E-Journal of Advanced Maintenance Vol.7-2 (2015) 189-198
Japan Society of Maintenology

Sustainability of compressive residual stress by stress improvement
processes
Satoru NISHIKAWA1,* Shigeru OKITA2 and Atsunori YAMAGUCHI3
1

Japan Power Engineering and Inspection Corporation, 14-1 Bentencho, Tsurumi-ku Yokohama,230-0044, Japan
2
Japan Power Engineering and Inspection Corporation, 14-1 Bentencho, Tsurumi-ku Yokohama,230-0044, Japan (Present address: The Japan Welding Technology Center)3
Japan Power Engineering and Inspection Corporation, 14-1 Bentencho, Tsurumi-ku Yokohama,230-0044, Japan (Present address: Japan Society of Maintenology)
ABSTRACT
Stress improvement processes are countermeasures against stress corrosion cracking in nuclear power plantcomponents. It is necessary to confirm whether a compressive residual stress induced by stress improvement
processes can be sustained under operation environment. In order to evaluate stability of the compressive
residual stress in 60-year operating conditions, the 0.07% cyclic strains of 200 times at 593 K were applied to
the welded specimens, then a thermal aging treatment for 1.66×106 s at 673 K was carried out. As the result, it
was confirmed that the compressive residual stresses were sustained on both surfaces of the dissimilar welds ofaustenitic stainless steel (SUS316L) and nickel base alloy (NCF600 and alloy182) processed by laser peening(LP), water jet peening (WJP), ultrasonic shot peening (USP), shot peening (SP) and polishing under 60-year
operating conditions.
KEYWORDS

ARTICLE INFORMATION

Stress improvement processes, Sustainability of the compressive residual
stress, Austenitic stainless steel, Nickel base alloy, LP, WJP, USP, SP,
Polishing

Article history:
Received 15 January 2015
Accepted 19 August 2015

1.

Introduction

One of the methods to prevent components of a nuclear power plant from stress corrosion cracking(SCC) is a stress improvement process such as peening techniques (laser peening, water jet peening,etc.) and polishing for applying compressive residual stress to a region where SCC is supposed tooccur. However, when a compressive residual stress is applied for prevention of SCC, it is necessary
to confirm persistence of the compressive residual stress upon the consideration of aged deteriorationof the component.
Until now, many studies have been conducted to investigate persistence of a compressive residual
stress. In these studies, the relaxation of a compressive residual stress were examined on the following
conditions:① Thermal relaxation by thermal aging treatment[1] – [4]② Stress (strain) loading[1], [5]-[8]③ Thermal relaxation by thermal aging treatment with stress (strain) loading[3], [5], [9]④ Thermal relaxation by thermal aging treatment in the presence of welding residual stress [3], [4],[7]⑤ Stress (strain) loading in the presence of welding residual stress[10], [11]⑥ Thermal relaxation by thermal aging treatment in welding residual stress with stress (strain)loading [3], [12]The examined materials are SUS304(Type304), SUS316L(Type316L), NCF600(UNS N06600),*

Corresponding author, E-mail: nishikawa-satoru@japeic.or.jp

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

189

S. Nishikawa, et al./ Sustainability of compressive residual stressby stress improvement processes

alloy132 and alloy182.
From the results of the studies, it was confirmed that a compressive residual stress is retained oneach of the conditions ①–⑥, but a consensus opinion about persistence of a compressive residual
stress has not been reached so far because the test condition and the specimen type were different foreach stress improvement process.
In this study, we investigated persistence of a compressive residual stress by performing stressrelaxation tests under the same condition based on the environment of a nuclear power plant using
specimens of the same size as the specimens were treated with different stress improvement
processes; laser peening (LP), water jet peening (WJP), ultrasonic shot peening (USP), shot peening(SP) and polishing.

2.

