It is important to eliminate blowholes or pinholes in iron and steel, but the mechanism of growth of them has not been suffciently studied yet. As a first step to study this mechanism, it is necessary to analyse the gases existing in blowholes or pinholes. For this purpose, a vacuum cutting apparatus was designed and set up instead ot drilling the sample in water or oil.
As already stated, a vacuum cutting apparatus was a lathe which worked in hjgh vacuum, and was composed of three parts, that is, high-vacuum part, 1ow-vacuum part, and atmospheric part. During the cutting with this apparatus, there were two kinds of gases extracted, one was discharged from blowholes or pinholes, the other was evolved from chips because of their high temperature. To prevent the mixing, the apparatus was designed so that its cutting velocity to be as slow as possible. Furthermore, to eliminate gases which evaporated from oil in vacuum, 1ow-pressure oil was used.
Blowhole samlples were cast from molten steel which absorbed oxygen or hydrogen and then cut in a vacuum-cutting apparatus. The gases discharged from blowholes were collected by a mercury diffusion pump and a jet mercury pump, then analysed by an orsat microanalysis apparatus. Chemical compositions of gases in blowholes were as fo1lows.<Br>1) Gases in blowholes were for the most part hydrogen and nitrogen, while CO, CO2, CH4 were of very small quantities.
2) Free oxygen gas existed for the small amount in the blowho1es.
3) Gases in blowholes which grew in non-deoxidizing steel were for the most part nitrogen, while hydrogen was of very small amount. On the other hand, gases in blowholes which grew indeoxidizing steel were for the most part hydrogen while nitrogen was very small in amount.
From this composition of gases, it was presumed that the CO gas existed in blowholes (which evolved from molten steel) decomposed in cooling; and the pressure in blowholes reduced; and then hydrogen and nitrogen diffused in to blowholes from the steel around.