E-Journal of Advanced Maintenance Vol.2 (2010) 50-64
Japan Society of Maintenology

Evaluation of Weld Residual Stress near the Cladding and J-weld in
Reactor Pressure Vessel Head for the assessment of PWSCC Behavior
Jinya KATSUYAMA1,*, Makoto UDAGAWA1, Hiroyuki NISHIKAWA1,†,
Mitsuyuki NAKAMURA1,‡ and Kunio ONIZAWA1
1

Nuclear Safety Research Center, Japan Atomic Energy Agency, 2-4 Shirakata-shirane, Tokai-mura,Naka-gun, Ibaraki 319-1195, Japan

ABSTRACT
Weld residual stress is one of the most important factors in order to assess the structural integrity ofsafety-related components such as reactor pressure vessel (RPV) in long-term operation of nuclear power plantsince the residual stress significantly affects crack initiation and growth behaviors. The inner surface of the RPV
made of low alloy steel is protected against corrosion by weld-overlay cladding of austenitic stainless steel. At
the J-weld of the vessel head penetrations, Ni-based alloys are used for weld material. The residual stresses
generated within the cladding, J-weld and base material were measured as-welded and after PWHT conditions
using the deep-hole-drilling method. Thermal-elastic-plastic-creep analyses considering the phasetransformation were also performed to evaluate the weld residual stress. By comparing analytical results withthe measured ones, it was shown that there was roughly a good agreement of residual stress distribution within
the cladding, J-weld and base metal. It was also suggested that taking the phase transformation during welding
and PWHT into account was important to improve the accuracy of weld residual stress analysis. Using theresidual stress distributions, fracture mechanics analyses for primary water stress corrosion cracking (PWSCC)
have been performed using probabilistic fracture mechanics analysis code. Effects of the weld residual stressand scatter of PWSCC growth rate on the crack penetration were evaluated through some case studies.
KEYWORDS

ARTICLE INFORMATION

weld residual stress, reactor pressure vessel, PWSCC, weld-overlay
cladding, J-groove welding, deep-hole-drilling method, finite element
analysis, structure integrity, crack growth

Article history:
Received 20 May 2010
Accepted 21 July 2010

1. Introduction
In order to assess the structural integrity of safety-related components in long-term operated lightwater reactors (LWRs), weld residual stress is one of the most important factors since the residual
stress distribution significantly affects the structural integrity related to crack initiation and growth
behaviors. This is because that high tensile stresses caused by the residual stress and/or operationloads result in high stress intensity factor (SIF) calculated for structural integrity assessment when acrack is found. To evaluate the residual stress distribution is also necessary to apply and improve therepair techniques by welding. Reactor pressure vessel (RPV) is one of the most important components
for safe long-term operation of nuclear power plants. The inner surface of the RPV, which is made of
low alloy steel of ASME SA533B-1, is protected against corrosion by weld-overlay cladding ofaustenitic stainless steel. In such dissimilar metal welds, residual stress is produced due to thecladding process and the difference of the thermal expansion coefficients between austenitic cladding
and ferritic base metal. The residual stress near the cladding of beltline region may have an influenceon the integrity of RPV during pressurized thermal shock (PTS) events. In addition to this, a numberof cracks due to primary water stress corrosion cracking (PWSCC) have been observed on vessel head*

Corresponding author, E-mail: katsuyama.jinya@jaea.go.jp
Present address: Mizuho Information & Research Institute, Inc., 2-3 Nishiki-cho, Kanda, Chiyoda-ku, Tokyo
101-8443, Japan‡
Present address: Research Organization for Information Science & Technology, 2-4 Shirakata-shirane,
Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan†

