고온질화처리를 통한 304L 스테인리스강의 내수소취성 개선

다국어 초록 (Multilingual Abstract)

As the transition to new energy sources has become inevitable for achieving carbon neutrality, hydrogen energy, which emits only pure water during the reaction, has emerged as a new focus of interest. Therefore, developing materials that ensure safety and reliability in hydrogen environments is essential. Among various materials, austenitic stainless steels with an face-centered cubic (FCC) structure have attracted attention as hydrogen infrastructure materials because they exhibit significantly lower hydrogen permeability and diffusivity, while having higher hydrogen solubility, compared with other stainless steels having body-centered cubic (BCC) or body-centered tetragonal (BCT) structures. However, in the case of 304L stainless steel, which contains a relatively low amount of nickel, strain-induced martensitic transformation occurs under high-pressure hydrogen environments. Although this transformation increases the strength of the material, the difference in hydrogen solubility between phases forces hydrogen toward the interface, which becomes a cause of brittle fracture. In this study, high-temperature solution nitriding (HTSN) was applied to dissolve nitrogen into 304L stainless steel in order to improve strength and simultaneously enhance FCC stability to reduce hydrogen embrittlement sensitivity.
Commercial 0.8 T 304L stainless steel was used, and nitriding was conducted at 1200°C in a nitrogen atmosphere at atmospheric pressure. Afterwards, nitrogen diffusion behaviour, hardness, microstructural changes, tensile properties, and fracture surfaces were analyzed, followed by an evaluation of hydrogen embrittlement sensitivity. When high-temperature solution nitriding was performed for more then 180 minutes, despite grain growth, complete nitrogen dissolution resulted in an increase of approximately 200 MPa in yield strength and approximately 240 MPa in tensile strength, and hardness increased by 110 HV0.05. When high-temperature solution nitriding was carried out for 60 minutes or longer, hydrogen embrittlement sensitivity improved by about 70% (based on relative elongation loss, REL), and only ductile fracture surfaces were observed. In addition, nitrogen dissolution impeded hydrogen diffusion, reducing the internal hydrogen content of the material by one-third.

목차 (Table of Contents)

• 제 1 장 1
• 1.1 서론 1
• 1.2 연구 목적 4
• 제 2 장 이론적 배경 5
• 2.1 오스테나이트계 스테인리스강 5
• 2.1.1. 300 계열 오스테나이트계 스테인리스강 5
• 2.1.2. 오스테나이트계 스테인리스강에서 질소의 영향 7
• 2.2 수소취성(Hydrogen embrittlement, HE) 13
• 2.2.1 수소취성 매커니즘 13
• 2.2.2 스테인리스강의 수소 취성 16
• 2.3 고온질화처리 20
• 제 3 장 합금 및 실험 방법 26
• 3.1 합금 및 열처리 26
• 3.2 기계적 시험 27
• 3.3 수소 장입 30
• 3.4 Thermal desorption analysis (TDA) 31
• 3.5 질소 고용 프로파일 분석 31
• 3.6 미세조직 및 파단면 분석 32
• 3.7 열역학 계산 34
• 3.7.1. 질소 용해도 (N solubility) 계산 34
• 3.7.2. 질소 확산 예측 시뮬레이션 35
• 제 4 장 실험결과 및 토의 38
• 4.1 질소 용해도(N solubility) 계산 38
• 4.2 초기 미세조직 40
• 4.3 기계적 거동 42
• 4.4 내수소취성 평가 51
• 제 5 장 연구 결론 59
• REFERENCES 61