Evaluating the Risk of Corona Discharge in Superalloy Vacuum Induction Melting Furnace Applications
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Copyright Ó 2019 American Foundry Society https://doi.org/10.1007/s40962-019-00392-3
Abstract Corona discharge, or gas breakdown, is undesired in superalloy vacuum induction melting (VIM) applications as it can damage insulation, produce ozone and subsequently oxidize and contaminate the melt. Gas breakdown also introduces uncontrolled variations to the casting process and increases the risk of needing to scrap the end product. To reduce the risk of corona, a rule-of-thumb of 150 to 1000 micron (150 mTorr to 1 Torr) was developed in the past as a No-Go Pressure Zone. Operating within this zone has been observed to create an unwanted, readily ionizable environment. An analytical model is developed within this work to evaluate the historical rule-of-thumb. The analytical model incorporates the following elements: Paschen’s law, gas compressibility, gas and insulation permittivity, and electric field non-uniformity. The model is then applied
to a sampling of VIM furnace designs to evaluate the shifts of the No-Go Pressure Zone due to variations in configuration and other operating conditions. In the end, the objective is to improve VIM furnace design and operation to reduce the risk of corona discharge—thereby improving the casting process, practice of alloy melting and quality of the end product.
Introduction
intensity.15 Paschen’s law explores the physical mechanism that leads to gas breakdown and is further discussed within ‘‘Paschen’s Law’’ section.
Background A corona discharge, also called gas breakdown, occurs through an ionized fluid from an electrically charged conductor. As the potential gradient of the electric field around the conductor continues to increase, it will eventually cause electrical breakdown or arcing. This occurs when the electric field intensity surpasses a threshold value specific to the properties of the system. Corona develops before this breakdown and is often observed as a bluish glow. Corona discharges may be created by applying high voltage to a small electrode point or obtrusion. They may also occur by achieving appropriate environment conditions at a corresponding breakdown voltage limit. This relationship between voltage and environment can be characterized by Paschen’s law. More specifically, Paschen’s law compares breakdown voltage to gas pressure and the gap distance between anode and cathode that creates the electrical field
International Journal of Metalcasting
Keywords: corona discharge, breakdown voltage, Paschen’s law, Clausius–Mossotti relation Redlich–Kwong equation of state, non-uniform electric field, vacuum induction melting, furnace design, superalloys
As the gas breaks down, it produces light. The bluish glow that the corona discharge produces comes from electrons recombining with positive ions to reform neutral atoms. When the atoms reform, they release a photon of light.14 These released photons contribute to the secondary ionization processes that ionize more atoms and add to the corona discharge. The electrical breakdown that produces a corona discharge a
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