Tadao Ogawa

Numerical modeling of solidification and subsequent transformation of Fe-Cr-Ni alloys

A computational method for the analysis of phase transformation involving solidification was developed with the assumption of thermodynamic equilibria at interfaces. The region of interest was divided into finite segments, and solute diffusion across the segments was computed by the use of the direct finite difference method (FDM). Simultaneously, thermodynamic equilibrium at each interface was updated at every step of the diffusion analysis to determine the location of the interfaces. The temperature decrease and the increment of fraction solid were calculated based on thermal balance, including a heat extraction condition. Solid state transformation from δ to γ phase within each FDM segment was modeled by the use of a Clyne-Kurz (C-K) type analysis with assumptions of complete mixing of solutes in theδ phase and limited back diffusion in theγ phase. The calculation results were compared with welding solidification experiments in the iron-chromium-nickel ternary system. Good agreement was obtained with respect to solute distribution and residual fraction ofδ phase over different compositions and solidification modes of the alloys used.

Effect of impurity content in stainless steel on resolidified surface condition after disruption load

Plasma disruptions in tokamaks may cause melting of the surface layer of first walls. If the layer resolidifies without any movement, the same layer repeatedly melts and protects the rest of the wall. A number of stainless steel samples were irradiated by a neutral beam injector. After irradiation tests with the same condition, some had wavy resolidification surfaces with dents, while some had smooth surfaces. In the former samples, the amplitude of the roughness increased by repetitive heat loads. The melt layer moved to the convex areas, which deepened the dents on each occasion. Sulphur was found to be one of the harmful elements which promotes the formation of the wavy surface. It is recommended to be kept below 0.005 wt% for obtaining a uniform resolidification surface. Even if samples have a low sulphur content, a high oxygen content also makes the surface rough. Other impurities have some effects, but they have not been clarified yet.

Surface ripple of resolidified stainless steel after disruption load

The lifetime the first wall in fusion reactor depends on its durability against plasma disruption loads that may cause the melting and evaporation of the surface layer. Irradiation tests for stainless steels were made using a neutral beam injector. It was found that high sulfur and oxygen contents in the steels made the resolidification surfaces rough and wavy, while high calcium, aluminum and titanium contents were revealed to effectively offset the sulfur and oxygen. These impurity contents affect the temperature coefficient of surface tension in liquid iron. The surface temperature in the wider sections declines more slowly than in the thinner parts, therefore, the surface temperature is nonuniform. When the liquid iron has a positive temperature coefficient of surface tension, the surface tension in the denser parts becomes relatively greater than in the thinner parts. Consequently, the former expands and the thinner section contracts.