E-Journal of Advanced Maintenance Vol. 13 No. 1 (2021) 1-11
Japan Society of Maintenology

Tempering Effect Prediction of Temper Bead Repair Technology for BMI
Penetration Nozzles
Lina YU1,*, Kazuyoshi SAIDA1, Kazutoshi NISHIMOTO1 and Naoki CHIGUSA2
1Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, JapanThe Kansai Electric Power Co., Inc., 8 Yokota, 13 Goichi, Mihama-cho, Mikata-gun, Fukui
919-1141, Japan

2

ABSTRACTThe aged reactor vessel bottom mounted instrument (BMI) penetration nozzles of nuclear power plant needs tobe repaired or maintained. Temper bead welding is one effective repair welding methods instead of post weld
heat treatment. For temper bead welding, hardness and toughness are the key criteria to evaluate the tempering
effect. A neural network-based method for hardness and toughness prediction in heat affected zone of low-alloysteel has been constructed to evaluate the tempering effect in temper bead welding by the authors. For the BMI
penetration nozzle with different mounting angles (0°, 15°, 30°and 45°), the hardness and toughness distribution
in heat affected zone of temper bead welding in the cross sections of BMI penetration nozzle was predictedbased on the thermal cycles numerically obtained by Finite Element Method (FEM). Through this method, thetempering effect of the temper bead repair technology can be predicted before the actual repair welding, thus theappropriate welding conditions would be able to optimize before the actual repair welding, which was very
useful for the repair welding for BMI penetration nozzles of nuclear power plant.
KEYWORDS

ARTICLE INFORMATION

BMI, Hardness, Toughness, Temper bead welding, low-alloy steel, FEM

Article history:
Received October 2020
Accepted May 2021

1.

Introduction

A bottom mounted instrument (BMI) penetration nozzle is a tube welded into the reactor vessel
bottom. Its role is to allow the introduction of nuclear instrumentation into the core of the reactor. Aleak in the BMI tube or J-weld constitutes a break of the primary coolant system. BMI nozzles inbottom head of the nuclear reactor pressure vessel are one of the pressure boundaries in reactor
coolant system for inspection of weld cracking, also, the BMI nozzle welds form part of the primarypressure boundary and the weld filler material is of alloy 600 which is known to be susceptible toprimary water stress corrosion cracking (PWSCC) which is one of important metallurgical problems
facing the nuclear industry [1,2]. Low alloy steel ASTM A533B possessing superior low-temperature
toughness and weldability is typically used as the material for pressurized water reactor vessels in
nuclear power plants. The aged nuclear power plants are needed to be repaired or maintained to assure
the safety and extend their lives. After repair welding, post weld heat treatment (PWHT) is normallyrequired to eliminate the residual stress and decrease the hardness. However, PWHT is sometimes
difficult to perform in operation in the case of repairing for large-scale structures. In practice, thetemper bead welding technique is an effective repair welding method instead of PWHT [3-5].
Temper bead welding is a kind of multi-pass welding, in which the tempering effect is caused byheat-arising from the subsequent multi-layer weld thermal cycles. For temper bead welding, hardness
and toughness are the key criteria to evaluate the tempering effect. The authors have previously
reported the hardness and toughness prediction systems of the temper bead welding technique usingneural network based on the experimentally obtained hardness and toughness database [6-7]. And theeffectivity for the plain surface has been proved.
However, for the repair welding of BMI penetration nozzle, the welding surface is not the plain
surface and sometime there are different angles between the BMI and the vessel bottom in practicaloperation. Therefore, in the present study, the hardness and toughness prediction in heat affected zone
* Corresponding author, E-mail: yulina@mapse.osaka-u.ac.jp
ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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L. Yu, et al./ Title of the Paper Tempering Effect Prediction of
Temper Bead Repair Technology for BMI Penetration Nozzles(HAZ) of temper bead welding of BMI penetration nozzle with different mounting angles (0°, 15°,30°and 45°) has been investigated based on the thermal cycles numerically obtained by Finite
Element Method. Through this method, the tempering effect of the temper bead repair technology canbe predicted before the actual repair welding, thus the appropriate welding conditions would be able
to optimize before repair welding, which was very useful for the repair welding for BMI penetration
nozzles of nuclear power plant.

