for a dissimilar metal weld of low alloy steeland Ni-base alloy
Takuya Ogawa*, Masao Itatani, Toshiyuki Saito, Takahiro Hayashi, Chihiro Narazaki, Kentaro TsuchihashiToshiba Corporation Power Systems Company, Power and Industrial Systems Research and Development Center, 8, Shinsugita-cho, Isogo-ku, Yokohama 235-8523, Japan
Keywords:Fracture assessmentDissimilar metal weldElastic-plastic fracture toughnessPlastic collapseElastic-plastic fracture mechanics
* Corresponding author. Tel.: þ81 45 770 2323; faxE-mail address: [email protected] (T. O
0308-0161/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.ijpvp.2011.10.012
a b s t r a c t
Recently, instances of SCC in Ni-base alloy weld metal of light water reactor components have beenreported. Despite the possibility of propagation of SCC crack to the fusion line between low alloy steel(LAS) of pressure vessel and Ni-base alloy of internal structure, a fracture assessment method ofdissimilar metal welded joint has not been established. The objective of this study is to investigatea fracture mode of dissimilar metal weld of LAS and Ni-base alloy for development of a fractureassessment method for dissimilar metal weld. Fracture tests were conducted using two types ofdissimilar metal weld test plates with semi-elliptical surface crack. In one of the test plates, the fusionline lies around the surface points of the surface crack and the crack tips at the surface points haveintruded into LAS. Material ahead of the crack tip at the deepest point is Ni-base alloy. In the other, thefusion line lies around the deepest point of the surface crack and the crack tip at the deepest point hasintruded into LAS. Material ahead of the crack tip at the deepest point is LAS. The results of fracture testsusing the former type of test plate reveal that the collapse load considering the proportion of ligamentarea of each material gives a good estimation for fracture load. That is, fracture assessment based onplastic collapse mode is applicable to the former type of test plate. It is also understood that a fractureassessment method based on the elastic-plastic fracture mode is suitable for the latter type of test plate.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, stress corrosion cracking (SCC) of Ni-base alloy weldmetal in light water reactor (LWR) power plants has raised greatconcerns about the safety of operation [1e7]. It is considered thatSCC crack initiates in Ni-base alloy weld metal and has the possi-bility to propagate to the fusion line between low alloy steel (LAS)of reactor pressure vessel (RPV) and Ni-base alloy in the structuressuch as shroud supports of boiling water reactors (BWR) andnozzles of pressurized water reactors (PWR). Fig. 1 shows sche-matic presentations of the dissimilar weld joint configurations andSCC cracks in shroud support of BWR [8] and a nozzle of PWR [9]. Inthe Rules on Fitness-for-Service for Nuclear Power Plants (JSME FFSCode) [10] of the Japan Society of Mechanical Engineers Code,a flaw assessment method based on linear elastic fracturemechanics is required for RPV. On the other hand, the applicabilityof the elastic-plastic fracture mechanics evaluation method to RPVhas been currently investigated by the Board on Nuclear Codes andStandards of the American Society of Mechanical Engineers.
: þ81 45 770 2211.gawa).
All rights reserved.
However, the effect of Ni-base alloy on the fracture behavior of LASis unclear and the fracture assessment method of dissimilar metalwelded joint has not been established.
The objective of this study is to investigate a fracture mode ofdissimilar metal weld (DMW) of LAS �and Ni-base alloy fordevelopment of a fracture assessment method applicable to DMW.Fracture resistance characteristics (J � Da curve) and applicablefracture assessment method have been studied using fracture testspecimens taken from DMW joints.
2. Strength characteristics of DMW
2.1. Tensile test
Materials used in this study are SQV2A and alloy 182, which aredesignated as JIS G 3120 SQV2A and JIS Z 3224 DNiCrFe-3,respectively. Mechanical properties of each material were ob-tained by tensile tests at room temperature (R.T.). Tensile speci-mens were machined from the same welded joint for CT specimenshown in the next section. Mechanical properties of both materialsare shown in Table 1. RambergeOsgood parameters of both mate-rials are shown in Table 2. Young’s moduli for the two materials
Fig. 1. Schematic presentations of the dissimilar weld joint configurations and SCC cracks in LWR.
Table 2RambergeOsgood parameters of both materials.
