U.S. patent application number 09/682815 was filed with the patent office on 2003-04-24 for in situ electroslag refining hot start.
Invention is credited to Benz, Mark Gilbert, Carter, William Thomas JR., Knudsen, Bruce Alan, Zabala, Robert John.
Application Number | 20030075010 09/682815 |
Document ID | / |
Family ID | 24741255 |
Filed Date | 2003-04-24 |
United States Patent
Application |
20030075010 |
Kind Code |
A1 |
Benz, Mark Gilbert ; et
al. |
April 24, 2003 |
IN SITU ELECTROSLAG REFINING HOT START
Abstract
An electroslag refining apparatus includes upper and lower
integral crucibles, with the lower crucible having a drain. In situ
hot start is effected by depositing in the lower crucible a
pre-refined starter. The starter is melted in the lower crucible to
form a starter pool, and slag is deposited atop the starter pool
for being melted thereby to develop a slag pool thereatop. An ingot
electrode is lowered through the upper crucible to immerse a tip
thereof into the slag pool. The electrode is powered to effect
resistance heating of the slag pool to melt the electrode tip. The
slag and starter pools are increased in volume into the upper
crucible, with the drain then being opened to effect steady state
operation.
Inventors: |
Benz, Mark Gilbert; (Burnt
Hills, NY) ; Carter, William Thomas JR.; (Galway,
NY) ; Zabala, Robert John; (Schenectady, NY) ;
Knudsen, Bruce Alan; (Amsterdam, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY
GLOBAL RESEARCH CENTER
PATENT DOCKET RM. 4A59
PO BOX 8, BLDG. K-1 ROSS
NISKAYUNA
NY
12309
US
|
Family ID: |
24741255 |
Appl. No.: |
09/682815 |
Filed: |
October 22, 2001 |
Current U.S.
Class: |
75/10.24 ;
266/200; 266/44 |
Current CPC
Class: |
F27B 14/20 20130101;
F27D 9/00 20130101; F27D 2009/0013 20130101; F27D 11/04 20130101;
F27B 14/14 20130101; F27B 14/063 20130101; Y02P 10/253 20151101;
C22B 9/18 20130101; Y02P 10/25 20151101; F27D 3/1536 20130101; F27D
19/00 20130101; F27D 11/10 20130101; F27D 3/1509 20130101 |
Class at
Publication: |
75/10.24 ;
266/200; 266/44 |
International
Class: |
C21C 005/52; C21B
013/00 |
Claims
1. A method for starting an electroslag refining apparatus
including upper and lower integral crucibles, with said lower
crucible having a drain, said method comprising: depositing in said
lower crucible a pre-refined starter; melting said starter in said
lower crucible to form a starter pool; depositing slag atop said
starter pool for being melted thereby to develop a slag pool atop
said starter pool; lowering an ingot electrode through said upper
crucible to immerse a tip thereof into said slag pool; powering
said electrode to effect resistance heating of said slag pool to
melt said electrode tip; and continuing depositing said slag and
lowering said electrode to increase volumes of said slag and
starter pools upwardly into said upper crucible.
2. A method according to claim 1 wherein said starter melting step
comprises induction heating said starter in said lower crucible to
melt said starter to form said starter pool.
3. A method according to claim 2 wherein: said slag includes slag
particles; and said starter includes starter particles melted to
form said starter pool having an initial height selected to prevent
said deposited slag particles from submerging through said starter
pool and being entrapped adjacent said drain.
4. A method according to claim 2 wherein said electrode is lowered
through said upper crucible into said lower crucible to initially
immerse said tip into said slag pool while maintaining a clearance
between said tip and said lower crucible.
5. A method according to claim 4 wherein said electrode decreases
in size toward said tip to match size with said slag pool in said
lower crucible during said initial immersion.
6. A method according to claim 2 further comprising forming a
starter skull atop said lower crucible and over said drain to
restrain draining of said starter pool therethrough.
7. A method according to claim 6 further comprising cooling said
drain to solidify from said starter pool said starter skull to line
said lower crucible atop said drain.
8. A method according to claim 7 further comprising opening said
drain upon said slag and starter pools rising into said upper
crucible to discharge therefrom a melt stream refined from said
electrode.
9. A method according to claim 8 wherein said drain is opened by
induction heating said starter skull at said drain to melt said
skull for discharge therethrough as said melt stream.