Experimental procedure

2.1. Preparation of evaluated specimens
Table 1 shows the chemical composition of each material prepared for making the dissimilar
welded joints used in stress relaxation tests as the specimens. Figure 1 shows the schematic of these
dissimilar welded joints. The two base metals SUS316L and NCF600 were joined by shielded metalarc welding with the weld metal Alloy182 to form the dissimilar welded joints of 15 mm in thickness,
200 mm in width and 250 mm in length. When the welding was performed, the electrode wasconnected to the positive terminal of a direct current power source. The current was set to 110–140 A.
The welding speed was set to 2.0–4.1 mm/s. The weld was made in five layers with nine passes. The
electrode was moved carefully not to form electrode stop/start points around the center of thespecimens, where residual stress would be evaluated. The surface of the welded joints was made flatby mechanically grinding the weld reinforcement, and the plate thickness was made 12.1 mm bymechanically grinding the surface of the initial layer side. The evaluated specimens (12.1 mm inthickness, 45 mm in width and 180 mm in length) were cut out from the welded joints in such a waythat the weld line was located along the center line in the longitudinal direction of the specimens. The
final layer side of the specimens was electrolytically polished by more than 0.1 mm in order toremove the machined layer of the surface. The stress improvement processes were performed on a 40
mm×45 mm area of the electrolytically polished surface of the specimens as shown in Figure 2. The
stress improvement processes performed were LP, WJP, USP, SP and polishing. Table 2 shows theconditions of LP, WJP, USP and SP. In the following sections, the effects of these stress improvement
processes on the persistence of the compressive residual stress are described.
Table 1 Chemical composition of the base metal and the deposited metal used (mass%)
Material
C
Si
Mn
P
S
Ni
Cu
Cr
Fe
Mo Nb+Ta Ti
OthersSUS316L
0.015 0.66 1.21 0.035 0.003 12.24 - 17.41 Bal. 2.08
NCF6000.08 0.23 0.25 0.008 0.001 73.54 0.07 16.77 9.05
Alloy182
0.058 0.76 7.38 0.007 0.004 68.3 0.03 14.30 7.08
1.52 0.37<0.50

Alloy 182
15

SUS316

NCF600250

L200
Figure 1 Schematic of a dissimilar welded joint from which specimens for thesustainability evaluation of compressive residual stress were cut

190

S. Nishikawa, et al./ Sustainability of compressive residual stressby stress improvement processes

Peened or polished area
SUS316L
Alloy182

45

NCF60040

12180
Figure 2 Schematic of peened and polished area on the specimen

Table 2 Conditions of the LP, WJP, USP and SP

LP

Pulse energy
Spot diameter
Number of pulses

WJP

USP

SP

Injection quantity
Injection time
Injection distance
Injection angle
Nozzle diameter
Amplitude
Ball diameter
Projection time
Material of cut wire
Cut wire contour
Projection angle

60 mJ
0.7 mm7000 pulse/cm2 (SUS316L)
1800 pulse/cm2 (NCF600, alloy182）
800 cm3/s
24 s/cm
140 mm
90 °
2 mm
50 μm
4 mm
150 s
SUS304
φ0.8 mm×0.8 mmL
90 °2.2. Strain loading
Strain was applied multiple times to the specimens in order to evaluate the influence of the stressvariation due to the plant start/stop operations. The yield stress of SUS316L, NCF600 and alloy182
decreases with the increasing temperature[12]. Therefore, the loading with the operating temperatureand the stress occurred at the start-up of a nuclear power plant should be severe condition for therelaxation of the compressive stress. Because applying a 0.07% strain at the PWR operating
temperature 593 K provides a stress equal to or more than the stress arising in the start-up of a plant, a
0.07% strain was cyclically applied to the longitudinal direction of the specimens at 593 K. The strain
rate was set to 6.67×10-5 s-1. The strain profile was chopping wave. The number of strain loading ofspecimens were 200 times considered as the cycle number more than the design number assumed 60
years operation. In addition, operating temperature in the BWR power plant is at 563 K and the strain
equivalent to the above is caused. It can be proposed that the compressive residual stress is sustained
under the BWR environment, if the compressive residual stress is sustained with these conditions.
2.3. Thermal aging treatment
In addition to stress loading, thermal relaxation by a long term aged at operating temperature isconsidered as a factor of sustainability of compressive stress by a stress improvement process during
plant operating. To examine the sustainability of the compressive residual stress considering both191

