Vacuum Degassing Processes for Liquid Steel Jul12

Vacuum Degassing Processes for Liquid Steel...

Vacuum Degassing Processes for Liquid Steel During the primary steelmaking process, gases like oxygen (O2), hydrogen (H2) and nitrogen (N2) dissolve in the liquid steel. These gases have a harmful effect on the mechanical and physical properties of steel. Dissolved O2 from liquid steel cannot be removed as molecular O2 and its removal is termed as deoxidation. The term degassing is used for the removal of H2 and N2 gases from liquid steel. Since the degassing process of liquid steel is carried out under vacuum, it is also known as vacuum degassing of liquid steel. Vacuum degassing processes are carried out in steel teeming ladles. Removal of H2 and N2 gases from liquid steel is necessary since both of these gases harm the properties of steel. Solubility of H2 in steel is low at ambient temperature. Excess H2 is rejected during solidification and results in pinhole formation and results into the porosity in solidified steel. Few ppm (parts per million) of H2 gas causes blistering and loss of tensile ductility. In case of N2 gas, maximum solubility of N2 in liquid iron is 450 ppm and less than 10 ppm at room temperature. During solidification excess N2 is rejected which can cause formation of either blow holes or nitrides. Excess N2 also causes embrittlement of heat affected zone during welding of steels and also impairs cold formability of steel. It was only in the early 1950s that the problem of producing steel with minimum gas content was solved through the development of a method of vacuum treatment of liquid steel in the ladle before its teeming.  The method was proposed by the scientists AM Samarin and LM Novik of erstwhile USSR in 1940. It was first tried industrially in the then USSR in 1952 at the Enakievskii metallurgical plant. In 1954...