10. A method according to claim S wherein said induction heating at
said drain and at said lower crucible are independent.
11. An electroslag refining apparatus comprising: upper and lower
integral crucibles, with said lower crucible having a drain; means
for depositing in said lower crucible a starter of pre-refined
metal; means for melting said starter in said lower crucible to
form a starter pool; means for depositing slag atop said starter
pool for being melted thereby to develop a slag pool atop said
starter pool; means for lowering an ingot electrode through said
upper crucible 12 to immerse a tip thereof into said slag pool;
means for powering said electrode to effect resistance heating of
said slag pool to melt said electrode tip; and wherein said slag
depositing means and electrode lowering means are configured to
continue depositing said slag and lowering said electrode to
increase volumes of said slag and starter pools upwardly into said
upper crucible.
12. An apparatus according to claim 11 wherein said melting means
comprise a first induction heater surrounding said lower crucible
to melt said starter to form said starter pool.
13. An apparatus according to claim 12 further comprising means for
forming a starter skull atop said lower crucible and over said
drain to restrain draining of said starter pool therethrough.
14. An apparatus according to claim 13 further comprising means for
cooling said drain to solidify from said starter pool said starter
skull to line said lower crucible atop said drain.
15. An apparatus according to claim 14 further comprising means for
opening said drain upon said slag and starter pools rising into
said upper crucible to discharge therefrom a melt stream refined
from said electrode.
16. An apparatus according to claim 15 further comprising a second
induction heater surrounding said drain for induction heating said
starter skull at said drain to melt said skull for discharge
therethrough as said melt stream.
17. An apparatus according to claim 16 wherein said first and
second induction heaters are separate and independent.
18. An apparatus according to claim 17 wherein: said lower crucible
is conical; and said electrode decreases in size toward said tip to
match size with said slag pool in said lower crucible during said
initial immersion.
19. An apparatus according to claim 18 wherein said electrode tip
is conical to maintain a clearance with said conical lower
crucible.
20. An apparatus according to claim 17 wherein said first induction
heater extends upwardly along said lower crucible above said drain
to form said starter pool having an initial height selected to
prevent said deposited slag particles from submerging through said
starter pool and being entrapped adjacent said drain.
Description
BACKGROUND OF INVENTION
[0001] The present invention relates generally to electroslag
refining, and, more specifically, to electroslag refining of
superalloys.
[0002] Electroslag refining is a process used to melt and refine a
wide range of alloys for removing various impurities therefrom.
U.S. Pat. No. 5,160,532, Benz et al., discloses a basic electroslag
refining apparatus over which the present invention is an
improvement. Typical alloys which may be effectively refined using
electroslag refining include those based on nickel, cobalt,
zirconium, titanium, or iron. The initial, unrefined alloys are
typically provided in the form of an ingot which has various
defects or impurities which are desired to be removed during the
refining process to enhance metallurgical properties thereof
including oxide cleanliness, for example.
[0003] In a conventional electroslag apparatus, the ingot is
connected to a power supply and defines an electrode which is
suitably suspended in a water cooled crucible containing a suitable
slag corresponding with the specific alloy being refined. The slag
is heated by passing an electric current from the electrode through
the slag into the crucible, and is maintained at a suitable high
temperature for melting the lower end of the ingot electrode. As
the electrode melts, a refining action takes place with oxide
inclusions in the ingot melt being exposed to the liquid slag and
dissolved therein, Droplets of the ingot melt, fall through the
slag by gravity, and are collected in a liquid melt pool at the
bottom of the crucible. The slag, therefore, effectively removes
various impurities from the melt to effect refining thereof.
[0004] The refined melt may be extracted from the crucible by a
conventional induction-heated, segmented, water-cooled copper guide
tube. The refined melt extracted from the crucible in this manner
provides an ideal liquid metal source for various solidification
processes including, for example, powder atomization, spray
deposition, investment casting, melt-spinning, strip casting, and
slab casting.
[0005] In the exemplary electroslag apparatus introduced above, the
crucible is conventionally water-cooled to form a solid slag skull
on the surface thereof for bounding the liquid slag and preventing
damage to the crucible itself as well as preventing contamination
of the ingot melt from contact with the parent material of the
crucible, which is typically copper. The bottom of the crucible
typically includes a water-cooled, copper cold hearth against which
a solid skull of the refined melt forms for maintaining the purity
of the collected melt at the bottom of the crucible. A discharge
guide tube below the hearth is also typically made of copper and is
segmented and water-cooled for also allowing the formation of a
solid skull of the refined melt for maintaining the purity of the
melt as it is extracted from the crucible.
