Haynes 282 초내열합금의 다른 열처리 인자 적용 및 장기 열간 노출에 따른 미세조직과 기계적 특성 고찰 = Effect of Heat Treatment and Long-term Thermal Exposure on Microstructure Evolution and the Mechanical Properties of Haynes 282 Superalloy

다국어 초록 (Multilingual Abstract)

In this study, we investigated the microstructure evolution and the mechanical properties of Haynes 282 superalloy in tension at 750 ˚C. Especially, The well-known strengthening phase in superalloy which is called γ′ precipitate was observed differently as various heat treatment conditions. Also, the thermal stability is very important factor which should be considered for application in thermal power plant. Therefore, long-tern thermal exposure was conducted at 750 ℃ up to 5000hr for all the heat treatment conditions. We conducted the standard 2step aging heat treatment (1010 ˚C/2h + 788 ˚C/8h) and newly suggested 1step aging heat treatment (800 ˚C/4h) which have advantages in time reduction and economics. Moreover, three different cooling rates (WQ, AC, FC) were conducted to study mass effect which can occur in large scale products. The γ′precipitates were observed and calculated average size and distribution after heat treatment. The average γ′precipitate size for standard 2step aging heat treatment was 17.3 nm. In case of 1step aging heat treatment, the average γ′precipitate size was increased along the decrement of cooling rate as 12.1 nm (WQ), 15.4 nm (AC) and 39.2 nm (FC). And, the γ′precipitate exhibit spherical shape. The tensile properties were measured at 750 ℃. The yield strength exhibit similar values (over 600 MPa) and for all conditions. Only a slight decrease of strength is observed, as expected, when precipitates were larger. However, the plastic behaviors were significantly different in stress-strain curve. Different strain hardening was observed. It implies that the different deformation mode was occurred. The deformation mechanism was investigated by TEM analysis. In all the cases, we observed a mixed mode of deformation such as climbing, shearing and bypassing. However, only 1step aging after FC condition showed shearing mechanism as a dominant mechanism which can be evidence of stress-strain curve. To approach theoretically, the strength increment associated to each mechanism was measured by using precipitation strengthening model. And, the results were well agreed with the observed TEM micrographs. For all the conditions calculated the existence of mixed mode of deformation mechanism. However, only 1step aging after FC condition calculated as showing shearing mechanism as a dominant mechanism. To idenfiy the thermal stability, the thermal exposure at 750 ℃ for up to 5000hr was conducted. The formation of deleterious phase was not detect even for the 5000hr exposure. The mechanical properties were very satisfying despite long-term exposure in all the cases. The yield strength was higher than 600MPa for all heat treatment conditions. This suggests that the newly suggested 1step aging heat treatment can replac with standard 2step aging heat treatment. There was significant coarsening of γ′precipitates during thermal exposure. However, they still remained sherical shape and the size was 79.6 nm for standard 2step aging heat treatment and 73.5 nm (WQ), 73.5 nm (AC), 83.1 nm (FC) for 1step aging heat treatment. The shape of γ′precipitates is important factor to evaluate the degradation. Therefore, the spherical shape implies that the γ′precipitates is maintaining the coherent interface with γ matrix suggesting high stability of the microstructure of Haynes 282 superalloy at 750 ℃. The coherency was analyzed after 5000hr at 750 ℃ for the 1step aging after FC condition. The perfect coherency along the interface between the γ materix and γ′precipitates. This coherency is believed to significantly contribute to the high resistance to thermal degradation of Haynes 282.

목차 (Table of Contents)

• 목차 01
• List of tables and figures 03
• I. 서 론 07
• II. 이론적 배경 11
• 1. Ni기 초내열합금 11
• 1) 초내열합금의 종류 및 특징 11
• 2) Ni기 초내열합금의 주요 강화기구 및 석출상 15
• 2. Haynes 282 합금의 주요 상 및 기계적 특성 22
• 1) Haynes 282 합금의 개요 22
• 2) Haynes 282 합금의 주요 상 26
• 3) Haynes 282 합금의 열처리 및 고온 기계적 특성 28
• III. 실험 방법 31
• 1. 시편 준비 및 열처리 31
• 2. 고온 인장 시험 32
• 3. 미세구조 분석 33
• IV. 결과 및 고찰 36
• 1. 열처리 인자에 따른 γ′석출물 변화 및 인장 특성 36
• 1) 열처리 인자에 따른 Haynes 282의 γ′ 석출물 비교 분석 36
• 2) 열처리 인자에 따른 Haynes 282의 고온 인장 특성 비교 분석 40
• 3) 열처리 인자에 따른 Haynes 282의 변형거동 비교 분석 44
• 2. 각 열처리 후 장기 열간 노출에 따른 미세조직 및 고온인장 특성 비교 분석 50
• 1) 장기 열간 노출에 따른 미세조직 분석 50 2) 장기 열간 노출에 따른 기계적 특성 비교 분석 55
• 3) 장기 열간 노출에 따른 상 안정성 평가 60
• V. 결 론 65
• 참고문헌 67
• Abstract & Keywords 73