U.S. patent number 3,849,584 [Application Number 05/409,329] was granted by the patent office on 1974-11-19 for plasma arc torch.
Invention is credited to Alfred Iosifovich Bukalo, Anatoli Ivanovich Chvertko, Victor Iosifovich Lakomsky, Gary Alexandrovich Melnik, Boris Evgenievich Paton.
United States Patent |
3,849,584 |
Paton , et al. |
November 19, 1974 |
PLASMA ARC TORCH
Abstract
A plasma arc remelting furnace system with improved components
including mechanisms for independently vertically feeding and
oscillating the metal blank through the top of the furnace.
Multiple plasma arc torches (plasmatrons) are installed in a sealed
chamber and adjustable operators enable angular disposition of
individual torches. The torches have improved torch nozzles with
heat sink construction enabling operation at higher temperatures,
better stabilized plasma arcs and longer torch life. The system can
utilize ingot molds of various cross section shapes, i.e., round,
square, rectangular and polygonal and, depending upon mold shape,
the number of torches can differ. Operating circuits are provided
for use with a plurality of different numbers (up to eight) of
torches, either with direct current or alternating current power
sources, and possible variations of circuits will enable using a
larger number of torches.
Inventors: |
Paton; Boris Evgenievich (Kiev,
SU), Lakomsky; Victor Iosifovich (Kiev,
SU), Melnik; Gary Alexandrovich (Kiev, SU),
Chvertko; Anatoli Ivanovich (Kiev, SU), Bukalo;
Alfred Iosifovich (Kiev, SU) |
Family
ID: |
23620014 |
Appl.
No.: |
05/409,329 |
Filed: |
October 24, 1973 |
Current U.S.
Class: |
373/21; 373/18;
219/121.36 |
Current CPC
Class: |
H05B
7/10 (20130101); C22B 9/226 (20130101); H05B
7/00 (20130101); H05H 1/34 (20130101); F27D
11/08 (20130101); H05H 1/3478 (20210501) |
Current International
Class: |
F27D
11/08 (20060101); C22B 9/16 (20060101); C22B
9/22 (20060101); H05H 1/34 (20060101); H05B
7/10 (20060101); H05B 7/00 (20060101); H05H
1/26 (20060101); H05b 007/18 () |
Field of
Search: |
;13/9,9P,18
;219/121P |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Envall; R. N.
Attorney, Agent or Firm: Strauch, Nolan, Neale, Nies &
Kurz
Claims
What is claimed and desired to be secured by Letters Patent is:
1. A plasma arc torch with a center electrode and a nozzle end
structure, said nozzle end structure comprising: an elongate,
hollow body coaxially surrounding and spaced from the lower end of
said electrode; a plurality of circumferentially spaced apart
electrically conductive heat sink inserts mounted in the interior
of said body, around and radially spaced from said electrode, each
said insert having a first portion radially adjacent said electrode
lower end and second portion axially disposed beyond the terminal
end of said electrode; said body providing an annular nozzle
orifice having end means defining a nozzle orifice with a smoothly
curved, convergent profile surface within said body for providing a
substantially laminar flow of plasma gas past said electrode and
inserts.
2. A plasma arc torch as defined in claim 1, wherein said smoothly
curved, convergent profile surface starts at a location upstream of
said inserts and merges into an essentially straight flow path
adjacent the upper ends of said inserts to the terminal end of the
nozzle body.
3. A plasma arc torch as defined in claim 1, wherein at least one
of said inserts has a portion axially disposed to project beyond
the terminal edge of said body.
4. A plasma arc torch as defined in claim 3, wherein the orifice
diameter of said nozzle is in a range of from 10-30 mm.
5. A plasma arc torch as defined in claim 3, wherein all said
inserts have portions axially disposed to project beyond the
terminal edge of said body.
6. A plasma arch torch as defined in claim 3, wherein the distance
of projection of said inserts beyond the terminal edge of said body
is within a range of from 10-15 mm.
