氏名　ANCHALEE SAENGSAI
学位の種類　博士（工学）
学位記番号　博甲第629号
学位授与の日付　平成24年8月31日
学位論文題目　SCC and Fretting Fatigue Behavior of Sensitized SUS304 Stainless Steel in Pressurized High Temperature Water　（高温高圧水中下の鋭敏化SUS304ステンレス鋼のSCCおよびフレッティング疲労挙動）
論文審査委員
　主査　教授　武藤　睦治
　副査　教授　井原　郁夫
　副査　准教授　宮下　幸雄
　副査　講師　大塚　雄市
　副査　新潟大学　工学部　准教授　大木　基史

• 論文目次
• 論文要旨
• 論文審査の結果の要旨

［平成24（2012）年度博士論文題名一覧］ ［博士論文題名一覧］に戻る．

Table of Contents　page
Chapter 1:　Introduction
　1.1　Introduction to Stainless Steels　p.1
　1.2　Introduction to Nuclear Powerplants　p.5
　1.3　Degradation of Austenitic Stainless Steels by Corrosion in Nuclear Power Applications　p.9
　1.4　License Renewal and Ageing Management of Nuclear Powerplants in Japan　p.12
　1.5　Research of SCC of Austenitic Stainless Steels under BWR environments　p.14
　1.5.1　SCC growth and reference curves　p.14
　1.5.2　SCC initiation and early propagation　p.17
　1.6　Research of Corrosion Fatigue of Austenitic Stainless Steels under BWR environments　p.19
　1.7　Scope of the Present Work　p.20
　1.8　References　p.23
Chapter 2:　Nucleation of Small Surface Pre-Crack in Sensitized SUS304 Stainless Steel by Fretting Fatigue under Pressurized High Temperature Water
　2.1　Introduction　p.28
　2.2　Material and Specimen Preparation
　2.2.1　Material　p.30
　1.5.2　Specimen preparation　p.33
　2.3　Testing Machines and Equipment
　2.3.1　Fatigue testing machine　p.34
　2.3.2　Autoclave　p.35
　2.3.3　Fretting fatigue assembly　p.37
　2.4　Modification of Fretting Fatigue Technique for SUS304 Stainless Steel　p.38
　2.5　Production of Oxide Layer on SUS304 Stainless Steel Specimen　p.40
　2.6　Procedure for Production of Pre-Crack Specimen
　2.6.1　Fretting fatigue in an elastic region　p.47
　2.6.2　Fretting fatigue in a plastic region　p.47
　2.6.3　Interrupted fretting fatigue　p.48
　2.7　Results and Discussions
　2.7.1　Fretting fatigue in an elastic region　p.48
　2.7.2　Fretting fatigue in a plastic region　p.50
　2.7.3　Interrupted fretting fatigue　p.52
　2.8　Conclusions　p.53
　2.9　References　p.54
Chapter 3:　SCC Behavior of Sensitized SUS304 Stainless Steel under Recirculated Pressurized High Temperature Water
　3.1　Introduction　p.58
　3.2　Experimental Procedure
　3.2.1　Material and specimen preparation　p.59
　3.2.2　Testing system　p.62
　3.2.3　Models and equations for calculation　p.64
　3.2.4　Testing condition　p.65
　3.2.5　Surface observation　p.65
　3.3　Results and Discussions
　3.3.1　Observation of small surface pre-cracks before constant load test　p.65
　3.3.2　Constant load test　p.67
　3.3.3　Constant load test with contact pad equipped　p.74
　3.4　Conclusions　p.75
　3.5　References　p.75
Chapter 4:　SCC Behavior of Sensitized SUS304 Stainless Steel under Pressurized High Temperature Water with High Oxygen Content and High Conductivity
　4.1　Introduction　p.58
　4.2　Experimental Procedure
　4.2.1　Material and specimen preparation　p.80
　4.2.2　Testing environment　p.83
　4.2.3　Models and equations for calculation　p.84
　4.2.4　Testing condition　p.85
　4.2.5　Surface observation　p.86
　4.3　Results and Discussions
　4.3.1　Observation of small surface pre-cracks before constant load test　p.86
　4.3.2　Constant load test　p.87
　4.3.3　Effect of pre-existing oxide layer　p.91
　4.4　Conclusions　p.93
　4.5　References　p.94
Chapter 5:　Fretting Fatigue Behavior of Sensitized SUS304 Stainless Steel under Pressurized High Temperature Water with High Oxygen Content and High Conductivity
　5.1　Introduction　p.97
　5.2　Experimental Procedure
　5.2.1　Material and specimen preparation　p.99
　5.2.2　Testing system　p.101
　5.2.3　Testing condition　p.104
　5.2.4　Surface observation　p.105
　5.3　Results and Discussions
　5.3.1　Plain fatigue　p.105
　5.3.2　fretting fatigue　p.106
　5.3.3　Effect of stress ratio on fretting fatigue behavior　p.111
　5.4　Conclusions　p.113
　5.5　References　p.114
Chapter 6:　Conclusion and Suggestion for Future Work　p.117