S. Nishikawa, et al./ Sustainability of compressive residual stressby stress improvement processes

operating temperature and stress loading, specimens were subjected to 200 cycles of strain andsubsequent thermal aging treatment simulating the actual long-term operation. Assuming that PWRs
are operated for 60 years at an operating rate of 100%, their components are subjected to a thermal
history of 593 K for 1.89×109 s. Using a Larson-Miller parameter, this condition is converted to an
acceleration condition of 673 K for 1.66×106 s, which is used in the thermal aging treatment in thisstudy. The operating temperature of BWRs is 563 K, which is lower than that of PWRs. Therefore, if
the compressive residual stress is sustained under the above acceleration condition, it will be sustained
under the operating conditions of BWRs.
2.4. Residual stress measurement
Specimens were subjected to 1, 2, 5, 10, 50, or 200 cycles of strain and subsequent thermal aging
treatment. The residual stress of thus-obtained specimens was measured by X-ray diffraction. Table 3
summarizes the conditions for the measurement of residual stress by the 2θ-sin2φ method.[14] The
residual stress was measured at one point on each of SUS316L, NCF600, and alloy 182; however,measurement was carried out twice at the same point on alloy 182 in consideration of the variabilityof measured results. Figure 3 shows the measurement points of residual stress. The surface residual
stress was measured at a position 2 mm from the fusion line in the heat-affected zone of SUS316L and
NCF600 and at the center of alloy 182. According to the results obtained by Masaki [6], the residual
stress changes negligibly in the direction perpendicular to that of stress loading and is relaxed in the
direction of stress loading. Therefore, the residual stress in the longitudinal direction of specimens
was measured in this study. In addition, the surface of the specimens obtained after 200 cycles ofstrain and subsequent thermal aging treatment was subjected to electrolytic polishing, and the residual
stress at a depth of 10–200 μm was measured to evaluate the sustainability of the compressive residual
stress inside the specimens. For specimens subjected to stress improvement processes other thanpolishing (i.e., LP, WJP, USP, and SP), the residual stress was measured at depths of 100 and 200 μm
for SUS316L and NCF600, respectively. For specimens subjected to polishing, the residual stress wasmeasured at a depth of 10 μm for both SUS316L and NCF600 because polishing affects thecompressive residual stress up to a depth of 50 μm[15]. As shown in Figure 3, the measurement pointof the residual stress for the specimens after the thermal aging treatment was located symmetrical tothose for the specimens obtained after 200 cycles of strain application because a concave was formedon the surfaces of SUS316L and NCF600 by electrolytic polishing.
Table 3 Measurement condition of residual stressby X-ray diffraction
Condition
Characteristic
X-ray
Tube voltage／tube current
Electrolytic
polishing area

Collimator area

Diffraction plane
Diffraction angle（2θ）

SUS316L NCF600

Peened or polished area

Alloy182

Cr-Kβ

SUS316L
2

40 kV/30 mA
Alloy182

φ3 mm

―

φ2 mm

φ2 mm
（Without
WJP and LP）
φ4 mm
（WJP
and LP）
311

148.5 °

2
10

10

NCF60040
180●：Measurement point of residual stress until 200 cycles of 0.07% strain○：Measurement point of residual stress after thermal aging treatment

Figure 3 Schematic of measurement point ofresidual stress by X-ray diffraction

153.6 °192

S. Nishikawa, et al./ Sustainability of compressive residual stressby stress improvement processes

3.

Results and discussion

3.1.

Sustainability of surface residual stress of SUS316L and NCF600

Figure4 shows the surface residual stress after 200 cycles of strain application and subsequent
thermal aging treatment at measurement positions 2 mm from the fusion line on SUS316L and
N
C
F
6
0
0
300

SUS316L（As welded）
NCF600（As welded）
SUS316L
NCF6000

Residual stress (MPa）Residual stress (MPa）

300

-300

-600

-900
1

0

-300

-600

-900

After thermal

00.1

SUS316L（As welded）
NCF600（As welded）
SUS316L
NCF600After thermal

0
0.1

aging treatment
10
100
1000
10000
Number of strain cycles

1

aging treatment
10
100
1000
10000
Number of strain cycles

(a)

(b)

300

SUS316L（As welded）
NCF600（As welded）
SUS316L
NCF6000

Residual stress (MPa）Residual stress (MPa）

300

-300

-600

-900
1

0

-300

-600

-900

After thermal

00.1

SUS316L（As welded）
NCF600（As welded）
SUS316L
NCF6000
0.1

aging treatment
10
100
1000
10000
Number of strain cycles

After thermal

1

aging treatment
10
100
1000
10000
Number of strain cycles

(c)