[0006] A plurality of water-cooled induction heating electrical
conduits surround the guide tube for inductively heating the melt
thereabove for controlling the discharge flow rate of the melt
through the tube. In this way, the thickness of the skull formed
around the discharge orifice in the guide tube may be controlled
and suitably matched with melting of the ingot for obtaining a
substantially steady state production of refined melt which is
drained by gravity through the guide tube.
[0007] In order to achieve steady state operation of the
electroslag refining apparatus, the apparatus must be suitably
started without introducing undesirable contamination or
impurities. In a conventional cold start method, a solid starter
plate is fixed into position at the bottom of the crucible and
above the discharge guide tube. The starter plate is formed of the
same material as the ingot electrode except that it has been
pre-refined and suitably machined for integral assembly into the
electroslag refining apparatus. It is therefore relatively
expensive and introduces additional complexity to the overall
apparatus.
[0008] In order to effect a cold start, the electrode is positioned
closely atop the starter plate, and conventional slag in
particulate form is deposited atop the starter plate around the
electrode. An electrical current is passed through the electrode to
the starter plate and then through the atmosphere to cause an
electrical arc to jump therebetween. The heat from the arc melts
the surrounding solid slag. When sufficient slag is melted, the
electrode is lowered into the slag to extinguish the arc, at which
time power to the electrode effects direct resistance heating of
the slag pool for increasing its temperature.
[0009] The heated slag pool then continues to melt the tip of the
electrode and the starter plate until a hole is melted through the
starter plate and liquid metal fills the crucible atop the guide
tube. The hole through the starter plate enlarges until it reaches
the outer perimeter of the plate, and resulting refined metal and
slag skulls line the crucible and the guide tube. Steady state
operation is reached when the rate of melting of the electrode and
discharge flowrate from the guide tube are substantially equal.
[0010] Although the starter plate is initially formed of
pre-refined metal, the electric arcing cold start introduces
undesirable nitrides therefrom which are not removed by the
electroslag refining process. If the starter plate melts too early
during the startup process, unrefined metal or slag may flow
through the guide tube causing undesirable impurities in the
discharge stream.
[0011] Alternatively, the electroslag refining apparatus may be
brought to steady state operation using a conventional hot start.
In this procedure, slag is melted in an external furnace and
deposited into the electroslag refining crucible to allow immediate
resistance heating of the slag and corresponding melting of the
electrode. However, a starter plate is still required to avoid
transfer of undesirable slag through the guide tube. Although the
undesirable nitrides are not introduced, the system is relatively
more complex and expensive in view of the required external
furnace, and the requirements of the pre-refined and machined
expensive starter plate.
[0012] Accordingly, it is desired to provide an improved
electroslag refining apparatus and method for in situ hot start
thereof.
SUMMARY OF INVENTION
[0013] An electroslag refining apparatus includes upper and lower
integral crucibles, with the lower crucible having a drain. In situ
hot start is effected by depositing in the lower crucible a
pre-refined starter. The starter is melted in the lower crucible to
form a starter pool, and slag is deposited atop the starter pool
for being melted thereby to develop a slag pool thereatop. An ingot
electrode is lowered through the upper crucible to immerse a tip
thereof into the slag pool. The electrode is powered to effect
resistance heating of the slag pool to melt the electrode tip. The
slag and starter pools are increased in volume into the upper
crucible, with the drain then being opened to effect steady state
operation.
BRIEF DESCRIPTION OF DRAWINGS
[0014] The invention, in accordance with preferred and exemplary
embodiments, together with further objects and advantages thereof,
is more particularly described in the following detailed
description taken in conjunction with the accompanying drawings in
which:
[0015] FIG. 1 is a schematic representation of an electroslag
refining apparatus in accordance with an exemplary embodiment of
the present invention for effecting in situ hot starting.
[0016] FIG. 2 is a schematic representation of upper and lower
crucibles found in the apparatus illustrated in FIG. 1 in which hot
starting is initiated using a pre-refined starter.
[0017] FIG. 3 is a view like FIG. 2 wherein the starter is melted
by induction heating to form a starter pool.
[0018] FIG. 4 is a view like FIG. 3 wherein slag is deposited atop
the starter pool for being melted thereby to form a slag pool.