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

50

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

penetrations (VHPs) [1]. At the J-weld of the VHPs, Ni-based alloys are used for weld material. In thedissimilar metal welds, the initiation and propagation of PWSCC resulting in leakage of reactor
coolant may strongly depend on the weld residual stress distribution. Therefore, it is very important toevaluate the weld residual stress with high accuracy for the structural integrity assessment of RPV.
The simulation on the weld residual stress is usually used to evaluate the residual stress
distribution related to the structural integrity. Many numerical simulations based on finite element
analysis (FEA) have been performed to evaluate the weld residual stress of weld-overlay cladding.
Siegele and Brand have performed FEA simulations to evaluate the stress fields of cladded plates [2].
They indicated results that weld-overlay cladding generated tensile stress in the cladding andcompressive stress in the base metal close to the cladding. In our previous study, FEA has been
performed, which treats weld-overlay cladding and PWHT, to evaluate the residual stress distribution
in and near the cladding [3-6]. Experimental measurements based on sectioning and deep-hole-drilling(DHD) techniques have also been conducted to confirm the accuracy of numerical simulations. The
results showed that the stress distribution obtained from the FEA almost agreed with the experimental
results, e.g., tensile residual stress reaching to yield stress was generated in the cladding due to thedifference in the thermal expansions between cladding and base metals. However, there is a difference
in residual stress distributions between results from Siegele and Brand and our results, particularly inbase metal close to cladding, i.e. heat affected zone (HAZ). Weld residual stress distributions around
the J-weld have been also evaluated in some papers based on FEA and inherent strain method [7-9].
Residual stress distributions have been shown that, for example, tensile stresses are generated at the
inner surface of nozzle tube and around the J-weld. However, there are very few descriptions aboutboth effects of the cladding and J-weld on the residual stress found in the previous papers. Since
residual stress distribution in such regions may affect on structural integrity of vessel heads, the weld
residual stress due to cladding as well as J-weld is to be evaluated with high accuracy.
It is well known that various conditions of residual stress, SCC growth rate and flow stressrelated to PWSCC have scattered and therefore affect the structural integrity of RPV. Since
probabilistic fracture mechanics (PFM) analysis method can treat such scatter and uncertainties in the
structural integrity assessment, it is a promising method for assessing the structural integrity related to
PWSCC at a J-weld as an example. PFM analysis method is also expected to provide an advancementof maintenance such as optimizing inspection period from analysis on scatter of inspection accuracy.
In JAEA, we have been developing some PFM analysis codes based on a Monte Carlo method
focusing on degradation mechanisms for safety-related reactor components. The PFM analysis codefor PWSCC is called as PASCAL-NP (PFM Analysis of Structural Components in Aging LWR –NiSCC / PWSCC) [10].
In the present study, a welding simulation method for three-dimensional cladded plate specimen
based on FEA has been improved to obtain more reliable results of the weld residual stress
distribution. For the improvement, the phase transformation in base metal as well as creep behavior
was incorporated in the simulation of weld-overlay cladding and PWHT. The analytical results havebeen compared with the residual stress distributions obtained from DHD method to confirm theaccuracy of the welding simulation. Weld residual stress near the J-weld and cladding for J-groove
welded specimen with cladding has been then evaluated by using the improved three-dimensional
welding simulation and DHD measurements. Applying the evaluated residual stress distributions to
PASCAL-NP, some case studies have been also performed on crack growth and leakage behavior.

2. Experimental
2.1. Cladded and J-Groove Welded Specimens
Cladded specimens are fabricated using thick plates of JIS SQV2A (equivalent to ASME
SA533B-1; called SA533B-1) ferritic steel (500L x 500W x 125t mm) as shown in Fig. 1.
Weld-overlay cladding was performed by a single layer electro-slag welding (ESW) method in thisstudy. Welding conditions are listed in Table 1. In this case, the electrode of Type 309L stainless steel(SS) is used to make weld metal similar to be Type 304L SS. After ESW, PWHT of about 615oC for
6.0 h was conducted.

51

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

To simulate a J-weld of VHPs, J-groove welded specimens are also fabricated through machining,buttering, J-groove welding and PWHT using the cladded specimen. Weld materials of the alloy 82 of
Ni-based alloy are used for buttering and J-groove welding. Tungsten inert gas (TIG) welding is used
for buttering and J-groove welding. Schematic flow of fabricating of J-groove welded specimen, andtop-view of the J-weld specimen including welding directions of over-lay cladding and J-groove
welding are illustrated in Fig. 2. Total numbers of welding-passes for buttering and J-groove welding
are 40 and 107, respectively. PWHT of about 615oC for 1.2 h and 11 h were conducted after butteringand J-groove welding, respectively.

Table 1 Welding conditions of ESW.
ESW
Material of band electrode
Width of band electrode (mm)Welding current (A)Arc voltage (V)Welding speed (mm/s)
Heat input (kJ/mm)
Number of pass (-)

Type 309L SS
50
995
26.0
2.5
10.3
9

Fig. 1 Overlay welded test plate.

(i) Weld-overlay Cladding and PWHT

500(ii) Machining of penetration hole (R40)
and J-groove

Pass no.
9(iii) Buttering and PWHT

Welding direction of J-groove8

Location 1

7

Location 2(iv) Machining of penetration hole (R50)

500

6
5

(37º)

(24º)

4
3
2
1

(iv) Insert the nozzle tube, J-groove
welding and PWHT

Welding direction of cladding

Unit : mm(a) Fabricating process of J-weld specimen(b) Top-view of J-weld specimen

Fig. 2 Schematic image of fabricating process of J-groove welded specimen.

