Blowhole Formation in Fe-N System under Reduced Pressures

The blowhole formation in the Fe-N system during solidification under the reduced pressures has been studied by using a levitation melting apparatus. Molten irons containing various nitrogen contents were cast into the copper mold under various reduced pressures. Blowholes were evaluated by the radiographic inspection and the examination of sectioned portions of the ingot. The results are summarized as follows: 1) The contents of nitrogen for blowhole formation are decreased by reducing external pressure at solidification and are approximately 25% lower than the equilibrium solubility of nitrogen at the melting point of iron and furthermore are propotional to square root of the irexternal pressures in the range of 1 atm. to 10 torr, However, below 10 torr. external pressures, the critical nitrogen contents for blowhole formation are independent of their external pressures and take almost a constant value of 40 ppm. 2) The growing process of blowholes is discussed from the point of view of the amount of gas available from the enriched layer of solute at the solid-liquid interface during solidification.