Introduction

During steelmaking gases like oxygen, hydrogen and nitrogen dissolve in steel. The term degassing is employed to remove nitrogen and hydrogen from steel. Dissolved oxygen from steel melt cannot be removed as molecular oxygen. Removal of oxygen is termed deoxidation and is discussed in lecture 24. This lecture concerns with degassing.

Both nitrogen and hydrogen impair the mechanical properties of steel. The maximum solubility of nitrogen in liquid iron is 450ppm and less than 10ppm at room temperature. During solidification excess nitrogen is rejected which may form either blow holes or nitrides. Excess nitrogen causes embrittlement of heat affected zone of welded steels and impair cold formability. Hydrogen in steel impairs steel properties. Solubility of hydrogen in steel is low at ambient temperature. Excess hydrogen is rejected during solidification and results in pinhole formation and porosity in steel. Few ppm of hydrogen causes blistering and loss of tensile ductility. Thus removal of nitrogen and hydrogen from steel is necessary.