Reference to “CRC”, “IHSI”, “Internal Polishing”
E-Journal of Advanced Maintenance Vol.1, No.1. (May, 2009)

Introduction of Repair/ Maintenance Techniques for SCC in Primary Loop
Recirculation System Piping

Teruaki SATO1, Kazuyoshi YONEKURA1, Satoshi HONGO２,Shouji HAYASHI３, Hideyo SAITOU３
1Toshiba Corporation,8 Shinsugita-cho, Isogo-ku, Yokohama, Kanagawa-Prefecture, 235-8523,
Japan
2
IHI Corporation, 1, Shin-nakahara-cho, Isogo-ku, Yokohama, Kanagawa-Prefecture, 235-8501,
Japan
3
Hitachi-GE Nuclear Energy, Ltd., 2-2, Omika-cho, 5-chome, Hitachi-shi, Ibaraki-ken, 319-1221,
Japan

1.

Introduction
In recent years, Stress Corrosion Cracking (SSC) has been found at many welded joints ofPrimary Loop Recirculation (PLR) piping in Boiling Water Reactor (BWR) plants. The piping is
made up of low-carbon stainless steel (SUS316LC) as it was thought that SCC generated bysensitization would be prevented by using low-carbon material. However, the results of latermock-up tests and experiments confirmed that hardened surfaces increased susceptibility totransgranular SCC, which progressed to IGSCC in crack growth experiments under simulated
reactor water conditions. These results indicate that in areas where there is a high amount ofstress and cracking susceptibility is increased by hardness due to surface machining, even if
low-carbon stainless steels are used, transgranular SCC may generate, go down around to thedepth of surface machining, and then may further advance as IGSCC, depending on the amountof stress and the chemical conditions in which crack tips are located. It is known that SCC isgenerated due to three factors superposed, i.e. materials, environments, and stresses. Needless tosay, it is effective to eliminate those factors to suppress SCC generation.
This report introduces some repair methods and preventive maintenance techniques for SCC in
PLR piping.

2.

Examples of repair methods and preventive maintenance techniques for SCC in PLR
pipingThis section describes major examples for repair methods and preventive maintenancetechniques related to material improvement and residual stress relaxation for SCC in PLR piping.
2.1. CRC：Corrosion Resistant Cladding
2.2. Internal Polishing
2.3. IHSI: Induction Heating Stress Improvement
2.4. SHT：Solution Heat Treatment
2.5. HSW：Heat Sink Welding
2.6. Weld Overlay

ISSN-0000/09 © 2009 – JSM and the authors. All rights reserved.

1

T. Sato et al./ Introduction of Repair/ Maintenance Techniques
for SCC in Primary Loop Recirculation System Piping

2.1 CRC: Corrosion Resistant Cladding[1]The CRC method is used to reduce SCC susceptibility by cladding the wetted inner-surface ofsensitized portions near the welded joints of piping with non-sensitized deposited metal (Fig. 1).
There are various methods to clad the wetted part inside the piping with deposited metal (insideweld overlays) and the method typically used is shown in Fig. 2.
The SCC susceptibility is reduced by securing high ferrite content on the wetted inner-surfaceof the piping’s welded joints.
Fig.3 shows that generally in the low-carbon zone, no SCC is observed in the area whereferrite content is over about 5%[2].
Table 1 indicates that, in the SCC-resistance experiments, SCC are well suppressed in theregion the inside weld overlay method was performed.
溶接部
Pipe weld

Sensitized zone
鋭敏化領域(Heataffected zone)
Fig.1. Typical Welded Joints

Pipe weld

Sensitized zone

1st layer Cladding(Stress improvement such as SHTor Internal polishing shall becarried out after weld)

2nd layer Cladding(As weld )Fig.2. Concept of the CRC Method

Table.1. SCC-Resistance Effect with CRC Method
Item

Detail of weld joint

Type(a) Normalweld joint(b) 2-layer
inner cladding
technique withSHT(c) Modified
single-layer
inner cladding
technique

DDAＣＢ A

CBB test results
Maximum crack
depth (μm)

Ａ298

Ｄ0ＡＢＣＤ0000C0Ｃ*1: Testing temperature 289degC, Oxygen concentration 20 ppm, Testing time 100hours

2T. Sato et al./ Introduction of Repair/ Maintenance Techniques
for SCC in Primary Loop Recirculation System Piping

Fig.3. SCC Susceptibility vs. Ferrite Content

2.2Internal Polishing[3]Maximum SCC crack length (μm)

To relieve residual stress of the inner-surfaces of piping, top surface stress is shifted tocompressed sides by polishing the inside surface. This reduction in residual stress results in thesuppression of SCC generation. Fig.4 shows the results of CBB tests performed on several kindsof surface polished materials. It is confirmed that polishing is found to be an effective measurefor SCC suppression by reducing residual stress on the surfaces.

