JSM 7th Annual Conference at Omaezaki, Shizuoka
Planning and Consideration on SFR R&D Activities in China
GA 
Toshiba Corporation, Takao SASAYAMA
IHI Corporation, Satoshi HONGO
Vol.3, No.2, NT38
Confirmation test of IHSI for pipes with crack
KEYWORDS：
Crack, IHSI, Stress Improve, SCC, Residual Stress, Preventive maintenance
1. Technical summary
Induction Heating Stress Improvement (IHSI) is one of preventive maintenance technologies for stress corrosion crack (SCC) such as primary loop recirculation system of Boiling Water Reactor (BWR). Recently, IHSI has been used a number of BWR plants because it can improve welding residual stress confirmed by past tests. We apply preventive maintenance IHSI for only non crack area, so if there is a crack we have to choose alternative methods (ex, replacement) for preventive maintenance. If this study shows that crack progression is improved by test of IHSI for pipe with crack, it will able to get more choices for repairing strategy and IHSI will be able to applied for one of repairing methods as good for working conditions. It helps to consider maintenance and repairing plans of BWR plant reasonably. In this study, we confirmed the test of IHSI for pipe with crack.
The objective of IHSI is to introduce a large temperature difference between the inner and outer surfaces of a pipe in order to produce sufficient thermal stress to induce some plastic flow and consequently to obtain a compressive residual stress at the inner surface of the pipe. The large temperature difference can be introduced by heating a pipe from the outside with an induction coil while cooling water is supplied simultaneously to the inner surface of the pipe As shown in Fig.1, set the IHSI condition to make liner temperature distribution of the inner pipe thickness. Heating and cooling condition of construction: set the essential variables. During the heating, inner pipe has a tensile stress, then cool-down process it forms compressive stress field. Additionally, in case of the pipe has a crack, crack tip is received tensile loading from inner pipe’s tensile stress. It gives plastic deformation to crack tip without ductile fracture. After the heating, compressive stress field is formed around the crack tip. These effects inhibit progress of crack.
(1)Concept of IHSI
(2)Stress deformation and Temperature distributions Induced by IHSI
Fig.1 Concept of IHSI
2. Development phase
Phase 3 : Publicly-accepted Phase
3. Scope
• Components:
Pipes for BWR Primary Loop Recirculation system, etc
• Location:
HAZ of weld line
• Materials:
Stainless steels, SUS316, SUS304, etc
• Others:
Applicable crack depth is less than 3t/8
4. Features
4.1 Confirmation items of test
The trials were examined in terms of focusing on the following items.
① No adverse effect on cracks and joints.
② Improve crack tip stress, and suppresses progress of the crack.
③Confirm the effect of the improvement of residual stress around the crack by IHSI treatment.
④Ensure that mitigation IHSI has the equivalent effect of preventive maintenance IHSI on the non crack area.
4.2 Results
①Confirm that cracks and joints have no adverse effect.
Confirmed that IHSI treatment for pipe with crack was suppressed with crack during IHSI heating and has no adverse effect on mechanical properties of materials. The following are test conditions and test results.
●Test conditions
Test model: Diameter 600A, Thickness 38.6mm, Material SUS316 (LC)
Form of crack: EDM initial notch + fatigue crack (deepness: 3/8t, length: inner face 90°)
IHSI condition: three-time heating over the maximum heating temperature 650℃
Form of crack: EDM initial notch + fatigue crack (deepness: 3/8t, length: inner face 90°)
IHSI condition: three-time heating over the maximum heating temperature 650℃
(1)Results of the cross-section observations
Fig.2 shows results of the micro cross-section observations with crack after IHSI treatment. As shown Fig.2, Trace which is caused by ductile fracture in the process of IHSI treatment wasn’t confirmed.
Fig.2 Cross-sectional observation of the crack tip
(2) Measurements of the cross-section hardness.
Fig.3 shows results of the cross-section hardness with crack after IHSI treatment. As shown Fig.3, 270~296HV hardness area was confirmed, but high hardness area wasn’t identified.
Fig.3 Hardness measurements of the crack tip
From these results confirmed that IHSI for pipe with crack wasn’t any harmed to crack and joint scope of crack deepness: 3/8t, length: inner face 90°
②Confirm that IHSI treatment improves stress of crack tip and reduces the progression of the crack
Implemented IHSI on the sample which was forcibly made deeply-cracked like a SCC situation by MgCl2. After that, it was immersed in MgCl2 solution again. Then confirmed that crack was no progress by this test.
②Confirm that IHSI treatment improves stress of crack tip and reduces the progression of the crack
● Test conditions
Test model: Diameter 300A, Thickness 19.75mm, Material SUS316LTP
Form of crack : Deepness: 3/8t, length: inner face 90°by MgCl2
IHSI condition: one-time IHSI treatment (about 1.5 times temperature difference of test parameters ⊿Treq.)