52

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

2.2. Residual stress measurements
Sectioning method based on beam thinning technique was performed to measure thethrough-thickness distribution of residual stress for cladded specimen in as-welded and PWHT
conditions. Beam shape specimens for the measurements were cut out from the center of cladded
specimen by avoiding overlap regions of welded beads. Detail of experimental procedure of residual
stress measurement by sectioning method is described in previous paper [3].
Steep and complicated distribution in residual stresses would be obtained even after PWHT
around dissimilar metal welds, such as weld-overlay cladding, due to the difference of the thermal
expansions between base metal and weld metal [3, 4]. Therefore, the DHD measurements for cladded
specimens were carried out to evaluate the through-thickness distributions of residual stress forcladded and J-groove welded specimens. The DHD technique is a unique method to obtain detail ofthe residual stress by 0.2 mm pitch in depth. Details of the method are found in a separate paper [11].
A measurement point for the cladded specimen is the center of cladded specimen where corresponds
to the center of weld bead. Measurement points for the J-groove welded specimen are illustrated inFig. 2. Location 1 was selected to evaluate the residual stress distribution within the J-weld. Location
2 was selected to evaluate the residual stress distributions through cladding at the centers of cladded
beads.

3. Analysis
3.1. Finite Element Analysis (FEA)
To obtain weld residual stress distribution due to overlay cladding and J-groove welding
analytically, thermal-elastic-plastic-creep analysis is performed using Abaqus, which is acommercially available FEA code [12]. Welding methods assumed in the analyses are ESW for the
single layer weld-overlay cladding, and TIG welding for the buttering and J-groove welding.
3.1.1. FEA Model
Geometry of the model of cladded plate specimen for the welding simulation of weld-overlay
cladding is shown in Fig. 3. Shapes of the plate specimen and welded beads of the model were
defined based on fabricated conditions as described above. As shown in Fig. 4, a half symmetricmodel is used for welding simulation of J-groove welded specimen to save the calculation time of
FEA. In addition to this, welding simulations of buttering and J-groove welding are conducted basedon lumping method of the welded beads as shown in Fig. 4 (b) (Same color region is treated as a one
bead.); e.g. the number of J-groove welding passes is decreased from 107 of actual welding to 19 of
FEA. Elements used in thermal analyses and elastic-plastic-creep analyses are three dimensional solid
elements of DC3D8 and C3D8, respectively. Overlap of weld metal is neglected to simplify the
analysis in this study.

Fig. 3 FEA model for a cladded specimen.

53

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

14.8

Cladding

40.3
55

o

ButteringJ-weld32.2

3.6

Nozzle tube
RPV base metal(a) Overview of FEA model(b) Close up of J-weld (Unit: mm)

Fig. 4 FEA model for J-groove welded specimen.

3.1.2. Material Properties
Each material property is temperature dependent [3-6, 13]. In order to evaluate residual stress inwith higher accuracy, the phase transformation of base metal is taken into account based on thecontinuous cooling transformation (CCT) diagrams of SA533B-1. The CCT diagrams used in thisstudy consist of three CCT diagrams with different achieving maximal temperatures of 900, 1200 and
1400oC [14]. The relationship between stress and strain in plastic region is defined using thekinematic hardening law. Yield stress of each phase such as Martensite or Bainite within HAZ wasestimated from correlation between measured hardness distribution and phase fraction distribution
obtained from FEA. Thermal expansion coefficients for phases of base metal such as Martensite,
Bainite and Austenite were determined from our experimental measurements [15]. The material
properties of base metal were defined by considering the phase fraction which varied during welding.
Heat transfer and radiation are considered at each surface of the model using a coefficient of heattransfer of 10 W/(m2 K) and an emissivity of 0.7. Back stress and equivalent plastic strain are reset tozero above 1200 oC in the elastic-plastic analysis.
afQq(x,0,0)

x

c

ar

c'
c

y

b

Fig. 5 Wide double-ellipsoid model.

3.1.3. Analysis of Weld-Overlay Cladding
Thermal analysis is performed to simulate temperature distribution during weld-overlay cladding
by ESW method. A movement of welding heat source is simulated by sequential heat inputs toelements aligning along the welding direction. A double-ellipsoid model is usually applied to simulate
the behavior of arc weld [16]. However, band electrodes of 50 mm in width are used for weld-overlay
cladding. Therefore, in this study, the model has been modified by inserting the width of a bead in thecenter of the double-ellipsoid model. This “wide double-ellipsoid model” is depicted in Fig. 5.