□ WJP, H4 test specimen◇ Scoch Bright, H4 test specimen▲ Emery, H4 test specimen■ Grinder, H4 test specimen◆ Grinder, H3 test specimen

Grinding Only

WJP or scotch-brightpolishing after grinding
Emery polishing

Mean residual stress (MPa)Fig.4. Relief of Residual Stress on Surfaces by Polishingand its Effect on SCC Suppression

2.3

Induction Heating Stress Improvement (IHSI)To generate the same temperature difference found at the heat effective zone that goes
through the thickness of the pipe, the outer-surface is heated by the high-frequency induction
heating method and the inner-surface is simultaneously cooled by water. The thermal stressgenerated by this process relieves the inner-surface residual stress of the piping. Fig. 5 shows thegeneral idea of the IHSI method. The stress distribution, deformation, and the temperature
distribution at the time IHSI is executed are shown in Fig. 6. In the IHSI method, a large
temperature difference throughout the pipe wall emerges due to simultaneous cooling through3T. Sato et al./ Introduction of Repair/ Maintenance Techniques
for SCC in Primary Loop Recirculation System Piping

applying water on the inside surface of the welded joints and the heating of the outside surface toa specified temperature set by the high-frequency induction heating method. At this time, thecompressive yield and tensile yield is generated on the outside and the inside surface,respectively (Fig. 6 (a)). When heating is stopped (cooling on the inside is continued), thetemperature difference becomes smaller and stress on the outside surface that was generated in
the heating process changes to tensile stress. Stress on the inside surface turns into thecompression stress and remains as residual stress (Fig. 6 (b)). Through IHSI, residual tensilestress on the inside of welded pipes can be relieved or changed to compressed sides. As examplesof IHSI application ( 600A ), measurements of residual stress on the inner-surfaces along theaxial direction of the piping after the IHSI method was performed for typical grooves (thetemperature difference between the inside and the outside is 397℃), and for narrow grooves (thetemperature difference is 299℃) are shown in Fig.7 and Fig.8 respectively. The results show that
the IHSI method changes inner-surface residual stress to compressive residual stress in thevicinity of welding and thus, residual stress is reduced. Moreover, the method is found to be
more effective for narrow grooves when compared to typical grooves, as the reduction of insideresidual stress along the axial direction of piping is greater, even though the temperature
difference between the outside and the inside surface is smaller.
Heating coil

reCircumfe

dntial welwater
Cooling

electricity source
for induction heating

Fig.5. Concept of the ISHI Method[ UnderHeating ]
【加熱中】
Outer
surface -σy
配管外面

High
高
圧縮
Compression

Tensile
引張+σy

配管内面
Inner
surface

低Low

Yielding
σy：降伏応力Stress[ AfterCooling ]
【冷却後】
Outer
surface
配管外面

ΔＴ

Balanced
釣り合いpositionの位置

Tensile
引張Tensile
引張Compression
圧縮

圧縮
Inner配管内面
surface Compression

ΔＴ = 0Stressdistribution
応力分配

Deformation
変形Temperature
distribution
温度分布

Fig.6. Stress Distribution, Deformation, andTemperature Distribution at the Time of IHSIApplication

4T. Sato et al./ Introduction of Repair/ Maintenance Techniques
for SCC in Primary Loop Recirculation System Piping

900
0°方位内面_軸方向● 0 deg, axial,
inner surface
90°方位内面_軸方向■ 90deg, axial,
inner surface▲ 180deg,180°方位内面_軸方向
axial, inner surface◆ 270deg,270°方位内面_軸方向
axial, inner surface

700

Stress(MPa)
応力（MPa）

500
300
100-100-300-500-700-900-150-100-50

050

100

150

Distancefrom the weld center (mm)
溶接中心からの距離（mm）

Fig.7. Residual Stress Distribution on the Inside
Surface Along the Axial Direction of the Piping(Typical Groove)

900
0°内面_軸方向● 0 deg, axial,
inner surface
90°内面_軸方向■ 90deg, axial,
inner surface
180°内面_軸方向▲ 180deg, axial,
inner surface
270°内面_軸方向◆ 270deg, axial,
inner surface

700

Stress(MPa)
応力（MPa）

500
300
100-100-300
-500
-700
-900
-150-100-50

050

100150Distancefrom the weld center (mm)
溶接中心からの距離mm

Fig.8. Residual Stress Distribution on the Inside
Surface along the Axial Direction of the Piping(Narrow Groove)

2.4

Solution Heat Treatment (SHT)[4]

Chromium rich carbide precipitation from the grain boundary is decomposed when heated byhigh temperature ( over 1000℃ ) and dissolves uniformly into the base material, which causes
the chromium-depleted-layer around the grain boundary to disappear. This heating method,
called solution heat treatment (SHT), can eliminate sensitized areas in the material formed bywelding and, as shown in Fig. 9, reduces residual stress at the same time[5]. Note that a furnaceis needed for this method and it is recommended that it is applied to welded joints at factories.