Time of MgCl2 : Crack introduced 120hours, penetrated 120hours after IHSI.
Form of crack : Deepness: 3/8t, length: inner face 90°by MgCl2
IHSI condition: one-time IHSI treatment (about 1.5 times temperature difference of test parameters ⊿Treq.)
Time of MgCl2 : Crack introduced 120hours, penetrated 120hours after IHSI.
Crack’s deepness change was examined by phased array UT after IHSI and MgCl2, and confirmed that the progress of crack was suppressed by IHSI. Fig.4 shows distribution of crack deepness before MgCl2, Fig.5 shows distribution of crack deepness after MgCl2 .
As shown these results, the sample was immersed in MgCl2 for 120 hours similar environment of crack introduce after IHSI. However, the average deepness of crack was 5.25mm before MgCl2. On the other hand, the average deepness of crack was 5.22mm after MgCl2. So it indicated that distinguished deepness of crack wasn’t confirmed.
As shown these results, the sample was immersed in MgCl2 for 120 hours similar environment of crack introduce after IHSI. However, the average deepness of crack was 5.25mm before MgCl2. On the other hand, the average deepness of crack was 5.22mm after MgCl2. So it indicated that distinguished deepness of crack wasn’t confirmed.
Fig.4 Ultrasonic Test result before MgCl2
Fig.5 Ultrasonic Test result after MgCl2
(2) Results of the cross-section observations (after MgCl2)
Fig.6 shows results of the cross-section observation after MgCl2.
As these results, we confirmed that main crack was generally thick and crack tip was expanded and edged down by IHSI treatment. Additionally, we confirmed that comparatively low deepness crack (named sub-crack) aside from main crack. Those cracks were generally narrow and crack tips were not clear expanded compared with main cracks.
Above that, main crack was reduced stress of the tip, and confirmed that crack progress was inhibited. But, we couldn’t definitely confirm effects of IHSI for low deepness cracks. This time to introduce deep crack, we forcibly changed stress distribution from As welded so that it seemed to be given more cracks in a wide sphere than actual SCC. Therefore, a number of low deepness cracks were submerged by deep main cracks. And it came to be considered that effect of stress reduction wasn’t confirmed clearly by IHSI.
Those results were arisen from strict conditions than actual, but we thought that we couldn’t deny the possibilities of causing such as low deepness cracks after IHSI treatment.
Fig.6 shows results of the cross-section observation after MgCl2.
As these results, we confirmed that main crack was generally thick and crack tip was expanded and edged down by IHSI treatment. Additionally, we confirmed that comparatively low deepness crack (named sub-crack) aside from main crack. Those cracks were generally narrow and crack tips were not clear expanded compared with main cracks.
Above that, main crack was reduced stress of the tip, and confirmed that crack progress was inhibited. But, we couldn’t definitely confirm effects of IHSI for low deepness cracks. This time to introduce deep crack, we forcibly changed stress distribution from As welded so that it seemed to be given more cracks in a wide sphere than actual SCC. Therefore, a number of low deepness cracks were submerged by deep main cracks. And it came to be considered that effect of stress reduction wasn’t confirmed clearly by IHSI.
Those results were arisen from strict conditions than actual, but we thought that we couldn’t deny the possibilities of causing such as low deepness cracks after IHSI treatment.
Fig.6 Cross-sectional observation
③Confirm that the effect of reducing residual stress around crack by IHSI
To confirm efficacy of IHSI around inner pipe’s surface main crack, we checked the existence of new crack from two samples. One is implemented IHSI with EDM cask, and the other is no IHSI treatment sample (As weld).
To confirm efficacy of IHSI around inner pipe’s surface main crack, we checked the existence of new crack from two samples. One is implemented IHSI with EDM cask, and the other is no IHSI treatment sample (As weld).
●Test conditions
Test model: Diameter 300A, Thickness 19.75mm, Material SUSF316(LC)
Form of crack: EDM crack (deepness: 3/8t, lengths: inner face 90°)
IHSI condition: one-time IHSI treatment (about 1.5 times temperature difference of test parameters ⊿Treq.)
Time of MgCl2: penetrated 120hours after IHSI.
Test model: Diameter 300A, Thickness 19.75mm, Material SUSF316(LC)
Form of crack: EDM crack (deepness: 3/8t, lengths: inner face 90°)
IHSI condition: one-time IHSI treatment (about 1.5 times temperature difference of test parameters ⊿Treq.)
Time of MgCl2: penetrated 120hours after IHSI.
(1)Results of inner PT observations.