2. FEM models of BMI penetration
In order to simulate the actual repair welding of BMI penetration nozzle with the mounting angle of0°, FEM model of multi-pass circle welded samples were produced by Patran, as shown in Fig. 1. As
illustrated in Fig. 1 (b), the union between the bottom of the vessel (stainless steel SUS304 cladding
layer) and the BMI Nozzle (Alloy 600) was formed by a buttering and a J-groove weld with Alloy
690, and the other part is low-alloy steel A533B. The dimensions of the groove shape and thethickness of SUS304 cladding were shown in Fig. 1(b).
The welding conditions of the multi-pass welding were shown in Table 1. The welding speed wasset as 2 mm/s for all pass welding. The heat input of the first layer (1st pass ~ 11th pass) welding was
set as 700 J/mm, while the heat input from the second layer (12th pass ~ 50th pass) welding was
arranged as 1050 J/mm, which is higher than that of the first layer welding. And the built-up sequencein the cross section was illustrated in Fig. 2. For the heat input of 700 J/mm, the average bead widthand beat height was set as 4 mm and 2.6 mm respectively. While the average bead width and beatheight were changed to 4 mm and 4 mm for the heat input of 1050 J/mm. In order to better simulate
the thermal history of weld metal well, the mesh size was set as about 1.3mm, and the mesh size in
HAZ near to weld metal is little coarser than that of weld metal. On the other hand, the mesh in thebase metal far from the weld metal was much rougher to reduce the simulation time.

(a) 3D model(b) cross section of the modelFig. 1 Finite element model of BMI nozzle with the angle of 0°
Table 1 Welding conditions of multi-pass welding
Heat Input, Q (J/mm)
700
1st pass ~ 11th pass
Welding Speed, v (mm/s) 2.0
Heat Input, Q (J/mm)
1050
12th pass ~ 50th pass
Welding Speed, v (mm/s) 2.0

2L. Yu, et al./ Title of the Paper Tempering Effect Prediction of
Temper Bead Repair Technology for BMI Penetration Nozzles

Fig. 2 Built-up sequence

Because the actual BMI penetration nozzle always have some angle with the reactor vessel bottom,
the FEM models of BMI penetration nozzles with 15°, 30° and 45° mounting angles were also
constructed as illustrated in Fig. 3. Every overlay welding was performed by 2 passes of semi-circle,
with the welding directions shown in Fig. 4. And the hardness and toughness of the start section (θ =0°) and the end section (θ = 180°) were mainly investigated in this study.
The thermal cycles in multi-pass temper bead welding were calculated using FEM softwaredeveloped for welding simulation [8]. The temperature dependencies of physical properties of A533B,
Alloy 600, Alloy 690 and SUS304 were illustrated in Tables 2-3 and Figs. 5-6, respectively.

(a) 15°(b) 30°(c) 45°
Fig. 3 Finite element model of BMI nozzle with different mounting angles

Fig. 4 Welding direction from the top view3

L. Yu, et al./ Title of the Paper Tempering Effect Prediction of
Temper Bead Repair Technology for BMI Penetration Nozzles

Temperature
(℃)
20
200
400
600
800
1000
1300

Temperature
(℃)
20
100
200
300
400
500
600
700
800
900

Table 2 Temperature dependencies of physical properties of A533B steel
Possion’s
Specific
Thermal
Yield
Young’s
Thermalratio
heat
conductivity
Strengthmodulusexpansion(J/kg℃)(W/mm℃)
(MPa)
(GPa)
(－)
(1/℃)
445
0.039
478
210
0.3
12.0e-6517
0.0389455
202
0.3
12.7e-6
592
0.036
405
188
0.3
13.9e-6723
0.0317238
160
0.3
13.8e-6812
0.037875
115
0.3
12.6e-6658
0.03091793
0.3
12.6e-6721
0.03655
100.3
14.5e-6
Table 3 Temperature dependencies of physical properties of Alloy 600
Specific heat
Thermal conductivity
Young’s modulus
Possion’s ratio(J/kg℃)(W/mm℃)
(GPa)
(－)
444
0.0149
214
0.324
465
0.0159
210
0.319
486
0.0173
205
0.314
502
0.0190
199
0.306
519
0.0205
193
0.301
536
0.0221
187
0.300
578
0.0239
180
0.301
595
0.0257
172
0.305
611
0.0275
164
0.320
628
0.0285154
0.330

Fig. 5 Temperature dependencies of physical properties of Alloy 690.