5Stress relief ratio (%)

T. Sato et al./ Introduction of Repair/ Maintenance Techniques
for SCC in Primary Loop Recirculation System Piping

Heat treatment temperature (deg C)

Fig.9. Stress Relief vs. Solution Heat Treatment
Temperature

2.5

Heat Sink Welding (HSW)[4]As shown in Fig. 10, HSW is a method which the inside of pipes are welded while cooled byflowing water or sprays after partitions are formed by the in gas welding for up to two or threelayers. Using this method, thermal stress is generated by the temperature difference of the outside
and inside of piping that goes through the thickness of the material. This thermal stress reduces
tensile residual stress, which is the cause of SCC around the inside surface of the welded pipes.
The results shown in Fig. 11 confirm that HSW is more effective than conventional methods
used in gas welding to reduce tensile residual stress.

Welding in air (1st層目迄)to 2 or 3 pass)
気中溶接(2〜3)
内表面水冷
insideater (ng wCooli

Heat
Sink Welding
水冷溶接( Remaining
pass are welded(気中溶接終了後の残層)
by HSW after in-air welding)Fig.10. Concept of the HSW Method6T. Sato et al./ Introduction of Repair/ Maintenance Techniques
for SCC in Primary Loop Recirculation System Piping

500

In-airwelding
気中溶接

軸方向残留応力Axial
residual stress（MPa)(MPa)

400(600A,38.9t)

300
200
1000HSW
水冷溶接-100(700A,46t)-200-20-15-10-50

5101520

Distance fromweld metal center (mm)
溶接金属中心からの距離(mm)Fig.11. Effect of stress improvement by HSW

2.6

Weld Overlay[6]In weld overlays, high ferrite welded metal that is superior in SCC resistance is overlaid in abelt around the outside of welded joints in piping with SCC (Fig.12). Strength was secured in
the belt-shaped overlay welded part of the piping, thus covering up for lost strength in the part ofthe piping where the crack occurred (Fig. 13).

Weld Overlay (WOL)

Pipe Weld
Crack

Pipe

Fig.12. Concept of the Weld Overlay Method

肉盛溶接
WeldOverlay
金属部

WOLWOL構造強度部
structural member section

Crack
き裂WOL
WOL構造強度部
structural member section

WOL processed section
WOL施工部

Originalpipe
原配管部

Axial cross sectionof WOL面
WOL配管部の軸断
processed region

Fig.13. Outline of Piping Cross-Section for theWeld Overlay Method

7T. Sato et al./ Introduction of Repair/ Maintenance Techniques
for SCC in Primary Loop Recirculation System Piping

3.

Conclusion
As SCC is caused by the superposition of three factors, materials, stresses, and environments,
major methods and techniques for repairs and preventive maintenance that eliminate the affectsof the first two factors are discussed in this paper.
Further development of new techniques will be continued, while taking into account new
knowledge obtained from research on low-carbon stainless steel SCC.

References[1] Shinji Tanaka, Tadahiro Umemoto and Ryoichi Kume, ”Mitigation of Stress Corrosion Cracking
Utilizing
Corrosion
ResistantCladding”,
Ishikawajima-Harima
Engineering
Reviw,Vol.19,No.3,(May 1979)( in Japanese).
[2] Hughes N, Clarke W L, Delwiche D E, “Intergranular Stress-Corrosion Cracking Resistance ofAustenitic Stainless Steel Castings”.
[3] Kaneo Takamori et al, ”SCC Crack Initiation and Propagation of Low Carbonized Stainless Steelin High Temperature Pure Water”, Maintenology, Vol.3,No2(2004),( in Japanese).
[4] Conference material, 5th conference of task-force on structural integrity of Nuclear Power
Generation Facility, Sub-committee on Nuclear safety and Security, Research committee on
Natural resource and Energy, Agency for Natural Resource and Energy, METI.
[5] Hiroshi Taira, “Beginning and Actual use of Stainless Steal”, Nihon Kogyo Shuppan.
[6] Conference material, 6th conference of task-force on structural integrity of Nuclear Power
Generation Facility, Sub-committee on Nuclear safety and Security, Research committee on
Natural resource and Energy, Agency for Natural Resource and Energy, METI.

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