Vacuum Melting and Remelting Processes - ASM International

ASM Handbook Volume 15: Casting (#05115G)
Beam rot
on the pc
Programmed electron beams
for refining low-density
inclusions and maintaining a
flat. shallow inaot oool
Fig. 6 Schematic of the continuous flow melting process
means. Impurities denser than the melt, such
as tungsten carbide tool tips in titanium, are
removed by sedimentation. Inclusions with
densities such that efficient flotation or sedi-
mentation does not occur can be partially
removed by adhesion to the slag raft.
Hearth dimensions are based on the type
and amount of refining required. For exam-
ple, hearths for vacuum distillation should
be nearly square and relatively deep to
allow sufficient melt stirring. For flotation
refining, the hearth should be long and
narrow (for superalloys, approximately 10
mm, or 0.4 in., of hearth length for each 100
Vacuum Melting and Remelting Processes / 415
3n beam
feedstock
lat
9ntal bar feeding
per
anical
val of
~ions
kg/h, or 220 lb/h, of melt rate is recommend-
ed). Hearths for titanium alloy scrap recy-
cling can be relatively short if all the mate-
rials can be transported to the pool of the
hearth rather than to the ingot pool.
Feeding. Material feeding criteria include
100% homogenous material transportation
to avoid uncontrolled evaporation of alloy-
ing elements and correct feeding into or
above the hearth pool. Horizontal feeding of
compacted, premelted, or cast material is
most often used. Loose scrap and raw mate-
rial are used only when compaction is too
expensive. Feeding of liquid metal was used
Table 4 Comparison of the characteristics of drip melting and continuous
flow melting
Characteristic Refractory metals Reactive metals, superalloys, and specialty steels
Power density ............................. High Soft; smoothly distributed
Inclusions ............................. Irrelevant Must be removed
Ingot shape and structure ............... Round; coarse grain Round or flat; fine grain, segregation-free
Mass production ....................... Low High
Competitive economical processes ....... Vacuum arc remelting Vacuum arc remelting; electroslag remelting
Preferred method ...................... Drip melting Continuous flow melting
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in one of the first continuous flow melting
furnaces to produce a ferritic steel in a vacu-
um induction furnace (Ref 10). Postrefining
was carried out in a cascade of five hearths
1.5 m (60 in.) long and 1 m (40 in.) wide.
Casting and Solidification. The criteria
for material casting and solidification include
the shape of the final product and the solidi-
fication rate required to avoid ingot tears or
other defects and to ensure a homogeneous
ingot structure. The multiple casting of small
ingots is sometimes used, especially when
forging is impossible because of the brittle-
ness of the solidified material (for example
MCrAly wear-resistant coating alloys). The
casting of round and rectangular ingots and
slabs is common practice, and the continuous
casting of hollow ingots is also being used
(Ref I 1). The casting of segregation-free in-
gots and ingots with a fine grain size is under
development to improve the workability of
superalloys (Ref 12, 13).
Continuous Flow
Versus Drip Melting
Table 4 compares the essential features of
drip melting and continuous flow melting.
Generally, continuous flow melting is used
for all refractory metals, superalloys, and
specialty steels, especially when flotation or
sedimentation of inclusions is required.
Drip melting is used for refractory metals
because of their high melting points and the
resulting high heat losses to the water-
cooled copper crucible. Depending on pro-
duction quantity, double or triple drip melt-
ing may require less energy than a single
continuous flow melt of some materials,
such as niobium.
Refining and Production Data
Data on continuous flow electron beam
melting and refining in laboratory and pilot
production furnaces are given in Table 5. The
data demonstrate the effectiveness of the pro-
cess in reducing impurities and interstitial
elements. It can also be seen that the selective
evaporation of chromium from superalloys
can be controlled by the distribution of beam
power at the trough pool and by controlling
trough pool area and melt rate. The selective
evaporation of aluminum from Ti-6A1-4V al-
loy is much more difficult to control; addition-
al aluminum must be used to compensate for
the aluminum evaporated.
Equipment for Continuous
Flow Electron Beam Melting
The equipment required for continuous
flow melting is different from that used in
drip melting mainly because of the trough
and the somewhat larger melting chamber.
In addition, because of the materials often
melted in the continuous flow process (su-
peralloys and titanium alloys), additional
instrumentation is often provided. This may
include an ingot pool level control system,