　Austenitic stainless steel is one of the most common materials used in nuclear power applications because of its good strength with excellent corrosion resistance. However, after long term service in pressurized high temperature water, the material was subjected to stress corrosion cracking which leading to existing cracks in the service components, especially in the heat affected zone of the weldment. The existing cracks in aged nuclear components became social concern about structural reliability because these cracks might propagate in the future when service lifetime of nuclear power plant is planned to be extended. Therefore, propagation mechanism of these existing cracks must be clarified. In this research, to verify the propagation behavior of existing small cracks, SCC and fretting fatigue behavior of sensitized SUS304 stainless steel in pressurized high temperature water at 7.3 MPa and 288 ℃, which is the operating condition for BWR, was studied.　For studying SCC behavior, small surface intergranular cracks similar to those detected in the heat affected zone of the weldment of primary water recirculation loop for BWR were introduced on the specimen surface by using a fretting fatigue technique followed by constant load tests at various initial stress intensity factors （Ki） to investigate failure behavior of this material under constant load condition. For studying fretting fatigue behavior under pressurized high temperature water at 7.3 MPa and 288 ℃, tests were conducted and results were compared with fatigue without fretting. Effect of stress ratio on fretting fatigue behavior was also studied.
Chapter 1 “Introduction”: Overview of stainless steels, type of stainless steel used in nuclear power applications and its degradation under pressurized high temperature water environment was made. Overview of nuclear powerplant in Japan and its license renewal and ageing management was also indicated. Available researches on SCC propagation, SCC initiation and corrosion fatigue of austenitic stainless steels in pressurized high temperature which simulated BWR environment were reviewed. Importance and scope of the current research were addressed.
Chapter 2 “Nucleation of Small Surface Pre-Crack on the Sensitized SUS304 Stainless Steel by Fretting Fatigue in Pressurized High Temperature Water”: To study SCC behavior of small cracks similar to those detected in the heat affected zone of the weldment of nuclear power components, sensitized SUS304 stainless steel specimens with small surface pre-crack were required. For this purpose, fretting fatigue technique with combined effect of high tangential stress due to fretting fatigue and pressurized high temperature water was successfully applied for nucleating surface intergranular cracks along the outer edge of contact region. Fretting fatigue at stress amplitude of 190MPa and 10% of number of cycle to failure applied, small surface crack with crack depth of about 30?m （one to two grains） and surface length of about 240?m （about seven to eight grains） could be produced. These small surface pre-cracked specimens were later used for investigating subsequent SCC behavior in Chapter 3 and Chapter 4.
Chapter 3 “SCC Behavior of Small Surface Pre-Crack on Sensitized SUS304 Stainless Steel in Pressurized High Temperature Water”: Study of SCC behavior of sensitized SUS304 stainless steels with small surface pre-crack was conducted in pressurized high temperature water at 7.3 MPa and 288 ℃. Constant load tests at various initial stress intensity factors （Ki） were carried out and results showed that the threshold value for small SCC crack propagation was lower than 4.1 MPa・m1/2, which was significantly low compared to that for long SCC crack （10-20 MPa・m1/2）. Under tensile loading condition, cracking of pre-existing oxide layer on specimen surface occurred due to mismatch deformation between oxide layer and stainless steel matrix and leading to exposure of fresh metal to corrosive environment where further corrosion could occur. When the mechanism repeated, crack advance was expected to occur. Further study of this mechanism is required.
Chapter 4 “SCC Behavior of Small Surface Pre-Crack on Sensitized SUS304 Stainless Steel in Simulated BWR Environment”: Study of SCC behavior of sensitized SUS304 stainless steels with small surface pre-crack was conducted in pressurized high temperature water at 7.3 MPa and 288 ℃ where dissolved oxygen amount and conductivity was controlled to simulate BWR environment. Constant load tests at various initial stress intensity factors （Ki） were carried out. The specimens tested at Ki higher than 4.8 MPa・m1/2 were fractured in tensile overload manner, while those tested at Ki lower than 4.7 MPa・m1/2 could survive for more than 1000 h. Accelerated tests by contact pad equipped on surface pre-crack specimen during constant load test did not give significant different result. Specimen tested at Ki of 5.0 MPa・m1/2 were fractured in tensile overload manner, while those tested at Ki equal or lower than 4.7 MPa・m1/2 could survive longer than 200h.
Chapter 5 “Fretting Fatigue Behavior of Sensitized SUS304 Stainless Steel in Pressurized High Temperature Water”: Fretting fatigue behavior of sensitized SUS304 stainless steel under pressurized high temperature water at 7.3 MPa and 288 ℃ was investigated at a contact pressure of 100 MPa and a frequency of 20 Hz. From the experimental result, fatigue strength for fretting fatigue was almost coincided with that of plain fatigue due to cyclic hardening of material during specimen preparation. Different crack nucleation mechanisms were observed according to level of stress amplitude applied. High stress ratio reduced fatigue strength due to longer exposure time of crack tip to corrosive environment at high mean stress.
Chapter 6 “Conclusions and Suggestions for Future Work”: The main conclusions were summarized and suggestions for future works were addressed.

