Fabrication of Porous Iron by Unidirectional Solidification in Nitrogen Atmosphere

Abstract

Porous iron whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification of the melt in a pressurized mixture gas of nitrogen and argon. Nitrogen dissolved in the molten iron is rejected at the solid-liquid interface during the solidification due to the solubility difference of nitrogen between the liquid and solid. The gas pores are evolved from the nitrogen insoluble in the solid iron, which grow unidirectionally. The porosity is controlled by the partial pressures of nitrogen and argon during melting and solidification. The porosity decreases with increase of the partial pressure of argon at a given nitrogen pressure according to the Boyle’s law. At a constant total pressure of the mixture gas, the porosity increases with increasing partial pressure of nitrogen and no pores are formed during solidification below a critical partial pressure of nitrogen. The nitrogen concentration in the solid iron increases with increasing partial pressure of nitrogen. The solid-solution hardening has been observed in as-cast porous iron, while more significant hardening has also been found in the porous iron quenched from a high temperature 1273 K, which is due to the martensitic transformation.