Material RambergeOsgood parameters, ε/ε0 ¼ s/s0 þ a(s/s0)n
a n s0 (MPa) E (MPa) [8]
SQV2A 5.627 10.481 588 206000Alloy 182 3.589 8.268 465 214000
T. Ogawa et al. / International Journal of Pressure Vessels and Piping 90-91 (2012) 61e6862
were quoted from the Rules on Design and Construction for NuclearPower Plants (JSME D&C Code) [11] of the Japan Society ofMechanical Engineers Code. Young’s modulus of NCF600 describedin JSME D&C Code was substituted as Young’s modulus of alloy 182.
2.2. Elastic-plastic fracture toughness test
As shown in Fig. 2, 1TCT specimens were taken from the groovewelded joint consisting of SQV2A and alloy 182 after post-weldheat treatment (PWHT) at 615 C � 24 h in order for crack to grow
Table 1Mechanical properties of SQV2A and alloy 182.
Material Specimen ID sy, s0.2
(MPa)su
(MPa)sf
a
(MPa)εf
(%)f(%)
SQV2A JIS G 3120 Min. 345 550e690 e Min. 18 e
TP No.1 601 679 640 21 63TP No.2 575 680 628 22 61Average 588 680 634 22 62
Alloy 182 JIS Z 3224 Min. 245 Min. 550 e Min. 30 e
TP No.1 478 751 615 35 50TP No.2 452 725 589 34 40Average 465 738 602 35 45
a sf ¼ (sy þ su)/2 for SQV2A, sf ¼ (s0.2 þ su)/2 for alloy 182.
from alloy 182 to SQV2A, and JIc tests were conducted at R.T. Threetypes of CT specimens were prepared as shown in Fig. 3. Pre-cracktips were in alloy 182 near the fusion line (DMW(182) specimen),on the fusion line (DMW(fusion) specimen), and in the heat-
A
A 150
90
85
75
A-A cross-section
45
210 SQV2A
Alloy 182
Fig. 2. Groove welded joint and CT specimen preparation (dimensions are in mm).
Specimen type Width of SQV2A
(mm)
DMW(182) 18
DMW(fusion) 20
DMW(HAZ) 22
Detail of A
45o
2.5
A
25
20
60
37.5
25
50
62.5
3
2- 12.5
Alloy 182
Width ofSQV2A
SQV2A
φ
Fig. 3. Shape and dimensions of CT specimen (dimensions are in mm).
T. Ogawa et al. / International Journal of Pressure Vessels and Piping 90-91 (2012) 61e68 63
affected zone (DMW(HAZ) specimen). Width of SQV2A waschanged with respect to specimen type to keep pre-crack lengthconstant at approximately 30 mm. Side grooves were machinedafter introduction of the pre-crack.
Two specimens were tested for each specimen type. Fig. 4 showsJ � Da data with fitting curves of each type of DMW CT specimen.Blunting, 0.2 mm offset and 0.15 mm and 1.5 mm exclusion lines inFig. 4 were based on sf of SQV2A. Note that J-integral of DMW CTspecimens has been evaluated based on ASTM E1820-08 [12]generally used for homogeneous specimen, assuming materialconstants of SQV2A, in this study. Compared with DMW(HAZ) andDMW(fusion), DMW(182) is more scattered in the specimens, butthe reason for this scatter is not clear. Fig. 5 shows the macroscopicobservations of fracture surfaces of JIc test specimens. The locationsof pre-crack tips are verified for each specimen. In Fig. 5, arrowsshow the locations of pre-crack tips.
Table 3 shows the results of JIc tests. In Table 3, JIc validity wasjudged by the size requirement of both ASTM E1820-06 [13] andE1820-08.
Fig. 4. J � Da plots and fitting curves
B; b0>25JQ=sf ðE1820� 06Þ (1)
B; b0>10JQ=sf ðE1820� 08Þ (2)
When ASTM E1820-06 is applied, only DMW(HAZ) has invalidJIc. On the other hand, all JQ become valid JIc when the validity isjudged by ASTM E1820-08.
Fig. 6 shows the summary of J�Da curves. In Fig. 6, J�Da curvesof each DMW specimen were compared with those of homoge-neous specimens of SQV2A and alloy 182 [14]. Fitting curves are theaverage of two specimens. Fitting curves of DMW specimens liebetween those of SQV2A and alloy 182. The J � Da curve ofDMW(HAZ) specimen is almost the same as that of SQV2A, whereasthe J� Da curve of DMW(182) specimen is close to that of alloy 182.It is considered that effect of pre-crack tip material is dominantwith respect to the J � Da relationship. In DMW(fusion) specimen,JDa curve had a similar characteristic to that of alloy 182 in the lowDa region, and the data points transited from the J � Da curve of
for each type of DMW specimen.