7. A plasma arc torch as defined in claim 1, wherein said electrode
terminal end projects beyond the terminal end of said body.
8. A plasma arch torch as defined in claim 7, wherein the distance
of projection of said electrode terminal end is within a range of
from 2-3 mm.
9. A plasma arc torch as defined in claim 8, wherein the orifice
diameter of said nozzle is in a range of from 10-30 mm.
10. A plasma arch torch as defined in claim 1, wherein the diameter
of said electrode is at least 10 mm.
11. A plasma arc torch as defined in claim 10, wherein the current
of the arc operating power can be up to 2,000 amperes and the
electrode is solid.
12. A plasma arc torch as defined in claim 10, wherein the current
of the arc operating power can be above 2,000 amperes and the
electrode is of composite strand structure.
13. A plasma arc torch as defined in claim 1, wherein said inserts
are made from material having a heat conductivity coefficient of at
least 0.3 and a melting temperature of at least 2,500.degree.
C.
14. A plasma arc torch as defined in claim 13, wherein said inserts
are made from tungsten.
15. A plasma arc torch as defined in claim 1, wherein said inserts
are cylindrical.
16. A plasma arc torch as defined in claim 2, wherein at least said
upper ends of said inserts are chamfered.
17. A plasma arc torch as defined in claim 16, wherein both ends of
said inserts are chamfered.
18. A plasma arc torch as defined in claim 1 wherein the shortest
distance from said electrode lower end to said inserts is in a
range of from 2-7 mm.
19. A plasma arc torch as defined in claim 1, wherein said inserts
are cylindrical and the lower inner peripheral portion of said body
has partially cylindrical keyways which receive said inserts.
20. A plasma arc torch as defined in claim 19, wherein said partial
cylindrical keyways have cross-sections exceeding a semicircle.
21. A plasma arc torch as defined in claim 1, wherein said body
includes a hollow chamber and means are provided for circulating a
cooling fluid through said hollow chamber.
22. A plasma arc torch as defined in claim 21, wherein the terminal
end portion of said body disposed radially adjacent said electrode
lower end and adjacent said inserts is solid.
23. A plasma arc torch as defined in claim 22, wherein said body
comprises copper material.
24. A plasma arc torch as defined in claim 3, wherein said
electrode terminal end projects beyond the terminal end of said
body.
25. A plasma arc torch as defined in claim 24, wherein the distance
of projection of said electrode terminal end is within a range of
from 2-3 mm.
26. A plasma arc torch as defined in claim 25, wherein the distance
of projection of said inserts beyond the terminal edge of said body
is within a range of from 10-15 mm.
27. A plasma arc torch as defined in claim 22, wherein the shortest
distance from said electrode lower end to said inserts is in a
range of from 2-7 mm.
28. A plasma arc torch as defined in claim 27, wherein the orifice
diameter of said nozzle is in a range of from 10-30 mm.
29. A plasma arc torch as defined in claim 8, wherein the distance
of projection of said inserts beyond the terminal edge of said body
is within a range of from 10-15 mm.
30. A plasma arc torch as defined in claim 29, wherein the diameter
of said electrode is at least 10 mm.
31. A plasma arc torch as defined in claim 30, wherein the orifice
diameter of said nozzle is in a range of from 10-30 mm.
32. A plasma arc torch as defined in claim 30, wherein the current
of the arc operating power can be up to 2,000 amperes and the
electrode is solid.
33. A plasma arc torch as defined in claim 30, wherein the current
of the arc operating power can be above 2,000 amperes and the
electrode is of composite strand structure.
34. A plasma arc torch as defined in claim 3, wherein said inserts
are made from material having a heat conductivity coefficient of at
least 0.3 and a melting temperature of at least 2,500.degree.
C.
35. A plasma arc torch as defined in claim 34, wherein said inserts
are made from tungsten.
36. A plasma arc torch as defined in claim 3, wherein said inserts
are cylindrical.
37. A plasma arc torch as defined in claim 3, wherein at least said
upper ends of said inserts are chamfered.
