Fabrication of lotus-type porous carbon-steel by continuous casting technique in nitrogen atmosphere

Lotus-type porous carbon steel (AISI1018) with long cylindrical pores aligned in one direction was fabricated by continuous casting technique at various transference velocities in pressurized nitrogen atmosphere. The molten carbon steel dissolving nitrogen in a crucible was solidified continuously through a cooled mold as being pulled down at a given velocity. Intermittent transference was applied in order to prevent the blockage of melt during solidification in the mold. Long lotus-type porous rod can be fabricated by this technique. The size and the morphology of pores were significantly affected by the transference velocity, which changes the cooling condition during solidification. At a low transference velocity the solid-liquid interface was kept perpendicular to the transference direction, which is necessary for formation of pores aligned in one direction. With increase of the transference velocity, the pores became coarse and irregular in the center part of the porous carbon steel. At a high velocity the solid-liquid interface was deep concave and the pores grew in the radial direction toward the center. Collision and merging of the pores at the center part caused the coarsening. This is typical for metals with low heat conductivity such as carbon steel at a high velocity, because the center part is cooled down much slower than the surface.