수소챔버 기반 저속인장시험을 통한 API 5L X70 강재의 노치인장강도 평가 = Evaluation of the Notched Tensile Strength of API 5L X70 Steel by Slow Strain Rate Testing in a Hydrogen Chamber

국문 초록 (Abstract)

수소는 탈탄소 시대를 대비한 주요 에너지원으로 주목받고 있으며, 이에 따라 고압 수소의 안전하고 신뢰성 있는 이송을 위한 강재 배관 기술에 대한 체계적인 검토가 요구되고 있다.
현재 국내외 수소 수송 인프라에서 적용 가능한 재료로는 높은 강도와 우수한 용접성, 경제성을 동시에 갖춘 탄소강이 가장 유력한 후보로 인식되고 있으며, 그중 API 5L X70 강재는 고강도 강관으로서 수소 이송 배관 구조에 활용 가능한 재료 중 하나로 평가되고 있다. 그러나 해당 강재는 고압 수소 환경에 장기간 노출될 경우 수소취성(Hydrogen Embrittlement) 현상에 의해 기계적 건전성이 저하될 가능성이 제기되고 있으며, 이에 따른 기초적인 물성 변화에 대한 실험적 검증이 필요한 상황이다.
본 연구에서는 API 5L X70 강재를 대상으로, 수소환경이 인장 특성에 미치는 영향을 확인하기 위해 저속인장시험(SSRT: Slow Strain Rate Testing)을 수행하였다. 시험 항목은 노치인장강도(Notched Tensile Strength, NTS)로 한정하였으며, 응력 집중 효과를 고려하기 위해 노치가 가공된 시편을 사용하였 다. 모든 시험은 동일한 변형률 속도 조건을 유지한 상태에서 수행되었으며, 공기환경과 10 MPa 수준의 고순도 수소환경에서 각각 비교 시험을 실시하였 다.
시험 결과, 수소환경에서 측정된 평균 NTS는 공기환경 대비 약 21.1% 감소 하는 것으로 나타나, 수소환경 노출에 따른 강도 저하 경향이 정량적으로 확인되었다. 또한 시험 후 시편 파면에 대한 육안 관찰 결과, 수소환경 시편에서는 중심부를 기준으로 취성적인 파괴 양상이 두드러지게 관찰된 반면, 공기환경 시편에서는 상대적으로 연성적인 파단 형상이 확인되었다.
본 연구는 특정 강종(API 5L X70)을 대상으로 제한된 시험 조건 하에서 수소환경 노출에 따른 NTS 변화를 정량적으로 평가한 예비 시험 사례로서, 향후 수소 이송용 강재 배관의 재료 적합성 검토 및 추가적인 수소취성 평가 연구를 위한 기초 자료로 활용될 수 있을 것으로 판단된다.

목차 (Table of Contents)

• Ⅰ. 서 론 ······················································································· 1
• 1.1 연구 배경 및 목적 ··················································································· 1
• 1.2 연구 범위 및 방법 ··················································································· 5
• Ⅱ. 이론적 고찰 ·············································································· 8
• 2.1 수소취성의 개념 및 매커니즘 ······························································· 8
• 2.2 저속인장시험(SSRT)의 매커니즘 ······················································· 11
• 2.3 저속인장시험에 대한 선행연구 ··························································· 13
• Ⅲ. 시험 방법 ················································································ 16
• 3.1 시험 재료 및 시편 설계 ······································································· 16
• 3.2 시험 장비 및 수소환경 구성 ······························································· 21
• 3.3 시험 수행 절차 ······················································································· 26
• 3.4 시험 결과 분석 방법 및 평가 지표 ··················································· 29
• Ⅳ. 시험 결과 및 고찰 ································································ 31
• 4.1 NTS 측정 결과 및 비교 ······································································ 31
• 4.2 수소환경 영향에 대한 고찰 ································································· 44
• Ⅴ. 결 론 ························································································· 46
• 참고문헌 ······················································································· 48
• ABSTRACT ··············································································· 50
• 감사의 글 ····················································································· 52