Ans. It has been observed that hydrogen (even as low as 5 ppm) causes internal flakes in large steel sections, and by pouring the liquid steel in vacuum chamber, it is possible to reduce hydrogen content to 1 ppm. Such vacuum castings are used extensively for large forgings such as electrical rotors.
Sometimes to reduce oxygen content to 0.001%, carbon deoxidation is carried out by removal of oxygen as carbon monoxide. This treatment substantially decreases the number of non-metallic inclusions in the steel and is used for bearings and other high-quality steels.