　本論文は、「SCC and Fretting Fatigue Behavior of Sensitized SUS304 Stainless Steel in Pressurized High Temperature Water （高温高圧水中下の鋭敏化SUS304ステンレス鋼のSCC及びフレッティング疲労挙動）」と題し、6章より構成されている。
第1章「Introduction」では、本研究に関わる研究動向を概説するとともに、本研究の目的と範囲を述べている。
　第2章「Nucleation of Small Surface Pre-crack in the Sensitized SUS304 Stainless Steel by Fretting Fatigue under Pressurized High Temperature Water」では、フレッティングによる局所的な高応力の繰り返しを利用して、高温高圧下で少数回のフレッティング疲労繰返しを与えることにより、局所的に微小なSCCき裂を導入する方法を開発している。
　第3章「SCC Behavior of Sensitized SUS304 Stainless Steel under Recirculated Pressurized High Temperature Water」では、溶存酸素量を一定に保った循環高温高圧水下で、微小き裂を導入した鋭敏化ステンレス鋼SUS304のSCC試験を行い、微小き裂のSCCき裂進展限界が、長いき裂のそれよりも顕著な低下を示すことなどを明らかにしている。
　第4章「SCC Behavior of Sensitized SUS304 Stainless Steel under Pressurized High Temperature Water with High Oxygen Content and High Conductivity」では、高温高圧水を循環させず、溶存酸素量が次第に高くなる厳しい環境下で、微小き裂のSCCき裂進展試験を行い、溶存酸素量の増加に伴いSCC挙動が顕著となり、またき裂面の酸化も顕著となることなどを明らかにしている。
　第5章「Fretting Fatigue Behavior of Sensitized SUS304 Stainless Steel under Pressurized High Temperature Water with High Oxygen Content and High Conductivity」では、部材間の接触を伴う場合の疲労挙動を調べるため、高温高圧水中でのフレッティング疲労試験を行い、応力比が高い方が疲労強度が低く、動的SCCでの強度低下と同様の挙動を示すこと、機械的振動に相当する繰返し周波数が数Hz以上の条件では、フレッティング繰返しの効果の方がSCCの影響よりも支配的であることなどを明らかにしている。
　第6章「Conclusion and Suggestion for future work」では、以上の研究の結果を総括的にまとめるとともに、将来の展望について述べている。
よって、本論文は工学上及び工業上貢献するところが大きく、博士（工学）の学位論文として十分な価値を有するものと認める。