[0019] FIG. 5 is a view like FIG. 4 wherein an ingot electrode is
lowered atop the slag pool for resistance heating thereof to
increase the volumes of the starter and slag pools during hot
starting.
DETAILED DESCRIPTION
[0020] Illustrated schematically in FIG. 1 is an electroslag
refining apparatus 10 in accordance with an exemplary embodiment of
the present invention. The apparatus 10 includes a cylindrical
upper crucible 12 and a conical lower crucible 14 extending
integrally therebelow. The lower crucible 14 includes a central
downspout or drain 16 extending downwardly.
[0021] Suitably suspended in the upper crucible 12 is an ingot 18
of a suitable alloy for undergoing electroslag refining.
Conventional means 20 are provided for feeding or lowering the
ingot 18 into the upper crucible 12 at a suitable feed rate. The
lowering means 20 may have any suitable form including a drive
motor and transmission rotating a screw, which in turn lowers or
translates downwardly a support bar 20a fixedly joined at one end
to the top of the ingot 18.
[0022] The ingot 18 is formed of any suitable alloy requiring
electroslag refining such as nickel or cobalt based superalloys,
for example. A suitable slag 22 is provided inside the crucible 12
and may take any conventional composition for refining a specific
material of the ingot 18. A dispenser or means 24 is provided for
depositing the slag 22 into the crucible 12, with the slag 22
taking the exemplary form of relatively small particles or nuggets.
The slag dispenser 24 may take any conventional form including a
chute for conveying the slag 22 into the crucible. If desired, the
slag 22 may be manually inside the crucible 12.
[0023] The ingot 18 includes a tip 18a at its lower end, and
conventional means 26 are provided for melting the ingot tip 18a as
it is lowered and fed into the crucible 12. The tip melting means
26 is in the exemplary form of a suitable alternating or direct
current power supply electrically joined to the ingot 18 through
the support bar 20a by a suitable electrical lead 26a. Electrical
current is carried through the ingot 18, which defines an
electrode, and through the slag 22, in liquid form, to the crucible
12, with a return electrical lead 26b to the power supply. In this
way, the means 26 are effective for powering the ingot electrode 18
to effect resistance heating of the slag 22 in its liquid form to a
suitably high temperature to melt the electrode tip 18a suspended
therein for consuming the electrode 18 as it is lowered during the
electroslag refining process.
[0024] Suitable means 28 are provided for cooling the upper
crucible 12, and a portion of the lower crucible 14, from the heat
generated during the refining process. The upper and lower
crucibles 12,14 may take any conventional form including hollow
copper jackets disposed inflow communication with the cooling means
28 which circulate through the crucibles cooling water for removing
heat therefrom. The cooling means 28 therefore include a suitable
circulating pump and heat exchanger for removing heat as the water
is circulated through the crucibles.
[0025] The slag 22 is initially in solid form and is initially
melted in a startup process as described hereinbelow to develop a
molten slag pool 30. The slag pool 30 undergoes resistance heating
as electrical current passes from the electrode 18 through the slag
pool 30 and to the crucible 12 in the electrical path to the
powering means 26. The temperature of the slag pool 30 is thereby
increased to melt the electrode tip 18a which forms a pool 32 of
refined ingot material below the slag pool 30.
[0026] The refined pool 32 is denser than the slag pool 30, and as
the ingot electrode 18 is consumed at its tip by melting thereof,
the melt travels downwardly through the slag pool 30 which removes
impurities therefrom for effecting electroslag refining, with the
refined pool 32 accumulating the refined melt therein. Since the
crucibles 12,14 are water cooled, corresponding slag and refined
metal skulls 30a and 32a develop over the entire submerged inner
surfaces of the crucibles to provide a continuous lining separating
the copper crucibles from the refined melt pool 32 and slag pool
30. This prevents contamination of the refined pool 32 from the
copper crucibles themselves.
[0027] The lower crucible 14 preferably includes a
circumferentially segmented, water-cooled copper induction guide
(CIG) tube 34 at the bottom thereof which includes the drain 16 for
extracting or discharging the refined pool 32 therefrom as a molten
melt stream 32b. The refined discharge stream 32b may then be used
for any suitable application including, for example, powder
atomization, spray deposition, investment casting, melt-spinning,
strip casting, and slab casting.