54

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

Sequentially coupled thermal-elastic-plastic analysis has been performed to obtain stress distributions.
In the thermal-elastic plastic analysis, adhesion of weld metal is considered at the same time of passof heat source.
3.1.4. Analysis of Buttering and J-groove Welding
Whole processes for fabrication of J-groove welded specimen indicated in Fig. 2 such asmachining, buttering, PWHT after buttering, J-groove welding and PWHT are simulated by FEA. The
simulation is performed after mapping the residual stress distribution produced by ESW cladding and
PWHT (as described in 3.1.3 and 3.1.5, respectively) into the J-weld FEA model by using map
solution technique. Machining of penetration hole and J-weld groove are simulated by removing themeshes belonging to those regions. Movements of heat source during TIG welding for buttering andJ-groove welding are simulated based on the same methodology as described in 3.1.3 using a normal
double-ellipsoid model as a moving heat source [16]. Welding parameters of the double-ellipsoid
model are determined by adjustments that analytical peak temperatures during welding becomecomparable to experimentally measured ones. Through the analyses of buttering and J-groove welding,the phase transformation of base metal based on the CCT diagrams is taken into account as the same
way as weld-overlay cladding. Contact conditions without friction are applied to the interface between
the outer surface of nozzle tube and the inner surface of penetration hole.
3.1.5. Analysis of PWHT
Creep analyses are performed to simulate PWHT under condition of 615oC after each welding
process of weld-overlay cladding, buttering and J-groove welding. The durations of PWHT for afterweld-overlay cladding, buttering and J-groove welding are 6.5 h, 1.2 h and 11 h, respectively. Norton
creep law is adopted as creep constitutive equation in the analyses. The following equations are usedas creep constitutive equations for each material.
(SA533B-1)
ε& cr = 2.82 × 10 −17 × σ 5.24
cr
−28
9.88
&
(Type 309L SS)(1)
ε = 1.49 × 10 × σ(Alloy 82)
ε& cr = 1.68 ×10−22 × σ 6.11
Where ε& cr is the equivalent creep strain rate (1/s) and σ is the Mises equivalent stress (MPa). The
constants in the equations have been taken from experiments. Martensite produced by each welding
process can be fully tempered by PWHT [15]. Therefore, each mechanical property in Martensite is
gradually replaced into Bainite during PWHT to simulate the phase transformation of tempered Martensite.
3.2. Crack Growth Analysis
3.2.1. Analysis Methods in PASCAL-NP code
The crack growth rate (CGR) of PWSCC is known as scattered as shown in Fig. 6 [17].
Therefore, the structural integrity assessment including crack growth prediction becomesover-conservative when the upper bound CGR data is used in the deterministic manner. In the
proposed code case of the JSME Rules on Fitness-for-Service, the mean CGR curve is used forconsidering such conservatism in the assessment. In such a case, PFM analysis can be the most usefuland rational method to properly assess the structural integrity of nuclear reactor components withconsidering the scatters of experimental data and field experiences [18].
We have, therefore, used a probabilistic fracture mechanics analysis code PASCAL-NP for crack
growth analysis. PWSCC may initiate at the inner or outer surface of cylinder-type structure of
Ni-based alloys. PWSCC may also initiate at the surface of Ni-based alloy weldment such as J-groove
weld. The orientation of the crack is axial and/or circumferential depending on the stress distribution.
PASCAL-NP has a function to calculate the SIF for such various types of crack related to PWSCC.
Figure 7 indicates the analysis flow chart of PASCAL-NP based on the Monte Carlo method. Using
the flow chart, the leakage and/or break probabilities are evaluated after PWSCC growth analysis
considering the scatters and uncertainties in CGR and flow stress. Details of PASCAL-NP aredescribed in a separate paper [13].

55J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

Start(i) Setting the analytical conditions
・Piping geometry
・Crack type・Residual stress・Operating condition
・Initial crack size
・Total sampling number , etcoExp. (325 C)oAve. (325 C)o+2σ (325 C)o+1σ (325 C)o-1σ (325 C)o-2σ (325 C)

1E-7

1E-8

1

10

Monte Carlo Method

Crack growth rate, da/dt (mm/sec)

1E-6

100
0.5

Stress intensity factor, KI-9 (MPa m )Fig. 6 Crack growth rate against stress intensity
factor for PWSCC in Ni-based welds [17].

(ii) Sampling random valuables
・Crack growth rate
(log-normal distribution)
・Flow stress of material(weibull distribution)(iii) Evaluation of crack growth
behavior and integrity assessment
・Time history of crack size
・Leak / Break judgment

No

End of sampling ?
Yes(iv) Statistical outputs
Nozzle tube

End

Fig. 7 Analytical flow chart of PASCAL-NP [10].

