What is meant by arc stability, and what factors affect the stability of a vacuum arc furnace?
Arc stability in welding refers to the ability of the arc to maintain a steady burn without interruptions, drifting, or magnetic deflection. Whether the welding arc burns stably directly affects welding quality and whether the welding process can be successfully carried out. The main factors influencing arc stability include:
- Welder’s skill and technique: Proper control of the welding arc by the operator helps prevent arc interruptions.
- Welding power source:
- If the power source characteristics meet the arc burning requirements, the welding arc will be more stable; otherwise, stability is poor.
- DC welding power sources generally provide better arc stability than AC sources.
- Higher open-circuit voltage makes it easier to strike an arc and improves arc stability, though safety precautions are necessary.
- Welding current: Higher welding current increases arc temperature and ionization in the arc column, enhancing thermionic emission and making the arc burn more stably.
- Electrode coating: Electrodes containing substances with low ionization potential (such as potassium, sodium, calcium oxides) improve gas ionization and conductivity, stabilizing the arc. Conversely, coatings lacking these materials reduce stability.
- Arc length: A short arc can cause short-circuits, while an excessively long arc causes violent swings, reducing stability and producing spatter.
- Welding surface conditions, airflow, and arc deflection: Contaminants like oil, water, or rust reduce arc stability. Airflow, wind, or deflection of the arc also decreases stability.
Additional information about welding arcs:
The high heat generated by an electric arc can melt or vaporize all metals. In industry, arcs are used for welding, melting, or cutting metals—for example, plasma cutters, EDM machines, and steelmaking arc furnaces. Arc lamps and movie projectors also use arc principles.
An arc discharge can be divided into three regions: cathode region, arc column, and anode region. Its conduction mechanism is:
- The cathode emits electrons via field emission and thermionic emission.
- The arc column conducts electricity through collisions among thermally excited particles, producing free electrons and positive ions—a process called thermal ionization.
- The anode collects electrons and generally has less influence on the arc process.
In the arc column, electrons and ions recombine to form neutral particles or diffuse out of the column—a process called deionization. In a stable arc discharge, the ionization rate equals the deionization rate, forming ionization equilibrium, which can be described using the Saha equation.
Difference between consumable and non-consumable vacuum arc furnaces:
- Consumable vacuum arc furnace: The electrode itself is the material being refined. During discharge, the electrode and molten pool form the arc together, and the electrode melts during the process.
- Non-consumable vacuum arc furnace: The electrode is made of high-melting, conductive material and is designed to remain stable. Typically, the molten pool consists of the furnace charge.