표제지
목차
Abstract 13
Ⅰ. 서론 15
1.1. 연구 배경 및 목적 15
Ⅱ. 이론적 배경 24
2.1. Ni 계 초내열합금 24
2.1.1. Ni 계 초내열합금 개발 동향 26
2.1.2. 247LC 초내열합금 28
2.2. 초내열합금의 용접고온균열 30
2.2.1. FZ 응고균열 32
2.2.2. T-HAZ 연성저하균열 32
2.2.3. PMZ 액화균열 33
2.2.4. 액화균열 메커니즘 36
2.2.5. PMZ 액화균열 거동 평가법 37
2.2.6. Ni 계 초내열합금에 대한 레이저 용접 선행 연구 39
Ⅲ. 사용재료 및 실험방법 42
3.1. 사용재료 42
3.2. 실험방법 42
3.2.1. GTAW 42
3.2.2. Spot-Varestraint 시험 48
3.2.3. 싱글모드(Single-Mode, SM) 레이저 용접 51
3.2.4. 미세조직 분석법 54
Ⅳ. 247LC 초내열합금 용접 열영향부 액화균열 제어인자 규명 56
4.1. 247LC 터빈 블레이드 용접부의 액화균열 발생 거동 및 메커니즘 56
4.1.1. 247LC 터빈 블레이드 주조 미세조직 56
4.1.2. 블레이드 용접부 고온균열 발생 거동 56
4.1.3. 247LC 초내열합금 PMZ 액화균열 발생 인자 59
4.2. 열영향부 미세조직에 따른 LCTR 변화 80
4.2.1. 용접 전열처리에 따른 액화균열 파면부 형상 변화 88
4.2.2. 용접 전열처리에 따른 액상화 온도의 변화 93
4.2.3. 용접 전열처리에 따른 247LC 초내열합금 LCTR 축소 검증 101
4.2.4. 저입열 GTAW를 통한 액화 무균열 용접부 확보 가능성 검토 103
4.3. SM 레이저 PMZ 액화균열 거동 105
4.3.1. SM 레이저 PMZ 미세조직 및 고온균열 특징 105
4.3.2. Pipeline 확산 메커니즘 110
4.3.3. 레이저 빔 Wobble을 이용한 액화균열 억제 검토 117
Ⅴ. 결론 120
참고문헌 123
게재 및 발표논문 136
Table 1. Effect of various alloying elements on phase stability in Ni-base alloys. 25
Table 2. Chemical compositoin of 247LC and Haynes230 44
Table 3. Conditions of GTAW for turbine blade manufacturing. 46
Table 4. Conditions of GTAW. 46
Table 5. Conditions of the spot-Varestraint test with GTAW. 50
Table 6. Specifications of the thermovision camera observation. 50
Table 7. Specification of single-mode fiber laser. 53
Table 8. Conditions of single-mode fiber laser welding. 53
Table 10. Representative chemical composition analyzed by WDS for γ/MC interface of crack surface 75
Table 11. Representative chemical composition analyzed by WDS for liquation cracking surface of as-cast 247LC after spot-Varestraint test 96
Table 12. Representative chemical composition analyzed by WDS for liquation cracking surface of Pre-WHT② 247LC after spot-Varestraint test 99
Fig. 1. Schematic of LNG combined cycle power plant. 20
Fig. 2. Schematic of gas Turbine. 21
Fig. 3. Development history of gas turbines. 22
Fig. 4. Flow chart of this study. 23
Fig. 5. Trends in Ni superalloy development. 27
Fig. 6. Typical microstructure of 247LC superalloy. 29
Fig. 7. Classification of weldments. 31
Fig. 8. Schematic of Microstructure around the weld pool: (a) Phase diagram, (b) Heat history, (c) Microstructure of phase around welds. 35
Fig. 9. Classification of Hot cracking test. 38
Fig. 10. Heat input and energy density in previous studies. 41
Fig. 11. Condition of pre-weld heat treatment. 44
Fig. 12. Schematic of the turbine blade by GTAW. 45
Fig. 13. (a) GTAW appearance, (b) Schematic of the GTAW. 47
Fig. 14. (a) Varestraint appearance, (b) schematic of the spot-Varestraint test with GTAW. 49
Fig. 15. (a) SM laser appearance, (b) schematic of the SM laser welding. 52
Fig. 16. As-cast microstructure and elemental distribution of the as-cast 247LC turbine blade, analyzed by EPMA. 57
Fig. 17. Cross-sectional macrostructure of the as-cast 247LC turbine blade welds. 58
Fig. 18. BSE image of and elemental distribution by EPMA in the PMZ of the as-cast 247LC turbine blade weld. 60
