Ho Jung Lee

Thermal Performance of Multilayer PVD Tungsten Coating for the First Wall Application in Nuclear Fusion Devices

The thermal performance of multilayer PVD tungsten and molybdenum coatings on graphite was studied for the application of first wall in nuclear fusion devices. The coatings with a thickness of 5 μm and different numbers of W/Mo layer were prepared and a series of plasma thermal loads from 1.3 to 4.1 MWm-2 was applied. Microstructure changes caused by thermal loadings were analyzed. Scratch test was then conducted to quantify the changes in the adhesion. The microstructural analysis and scratch test results showed that the thermal performance of the coatings varied depending on the degree of heat loads and coating structure. A molybdenum interlayer improved the coating adhesion while the plasma heat resistance of the double-layer W/Mo coating was slightly better than the others.

Evaluation of Tensile Property of Austenitic Alloys Exposed to High-Temperature S-CO2Environment

Super-critical CO2 (S-CO2) Brayton cycle has been considered to replace the current steam Rankine cycle in Sodium-cooled Fast Reactor (SFR) in order to improve the inherent safety and thermal efficiency. Several austenitic alloys are considered as the structural materials for high temperature S-CO2 environment.. Microstructural change after long-term exposure to high temperature S-CO2 environment could affect to the mechanical properties. In this study, candidate materials (austenitic stainless steels and Alloy 800HT) were exposed to S-CO2 to assess oxidation resistance and the change in tensile properties. Loss of ductility was observed for some austenitic stainless steels even after 250 h exposure. The contribution of S-CO2 environment on such changes was analyzed based on the characterization of the surface oxide and carburization of the materials in which 316H and 800H showed different oxidation behaviors. § 이 논문은 2014 년도 대한기계학회 재료 및 파괴부문 춘계 학술대회(2014. 4. 3.-4., 서귀포KAL호텔) 발표논문임 † Corresponding Author, chjang@kaist.ac.kr C 2014 The Korean Society of Mechanical Engineers

Evaluation of the thermal aging of δ-ferrite in austenitic stainless steel welds by electrochemical analysis

Cr-segregation by spinodal decomposition and G-phase precipitation were observed in δ-ferrite of austenitic stainless steel welds thermally aged at 400 °C for up to 20,000 h. A reversion heat treatment (R-HT) at 550 °C for 1 h dissolved the Cr-segregation in the aged welds while some intermetallic precipitates were present. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) analysis showed no significant differences among them. However, after selective etching of the austenite phase, the DL-EPR values of δ-ferrite phase steadily increased with aging time due to the growth of Cr-depleted regions by spinodal decomposition. The electrochemical behavior of δ-ferrite after R-HT condition was similar to that of unaged welds, indicating the intermetallic precipitates did not affect the corrosion resistance in this case. Overall, DL-EPR analysis of δ-ferrite phase provided better correlation with spinodal decomposition.