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L. Yu, et al./ Title of the Paper Tempering Effect Prediction of
Temper Bead Repair Technology for BMI Penetration Nozzles

Fig. 6 Temperature dependencies of physical properties of SUS304.

3. Neural Network-based hardness and toughness prediction systems
Because the mechanical properties such and hardness and toughness in HAZ subjected to various
kinds of thermal cycles is determined by the parameters: Tpi, CRi and TCTP [6,7], the prediction
systems of hardness/toughness were constructed with using a Neural Network (NN). Tpi is the peaktemperature of the ith pass thermal cycle, CRi means the cooling rate from 800℃ to 500℃ of the ith
pass thermal cycle (When the Tp of the thermal cycle is lower than 800℃, the cooling rate at 400℃ is
taken as CR). And the tempering effect of all the temper thermal cycles can be considered to be
expressed as a factor, TCTP (Thermal cycle tempering parameter), proposed by the authors, byextending the Larson-Miller parameter (LMP) to non-isothermal heat treatment [6].
The NN [9,10] method is a powerful tool which can process such complex data as involved in thepresent research. NN is a mathematical model or computational model that simulates the structure
and/or functional aspects of biological neural networks. In most cases, a NN is an adaptive systemthat changes its structure based on external or internal information that flows through the network
during the learning phase. Modern neural networks have become useful modeling tools for non-linear
statistical data. They are usually used to model complex relationships between inputs and outputs or
to find patterns in data.
The radial basis function (RBF) [11] is a powerful technique for interpolation of multidimensional
space in a NN. Fig. 7 illustrates the RBF-NN model. RBF networks typically have three layers: an
input layer, a hidden layer with a non-linear RBF activation function and a linear output layer. The
hidden layer can be described by a Gaussian basis function:(1)
where x is the input data, c is the center vector, and r is the euclidean distance. In the basic form all
inputs are connected to each hidden neuron.
The output O(xi) of the network is thus(2)
where n is the number of neurons in the hidden layer, cj is the center vector for neuron j, and wj are
the weights of the linear output neuron. The weights wj, cj, and r are determined in a manner that
optimizes the fit between O(xi) and the data. In the present study, the thermal cycle parameters (T pi,
CRi and TCTP) are the input data, and the hardness/toughness is the output data.
Therefore, based on the experimentally obtained hardness/toughness database, the neuralnetwork-based hardness and toughness prediction systems have been constructed by the authors [6,7].

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L. Yu, et al./ Title of the Paper Tempering Effect Prediction of
Temper Bead Repair Technology for BMI Penetration Nozzles

Fig. 7 Radial basis function neural network model

4. Temperature analysis of multi-pass welding in BMI penetration nozzle by
FEM simulation
The temperature distributions produced by multi-pass thermal cycles in welds during buttering andJ-welding in BMI penetration nozzle were calculated using three-dimensional finite element analysis
code, developed specifically for welding simulation. Figs. 8-11 presented the calculated peaktemperature distribution of BMI penetration nozzles with the mounting angles varied as 0°, 15°,30°and 45°, respectively. Peak temperatures were presented in different color scale according to thedifferent temperature levels. And the peak temperature was shown from the 3D view and the top view.
And from the top view, there was no marked difference in the peak temperature distribution after
multi-pass welding when the mounting angles varied from 0° to 45°.
[℃]

[℃]

(a) 3D view(b) Top viewFig. 8 Simulated peak temperature distribution in BMI nozzle after multi-pass welding
with the mounting angle of 0°6

L. Yu, et al./ Title of the Paper Tempering Effect Prediction of
Temper Bead Repair Technology for BMI Penetration Nozzles
[℃]

[℃]

(a) 3D view(b) Top viewFig. 9 Simulated peak temperature distribution in BMI nozzle after 50 pass welding
with the mounting angle of 15°
[℃]

[℃]

(a) 3D view(b) Top viewFig. 10 Simulated peak temperature distribution in BMI nozzle after 50 pass welding
with the mounting angle of 30°
[℃]

[℃]

(a) 3D view(b) Top viewFig. 11 Simulated peak temperature distribution in BMI nozzle after 50 pass welding
with the mounting angle of 45°

