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9-12% Cr강의 용접부에 미치는 δ-ferrite의 영향
안성용,강남현,Ahn, Sung-Yong,Kang, Namhyun 대한용접접합학회 2013 대한용접·접합학회지 Vol.31 No.6
As the energy consumption increases rapidly, power generation needs the high energy efficiency continuously. To achieve the high efficiency of power generation, the materials used have to endure the higher temperature and pressure. The 9-12%Cr steels possess good mechanical properties, corrosion resistance, and creep strength in high temperature due to high Cr contents. Therefore, the 9-12%Cr steels are widely used for the high-temperature components in power plants. Even though the steels usually have a fully martensitic microstructure, they are susceptible to the formation of ${\delta}$-ferrite specifically during the welding process. The formation of ${\delta}$-ferrite has several detrimental effects on creep, ductility and toughness. Therefore, it is necessary to avoid its formation. As the volume fraction of ${\delta}$-ferrite is less than 2% in microstructure, it has the isolated island morphology and causes no significant degradation on mechanical properties. For ${\delta}$-ferrite above 2%, it has a polygonal shape affecting the detrimental influence on the mechanical properties. The formation of ${\delta}$-ferrite is affected by two factors: a chemical composition and a welding heat input. The most effective ways to get a fully martensite microstructure are to reduce the chromium equivalent less than 13.5, to keep the difference between the chromium and nickel equivalent less than 8, and to reduce the welding heat input.
Hanji Park(박한지),Namhyun Kang(강남현) 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.5
Many studies have investigated the hydrogen induced stress cracking (HISC) properties of base metal (BM), fusion zone (FZ), and heat affected zone (HAZ), respectively. However, it is important to evaluate the HISC behavior of the entire weldments because the actual weldments are applied with stress simultaneously during service. This study aims to investigate the role of the interaction of hydrogen and microstructures on the HISC for transverse welds containing BM, FZ, and HAZ. In-situ slow-strain-rate-testing (SSRT) with hydrogen charging was conducted for YS 360MPa and YS 550MPa grade steel welds. The in-situ SSRT changed the fracture location from BM to HAZ for the transverse weldments. We investigated the hydrogen trapping site in HAZ and initiation-propagation of HISC for various steels. Although some weld microstructures or softened HAZ are harmless under normal conditions in air, they should be critically investigated in a hydrogen atmosphere.
Al-Si 도금된 1.5 GPa 급 핫스탬핑 강의 수소확산
박철호(Cheolho Park),강남현(Namhyun Kang) 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
자동차의 경량화를 위해 요구되는 철강의 강도가 높아지면서 수소 지연 파괴가 드러나고 있다. 수소취성 감수성은 철강의 강도에 따라 증가하며, 특히 약 1,000MPa 이상인 고강도 강의 경우 일정 시간이 지나면 수소취성에 의한 균열이 발생하여 갑자기 절단되는 지연파괴(delayed fracture)가 일어난다. 특히 차량 구조부 고강도 강에 수소취성이 발생한 상태에서 충돌이 발생하는 경우 심각한 사고가 될 수 있다. 수소취성 가능성을 예측하고자, 도금유무에 따른 수소확산 거동을 측정하였으며, 핫스탬핑 전과 후의 미세조직의 변화 및 수소확산거동 차이를 확인하였다.