Fig. 5. Macroscopic observations of fracture surfaces of JIc test specimens.
Table 3Results of JIc tests using DMW CT specimen.
Specimen ID JQ (kJ/m2) Validity for JIc
E1820-06 E1820-08
DMW(182)-1 295 Valid ValidDMW(182)-2 393 Valid ValidDMW(fusion)-1 394 Valid ValidDMW(fusion)-2 391 Valid ValidDMW(HAZ)-1 582 Invalid ValidDMW(HAZ)-2 510 Invalid Valid
T. Ogawa et al. / International Journal of Pressure Vessels and Piping 90-91 (2012) 61e6864
alloy 182 to that of SQV2A with the increase of Da since dominantmaterial at the ductile crack tip transited from alloy 182 to SQV2A.
3. Fracture test of DMW
Two types of DMW test plates were prepared from the groovewelded joints. Fig. 7 shows the preparation of one of the test plates.The center region of the plate width is alloy 182 through the
Fig. 6. Summary of J � Da curves and fitting curves.
thickness and both sides of the plate are SQV2A; it is named NDDtest plate. Fig. 8 shows the preparation of the other test plate. Thesurface side of the test section in the test plate is alloy 182 and theback side is SQV2A; it is named LDD test plate. PWHT at615 �C � 24 h was applied to both welded joints before test platepreparation.
Semi-elliptical surface flawwas introduced at the center of platewidth and plate length by electrical discharge machining (EDM)and fatigue crack was introduced from EDM notch. Macroscopicphotographs of cross-section of crack for each specimen taken afterfracture tests are shown in Fig. 9. In Fig. 9, broken lines show theforefront of fatigue crack and chain lines show the fusion linebetween SQV2A and alloy 182. The depth and length of fatiguecrack measured after the fracture tests are shown in Table 4. Thedepth of fatigue crack is corrected since the plate thickness wasreduced after fracture test.
In NDD test plate, the fusion line lies around the surface pointsof the surface crack. Fatigue crack tips at surface points haveconsiderably intruded into SQV2A in NDD-1 and NDD-2, whereasthey have slightly intruded into SQV2A in NDD-3 so that NDD-3 hasrelatively large area of alloy 182. Width of alloy 182 on the surfaceside of test section in NDD test plates is also shown in Table 4. In
35
60
60
(35)
19
0
320
300
50
30
15
10
Alloy 182 SQV2A
Test plate
Fig. 7. Groove welded joint and NDD test plate preparation (dimensions are in mm).
30
10
43
60
60
(42)
205
320 300
50
Alloy 182 SQV2A
SQV2A
Test plate
Fig. 8. Groove welded joint and LDD test plate preparation (dimensions are in mm).
Table 4Dimensions of fatigue crack measured after fracture tests and width of alloy 182 onthe surface side.
Test plateID
Dimensions of fatigue crack Width of alloy182 on the surfaceside (mm)
a (mm) 2c (mm) a/c
NDD-1 7.8 17.7 0.88 13.3NDD-2 6.1 21.9 0.56 11.4NDD-3 6.3 21.9 0.58 21.8LDD-1 9.6 18.0 1.07 e
LDD-2 6.5 24.2 0.54 e
LDD-3 7.3 25.2 0.58 e
T. Ogawa et al. / International Journal of Pressure Vessels and Piping 90-91 (2012) 61e68 65
LDD test plate, the fusion line lies around the deepest point of thesurface crack. Crack tip at the deepest point has considerablyintruded into SQV2A in LDD-1, whereas it has slightly intruded intoSQV2A in LDD-2 and LDD-3. NDD-1 and LDD-1 have relatively deepsurface crack whose aspect ratio is almost 1.0.
Fracture tests were conducted using 300 kN universal materialstesting machine at R.T. Relationships between load and displace-ment of crosshead are shown in Fig. 10. After the maximum load(Pmax), load has decreased with increase of displacement, andfracture has occurred.