(d)

Residual stress (MPa）300

SUS316L（As welded）
NCF600（As welded）
SUS316L
NCF600

0

-300

-600

-900

After thermal

0
0.1

1

aging treatment
10
100
1000
10000
Number of strain cycles(e)Figure 4 Effect of 200 cycles of 0.07% strain at 593 K and subsequent thermal aging treatment for 1.66
6

×10 s at 673 K on surface compressive residual stress in SUS316L and NCF600: (a) LP, (b)WJP, (c) USP, (d) SP, and (e) polishing193

S. Nishikawa, et al./ Sustainability of compressive residual stressby stress improvement processes

subjected to each stress improvement process. Although the surface residual stress was tensile owing
to welding, it became compressive as a result of the stress improvement processes. The compressive
residual stress was sustained on the surfaces of SUS316L and NCF600 subjected to all five stress
improvement processes after 200 cycles of 0.07% strain application at 593 K and subsequent thermal
aging treatment for 1.66×106 s at 673 K. The compressive residual stress of many specimens was
significantly relaxed after the first cycle of strain application and remained almost the same up to 200
cycles of strain application, although the tendency of relaxation differed depending on the stress
improvement process used and the material. From this result, it was found that the compressive
residual stress was significantly relaxed owing to the redistribution of stress resulting from thetemperature increase and stress loading in the first cycle of strain application and that the change inthe compressive residual stress was negligible upon subsequent stress loading.
3.2.

Sustainability of surface residual stress of alloy182

Figure 5 shows the surface residual stress after 200 cycles of stress application and subsequent
thermal aging treatment at the center of alloy182 subjected to each stress improvement processes. The
surface residual stress became compressive after any stress improvement process. The compressive
residual stress was sustained on the surface of alloy182 after 200 cycles of 0.07% strain application at
593 K and subsequent thermal aging treatment for 1.66×106 s at 673 K. Although the compressive
residual stress was relaxed up to five cycles of strain application in the specimens subjected to WJP, it
was approximately −500 MPa after ten cycles of strain application. The reason behind the significant
change in the compressive residual stress of the specimens subjected to WJP compared with thosesubjected to other stress improvement processes is considered to be a large measurement error. The
crystal grains in alloy182 are larger than those in the base material [15] and crystals solidify in the<100> direction. Thus, the number of crystal grains included in the X-ray irradiation region decreases,
so that accuracy of X-ray stress measurement decreased. Therefore, in the specimens subjected to
WJP with a small area of surface processing[16], large crystal grains remain, leading to a large
measurement error and the large change in the compressive residual stress compared with specimens
subjected to other stress improvement processes. In the specimens subjected to USP and SP with anincreased area of surface processing, the large crystal grains of alloy182 are considered to be
downsized, leading to an increased number of crystal grains in the X-ray irradiation region and a highmeasurement accuracy of residual stress compared with that of specimens subjected to WJP.
3.3

Sustainability of the compressive residual stress inside the specimen

Figure 6 shows the result of the residual stress measurement in the depth of 10–200 μm from thesurface of SUS316L and NCF600 at 2 mm from the fusion line. The compressive residual stresses
were sustained in the specimens after 200 cycles of 0.07% strain application at 593 K and subsequent
thermal aging treatment for 1.66×106 s at 673 K. For the specimens subjected to stress improvement
processes other than polishing (i.e., LP, WJP, USP, and SP), the compressive residual stresses were
sustained in SUS316L at a depth of 100 μm and in NCF600 at a depth of 200 μm. For the specimen
subjected to polishing, the compressive residual stress was sustained in both SUS316L and NCF600
in the depth of 10 μm.
From the above findings, the compressive residual stress is sustained in welded specimens
comprising SUS316L, NCF600, and alloy 182 subjected to LP, WJP, USP, SP, and polishing under theconditions of 60 years operating of a nuclear power plant.

