표제지
목차
Abstract 7
1. 서론 23
2. 이론적 배경 26
2.1. 탄소성 파괴인성과 구속효과 26
2.2. 재결정 32
2.2.1. 재결정 원리와 응용 32
2.2.2. 재결정의 방법 34
2.3. 분극 및 전기화학적 측정법 36
2.3.1. 분극의 정의와 종류 36
2.3.2. 전기화학적 부식속도 측정법 40
2.4. 응력부식균열 42
2.4.1. 응력부실균열(stress corrosion cracking, SCC) 기구 42
2.4.2. 응력부식균열에 영향을 미치는 인자들 46
3. STS 316L의 파괴거동 49
3.1. 서언 49
3.2. 시험편 및 실험방법 50
3.2.1. 시험재료의 화학적 조성 50
3.2.2. 실험방법 54
3.3. 실험결과 및 고찰 63
3.3.1. 기계적 특성평가 63
3.3.2. R-곡선법에 의한 파괴인성 Jc 평가 67
3.4. 결론 81
4. STS 316L의 구속효과 A₂평가 82
4.1. 서언 82
4.2. 실험방법 83
4.3. 실험결과 및 고찰 88
4.3.1. 균열선단 근방 변위 측정 88
4.3.2. 균열선단 근방 구속효과 A₂ 평가 102
4.4. 결론 120
5. STS 316L의 재결정법에 의한 소성역 121
5.1. 서언 121
5.2. 시험편 및 실험방법 122
5.2.1. 인장 및 CT 시험편 122
5.2.2. 실험방법 124
5.3. 실험결과 및 고찰 128
5.3.1. 재결정 온도와 시간 결정 128
5.3.2. 진변형율과 재결정 조직 130
5.3.3. 재결정법에 의한 소성역 한계 132
5.3.4. 소성한계와 구속효과 A₂ 135
5.4. 결론 140
6. STS 316L의 해수환경 하에서 전기화학적 시험에 의한 최적방식전위 결정에 관한 연구 142
6.1. 서언 142
6.2. 시험편 및 실험방법 143
6.2.1. 시험용 재료 143
6.2.2. 실험방법 143
6.3. 실험결과 및 고찰 145
6.4. 결론 174
7. STS 316L의 저변형율 인장시험에 의한 응력부식균열과 소수취화 저항성 평가 176
7.1. 서언 176
7.2. 시험편 및 실험방법 177
7.2.1. 시험재료 및 형상 177
7.2.2 실험방법 177
7.3. 실험결과 및 고찰 180
7.3.1. 저변형율 인장속도 결정 180
7.3.2. 저변형율 인장실험 고찰 185
7.4. 결론 208
8. 총괄 210
참고문헌 214
Chapter 3 20
Table 3.1. Chemical compositions of STS 316L 50
Table 3.2. Tensile test results of STS 316L 63
Table 3.3. Mechamical properties of STS 316L 68
Table 3.4. Test input data for precrack at crack tip of CT specimen using MAX program 68
Table 3.5. J-integral and crack growth △a on CT specimen using R-curve method 73
Table 3.6. SZW measured by SEM according to different LLD 76
Chapter 4 21
Table 4.1. Stress exponents of high order terms and material constants for A₂ value analysis 87
Table 4.2. Measuring values of surface crack growth and CTOD according to gradually increasing LLD 89
Table 4.3. Results of displacement, δ value according to different y-direction measuring positions at mean crack tip 91
Table 4.4. Results of displacement, δ value according to different y-direction measuring positions at 1mm ahead of crack tip 92
Table 4.5. Results of displacement, δ value according to different y-direction measuring positions at 2mm ahead of crack tip 93
Table 4.6. Results of displacement, δ value according to different y-direction measuring positions at 3mm ahead of crack tip 94
Table 4.7. Results of A₂ value according to different y-direction measuring positions at crack tip 103
Table 4.8. Results of A₂ value according to different y-direction measuring positions at 1mm ahead of crack tip 104
Table 4.9. Results of A₂ value according to different y-direction measuring positions at 2mm ahead of crack tip 105
Table 4.10. Results of A₂ value according to different y-direction measuring positions at 3mm ahead of crack tip 106
Chapter 5 22
Table 5.1. Results of calculated grain size about recrystallization of various true strains under annealing heat treatment (930˚C, 30hrs) of STS 316L tensile test specimen 131
Table 5.2. Result of estimated plastic area near crack tip according to load line displacement of STS 316L CT specimen 134
Chapter 6 22
Table 6.1. Results obtained from Tafel analysis of STS 316L in natural sea water 149
Chapter 7 22
Table 7.1. Comparison of mechanical properties by SSRT in between air and sea water at 0.005mm/min strain rate 189
Table 7.2. Optimum corrosion protection potential obtained from SSRT in natural sea water 207
Chapter 2 11
Fig. 2.1. Opening stress in front of a crack tip 29
