TELEDYNE ALLVAC'S PLASMA COLD HEARTH FURNACE
One of the first design decisions made was to use plasma rather than
electron beam as the heat source. A fair comparison of the two methods is
complex and not within the scope of this report. The major factor
influencing the choice of plasma, however, was the high vacuum conditions
required for electron beam melting. The significant evaporation of alloying
elements that occurs under these conditions makes ingot chemistry control
very difficult and also results in a significant yield loss. Both of these
concerns are eliminated with plasma.
Teledyne Allvac's plasma furnace, supplied by Retech Inc, uses four 750
kilowatt transferred arc plasma torches. One torch heats each of three
hearth pools and the fourth heats the ingot top (Fig. 1). Metal flow from
the melting area is carried through two right angles and three shallow lips
before reaching the ingot. A steel wall provides a barrier between the
initial melting pool and the ingot mold. This turning of the flow through a
180 degree angle with an intervening barrier wall is an important feature
as it prevents unmelted material from moving directly into the ingot and
short circuiting the defect removal process. Plasma torch movement is
controlled through easily programmed patterns. The same microcomputer that controls torch movement also provides ingot withdrawal control.