STS 316L의 재결정법에 의한 2매개변수 파괴인성평가 및 전기화학적 특성

다국어 초록 (Multilingual Abstract)

Used in higher value-added heavy industrial businesses such as oil plant, shipbuilding industry and power plants, austenitic stainless steels have problems like pitting and micro crack which occur because of intergranular corrosion at material surface. Those causes severe damage to the whole structures nevertheless high toughness and mechanical strength in material. To minimize these defects, STS 316L which has low content of C and some of Mo, is preferred to application for offshore structures nowadays.
Fracture toughness, Jc and critical CTOD, δc by R-curve method, stretched zone width and the fracture behaviors of crack tip are experimentally estimated. The level of the constraint effects, A2 at a crack tip in CT specimen during a practical fracture toughness test are also determined by CTOD measured from author technique including δ5 technique, Experimentally, any deformation measurement close to the crack tip like CTOD yield a displacement that is a function of J and A2, In this paper, A2 is determined using the several CTOD or displacement v(r, θ), Using the CTOD determined from two marks, one at r=1, 1.5, 2, 2.5, 3mm above and at r=1, 1.5, 2, 2.5, 3mm below the crack tipfrom the crack tip opening on CT specimen. The CTOD between 1~3mm above and below at a crack tip, are used for determining A2 displacement δ(r, θ) within the range of the crack tip region, 0.9<r<4.0. With these processes, the fracture failure criteria that measures and evaluated both structures safety and durability can be predicted.
Moreover, plastic boundary has been visualized and stress conditions near the crack tip was represented using recrystallization heat treatment. Itis suggested that the constraint effects A2 evaluation has the limit locations of near crack tip opening displacement.
Fracture toughness, constraint effects A2 and plastic boundary configuration by recrystallization for STS 316L 10% side grooved CT spcimen have been studied in the plane strain conditions under mode I loading, and the results are as follows:
(1) The fracture toughness Jc by R-curve method was 414KJ/m2,
(2) A prominent location to measure effective constraint effects A2 at Jc is r=2mm, θ=90o from crack tip and constraint effects lA2l is 5.33.
(3) The plastic boundary of true strain εt = 0.05mm/mm by recrystallization heat treatment has been visually investigated, and with the comparison of estimation for constraint effects A2, the ranges of proper displacement measurement point at near crack tip were between 0mm and 1mm in x axis, between 1mm to 3mm in y axis.
Also, in case of offshore structures, they are exposed to both the severe corrosion environment with fracture of passive film by chloride ion and the stress of dynamic external forces and fatigue by wave and tide, Consequently, stress corrosion cracking can be occurred during the long service time. Electrochemical and slow strain rate tests have been used to evalate the durability on stress corrosion cracking and hydrogen embrittlement of STS 316L.
The results of electrochemical experiments and strain rate test for STS 316L in seawater are as follows :
(1) The polarization trend for STS 316L shows the effects of concentration polarization due to dissolved oxygen reduction reaction and activation polarization due to hydrogen gas generation. The turning point of two reactions in the polarization curve presented -0.92V(vs, Ag/AgCl).
(2) The dissolved oxygen reduction reaction is corresponding to corrosion protection potential. The results of cathodic polarization experiments, the low current density is observed at the range of -0.56V ~ -0.92V(vs, Ag/AgCl) in seawater.

목차 (Table of Contents)

• Abstract v
• List of Figures ix
• List of Table xviii
• 1. 서 론 1
• 2. 이론적 배경 4
• 2.1 탄소성 파괴인성과 수속효과 4
• 2.2 재결정 10
• 2.2.1 재결정 원리와 응용 10
• 2.2.2 재결정의 방법 12
• 2.3 분극 및 전기화학적 측정법 14
• 2.3.1 분극의 정의와 종류 14
• 2.3.2 전기화학적 부식속도 측정법 18
• 2.4 응력부식균열 20
• 2.4.1 응력부식균열 기구 20
• 2.4.2 응력부식균열에 영향을 미치는 인자들 24
• 3. STS 316L의 파괴거동 27
• 3.1 서언 27
• 3.2 시험편 및 실험방법 28
• 3.2.1 시험재료 특성 및 형상 28
• 3.2.2 실험방법 32
• 3.3 실험결과 및 고찰 41
• 3.3.1 기계적 특성평가 41
• 3.3.2 R-곡선법에 의한 파괴인성 Jc 평가 45
• 3.4 결론 59
• 4. STS 315L의 구속효과 A2 평가 60
• 4.1 서언 60
• 4.2 실험방법 61
• 4.3 실험결과 및 고찰 66
• 4.3.1 균열선단 근방 변위 측정 66
• 4.3.2 균열선단 근방 구속효과 A2 평가 80
• 4.4 결론 98
• 5. STS 316L의 재결정법에 의한 소성역 99
• 5.1 서언 99
• 5.2 시험편 및 실험방법 100
• 5.2.1 인장 및 CT 시험편 100
• 5.2.2 실험방법 102
• 5.3 실험결과 및 고찰 106
• 5.3.1 재결정 온도와 시간 결정 106
• 5.3.2 진변형율과 재결정 조직 110
• 5.3.3 재결정법에 의한 소성역 한계 113
• 5.3.4 소성한계와 구속효과 A2 120
• 5.4 결론 126
• 6. STS 316L의 해수환경 하에서 전기화학적 시험에 의한 최적방식전위 결정에 관한 연구 128
• 6.1 서언 128
• 6.2 시험편 및 실험방법 129
• 6.2.1 시험용 재료 129
• 6.2.2 실험방법 129
• 6.3 실험결과 및 고찰 131
• 6.4 결론 160
• 7. STS 316L의 저변형율 인장시험에 의한 응력부식균열과 수소취하 저항성 평가 162
• 7.1 서언 162
• 7.2 시험편 및 실험방법 163
• 7.2.1 시험용 재료 및 형상 163
• 7.2.2 실험방법 163
• 7.3 실험결과 및 고찰 166
• 7.3.1 저변형율 인장속도 결정 166
• 7.3.2 저변형율 인장시험 고찰 171
• 7.4 결론 194
• 8. 총괄 196
• 참고문헌 200