38. A plasma arc torch as defined in claim 37, wherein both ends of
said inserts are chamfered.
39. A plasma arc torch as defined in claim 3, wherein the shortest
distance from said electrode lower end to said inserts is in a
range from 2-7 mm.
40. A plasma arc torch as defined in claim 3, wherein said body
includes a hollow chamber and means are provided for circulating a
cooling fluid through said hollow chamber.
41. A plasma arc torch as defined in claim 40, wherein the terminal
end portion of said annular body disposed radially adjacent said
electrode lower end and adjacent said inserts is solid.
42. A plasma arc torch as defined in claim 41, wherein said body
comprises copper material.
43. A plasma arc torch as defined in claim 42, wherein said inserts
are made from tungsten.
44. A plasma arc torch as defined in claim 1, wherein the orifice
diameter of said nozzle is in a range of from 10-30 mm.
45. A plasma arc torch with a center electrode and a nozzle end
structure comprising: an elongate hollow body coaxially surrounding
and spaced from the lower end of said electrode; and a plurality of
circumferentially spaced apart electrically conductive heat sink
inserts mounted in the interior of said body, around and radially
spaced from said electrode, each said insert having a portion
radially adjacent said electrode and an end of at least one of said
inserts projecting axially beyond the terminal end of said
body.
46. A plasma arc torch as defined in claim 45, wherein the orifice
diameter of said nozzle is in a range of from 10-30 mm.
47. A plasma arc torch as defined in claim 45, wherein all said
inserts have portions axially disposed to project beyond the
terminal end of said body.
48. A plasma arc torch as defined in claim 47, wherein the distance
of projection of said inserts beyond the terminal edge of said
annular body is within a range of from 10-15 mm.
49. A plasma arc torch as defined in claim 45, wherein the terminal
end of said electrode projects beyond the terminal end of said
body.
50. A plasma arc torch as defined in claim 49, wherein the distance
of projection of said electrode terminal end is within a range of
from 2-3 mm.
51. A plasma arc torch as defined in claim 50, wherein the orifice
diameter of said nozzle is in a range of from 10-30 mm.
52. A plasma arc torch as defined in claim 45, wherein the diameter
of siad electrode is at least 10 mm.
53. A plasma arc torch as defined in claim 52, wherein the current
of the arc operating power can be up to 2,000 amperes and the
electrode is solid.
54. A plasma arc torch as defined in claim 52, wherein the current
of the arc operating power can be above 2,000 amperes and the
electrode is of composite strand structure.
55. A plasma arc torch as defined in claim 45, wherein said inserts
are made from material having a heat conductivity coefficient of at
least 0.3 and a melting temperature of at least 2,500.degree.
C.
56. A plasma arc torch as defined in claim 55, wherein said inserts
are made from tungsten.
57. A plasma arc torch as defined in claim 45, wherein said inserts
are cylindrical.
58. A plasma arc torch as defined in claim 57, wherein at least the
upper ends of said inserts are chamfered.
59. A plasma arc torch as defined in claim 45, wherein the shortest
distance from said electrode lower end to said inserts is in a
range of from 2-7 mm.
60. A plasma arc torch as defined in claim 59, wherein the orifice
diameter of said nozzle is in a range of from 10-30 mm.
61. A plasma arc torch as defined in claim 45, wherein said inserts
are cylindrical and the inner peripheral portion of said body has
partially cylindrical keyways which receive said inserts.
62. A plasma arc torch as defined in claim 45, wherein said body
includes a hollow chamber and means are provided for circulating a
cooling fluid through said hollow chamber.
Description
BACKGROUND OF THE INVENTION
The present invention was developed to provide improved operation
and longer life for plasma arc remelting systems used to make metal
ingots and components of such systems. Together with the improved
structure the invention contemplates improvements in the methods
for the operation of such systems.