RPV base metal

Ri

T

t

WL

aL

J-weld

Crack

a
O

b

Items

(mm)

Inner radius of nozzle, Ri

35.2

Nozzle thickness, t
Vessel head thickness, T

14.8
183

WL
aL

55.1
32.2

Crack depth, aCrack half length, b

0.3
0.9

Fig. 8 Geometry of J-weld for PFM analysis.

3.2.2. Analytical Conditions
In order to evaluate the effects of scatter in the CGR of PWSCC, PFM analysis is performed at
the J-weld of a VHP. Total iteration number of simulations based on Monte Carlo method was
specified as 1,000,000 in this study so as to be enough for the evaluation of PWSCC growth behavior
with the scatter of CGR. Both vessel head thickness, T, and the distance from nozzle inner surface tothe base metal of vessel head, WL, are set as typical dimensions as shown in Fig. 8. In this study, WL
was set to 55.1 mm based on the fabricated J- weld specimen as was shown in Fig. 4. T was set to 183
mm from a dimension of actual vessel head thickness. Operating temperature and pressure are set to318 ºC and 15.4 MPa, respectively [19]. The material and temperature dependencies on CGR are
considered in the analysis. The mean CGR of Ni-based alloy weld metal is higher than that of base56

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

metal [17, 20]. Furthermore, the scatter of CGR of weld metal is smaller than that of base metal [20].
In PFM analysis, these scatters of CGR are expressed by log-normal distribution. A SIF calculator
incorporated into PASCAL-NP based on API [21] is applied to the crack growth analysis. Initial crack
depth is set as the size that the SIF at the deepest point of crack exceeds slightly the threshold value,
Kth, of 9.0 MPa m . The aspect ratio, that is length per depth, of initial semi-elliptic crack is assumed
to be six which is a typical value for a postulated crack in the assessment of structural integrity. In thisstudy, the depth and half length of initial semi-elliptic crack were defined as 0.3 mm and 0.9 mm,
respectively. The J-weld is judged as the occurrence of leakage when the deepest point of crack
reached the full thickness of J-groove weld (32.2 mm).

4. Results and Discussion
4.1. Residual Stress Produced by Weld-Overlay Cladding
In residual stress analysis considering phase transformation of base metal, the accuracy ofthermal analysis is important since the transformation temperature and phase fraction strongly dependon cooling rate during welding. Shown in Fig. 9 (a) are temperature histories during weld-overlay
cladding obtained from experiment and analysis. The shape of molten pool obtained from analysis isshown in Fig. 9 (b). Penetration depth measured by macroscopic observation was around 0.4 mm for
the ESW-cladded specimen. By adjusting the heat input parameter Q and shape parameters a, b and c,
the molten pool shape simulated for ESW was found to be in good agreement with experiments.
Temperature history such as peak temperature and cooling rate also agreed well as shown in the
figure.
1000

Experiment
Analysis

865
836

o

Temperature ( C)

800

600

400

Penetration depth ~ 0.4mm

200

0

0

5000

10000

15000

Time (s)(a) Temperature history(b) Molten pool shape

Fig. 9 Temperature history by FEA and experiment measured at the same positions of 15mm fromcladded surface, and molten pool shape from FEA. (Gray region corresponds to molten pool.)Calculated residual stresses after ESW cladding are shown in Fig. 10. The longitudinal stresswhich is parallel to the welding direction became tensile in the cladding and compressive just below
the cladding, and tensile below the compressive area in the plate. Residual stress at the interface area
between weld beads in base metal indicated the maximum value of tensile stress. These analysis
results qualitatively correspond to an experimental one measured by R. Kume et al. [22]. It can beseen that tensile stress became high as welding pass increased. This is because tensile stress in thelongitudinal direction generated by preceding welding was reduced by subsequent welding. This has
also been agreed with the results reported by Siegele and Brand [2].
Figure 11 represents the through-thickness distributions of longitudinal residual stress at thecenter of ESW specimen obtained from FEA with and without considering phase transformation. An
experimental result measured by sectioning method [3] is also shown in the figure. It is clear that theaccuracy of FEA was significantly improved by taken the phase transformation into account. In
particular, the phase transformation improved to simulate compressive residual stress within basemetal close to the cladding. Such compressive stress could be occurred by the phase transformation

57

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

behavior. In the case of SA533B-1 steel, Martensitic and Bainitic phases with different yield stresslevels and with different volumes precipitate from Austenite phase during cooling. Due to the volume
changes from Austenite to Martensite and Bainite as well as internal constraint from the plate,compressive stress could be generated. In order to evaluate the residual stress with high accuracy,
therefore, it is important to take the phase transformation into account. Similar investigation has beenfound in reference [23] by Y. Mikami et al. for a fillet welding T joint.
Weld sequence(a) Longitudinal
Weld sequence

(MPa)
+300
+200
+100
+0.0
-100
-200
-300

(b) Transverse
Fig. 10 Longitudinal and transverse residual stresses after cladding at the center of cladded specimen.