Fig. 2.2. Concentration polarization curve 38
Fig. 2.3. Combined polarization curve 39
Fig. 2.4. Polarization curves explained by Tafel's extrapolation 41
Fig. 2.5. Schematic diagram of the required conditions for SCC 43
Fig. 2.6. Schematic of processes occurring for SCC in aqueous solution containing chloride 45
Fig. 2.7. Schematic representation of carbide precipitation at a grain boundary during sensitization to intergranular corrosion in stainless steel 48
Chapter 3 11
Fig. 3.1. The orientation of CT and tensile test specimens 52
Fig. 3.2. Dimensions and configurations of tensile test and CT specimens 53
Fig. 3.3. Schematic diagram of the fracture toughness test system. 57
Fig. 3.4. R-curve method 61
Fig. 3.5. JQ determination by R-curve method 62
Fig. 3.6. Engineering stress-strain curves for STS 316L tensile test specimens 64
Fig. 3.7. (a) Engineering and true stress-strain curves, (b) True stress-strain curve for STS 316L tensile test specimen 66
Fig. 3.8. Load vs load line displacement curves with increasing LLD from 1mm to 4mm to determine Jc value using R-curve method 69
Fig. 3.9. Load vs load line displacement curves with increasing LLD from 5mm to 8mm to determine Jc value using R-curve method 70
Fig. 3.10. Load vs load line displacement curves with increasing LLD from 9mm to 10mm to determine Jc value using R-curve method 71
Fig. 3.11. J-△a curve for STS 316L CT specimen to determine Jc 74
Fig. 3.12. Optical microscope fractography on facture surface for measuring stretched zone width 75
Fig. 3.13. Fracture surface micro-configuration and SZW fractography at LLD 4mm 77
Fig. 3.14. Fracture surface micro-configuration and SZW fractography at LLD 5mm 78
Fig. 3.15. Fracture surface micro-configuration and SZW fractography at LLD 8mm 79
Chapter 4 12
Fig. 4.1. Measuring points of displacement, δ to calculate the constraint effect, A₂ value 85
Fig. 4.2. CTOD vs. surface crack growth curve for CT specimen to determine δc 90
Fig. 4.3. Displacement vs. LLD curves according to different y-direction measuring position at crack tip and 1mm from crack front 96
Fig. 4.4. Displacement vs. LLD curves according to different y-direction measuring position at 2mm and 3mm from crack front 97
Fig. 4.5. Displacement vs. LLD curves according to different x-direction measuring position at 1mm and 1.5mm of parallel line with crack propagation 99
Fig. 4.6. Displacement vs. LLD curves according to different x-direction measuring position at 2mm and 2.5mm of parallel line with crack propagation 100
Fig. 4.7. Displacement vs. LLD curves according to different x-direction measuring position at 3mm of parallel line with crack propagation 101
Fig. 4.8. -A₂ vs. LLD curves according to different x-direction measuring position at 1mm and 1.5mm of parallel line with crack propagation 107
Fig. 4.9. -A₂ vs. LLD curves according to different x-direction measuring position at 2mm and 2.5mm of parallel line with crack propagation 108
Fig. 4.10. -A₂ vs. LLD curves according to different x-direction measuring position at 3mm of parallel line with crack propagation 109
Fig. 4.11. -A₂ vs. LLD curves according to different y-direction measuring position at crack tip and 1mm from crack front 111
Fig. 4.12. -A₂ vs. LLD curves according to different y-direction measuring position at 2mm and 3mm from crack front 112
Fig. 4.13. J-integral vs. -A₂ vs. LLD curves according to different y-direction measuring position at 1mm and 1.5mm of parallel line with crack propagation under 3mm~5mm LLD control 114