Installations in the prior art used for the production of ingots in
plasma arc remelting teach use of a cooled mold with a vertically
movable bottom part for lowering the ingot being made. The mold is
positioned within the lower portion of a hermetically sealed
chamber and the installations utilize one or several plasma torches
connected to a source of electrical energy. A suitable power
operated mechanism connected to the bottom part provides for moving
that part and extracting the formed ingot. Reference can be made to
U.S. Pat. Nos. 3,147,329 and 3,496,280 for explanations of
plasmatron operation and plasma arc remelting.
One known installation of this type is disclosed in British Pat.
No. 1,237,155 based, in part, on prior development work of several
of the applicants hereof. A serious problem encountered was that
the plasma arcs frequently burned through the water cooled torches
and/or the mold, thereby releasing the coolant fluid into the
evacuated space of the chamber and causing serious explosions due
to the presence of high temperature molten metal therein. In that
installation plasma torches having a fixed position with respect to
the mold were provided for melting a metal blank which was lowered
into the remelting chamber. Difficulties with the thermal balance
in this installation were encountered and overcome. Another problem
was that the plasma arcs did not occupy the same paths between the
plasma torches and the mold in successive runs. As a result the
metal blank was not uniformly melted in this apparatus.
Many of these disadvantages were overcome by development of the
improved system disclosed in part in a U.S.S.R. publication
entitled Stahl, No. 6, 1971 which teaches top feeding and revolving
of a blank in a plasmatron furnace as well as angular adjustment of
at least one of several torches arranged to direct the plasma arc
flame downwardly toward the lower end of the blank and against the
upper end of the water-cooled mold. Those improvements enabled
operation wherein at least one of the plasma arc torches is
adjustably mounted via a ball and socket joint in the chamber so
that the position of its plasma arc flame can be adjusted with
respect to the mold and wherein the metal blank being melted can be
rotated as well as lowered axially into the remelting zone within
the chamber.
Radial arrangement of several plasmatrons around a crystallizer
allows the placement of heat sources evenly around the molten pool,
or bath, of metal which exists at the top of the ingot being
formed. Precise regulation of the heating of all sections of the
bath is obtained by changing the circumferential distances between
the plasmatrons. It is known, that at low remelting rates, 70 to 80
percent of the heat released by the solidificating ingot is removed
to the water-cooled copper crystallizer through its contact strip
with the bath. In heating the bath by plasma flames placed along
the periphery of the bath it is easy to obtain its flat shape.
Also, at a certain inclination of the plasmatrons to the bath, one
can make the liquid metal revolve, at a desired rate, around the
vertical axis by using the energy of plasma jets.
The experience in operation of plasma-arc furnaces with a radial
arrangement of plasmatrons around the crystallizer has shown, that
through control of the heating of the bath by changing the
peripheral distances between the plasmatrons, one can obtain in the
same furnace (by changing only the crystallizer and priming) round,
square, rectangular and other shaped ingots from the same blank,
for instance, of round cross section.
Another significant advantage of the multi-plasmatron furnace with
axial feed of the blank is an essential (almost 70 percent)
radiation screening of the plasma jets and bath by the blank.
Blanks larger than 150 mm in diameter are melted close to free
surface of the bath and their melted face takes a flat or a concave
form, thus causing an increase in the efficiency of the remelting
process. In this case the demands put on the quality of the blanks
are less rigid than on blanks used in furnaces with sidewise feed
or blanks, where usually the blanks must be much thinner than the
ingot and therefore their manufacture consumes more labor. Blanks
for multi-plasmatron furnaces can be of round or square cross
section, or they can be composed of end and side scrap of sheet. In
the case of remelting a loose material in a furnace with a radial
arrangement of plasmatrons, the material is fed to the middle of
the bath, securing a good and complete melting of the fed
material.
A non-uniform temperature field is generated during melting of a
metal in a water-cooled copper crucible by intensive heat-energy
flow from plasmatrons. Temperature gradients can reach
200.degree./cm. The non-uniform temperature field generates
free-convexion macroflows, causing a stirring of the metallic bath
with an intensity directly proportional to the number of
plasmatrons installed in the furnace. This stirring promotes a
chemical homogenization of the molten metal and accelerates the
reactions which take place in the diffusion zone. Therefore, in the
multi-plasmatron furances ingots of higher quality can be obtained
than in single-plasmatron furnaces, not only because of the thermal
conditions of the process, but also in connection with a more
favorable diffusion kinetics of metal-refining reactions.