Longitudinal residual stress (MPa)400
Sectioning
FEA
FEA (w/o Phase trans.)

300
200
100
0
-100
-200
-300

Cladding
0

20

40

60

80

100

120

140

Distance from cladded surface (mm)Fig. 11 Through-thickness distribution of weld residual stress after cladding for cladded specimen.

Figure 12 shows residual stress distribution at the center of welding line after PWHT condition
for ESW cladded specimen. Residual stress within base metal near the cladding was dramaticallyrelaxed by PWHT although residual stress around the interface between the weld beads still remained
tensile stress. In the cladding, high tensile stress almost equivalent to yield stress was generated due tothe difference in thermal expansions between cladding and base metal.
The through-thickness distribution of longitudinal residual stress at the center of cladded
specimen of ESW evaluated from FEA, sectioning and DHD techniques are plotted in Fig. 13. In thecladding, high tensile stress around 280 MPa was obtained from FEA and each experimental
measurement. The residual stress distribution indicates very steep change across the fusion line fromtensile stress in cladding to compressive stress in base metal when the phase transformation isconsidered. The DHD measurement also showed compressive stress around the same area. On theother hand, FEA results without considering the phase transformation provides no compressive stress58

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

in base metal near cladding. Therefore, the compressive residual stress in base metal near the cladding
is due to the phase transformation. In addition to the improvement of welding simulation of
weld-overlay cladding, the creep analysis has also simulated the phase transformation of tempered
Martensite as mentioned in the section of PWHT analysis procedure. It is suggested from the resultsthat taking the phase transformation into account is necessary to obtain highly accurate residual stresseven after PWHT.
Weld sequence(a) Longitudinal
Weld sequence

(MPa)
+400
+300
+200
+100
+0.0
-100
-200

(b) Transverse
Fig. 12 Longitudinal and transverse residual stresses after PWHT at the center of cladded specimen.

Longitudinal residual stress (MPa)400
Sectioning
DHD
FEA
FEA (w/o Phase trans.)

Cladding

300
200
100
0
-100
-200

0

20

40

60

80

100

120

140

Distance from cladded surface (mm)Fig. 13 Through-thickness distribution of residual stress after PWHT for cladded specimen.

4.2. Residual Stress Produced by Buttering and J-groove Welding
Residual stress distributions in as-welded and after PWHT conditions for J-weld model are
shown in Figs. 14 and 15, respectively. Hoop and radial directions correspond to longitudinal andtransverse ones of J-groove welding since the figure is drawn based on the cylindrical coordinate
system of the nozzle tube. Tensile radial stress in the cladding surface surrounding J-weld can be seen.
The tensile stress area became larger after PWHT. On the other hand, very high tensile hoop stress atthe surface region of J-weld was decreased after PWHT. Through-thickness distributions of radial andhoop residual stresses inside and near the J-weld are shown in Figs. 16 and 17, respectively, obtained
from the FEA. Locations 1 and 2 correspond to the measuring points plotted in Fig. 2. Experimental
results measured by DHD method are also plotted in these figures. It can be found that residual stress
distributions evaluated by FEA and DHD method agree roughly, although some differences ofresidual stress values within the J-weld exists as shown in Figs. 16 (a) and 17 (a). For example, the59

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

hoop stress from FEA is ~200 MPa higher than DHD measurement in the J-weld. The differences canbe due to the effect of lumping of welded bead. The use of relatively large element size near the
J-weld may also cause the differences. Therefore, the causes of differences are studied further byimproving FEA models and methods. The error in DHD measurements also has to be considered.
Nevertheless, it is noted that residual stresses within the cladding and base metal in PWHT condition
obtained from FEA agree very well with the experimental measurement as shown in Figs. 16 (b) and17 (b). It is concluded that weld residual stress can be predicted with high accuracy by considering the
fabricating process of cladding, buttering, J-groove welding and PWHT as well as phasetransformation of base metal.
From the results shown in Figs. 14 to 17, tensile stress within the cladding, which had been
generated by weld-overlay cladding and PWHT, remains through the fabricating process of J-groove
welding and PWHT. Inside the J-weld, high tensile stress in the hoop direction was generated byJ-groove welding. After PWHT, the results from DHD measurement showed that the tensile stress inboth directions within the J-weld decreases to less than about 200 MPa. Relaxation of the tensilestress can be also shown by creep analysis although the degree of decrease in the tensile stress wasless than that of DHD measurement. Although there are such differences in residual stress values
between FEA and experiment, it is noted that tensile stress in J-weld remains even after PWHT in thesame manner as tensile stress within the cladding. It is also interesting that higher tensile stress is seen
in buttered region, where is drawn by orange in Figs. 16 (a) and 17 (a), as compared with that of
J-weld. The cause of this difference has not been made clear yet. As shown in Figs. 16 (b) and 17 (b),both results from FEA and DHD measurement suggested that tensile stress values within the cladding
are increased by conducting PWHT. Therefore, the PWHT for the relief of the residual stress due toJ-groove welding may have an influence on the increase in tensile stress within the cladding around
the J-weld.