[0028] The guide tube 34 is conventionally configured and water
cooled so that the refined skull 32a extends downwardly through the
drain 16 and defines an orifice 16a through which the melt stream
32b may be discharged without contamination from the copper guide
tube 34 itself. The thickness of the skull 32a at the drain 16 may
be controlled to control the size of the orifice 16a and in turn
control the discharge flow rate of the melt stream 32b in a
conventional manner.
[0029] More specifically, the guide tube 34 includes a plurality of
circumferentially spaced apart guide fingers having a suitable
electrical insulation therebetween. The fingers are preferably
hollow for circulating cooling fluid such as water therethrough
during operation. A first or upper induction heater 36
circumferentially surrounds the lower crucible 14 above the drain
16. And, a second or lower induction heater 38 circumferentially
surrounds the drain 16 below the first heater 36. Both first and
second induction heaters 36,38 are conventional in configuration
and separately include respective pluralities of hollow
electrically conducting tubes operatively joined to conventional
first and second power supplies 36a and 38a.
[0030] The power supplies 36a,38a are effective for providing
electrical current to the corresponding heaters 36,38 for
inductively heating the melt pool 32 within the local area defined
by the guide tube 34. The power supplies 36a,38a also include
suitable means for circulating a cooling fluid such as water
through the hollow induction tubes of the first and second heaters
36,38 for cooling the heaters themselves as well as providing
additional cooling of the guide tube 34.
[0031] But for the present invention as described hereinbelow, the
electroslag refining apparatus 10 described above is conventionally
configured and operated for electroslag refining the ingot
electrode 18 to produce the discharge stream 32b of refined metal
for use as desired. The first and second induction heaters 36,38
are conventionally operated for controlling the local heating and
cooling of the melt pool 32 above the guide tube 34, and
correspondingly controlling the diameter of the drain orifice 16a
to control discharge flow rate. However, in order to reach steady
state operation of the apparatus 10, the apparatus 10 must be
suitably started without introducing undesirable impurities which
would degrade the resulting discharge stream 32b.
[0032] In accordance with the present invention, an improved
method, and modification of the basic electroslag refining
apparatus 10, are provided to effect in situ hot start of the
apparatus 10 in a relatively simple and easily implemented method
without the need for a conventional starter plate. The startup
process may therefore be decreased in cost and complexity without
introducing undesirable contamination.
[0033] More specifically, the improved method and apparatus for
effecting hot starting of the refining apparatus 10 are illustrated
schematically in FIGS. 2-5, with FIG. 1 illustrating the subsequent
steady state operation of the apparatus 10. As initially
illustrated in FIG. 2, the hot start method begins with the empty
crucibles 12,14 in which a pre-refined starter 40 is initially
deposited automatically or manually. The starter is preferably in
the form of a plurality of loose, solid starter particles which are
deposited and accumulate at the bottom of the lower crucible 14
above the drain 16.
[0034] The starter 40 is the same material as that found in the
specific ingot electrode 18, and is preferably pre-refined. For
example, the starter particles 40 may be recycled chips obtained as
the scrap material from machining operations of previously refined
metal.
[0035] The chips are suitably cleaned to remove any undesirable
surface contamination therefrom prior to being deposited in the
lower crucible 14. The starter particles 40 may also be
manufactured specifically for that purpose.
[0036] In this way, the additional expense and machining associated
with a conventional starter plate are eliminated. And, the
crucibles 12,14 may now be configured without mounts or flanges for
supporting an integral starter plate now eliminated.
[0037] If desired, the drain 16 may be initially plugged by a
specifically configured starter particle 40a in the form of a ball
or sphere plug, for example, of suitably large diameter for
preventing its passage through the drain 16. The starter particles
40 may be suitably deposited into the lower crucible 14 by a
suitable dispenser or means 42 which deposits the starter particles
40 downwardly through the upper crucible 12 for accumulation in the
lower crucible 14. The dispenser 42 may include a suitable chute
for conveying the particles, which may be conveyed either
automatically or manually if desired.
[0038] In the next step illustrated in FIG. 3, the starter
particles 40 are suitably melted in the lower crucible 14 to form a
starter melt pool 40b. In this regard, the upper induction heater
36 may be electrically powered ON by its power supply 36a to
inductively melt the starter particles 40 through the walls of the
guide tube 34 to form the starter pool 40b.
[0039] In the setup step illustrated in FIG. 2, both heaters 36,38
are preferably powered OFF, and in the initial melting step
illustrated in FIG. 3, the upper heater 36 is powered ON, while the
lower heater 38 remains powered OFF.
