An investigation was made into the removal of hydrogen from the melt of medium carbon steel on 10 heat practices of vacuum, pouring under a pressure of 8–12 mmHg. The degassing process can be divided into three steps according to observation from high-speed photography. By vacuum degassing, hydrogen, nitrogen, and oxygen are reduced from an average of 3.5 to 1.7 ppm, from 70 to 60 ppm, and from 60 to 35 ppm, respectively. With the composition and the volume of purge gas which comes out of the steel in the vacuum tank, hydrogen, nitrogen, and oxygen in the melt should decrease by about 2, 9, and 15 ppm, respectively. The removal of hydrogen depends upon the formation and the growth of gas bubbles. In this case, the amount of oxygen has a large affect upon the removal of hydrogen. In other practices, it is recognized that a lower pressure such as 1–3 mmHg is more favorable for the removal of hydrogen than 8–12 mmHg from the view point of growth and formation of gas bubbles.

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