(MPa)
+500
+400
+300
+200
+100
+0.0
-100
-200

(a) Radial stress(b) Hoop stressFig. 14 Residual stress distribution in as-welded condition of J-weld specimen with cladding.

(MPa)
+500
+400
+300
+200
+100
+0.0
-100
-200

(a) Radial stress(b) Hoop stressFig. 15 Residual stress distribution after PWHT condition of J-weld specimen with cladding.

60

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

500

400

Buttering300

As weld (FEA)
After PWHT (FEA)
As weld (DHD)
After PWHT (DHD)

Radial residual stress (MPa)

Radial residual stress (MPa)

500

200
100
0
-100
-200

300
200
100
0
-100

Cladding

J-weld0

As weld (FEA)
After PWHT (FEA)
As weld (DHD)
After PWHT (DHD)

400

20

40

60

80

100

-200

120

Through-thickness distance from welded surface (mm)

0

20

40

60

80

100

120

Through-thickness distance from welded surface (mm)(a) Location 1 (J-weld)(b) Location 2 (Cladding)Fig. 16 Comparison of radial residual stresses between as-welded and PWHT conditions.
500

400

Buttering300

As weld (FEA)
After PWHT (FEA)
As weld (DHD)
After PWHT (DHD)

Hoop residual stress (MPa)

Hoop residual stress (MPa)

500

200
100
0
-100

As weld (FEA)
After PWHT (FEA)
As weld (DHD)
After PWHT (DHD)

Cladding

400
300
200
100
0
-100

J-weld
-200

0

20

40

60

80

100

120

Through-thickness distance from welded surface (mm)(a) Location 1 (J-weld)

-200

0

20

40

60

80

100

120

Through-thickness distance from welded surface (mm)(b) Location 2 (Cladding)Fig. 17 Comparison of hoop residual stresses between as-welded and PWHT conditions.

4.3. Crack Growth Analysis for PWSCC
Using the residual stress distribution within J-groove weld measured by DHD method, PWSCC
growth behavior was calculated by using PASCAL-NP. The stresses used in the analysis correspondto the hoop stress since a crack was assumed to exist along radial direction. In this case, it is assumed
that a crack initiated at the welded surface of J-weld propagates to the crevice region between vessel
head base metal and the nozzle. We note that a fourth-order polynomial curve fitting for the SIF
calculation based on API [21] was a good approximation of the hoop stress distribution within the
J-groove weld. Figure 18 indicates the results of crack growth behavior and distribution of crack
depth as a function of operation time calculated by (a) deterministic, (b) probabilistic and (c)probabilistic methods. It is obtained from Fig. 18 (a), for example, that CGR of average+1σ andaverage+2σ cases become around 3.4 and 1.9 times faster, respectively, than the average CGR case.
Large scatter of crack depth is recognized clearly from probabilistic analysis result shown in Fig. 18(b). This is due to the large scatter of CGR of PWSCC as indicated in Fig. 6. The average CGR
becomes very slow after around 10 years of operation as shown in Fig. 18 (c). This is possibly
explained from the facts indicated in Fig. 18 (a) that cracks which have higher CGR reach the depth of
J-weld groove in early stage of operation. For example, in case of CGR of average+1σ, leakage occurs
before ~4 years.

