Meaning
Refining processes used in secondary metallurgy reduce the concentrations of dissolved gases and impurities in molten steel to ensure high product quality. Ladle chemistry degassing removes hydrogen, nitrogen, and oxygen from the liquid steel under a deep vacuum before the metal is cast. This treatment is regulated under Chinese metallurgical standards, including JB/T 5000, to ensure the cast products meet mechanical safety criteria.
The vacuum level and agitation rate must be carefully controlled to achieve the required chemical composition.
Vacuum Agitation
Applying a strong vacuum over the molten metal causes dissolved gases to escape rapidly from the liquid solution. Bubbles of inert argon gas are injected from the bottom of the ladle to stir the steel and bring all portions of the melt into contact with the vacuum interface. This continuous movement accelerates the rate of gas removal and helps homogenize both the temperature and the chemical composition of the batch.
The stirring action also assists in separating non-metallic inclusions from the liquid steel.
Impurity Removal
Reducing dissolved hydrogen and nitrogen is critical to preventing internal cracking and brittleness in the final forged products. If the gas concentrations remain too high, the metal can experience hydrogen-induced cracking during cooling or under load. The degassing process allows manufacturers to achieve low gas levels, often below two parts per million for hydrogen.
Achieving this purity requires maintained vacuum pressures below a few millibars for a duration determined by the initial chemistry of the melt.
Quality Control
Assuring chemical composition involves taking samples of the molten metal before, during, and after the vacuum treatment to verify the progress of the refining action. Metallurgical technicians use optical emission spectrometry to measure the exact elemental concentrations in the samples. If the results show that gas levels are within the tolerated limits, the steel can be released for continuous casting.
This step prevents expensive casting errors, and any deviation must be corrected with further vacuum treatment to avoid the rejection of the entire heat of steel. The process is critical for producing alloys intended for high-pressure gas cylinders, wind turbines, or aerospace components. Detailed logs of the vacuum level, gas consumption, and intermediate chemical analysis are archived by the quality department to provide full traceability for external compliance audits and customer inspections.