[0040] However, both power supplies 36a,38a are operated for
circulating the cooling water through the respective heaters 36,38
for cooling thereof. The circulating coolant in the lower heater 38
is therefore effective for cooling the drain 16 for forming a
solidified starter skull 40c atop the lower crucible 14 and over
the drain 16 to restrain or prevent draining of the starter pool
40b therethrough.
[0041] Depending upon the heat input from the upper heater 36, the
starter plug 40a may remain solid or may initially melt along with
the other starter particles 40 until resolidification thereof
occurs from contact with the cold walls of the guide tube 34. In
either case, an effective starter skull 40c lines the upper surface
of the guide tube 34 and prevents discharge of the starter pool 40b
through the drain 16 during startup. Formation of the starter skull
40c has the additional advantage of preventing contamination of the
starter pool 40b by the copper guide tube 34.
[0042] Since the starter material may now be provided in particle
form in the preferred embodiment, the particles 40 may be
introduced gradually within the melting ability of the upper heater
36. The induction heating effect of the upper heater 36 is confined
to the conical region of the guide tube 34 bounded laterally and
vertically by the heating coils of the upper heater 36. In this way
localized heating of the starter particles 40 is effected for
developing localized melting of the particles to initiate formation
of the starter pool 40b.
[0043] As shown in FIG. 3, a sufficient amount of the starter
particles 40 is initially deposited in the lower crucible 14 and
melted by the upper heater 36 to form the starter pool 40b having a
predetermined volume, and elevation or height A atop the drain
16.
[0044] As shown in FIG. 4, the slag 22, preferably in particle
form, is next deposited atop the starter pool 40b for being melted
thereby to start the liquid slag pool 30b floating atop the starter
particle pool 40b. This may be accomplished by using the slag
dispenser 24 for dispensing the slag particles 22 downwardly
through the upper crucible 12 and into the lower crucible 14 atop
the starter pool.
[0045] The initial vertical height A.sub.1 of the starter pool 40b
is selected to prevent the deposited slag particles 22 from
submerging through the starter pool and being entrapped adjacent to
the drain 16. Since the slag particles 22 are relatively buoyant
compared to the liquid starter pool 40b, a suitable initial height
A.sub.1 of the starter pool is quickly obtained, with corresponding
solidified skulls of the starter and slag particles lining the
inner surface of the lower crucible 14 as the starting process
continues, until an initial volume or vertical height B.sub.1 of
the slag pool 30b is formed atop the starter pool 40b.
[0046] As shown in FIG. 5, the ingot electrode 18 may then be
lowered through the upper crucible 12 into the lower crucible 14 to
immerse the electrode tip 18a into the developing slag pool 30b.
The powering means 26 are then powered ON for powering the
electrode 18 to effect resistance heating of the slag pool to melt
the electrode tip 18a to thereby continue the hot start process by
adding melt from the electrode 18 to the developing starter pool
40b. Conventional resistance heating of the slag pool then takes
over from the induction heating provided by the upper heater 36 to
continue the in situ hot start operation.
[0047] By continuing depositing slag 22 and lowering the electrode
18, the respective volumes of the slag and starter pools 30,32 are
increased, with the respective vertical height B.sub.2,A.sub.2
rising upwardly into the upper crucible 12. A suitable electrical
controller 44 may be operatively joined to the lowering means 20
and the slag dispenser 24 for coordinating depositing of the slag
particles 22 and lowering of the electrode 18 for increasing the
respective heights of the slag and starter pools.
[0048] The additional melted slag 22 is added to the slag starter
pool to eventually define the steady state slag pool 30 having a
suitable volume and vertical height B.sub.3 as illustrated in FIG.
1. And, the addition of molten metal from the ingot electrode 18 to
the starter pool increases the volume thereof to define the
resulting melt pool 32 having a suitable steady state vertical
height A.sub.3 as illustrated in FIG. 1. The corresponding skulls
30a and 32a grow to completely line the submerged portions of the
upper and lower crucibles 12,14 around the respective pools
30,32.
[0049] In this way, both the starter particles 40 and slag
particles 22 are introduced in small volumes in the lower crucible
14 and suitably heated firstly by the upper induction heater 36 and
then by resistance heating through the electrode 18 to develop and
grow the respective slag and melt pools. The corresponding skulls
also develop along the inner surfaces of the crucibles 12,14 for
preventing contamination of the refined melt pool 32 from the
copper crucibles.
