Sanghoon Noh

Joining of ODS Steels and Tungsten for Fusion Applications

Liquid phase diffusion bonding between ODS steel and pure W was carried out and its joint strength was investigated for fusion applications. A block of high-Cr ODS ferritic steel and a W plate were diffusion bonded at 1240 °C for 1h with/without an insert material under an uni-axial compression load and a high vacuum atmosphere. Cross sectional microstructures of joint region were observed by scanning electron microscope and the mechanical properties of the joint region were evaluated by hardness test and torsion tests. Microstructure analysis revealed that high Cr ODS ferritic steel block and W plate with insert material was successfully diffusion bonded with free of voids. Shear strength of liquid phase diffusion bonded ODS steel and W was higher than that of directly solid state diffusion bonded ODS steel and W. This was attributed to residual strain which is resulted from the difference of thermal conductivity between the ODS steel and W.

Tensile Behavior of Transient Liquid Phase Bonded ODS Ferritic Steel Joint

The oxide dispersion strengthened (ODS) steel is one of the candidate structural materials for Gen. IV systems and DEMO reactor systems because of its excellent elevated temperature strength, corrosion and radiation resistance. Joining technology development of ODS steels is unavoidable to realize these advanced nuclear systems with huge and complex structures. However, joining of ODS steels with conventional melting–solidification processes is considered to cause detrimental effects on joint regions because of the possible modification of the fine homogeneous microstructure to rather coarse and inhomogeneous microstructure. Therefore, suitable joining techniques need to be developed with such a process that these featured microstructures are reasonably maintained after the processes. In this study, tensile behavior of TLPB ODS steel joint was investigated. Thin pure boron insert material was deposited by electron beam physical vapor deposition (EBPVD) to join ODS steel (Fe–15Cr–2W–0.2Ti–0.35Y2O3) blocks using uni-axial hot press. ODS steel was successfully bonded with free of voids at bonding interface with EBPVD bond. Tensile strength of the joint is similar with the base materials, while total elongation is decreased at 700°C accompanied by fracturing at the joint interface. The fracture is considered to be due to partial discontinuous microstructures aligned along the interface.

Relationship Between Microstructure and Mechanical Property of Transient Liquid Phase Bonded ODS Steel

The oxide dispersion strengthened (ODS) Steel is one of the candidate structural materials for Gen. IV fission systems and fusion DEMO reactor because of its excellent elevated temperature strength, corrosion and radiation resistance. To fabricate these advanced nuclear systems with huge and complex structure, bonding and welding is one of unavoidable issues. Thus, suitable bonding and welding techniques need to be developed with such a process that the microstructures with very small grain size and distribution of nano-oxide particles are not remarkably changed. Transient liquid phase bonding (TLPB) is a potential joining technique to minimize the disruption of these unique microstructures.