Fig. 19. BSE image and EPMA analysis of the migrated grain boundary region of the as-cast 247LC welds. 64
Fig. 20. Fracture surface of a liquation crack in the as-cast turbine blade welds. 65
Fig. 21. The surface of a liquation crack in the as-cast 247LC turbine blade welds by EPMA 66
Fig. 22. Equilibrium phase diagram, based on the chemical composition in Table 9 by Thermo-Calc. 68
Fig. 23. Equilibrium phase diagram of the 247LC superalloys by Thermo-Calc. 69
Fig. 24. The surface of a liquation crack in the as-cast turbine blade welds (for the γ-γ' eutectic colonies) by EPMA. 70
Fig. 25. Effect of B on liquation of γ/γ' eutectic colonies. 72
Fig. 26. Equilibrium phase diagram, based on the chemical composition in Table 10 by Thermo-Calc. 75
Fig. 27. Schematic of liquation cracking by constitutional liquation (a) γ-MC diagram, (b) constitutional liquation of MC. 76
Fig. 28. BSE image and EPMA analysis of the surface of a liquation crack in the as-cast 247LC turbine blade welds (for the MC-type carbides). 77
Fig. 29. Effect of B on liquation of MC-type carbides. 79
Fig. 30. Liquation cracking behavior of the (a) as-cast, (b) Pre-WHT① and (c) Pre-WHT② 247LC after the spot-Varestraint test. 82
Fig. 31. Relationship between the maximum crack length and bending strain of the as-cast 247LC. 84
Fig. 32. Relationship between the maximum crack length and the heat treatment condition. 85
Fig. 33. Representative temperature profile at the time of spot-Varestraint test. 86
Fig. 34. Relationship between LCTR and specimen condition. 87
Fig. 35. Microstructure of Base metal: (a) as-cast, (b) Pre-WHT① and (c) Pre-WHT② 247LC. 89
Fig. 36. Image quality, inverse pole figure and phase maps analyzed by EBSD for as-cast, Pre-WHT① and Pre-WHT② 247LC (magnification 2000x). 90
Fig. 37. Phase maps analyzed by EBSD for as-cast, Pre-WHT① and Pre-WHT② 247LC (magnification 200x). 91
Fig. 38. Fracture surface of a liquation crack for as-cast, Pre-WHT① and Pre-WHT② 247LC after spot-Varestraint test. 92
Fig. 39. The surface of a liquation crack in the as-cast 247LC by EPMA. 95
Fig. 40. Equilibrium phase diagram, based on the chemical composition in Table 11: (a) γ/γ' eutectic, (b) MC carbide by Thermo-Calc. 96
Fig. 41. BSE image and EPMA analysis of the surface of a liquation crack in Pre-WHT② 247LC. 98
Fig. 42. Equilibrium phase diagram, based on the chemical composition in Table 12: (a) γ+γ', (b) MC carbide by Thermo-Calc. 99
Fig. 43. Surface appearance and cross-sectional macrostructure of the welds, obtained by... 102
Fig. 44. Surface appearance and cross-sectional macrostructure of the welds, obtained by GTAW with different heat input, of Pre-WHT② 247LC. 104
Fig. 45. Surface appearance and cross-sectional macrostructure welds, obtained by SM laser weldilng... 106
Fig. 46. Image quality and inverse pole figure analyzed by EBSD for as-cast 247LC. 112
Fig. 47. BSE image and EPMA analysis of the surface of a Solidification and liquation crack in as-cast 247LC. 114
Fig. 48. (a) Surface appearance and cross-sectional macrostructure, (b) Image quality and inverse pole... 118