Fig.7 shows results of PT observations of inner pipe’s surface about As weld and IHSI samples. As shown Fig.7, As weld sample was found new cracks around the weld zone with or without EDM cracks.
Meanwhile, IHSI sample was found new cracks only close to EDM cracks. From these reasons, IHSI treatment improves welding residual stress expect for crack part, and these results indicate that the effect of that IHSI is equal to preventive maintenance one.
Fig.7 shows results of PT observations of inner pipe’s surface about As weld and IHSI samples. As shown Fig.7, As weld sample was found new cracks around the weld zone with or without EDM cracks.
Meanwhile, IHSI sample was found new cracks only close to EDM cracks. From these reasons, IHSI treatment improves welding residual stress expect for crack part, and these results indicate that the effect of that IHSI is equal to preventive maintenance one.
Fig.7 Penetrant test results inside surface after MgCl2
(2)Results of the cross-section observations
Fig.8 shows results of the cross-section observations of EDM crack tip of IHSI and As Welded after MgCl2.
As shown Fig.8, in case of As weld sample was confirmed that small cracks are come up from crack tip, however IHSI sample wasn’t confirmed any cracks such like As weld sample. Therefore, IHSI seems to improve the crack tip stress too.
From those things indicate that effect of IHSI close to cracks tend to fall in, compared to non crack area.
Fig.8 shows results of the cross-section observations of EDM crack tip of IHSI and As Welded after MgCl2.
As shown Fig.8, in case of As weld sample was confirmed that small cracks are come up from crack tip, however IHSI sample wasn’t confirmed any cracks such like As weld sample. Therefore, IHSI seems to improve the crack tip stress too.
From those things indicate that effect of IHSI close to cracks tend to fall in, compared to non crack area.
Fig.8 Cross-sectional observation of the crack tip after MgCl2
④Confirm that IHSI has equivalent effect of preventive maintenance IHSI on non crack area.
To confirm efficacy of IHSI aside from inner pipe’s surface crack, we affirmed the existence of new crack. The crack was introduced by MgCl2 after IHSI.
To confirm efficacy of IHSI aside from inner pipe’s surface crack, we affirmed the existence of new crack. The crack was introduced by MgCl2 after IHSI.
● Test conditions
Test Model: Diameter 300A, Thickness 19.75mm, Material SUSF316(LC)
Form of crack: EDM crack (deepness: 3/8t, lengths: inner face 90°)
IHSI condition: Once implement IHSI (about 1.5 times temperature difference of test parameters ⊿Treq.)
Time of MgCl2: 120hours after IHSI.
Test Model: Diameter 300A, Thickness 19.75mm, Material SUSF316(LC)
Form of crack: EDM crack (deepness: 3/8t, lengths: inner face 90°)
IHSI condition: Once implement IHSI (about 1.5 times temperature difference of test parameters ⊿Treq.)
Time of MgCl2: 120hours after IHSI.
(1) Observations of inner face PT
Fig.9 shows results of observations of inner face PT which was penetrated by MgCl2 after IHSI. As shown Fig.9, new crack wasn’t confirmed around non crack area. However, on part of the crack, we found new crack existence. From this, we can consider that effect of countermeasure IHSI around non crack area is equal to preventive maintenance IHSI.
Fig.9 shows results of observations of inner face PT which was penetrated by MgCl2 after IHSI. As shown Fig.9, new crack wasn’t confirmed around non crack area. However, on part of the crack, we found new crack existence. From this, we can consider that effect of countermeasure IHSI around non crack area is equal to preventive maintenance IHSI.
Fig.9 Penetrant test results inside surface after MgCl2
(2) Results of residual stress measurement
Fig.10 shows inner face axial welding residual stress of EDM crack part and non crack part on the identical joint. As shown Fig.10, part of crack wasn’t enough improvement to side of compression near the crack area. However, we confirmed the residual stress was improved enough to compression stress about 100 MPa even around the depo.
Fig.10 shows inner face axial welding residual stress of EDM crack part and non crack part on the identical joint. As shown Fig.10, part of crack wasn’t enough improvement to side of compression near the crack area. However, we confirmed the residual stress was improved enough to compression stress about 100 MPa even around the depo.
Fig.10 Residual Stress distribution for inside surface
From this, we can consider that effect of IHSI for non crack part on the identical joint is equal to preventive maintenance IHSI.
5. Examples of Application
Mitigation IHSI for pipes with crack was performed successfully at Japanese BWR plant in 2011
6. Reference
- [1]T.Sasayama and S.Hongo “Confirmation test of IHSI for pipe with crack” , 7th Annual Conference of the Japan Society of Maintenology, Omaezaki , pp.379 – 383, July 2010.( in Japanese)
7. Contact
Japan Society of maintenology (ejam@jsm.or.jp)