61

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

To evaluate the effect of residual stress within the J-weld on probability of leakage, comparison
was carried out using residual stress distributions obtained from FEA and DHD measurement asshown in Fig. 17 (a). Since there is a possibility that the residual stress distribution has a scatter of
beyond 100 MPa caused by uncertainties of welding parameters [24], the residual stress distributions
were used for the case studies, i.e., FEA stress for a higher case, and DHD measurement stress for alower case. Figure 19 shows (a) the cumulative probabilities of leakage and (b) the average value of
SIF at the deepest point of crack tip as a function of operating time. The average SIF shown in Fig. 19(b) was calculated as the arithmetic mean of SIF values of all cracks not having reached the fullthickness of J-groove weld. As shown in Fig. 19 (a), the probability of leakage based on FEA stressbecomes higher from the early stage than that of DHD measurement. Operating times when theprobabilities of leakage become 50% for FEA and DHD measurement are 3.5 and 7.8 years,
respectively. The higher probability of leakage from FEA is caused by higher average SIF value for
FEA than that of DHD measurement as shown in Fig. 19 (b). After 1.5 years from operation start, forinstance, SIF based on FEA stress reaches about three times higher than DHD measurement. The
difference in SIF value is due to the difference in the hoop stress distribution in J-weld of around 200
MPa between FEA and DHD measurement as shown in Fig. 17 (a). Therefore, it is necessary toevaluate the residual stress distribution within the J-weld with high accuracy for structural integrity
assessment related to PWSCC behavior.
At present, PASCAL-NP can consider uncertainties of CGR and flow stress of the materials ofinterest. In case of PWSCC, taking uncertainties of weld residual stress, inspection accuracy andinitial crack sizes into account is necessary for the structural integrity assessment based on PFM
analyses. Further improvement of PASCAL-NP is currently ongoing so as to be applied to any
countermeasures for PWSCC.

Average of crack depth (mm)

30
25
20
15
Crack growth rate
Average+2σ
Average+1σ
Average +1s
Average-1σ
Average-2σ

10
5
0

0

5

10

15

20

25

30

Operating time (year)(a) Crack depth from deterministic analysis

(%)
30

30

Average of crack depth (mm)

30.00
13.47
6.049

Crack depth (mm)

25

2.716
1.220

20

0.54770.246015

0.11040.04959
0.02227100.01000

5
0
0

5

10

15

20

25

25
20
15
10
5
0

30

Operating time (year)

0

5

10

15

20

25

30

Operating time (year)(b) Probabilistic density of crack depth(c) Average crack depth by PFM analysis

Fig. 18 Crack growth behavior and distribution of crack depth as a function of operating time calculated
based on the stress distribution from DHD measurement.

62

4.0

100

DHD
FEA
80

60

40

20

0

DHD
FEA
0

5

10

15

20

25

30

Operating time (year)

3.5

80

3.0
60
2.5
40
2.0
20

0

1.5

SIFFEA/SIFDHD
0

5

10

15

20

25

SIF(FEA) / SIF(DHD)100

Average SIF at the deepest point
0.5
of crack tip(MPa m )Cumulative probability of leakage (%)

J. Katsuyama, et al./ Evaluation of Weld Residual Stress near the Cladding and J-weldin Reactor Pressure Vessel Head for the assessment of PWSCC Behavior

1.030

Operating time (year)(a) Cumulative probability of leakage(b) Average SIF at the deepest point of crack tip

Fig. 19 Cumulative probability of leakage due to PWSCC and SIF at the deepest point as a function ofoperating time in J-weld calculated by PASCAL-NP.

5. Conclusion
Thermal-elastic-plastic-creep analyses considering the phase transformation of base metal havebeen performed based on three-dimensional finite element method to evaluate residual stress
distribution produced by weld-overlay cladding, J-groove welding and PWHT. Using the evaluatedresidual stress, the effect of residual stress on the structural integrity of RPV related to PWSCC has
been assessed by using PFM analysis code of PASCAL-NP. From these analytical results, thefollowing conclusions are drawn:(1)
By taking the phase transformation during weld-overlay cladding and PWHT into
account, it was shown that residual stress distribution from FEA agreed well with that ofexperimental measurement. FEA results without considering the phase transformation
could not provide compressive stress in base metal near cladding. In order to evaluate
the weld residual stress near the cladding with high accuracy, the phase transformationof materials during not only welding but also PWHT should be considered.
(2)
To evaluate the residual stress in and near the J-weld, the consideration of the phasetransformation also has an influence on the improvement in accuracy of analysis results.
However, some differences in residual stresses between FEA and DHD measurement
within J-weld were found. From a viewpoint of FEA, the differences are necessary to beminimized by improving the FEA methods.
(3)
Within the cladding, tensile residual stress produced by weld-overlay cladding remains
after the fabrication process of J-groove welding including the PWHT.
(4)
The effect of scatter of CGR on PWSCC behavior was quantitatively evaluated based on
the crack growth analyses using the residual stress distributions within J-weld andoperating loads.
(5)
Since the CGR of PWSCC has a large scatter, it is suggested that the residual stressshould be evaluated with high accuracy for structural integrity assessment including
crack growth analysis.

Acknowledgement
This study was performed under the contract between Nuclear and Industrial Safety Agency of
the Ministry of Economy, Trade and Industry of Japan and the JAEA.

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