Blowholes in iron and steel are presumably due to CO, H2, N2 gases. In killed steel, so far as deoxidation is done fully, the pressure of CO gas in molten steel is very low and H2, N2 gases may become a cause of blowholes. It was explained in Report IV (Tetsu-to-Hagané, May 1956, Vol. 42, pp. 391-397) that when the H2 content was less than 8×10-4% H2, hydro- gen was not the main cause to blowhole formation if the molten steel was deoxidized fully. But in steel castings or large killed ingots, blowholes often grow in killed steel even if hydrogen content is 3-5×10-4%. Accordingly, to make clear the cause of blowhole formation in steel castings or large killed ingots, some experiments were done as follows.
1) The same molten steel was cast in steel mold and sand molds. These patterns were all of the same size and form. Sand molds were heated at 100°C×2h, 300°C×2h, 700°C×2h before casting. These cast samples were cut and analysed with a vacuum cutting apparatus and the results obtained were as follows.
a) It was made clear that moisture of sand molds was decomposed and became a cause of blowhole formation and in this pattern, heating temperature of sand molds at 100°C×2h was not sufficieat because of high hydrogen content in the sample.
b) When sand molds were left long from molding to casting, the moisture absorbed by sand molds from atmosphere was decomposed in casting and became a cause of blowhole for- mation.
2) Cylinders of 50φ×200mm were taken from the center of a 12 ton killed ingot, and analysed with a vacuum cutting apparatus, and the discharged gas was measured at room temperature. The results were as follows.
a) Discharged gas at room temperature was mostly composed of hydrogen and the diffu- sion constant of hydrogen was of the order of 10-7.
b) The hydrogen content in the center of large ingots was calculated from the diffusion constaat as well as the results analysed with a vacuum cutting apparatus. The results obta-ined showed 12-15×10-4% H2, and these pressure in δ-iron was higher than 1 atm. and was possible to become a cause of blowhole formation.