Fig. 9. Macroscopic photographs of cross-sect
4. Fracture assessment
4.1. Assessment based on plastic collapse
Applicability of the fracture assessment method based on plasticcollapse was evaluated by comparison of experimental maximumload with collapse load as shown in Fig. 11. Collapse load wasevaluated in consideration of the proportion of ligament area ofeach material by Eq. (3). sf obtained by tensile tests and 2.7Sm ofeach material was used as s0, respectively.
Pc ¼ s0;SQV2A � ASQV2A þ s0;182 � A182 (3)
Plastic collapse load analyses for NDD test plates and LDD testplates with semi-elliptical surface crack were conducted by thefinite element method (FEM). Relationships between true stressand true strain of SQV2A and alloy 182 obtained by tensile testswere used in the analyses. Collapse load obtained by the twice
ion of crack for NDD and LDD test plates.
0
50
100
150
200
250
0 2 4 6 8 10
Displacement (mm)
Loa
d (k
N)
LDD-1LDD-2LDD-3
LDD test plate, R.T.
0
50
100
150
200
250
300
0 2 4 6 8 10 12Displacement (mm)
Loa
d (k
N)
NDD-1NDD-2NDD-3
NDD test plate, R.T.
Fig. 10. Relationships between load and displacement of crosshead.
0
100
200
300
400
Pm
ax ,
Pc (
kN)
PmaxPc by fPc by 2.7SmPc by TES method
LDD-1 LDD-2 LDD-30
100
200
300
400
Pm
ax ,
Pc (
kN)
PmaxPc by fPc by 2.7Sm
0
100
200
300
Pm
ax ,
Pc
(kN
)
Pc by 2.7SmPc by TES method
Pc by 2.7SmPc by TES method
NDD-1 NDD-2 NDD-3
Fig. 11. Results of comparison of the experimental maximum load with collapse load for NDD and LDD test plates.
T. Ogawa et al. / International Journal of Pressure Vessels and Piping 90-91 (2012) 61e6866
elastic slope (TES) method [11,15] was also used for fractureassessment.
In NDD test plate, it is found that almost all collapse loads giveconservative evaluation. Particularly, collapse load using sf of eachmaterial gives good estimation for Pmax.
In LDD test plate, although collapse load using 2.7Sm of eachmaterial gives conservative evaluation, other collapse loads arehigher than Pmax. It is considered that the fracture assessmentmethod based on plastic collapse is not applicable to LDD test plate.
4.2. Assessment based on Elastic-plastic fracture mechanics
Fracture assessment based on elastic-plastic fracture mechanics(EPFM) was conducted for LDD test plate to which plastic collapseevaluation was not applicable. Fracture load was evaluated using J-integral of the deepest point of surface crack by two methods. J-integral of the deepest point of surface crack was calculated byreference stress method as shown by Eq. (4) [16,17] generally used
0
500
1000
1500
0 0.5 1 1.5 2 2.5 3
Crack extension (mm)
J-i
nteg
ral (
kJ/m
)
Japp of LDD-2Japp of LDD-3Jmat of SQV2A
Assessment using J- a curve of SQV2A. Δ
a b
Fig. 12. Japp of LDD-2 a
for homogeneous specimen, assuming material constants (n, E, sy,a and n) of SQV2A, since the deepest point of surface crack was inSQV2A for all LDD test plates. Stress intensity factor (K) of thedeepest point of surface crack used in Eq. (4) was calculated by theequation of Raju-Newman [18].
J ¼ FK21� n2
E(4)
F ¼ Eεref
sref; sref ¼ sm
1� 2; sm ¼ P
2W$t; 2 ¼ ac
tðcþ tÞ;
εref ¼ syE
�srefsy
þ a
�srefsy
�n�
In one method in which ductile crack extension is not consid-ered, it was assumed that the fracture occurs when J-integral of thedeepest point of surface crack exceeds JQ of SQV2A. In the other
0
500
1000
1500
0 0.5 1 1.5 2 2.5 3
Crack extension (mm)
J-i
nteg
ral (
kJ/m
)
Japp of LDD-2Japp of LDD-3Jmat of DMW(HAZ)
Assessment using J- a curve of DMW(HAZ). Δ
nd LDD-3 and Jmat.