Fig. 4.14. J-integral vs. -A₂ vs. LLD curves according to different y-direction measuring position at 2mm and 2.5mm of parallel line with crack propagation under 3mm~5mm LLD control 115
Fig. 4.15. J-integral vs. -A₂ vs. LLD curves according to different y-direction measuring position at 3mm of parallel line with crack propagation under 3mm~5mm LLD control 116
Fig. 4.16. Critical distance zone by chao vs. -A₂curves according to different y-direction(θ=90˚) measuring position at load line displacement of 4mm 118
Chapter 5 14
Fig. 5.1. Dimensions and configuration of true stress-strain tensile test specimeu 123
Fig. 5.2. Dimesions and configuration of CT specimen after fracture toughness test for observing plastic zone by recrystalization 123
Fig. 5.3. Decision of heat treatment temperature on the gamma loop of stainless steel 125
Fig. 5.4. Decision of heat treatment time for recrystallization 126
Fig. 5.5. Microstructures of changed temperature due to determination of recrystallizstion temperature for 24hrs heat treatment of STS 316L(내용없음) 15
Fig. 5.6. Recrystallized microstuctures to determine recrystallization heat tretment time at 930℃ fo STS 316L(내용없음) 15
Fig. 5.7. Microstrutures of carious true strains, εt under recrystallization annealing heat treatment. (930℃, 30hrs)(이미지참조)(내용없음) 15
Fig. 5.8. Plastic region near crack tip according to load line displacement of STS 316L CT specimen(내용없음) 15
Fig. 5.9. Microsturctures of plastic region mear crack tip at surface of net thickness under LLD 4mm(내용없음) 15
Fig. 5.10. Microsturctures of plastic region mear crack tip at quater of net thickness under LLD 4mm (내용없음) 15
Fig. 5.11. Microsturctures of plastic region mear crack tip at center of net thickness under LLD 4mm(내용없음) 15
Fig. 5.12. Compared plastic zone with measuring points of constraint effect A₂for STS 316L(내용없음) 15
Fig. 5.13. A₂ behaviors of both plastic and included elastic regions for STS 316L 138
Chapter 6 16
Fig. 6.1. Experimental apparatus for electrochemical tests 144
Fig. 6.2. Variation of open circuit potential for STS 316L specimen for 84,600s in natural sea water 146
Fig. 6.3. Anodic polarization curve for STS 316L specimen in natural sea water 147
Fig. 6.4. Cathodic polarization curve for STS 316L specimen in natural sea water 148
Fig. 6.5. Polarization curve to Tafel analysis of STS 316L specimen in natural sea water 150
Fig. 6.6. Time-current density curves during potentiostatic experiments at range of -0.2V ~ 0.4V for STS 316L in natural sea water 152
Fig. 6.7. Time-current density curves during potentiostatic experiments at range of 0.6V ~ 1.3V for STS 316L in natural sea water 153
Fig. 6.8. Time-current density curves during potentiostatic experiments at range of 1.4V ~ 2.0V for STS 316L in natural sea water 155
Fig. 6.9. Comparison of current density curves after 1,200s potentiostatic experiments for STS 316L at various anodic potential in natural sea water 156
Fig. 6.10. Surface photographs after 1,200s potentiostatic experiments for STS 316L at various anodic potential in natural sea water 157
Fig. 6.11. Surface photographs after 1,200s potentiostatic experiments for STS 316L at -0.2V~0.2V in natural sea water 159
Fig. 6.12. Surface photographs after 1,200s potentiostatic experiments for STS 316L at 0.4V~0.9V in natural sea water 160
Fig. 6.13. Surface photographs after 1,200s potentiostatic experiments for STS 316L at 1.0V~1.3V in natural sea water 161