194

300

300

0

0

Residual stress (MPa）Residual stress (MPa）S. Nishikawa, et al./ Sustainability of compressive residual stressby stress improvement processes

-300
-600
-900
-1200
1

-600
-900
-1200

After thermal

0
0.1

-300

aging treatment
10
100
1000
10000
Number of strain cycles

After thermal

0
0.1

1

aging treatment
10
100
1000
10000
Number of strain cycles

(b)
300

0

0

Residual stress (MPa）

300

-300
-600
-900
-1200
1

-300
-600
-900
-1200

After thermal

0
0.1

aging treatment
10
100
1000
10000
Number of strain cycles

After thermal

0
0.1

1

aging treatment
10
100
1000
10000
Number of strain cycles

(c)

(d)
300

Residual stress (MPa）Residual stress (MPa）

(a)

0
-300
-600
-900
-1200

After thermal

0
0.1

1

aging treatment
10
100
1000
10000
Number of strain cycles(e)Figure 5 Effect of 200 cycles of 0.07% strain at 593 K and subsequent the thermal aging treatment
6

for 1.66×10 s at 673 K on surface compressive residual stress in alloy182: (a) LP, (b) WJP,(c) USP, (d) SP, and (e) polishing195

S. Nishikawa, et al./ Sustainability of compressive residual stressby stress improvement processes

300

300
SUS316L（100 μ m depth）

SUS316L（100 μ m depth）
NCF600（200 μ m depth）

Residual stress （MPa）

Residual stress （MPa）

NCF600（200 μ m depth）
0

-300

-600

-900

0

-300

-600

-900
200 cycles of strain +thermal aging treatment

200 cycles of strain

200 cycles of strain

(a)

(b)

300

300
SUS316L（100 μ m depth）

SUS316L（100 μ m depth）

NCF600（200 μ m depth）

NCF600（200 μ m depth）

Residual stress （MPa）

0

-300

-600

-900

0

-300

-600

-900
200 cycles of strain +thermal aging treatment

200 cycles of strain

200 cycles of strain

200 cycles of strain +thermal aging treatment

(d)

(c)

300
SUS316L（10 μ m depth）
NCF600（10 μ m depth）

Residual stress （MPa）

Residual stress （MPa）

200 cycles of strain +thermal aging treatment

0

-300

-600

-900
200 cycles of strain

200 cycles of strain +thermal aging treatment(e)Figure 6 Effect of 200 cycles of 0.07% strain at 593 K and subsequent thermal aging treatment for
6

1.66×10 s at 673 K on compressive residual stress in depth point: (a) LP, (b) WJP, (c) USP,(d) SP, and (e) polishing196

S. Nishikawa, et al./ Sustainability of compressive residual stressby stress improvement processes

4.

Conclusions

In this study, the sustainability of the compressive residual stresses in welded specimens subjected
to stress improvement processes (i.e., LP, WJP, USP, SP, and polishing) to prevent components of a
nuclear power plant from SCCs was examined considering 60-year operation of actual reactors. The
obtained results are the following:
1)
The compressive residual stresses were sustained on the surface of welded specimens
comprising SUS316L, NCF600, and alloy 182 subjected to stress improvement processes after
200 cycles of 0.07% strain application at 593 K and subsequent the thermal aging treatment for
1.66×106 s at 673 K.
2)
For specimens subjected to LP, WJP, USP, and SP, the compressive residual stresses were
sustained in SUS316L at a depth of 100 μm and in NCF600 at a depth of 200 μm after 200
cycles of 0.07% strain application at 593 K and subsequent the thermal aging treatment for
1.66×106 s at 673 K.
3)
For welded specimens subjected to polishing, the compressive residual stresses were sustainedin both SUS316L and NCF600 at a depth of 10 μm after 200 cycles of 0.07% strain application
at 593 K and subsequent the thermal aging treatment for 1.66×106 s at 673 K.

Acknowledgement
This study is a part of joint research results by ten electric power corporations (The Kansai ElectricPower Company, Hokkaido Electric Power Co., Inc., Tohoku Electric Power Co., Inc., Chubu ElectricPower Co., Inc., Hokuriku Electric Power Co., Inc., Chugoku Electric Power Co., Inc., Kyushu
Electric Power Co., Inc., The Japan Atomic Electric Power Co., Inc. and Electric Power Development
Co., Inc.) and three plant manufacturing corporations (Toshiba Corporation, Hitachi-GE Nuclear
Energy, Ltd. and Mitsubishi Heavy Industries, Ltd.). Discussion of these persons concerned isthanked heartily.

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