0
100
200
300
400P
max
, P
J (k
N)
PmaxPJ not considering crack extensionPJ considering crack extension using J- a curve of SQV2APJ considering crack extension using J- a curve of SQV2APJ considering crack extension using J- a curve of DMW(HAZ)
LDD-1 LDD-2 LDD-3
Fig. 13. Results of comparison of the experimental maximum load with estimatedfracture load for LDD test plates.
T. Ogawa et al. / International Journal of Pressure Vessels and Piping 90-91 (2012) 61e68 67
method in which ductile crack extension is considered, criteria forfracture assessment are shown in Eq. (5). Japp is J-integral of thedeepest point of surface crack by Eq. (4). J-Da curves of homoge-neous SQV2A CT specimen and DMW(HAZ) CT specimenwere usedas the Jmat, respectively. The latter method was not applied to LDD-1 since LDD-1 did not have enough ligament through-thickness.Fig. 12 shows Japp and Jmat. In this case, fracture occurs at
Japp ¼ Jmat ;vJappva
¼ dJmat
da(5)
Fig. 13 shows the comparison of the experimental maximumload with estimated fracture load based on EPFM. It is found thatthe conservative evaluation is possible by the method consideringductile crack extension and the accurate evaluation is possible bythe method without considering ductile crack extension. It isconsidered that fracture assessment based on EPFM is suitable forLDD test plate.
4.3. Discussion
As a result of the fracture assessment, it was found thatassessment based on plastic collapse was applicable to NDD testplate and assessment based on EPFM was applicable to LDD testplate, respectively. It was considered that the fracture behavior ofthe material in depth direction is one of the dominant factors forfracture of DMW test plate. It has been reported that the fracturetests of Ni-base alloy weld metal showed plastic collapse behaviorfor various sizes of the test specimens [19e21]. On the other hand,the applicability of flaw acceptance criteria based on EPFM to LAS isunder investigation [21].
5. Conclusions
Elastic-plastic fracture toughness tests using CT specimen andfracture tests using plate specimen taken from DMW joint con-sisting of SQV2A and alloy 182 were conducted in order to developa fracture assessment method applicable to the case that crackinitiated in Ni-base alloy and propagated to near the fusion linebetween low alloy steel and Ni-base alloy. The following resultswere obtained.
(1) When pre-crack tip is in the heat-affected zone, J � Da curve ofDMW specimen is almost the same as that of homogeneousSQV2A. When pre-crack is in alloy 182 near the fusion line,J � Da curve of DMW specimen is close to that of alloy 182.
(2) When pre-crack tip is on the fusion line, J�Da relation of DMWspecimen transits from J � Da curve of alloy 182 to that ofhomogeneous SQV2A by crack extension.
(3) When crack tip is near the fusion line andmaterial ahead of thedeepest point of surface crack is alloy 182, collapse loadconsidering the proportion of ligament area of each materialusing sf gives accurate evaluation. Further, conservative eval-uation is possible by collapse load considering the proportion ofligament area of eachmaterial using 2.7Sm. It is considered thatfracture assessment basedonplastic collapsemode is applicableand a good estimation is possible by the simple equationconsidering the proportion of ligament area of each material.
(4) When crack tip is near the fusion line andmaterial ahead of thedeepest point of surface crack is SQV2A, the fracture assess-ment method based on elastic-plastic fracturemode is suitable.It is possible to evaluate fracture load conservatively byconsidering ductile crack extension.
Nomenclature
sy Yield stresss0.2 0.2% proof stresssu Ultimate tensile stresssf Flow stress from tensile tests0 Effective yield stressεf Elongationf Reduction of areaa RambergeOsgood parametern RambergeOsgood parameterE Young’s modulusn Poisson’s ratioJQ Candidate value of JIcJIc Elastic-plastic fracture toughnessB Thickness of CT specimenb0 Initial ligament length of CT specimenDa Amount of ductile crack extension in JIc testa Depth of semi-elliptical surface flaw2c Length of semi-elliptical surface flawa/c Aspect ratio2W Width of DMW test platet Thickness of DMW test plateP LoadPmax Maximum load in fracture testPc Collapse loadPJ Fracture load estimated based on EPFMSm Design stress intensity in design and construction ruleK Stress intensity factor at the deepest pointJ J-integral at the deepest points0, SQV2A Effective yield stress of SQV2As0, 182 Effective yield stress of alloy 182ASQV2A Ligament area of SQV2AA182 Ligament area of alloy 182Japp Applied J-integralJmat Fracture resistance
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