Characteristics of the molybdenum-depositing arc and the metal-arc melting process

The arc melting of metals such as molybdenum, using the consumable-electrode process, is described. After a discussion of the nature, the shape and the stability of the arc, consideration is given to its behaviour inside a water-cooled copper mould when using a.c. and d.c. power supplies both under vacuum and in an argon atmosphere. The effect these factors have on ingot quality and on mould life is also discussed. The cause and detrimental effects of an unintentional electrical discharge which occurred in a prototype melting-plant are described in detail, and suggestions are made for minimizing the occurrence of the phenomenon. Experiments have been conducted to determine certain relationships between arc voltage, gas pressure and arc length for the high-intensity low-pressure molybdenum-depositing arc. The shapes of burn-offs are related to the current supply used: electrode-negative burn-offs are of two principal types, one conical and the other characterized by two drip-points; electrode-positive burn-offs have hemispherical ends; and a.c. burn-offs have almost flat ends. A theory is suggested to explain the configurations. The relationship between burn-off rate and current has been determined over a wide range of conditions; globule size was found to be related to the type of power supply used, and an explanation of this is included.