Process Overview
Vacuum degassing removes dissolved gases — primarily hydrogen, nitrogen, and excess oxygen — from liquid steel under sub-atmospheric pressure. At steelmaking temperatures (~1,600 °C), the solubility of hydrogen and nitrogen in liquid steel is proportional to the square root of their partial pressure above the melt (Sievert's Law). Reducing the pressure from atmospheric (1 bar) to 0.5–3 mbar reduces equilibrium hydrogen solubility from approximately 7 ppm to 1–2 ppm, enabling removal to levels demanded by the most critical steel grades.
Vacuum degassing is mandatory for grades where dissolved gases cause product failures in service: hydrogen induces "white spots" and delayed fracture in heavy plate and forgings; nitrogen causes strain ageing embrittlement in deep-drawing automotive sheet; excess oxygen forms oxide stringers that impair fatigue life in bearing, spring, and tire cord steels. As steel quality specifications have tightened progressively over the past 40 years, vacuum degassing has moved from a specialist operation for a narrow premium segment to a standard step in the production of flat-rolled, heavy plate, and linepipe products.
The two dominant vacuum degassing technologies — the RH degasser (Ruhrstahl-Heraeus) and the Tank/Vessel Degasser (VD/VOD) — both achieve the same thermodynamic goal (low pressure above the melt) but by fundamentally different engineering approaches.
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