Fig. 6.14. Surface photographs after 1,200s potentiostatic experiments for STS 316L at 1.4V~2.0V in natural sea water 162
Fig. 6.15. Time-current density curves during potentiostatic experiments at range of -0.4V ~ -0.75V for STS 316L in natural sea water 163
Fig. 6.16. Time-current density curves during potentiostatic experiments at range of -0.8V ~ -1.2V for STS 316L in natural sea water 164
Fig. 6.17. Comparison of current density curves after 1,200s potentiostatic experiments for STS 316L at various cathodic potential in natural sea water 166
Fig. 6.18. Surface photographs after 1,200s potentiostatic experiments for STS 316L at various cathodic potential in natural sea water 167
Fig. 6.19. Surface morphologies of STS 316L after 1,200s potentiostatic experimental at -0.4V ~ -0.65V in natural sea water 169
Fig. 6.20. Surface morphologies of STS 316L after 1,200s potentiostatic experimental at -0.7V ~ -0.85V in natural sea water 170
Fig. 6.21. Surface morphologies of STS 316L after 1,200s potentiostatic experimental at -0.9V ~ -1.2V in natural sea water 171
Fig. 6.22. Comparison of current density curve after 1,200s potentiostatic experiment for STS 316L at various polarization potential in natural sea water 172
Chapter 7 18
Fig. 7.1. Schematic diagram of STS 316L specimen for SSRT 178
Fig. 7.2. Experimental equipments for slow strain rate test 179
Fig. 7.3. Stress-elongation curves at various strain rates in natural sea water 181
Fig. 7.4. Comparition of yield strength and max. tensile strength by SSRT at various strain rates in natural sea water. 182
Fig. 7.5. Comparison of elongation and time to fracture by SSRT at various strain rates in natural sea water 183
Fig. 7.6. Surface morphologies of fractured specimens after SSRT at various strain rates in natural sea water 184
Fig. 7.7. Stress-Elongation curves in sea water and air by SSRT 186
Fig. 7.8. Stress-elongation curves by SSRT at free corrosion, 1.2V, 0.5V and 0.4V in natural sea water 187
Fig. 7.9. Stress-elonagation curves by SSRT at free corrosion, 0.1V, -0.25V and -0.4V in natural sea water 188
Fig. 7.10. Stress-elongation curves by SSRT at free corrosion, -0.6V, -0.7V and -0.8V in natural sea water 190
Fig. 7.11. Stress-elongation curves by SSRT at free corrosion, -0.85V, -0.9V and -0.95V in natural sea water. 191
Fig. 7.12. Stress-elongation curves by SSRT at free corrosion, -1.0V, -1.2V and -2.0V in natural sea water 193
Fig. 7.13. Maximum tensile strength with applied potential by SSRT in natural sea water 194
Fig. 7.14. Yield strength with applied potential by SSRT in natural sea water 196
Fig. 7.15. Elongation with applied potential by SSRT in natural sea water 197
Fig. 7.16. Time to fracture with applied potential by SSRT in natural sea water 198
Fig. 7.17. Photographs of fractured specimen after SSRT in air, free corrosion and applied potential of -0.7~1.2V in natural sea water 200
Fig. 7.18. Photographs of fractured specimen after SSRT applied potential of -0.8 ~ 2.0V in natural sea water 201
Fig. 7.19. Surface morphology of fractured specimens after SSRT in air, free corrosion and applied potential of 0.4~1.2V in natural sea water 204
Fig. 7.20. Surface morphology of fractured specimens after SSRT applied potential of -0.7~0.1V in natural sea water 205
Fig. 7.21. Surface morphology of fractured specimens after SSRT applied potential of -0.8~-2.0V in natural sea water 206