Dong-Su Bae

Improvement of high temperature strength and low temperature toughness of high manganese-chromium austenitic steels

Abstract High Mn-Cr austenitic steels are still considered to be an important high temperature structural material from the point of view of fast-induced radioactivity decay (FIRD) and non-magneticity. The objective of the present study is to investigate the mechanical properties of 12% Cr-15% Mn austenitic stainless steels and to compare these properties with those of the reference materials of JPCAs and JFMS, which are being investigated for the development of fusion reactor structural materials in Japan. The effects of the alloying elements V, Ti, Ta, etc. were investigated to determine the improvement of mechanical properties. Tiny precipitates of VN and Ti(C, N) raised the high-temperature strength considerably. Content of 0.1 to 0.2% C, however, formed very coarse precipitates of M 23 C 6 type carbide on the grain boundaries, which deteriorated low temperature toughness inducing intergranular fracture. Microstructural evolution during long-term aging was also investigated.

Effects of nitride distribution on internal stress during high temperature deformation of 12%Cr-15%Mn austenitic steels

Many investigations on internal stress of various materials have been performed as the internal stress is considered to be an important factor to control high temperature deformation. The present authors have researched the effect of very fine and dense vanadium nitride (VN) precipitates on internal stress and effective stress which were measured by a strain dip test during high temperature creep of 12%Cr-15%Mn austenitic steels. In the present study, the authors have investigated the difference of the response to a strain dip test among the 12%Cr-15%Mn materials which are alloyed with small amounts of V, Ti and Ta and have different distributions of nitrides formed by an aging treatment, and have discussed the effect of nitride distribution on the interaction of dislocations with the precipitates.

Phase instability and toughness change during high temperature exposure of various steels for the first wall structural materials of a fusion reactor

Abstract The objective of the present research is to clarify the phase instability, particularly, the precipitation behavior of carbide and nitride during the long term aging in the non-irradiation state of the materials proposed for the first wall structural component of fusion reactors, such as a type 316 austenitic steel, its modified steels, ferritic heat resisting steels and reduced radio-activation materials. The effect of the precipitation behavior on the toughness is also investigated. It is noticed that the toughness was much deteriorated by the formation of large amounts of coarse carbides within grains and on grain boundaries during 2.88 × 10 4 ks (8000 h) aging at 873 K and that intergranular fracture occurred by the impact test at room temperature even in the type 316 steel.