[0050] The powering means 26 may be further operated to
additionally heat the slag pool 30 using resistance heating to a
suitable temperature, having superheat for example, so that with
suitable volumes of the slag and melt pools 30,32, steady state
operation of the electroslag refining apparatus 10 may begin.
[0051] Accordingly, after the slag and starter pools have suitably
risen into the upper crucible 12, the drain 16 may be suitably
opened to discharge therefrom the melt stream 32b as illustrated in
FIG. 1. The drain 16 is preferably opened by induction heating the
starter skull 40c atop the drain 16 to melt the starter skull 40c
for forming the drain orifice 16a therein for discharging by
gravity the melt stream 32b. The first and second induction heaters
36,38 are separate and independent for separately controlling
heating and cooling within the corresponding portions of the guide
tube 34. In this way, the respective skulls formed along the inner
surfaces of the guide tube 34 may be controlled.
[0052] The upper induction heater 36 is specifically configured in
accordance with the present invention to have a suitable vertical
extent along the guide tube 34 for initially melting the starter
particles 40 for effecting a suitably high starter pool 40b.
Because of this additional function, the vertical extent of the
upper heater 36 is greater than that in a conventional heater that
is not used for starting. The upper heater 36 is otherwise
conventionally used during steady state operation for controlling
the local heating of the melt above the guide tube 34 and
controlling the thickness of the refined skull 32a. And, the lower
heater 38 may be conventionally configured and operated for steady
state operation. And for hot start, it is preferably powered OFF,
and provides solely internal cooling through the hollow conduits
thereof for maintaining closed the drain 16 as the slag and starter
pools accumulate in volume and height.
[0053] The modifications to the electroslag refining apparatus 10
disclosed above now enable that apparatus to effect in situ hot
starting without the need for the conventional solid disk, machined
starter plate, or arc heating to melt the slag, or an external
furnace therefor. The starter material is now independent from the
refining apparatus itself, and does not form a part thereof.
Instead, the starter material is merely a consumable element, and
in simple particle form allows continuous formation of the starter
pool with a corresponding protective skull lining to prevent
contamination thereof. Upon reaching steady state, the melt pool 32
may be suitably discharged through the drain 16 for conventional
use as desired.
[0054] As illustrated in FIG. 1, the ingot electrode 18 is
preferably in cylindrical form and has a maximum outer diameter for
fitting within the available space in the upper crucible 12. Since
the lower crucible 14 is preferably conical, the use of a
conventional cylindrical ingot electrode 18 is not desirable since
it may prematurely contact the inner surface of the lower crucible
14 as it is lowered for hot starting.
[0055] Accordingly, in accordance with another feature of the
present invention, the electrode tip 18a as illustrated in phantom
in FIG. 1 and in solid line in FIG. 5, is initially sized and
configured to maintain a suitable horizontal and vertical clearance
between the tip 18a and the lower crucible 14 during startup
immersion of the tip 18a into the starter pool 40b. The electrode
18 therefore preferably decreases in size or outer diameter toward
the tip 18a to closely match the size or diameter of the developing
slag pool 30b in the lower crucible 14 during startup immersion as
illustrated in FIG. 5. The preferred configuration of the electrode
tip 18a before it is consumed, therefore preferably matches both
the configuration of the lower crucible 14 and developing slag pool
30b therein for maintaining a suitable clearance around the
circumference of the electrode 18 in the lower crucible 14 for
preventing undesirable contact therewith.
[0056] The electrode tip 18a should preferably contact only the
slag pool 30b during immersion so that it may be melted by the slag
pool 30b and add to the volume of the starter pool 40b. Once the
slag pool 30b exceeds the elevation of the transition between the
conical lower crucible 14 and the cylindrical upper crucible 12,
the ingot electrode 18 may have its maximum outer diameter to
maximize melt production since the shape of the tip 18a thereof is
no longer relevant after the initial in situ hot starting.
[0057] While there have been described herein what are considered
to be preferred and exemplary embodiments of the present invention,
other modifications of the invention shall be apparent to those
skilled in the art from the teachings herein, and it is, therefore,
desired to be secured in the appended claims all such modifications
as fall within the true spirit and scope of the invention.
[0058] Accordingly, what is desired to be secured by Letters Patent
of the United States is the invention as defined and differentiated
in the following claims in which we claim:
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