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L. Yu, et al./ Title of the Paper Tempering Effect Prediction of
Temper Bead Repair Technology for BMI Penetration Nozzles

5. Predicted hardness distribution in the cross section of BMI penetration
nozzle with different mounting angles
Based on the calculated thermal history at all grid nodes of the mesh, the hardness at the grid nodesof the cross section of BMI penetration nozzle was calculated, using the previously constructed neuralnetwork-based hardness prediction system by the authors. Thus calculated hardness distribution in the
cross sections of BMI penetration nozzle with different mounting angles (0°, 15°, 30°and 45°) was
visualized as color chart maps in Fig. 12. In this study, the hardness was investigated by the Vickers
hardness measured at a load of 9.8N for 20s.
Fig. 12(a) illustrates the hardness distribution in the cross sections (θ = 0° and θ = 180°) of BMI
nozzle with the mounting angle of 0° after multi-pass welding (buttering and J-welding). Weld metal(WM) of Alloy 690 was shown in red color, and base metal (BM) of low-alloy steel A533B, SUS304
and Alloy 600 were shown in different grey colors, respectively. The hardness in HAZ of A533B steel
was shown with rainbow colors depending on the different hardness levels. It can be seen that there
are hardened ranges near the WM, but all the hardness in HAZ of BMI nozzle was lower than 350HV
with the mounting angle of 0°. Similarly, when the mounting angle were changed from 15° to 45° asshown in Fig. 12(b)-(d), the hardness in the cross sections of BMI nozzle were lower than 350HV (therequired specification in industry [6]). This illustrated the effectiveness of temper bead welding on
decreasing hardness, and it proved that the chosen welding conditions of multi-pass welding wereappropriate. And there was no remarked difference in the hardness distribution when the mounting
angles changed from 0° to 45°, indicating that the mounting angle has insignificant effect on thehardness in the cross sections of BMI penetration nozzle.

(a) 0°(b) 15°(c) 30°(d) 45°
Fig. 12 Calculated hardness distribution in the cross sections of BMI nozzle with different mounting
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L. Yu, et al./ Title of the Paper Tempering Effect Prediction of
Temper Bead Repair Technology for BMI Penetration Nozzles

6. Predicted toughness distribution in the cross section of BMI penetration
nozzle with different mounting angles
Similarly, the toughness at every grid nodes of the cross section of BMI penetration nozzle wascalculated based on the calculated thermal history at the grid nodes of the mesh, using the previously
constructed neural network-based toughness prediction system. The calculated toughness distribution
in the cross sections of BMI penetration nozzle with different mounting angles was visualized as color
chart maps in Fig. 13. In this study, the toughness was investigated by the absorbed energy of Charpy
impact test. Charpy impact test was carried out at 261K, which is the required test temperature ofCharpy impact validity qualification test for temper bead welding in nuclear power industries. In order
to avoid FPD (Fracture path deviation), side-notched Charpy V-notch specimens were prepared fromsamples subjected to the simulated thermal cycles. It should be noted that in the required specification
in industry, there is absorbed energy 68J for standard Charpy impact test sample, which is equal to
40J for side-notched Charpy impact test sample used in the authors’ study [7].

(a) 0°(b) 15°(c) 30°(d) 45°
Fig. 13 Calculated toughness distribution in the cross sections of BMI nozzle with different mounting
anglesWhen the mounting angle was 0°, the calculated toughness distribution in the cross sections (θ = 0°and θ = 180°) of BMI nozzle after multi-pass buttering and J-welding was illustrated in Fig. 13(a).
WM of Alloy 690 was shown in red color, and BM of A533B steel, SUS304 and Alloy 600 wereshown in different grey colors, respectively. The toughness in HAZ of A533B steel was shown withrainbow colors depending on the different toughness levels. It can be seen that there was low
toughness zone near the WM, but all the toughness in HAZ of BMI nozzle was higher than 40J with9

L. Yu, et al./ Title of the Paper Tempering Effect Prediction of
Temper Bead Repair Technology for BMI Penetration Nozzles

the mounting angle of 0°. Similarly, when the mounting angle were changed from 15° to 45° as shownin Fig. 13 (b)-(d), the toughness in the cross sections of BMI nozzle was found be higher than 40J (therequired specification in industry), even when the mounting angle was changed to 45°. The calculated
toughness results illustrated the effectiveness of temper bead welding on increasing toughness, and it
proved that the chosen welding conditions of multi-pass welding were appropriate.
However, according to the toughness distribution with the mounting angle changed to 30° and 45°
as shown in the magnified part of Fig. 13 (c)-(d), it was found that some low toughness zone (theabsorbed energy between 40J and 80J) occurred in HAZ of the low-alloy steel A533B near to the
SUS304 cladding layer in the section with θ of 180°, especially for the case of the 45° mounting angle.
This indicated that the mounting angle has significant effect on the toughness in the cross sections of
BMI penetration nozzle, which was different from hardness. Therefore, both hardness and toughness
in the HAZ of repair welding need to be investigated to evaluate the tempering effect of temper beadwelding for BMI penetration nozzle.

7. Benefits of the proposed method
It’s well known that the BMI penetration nozzles have different geometric variables such as
mounting angle, weld shape, nozzle thickness, nozzle outer radius, et al., according to the nozzle
location and plant-specific design [1]. The geometric variables may affect the residual stresses as wellas the mechanical property behaviors. In the previous study of Kim et al. [1], the geometric variablesof BMI penetration nozzle such as mounting angle, nozzle thickness, seemed to have significant effecton the PWSCC initiation potential. However, in the study of Mochizuki et al. [2], the FEM residual
stress analysis showed that the mounting angles and welding pass sequence conditions have small
effect on the residual stress of J-welding of BMI penetration nozzles. Beside of the residual stresses,
the mechanical properties are the key criteria to evaluate the temper effect of the repairing welding for
BMI penetration nozzles. Therefore, the mechanical properties, such as hardness and toughness in thesections, which present the effects of various geometric variables, are very important to evaluate thetemper effect of the repairing welding. In order to control the microstructure and the mechanicalproperties after welding, the appropriate welding conditions are necessary to be selected before theactual repair welding.
In this study, based on the FEM thermal history simulation of multi-pass circle welding, thehardness and toughness in the cross sections of BMI penetration nozzle with different mounting
angles (0°, 15°, 30°and 45°) was successfully predicted using the proposed method. And the results
showed that the mounting angle had insignificant effect on the hardness, however it had significate
effect on the toughness in the cross sections of BMI penetration nozzle. Therefore, both hardness andtoughness in the HAZ of repair welding need to be investigated to evaluate the tempering effect oftemper bead welding for BMI penetration nozzle.
Using the proposed method, the hardness and toughness in the BMI penetration nozzle withdifferent geometric variables such as mounting angle, nozzle thickness, et al., could be predicted prior
to the actual multi-pass repair welding. It means that if the calculated hardness in HAZ is higher thanthe critical value, for instance more than 350HV (the required specification in industry), or the
calculated toughness is lower than the critical value of 40J (the required specification in industry forside-notched Charpy sample), the welding conditions should be modified. Thus, the appropriate
welding conditions can be selected prior to the actual repair welding for BMI penetration nozzle withdifferent geometric variables. Therefore, the presently proposed method is greatly useful and effective
for assessment of tempering effect of temper bead welding techniques for BMI penetration nozzle ofthe nuclear power plants.

8. Conclusions
A neural network-based prediction system for the hardness and toughness in BMI penetration
nozzle with different mounting angles when temper bead welding technique was applied has been
investigated. The following conclusions can be drawn:(1) Based on the thermal history numerically obtained by FEM simulation, the hardness distribution
and toughness distribution in the cross section of BMI penetration nozzles with different
mounting angles (0°, 15°, 30°and 45°) was calculated, using the neural network-based
hardness/toughness prediction system.

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L. Yu, et al./ Title of the Paper Tempering Effect Prediction of
Temper Bead Repair Technology for BMI Penetration Nozzles(2) The predicted hardness and toughness results showed that the mounting angle had insignificant
effect on the hardness, however it had significate effect on the toughness in the cross sections of
BMI penetration nozzle. Therefore, both hardness and toughness in the HAZ of repair welding
needed to be investigated to evaluate the tempering effect of temper bead welding for BMI
penetration nozzle.
(3) The proposed method was greatly useful and effective for assessment of tempering effect oftemper bead welding techniques for BMI penetration nozzle of the nuclear power plants.

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