Experience gained from those previously known plasma arc remelting
systems brought to light various difficulties in obtaining
appropriate control over torch adjustment, operational functioning
of the feed and revolving structures and emphasized the very short
torch life, one of the major problems of plasma arc torches used in
furnace systems for remelting metals.
The location of plasmatrons around the crystallizer of the furnace
does ensure better operational conditions than in the case of axial
arrangement, where the whole, or almost the whole, capacity of the
furnace is concentrated in a single plasmatron. Nevertheless, to be
economically feasible and acceptable, the plasmatrons used in
metallurgy, whether in single or multi-torch furnaces, unlike
plasma generators for welding, cutting, surfacing, etc., must
possess a considerable resource in working capacity and be reliable
in operation; in this respect the ablation of the tungsten cathode
and the failure of the nozzle must be eliminated or reduced to a
minimum. The stronger is the current of the power source, the more
difficult it is to secure high working capacity of the plasmatron.
Prior art plasma arc torches have been water cooled, the nozzles
have been water cooled and even the center electrode has been water
cooled but still the tungsten cathodes and the nozzle structures
fail in a short time, often before an ingot is completed.
SUMMARY OF THE INVENTION
The present invention consists of an installation for the
production of metal ingots including a hermetically sealable
chamber which contains an ingot forming mold and mounted through
the chamber walls are a plurality of plasma arc torches. The
torches are mounted for adjustment along their axis as well as
being swivelled in the chamber wall enabling precise location and
shifting of the plasma arc flame issuing from the torch with
respect to the other torches and with respect to the mold. Also
provided are an electric power supply source for powering the
torches and a device for maintaining a metal charge such as a blank
arranged within the chamber to be melted in the plasma arc, the
molten metal from the charge being collected and solidified in a
mold to form an ingot. The torches, the chamber, the mold and even
the charge maintaining device are preferably made with hollow walls
and fluid cooling is provided. A mechanism for rotating a metal
blank being melted is provided and can be operated simultaneously
or selectively with respect to directional feed of the blank along
its axis. When the metal charge being melted is in the form of
metal particles, the blank feeding mechanism can be omitted.
Primary objects of this invention reside in providing an improved
plasma arc remelting torch and torch nozzle construction.
A primary novel component is a plasma arc remelting torch
(plasmatron) with a novel nozzle area construction which increases
the stable, plasma arc producing service life of the torch
electrode and the torch nozzle end, these being the critical
components of the torch.
In conjunction with the preceeding, primary novel component,
further objects of the invention reside in a plasmatron nozzle
construction utilizing one or more of the following constructional
details: smooth gas flow path profile to avoid turbulence as well
as shock formations at supersonic flow; torch electrode projection
(preferably 2-3 mm) from nozzle body; use of heat sink inserts
within the nozzle orifice and surrounding the electrode; press-fit
heat seat inserts enabling ease of replacement; projection of heat
sink inserts (preferably 10-15 mm) from the terminal end of the
nozzle; torch electrode and heat sink inserts both projecting from
the nozzle terminal end with inserts projecting beyond the
electrode; nozzle being fluid cooled but with no cooling at the
terminal end; nozzle made from copper with high heat conductivity
and including heat sink inserts of other material having not only a
high degree of heat conductivity but also a high melting
temperature; cylindrical inserts having chamfered ends; torch
electrode spaced from heat sink inserts at a preferred distance of
2-7 mm, electrode diameter being at least 8 mm, and nozzle orifice
having a diameter preferably within the range of 10-30 mm; and,
heat sink inserts preferably being made of tungsten or other
material having similar melting point and thermal conductivity
properties. Each of these aspects contributes to increased torch
life.
Further novel features and other objects of this invention will
become apparent from the following detailed description, discussion
and the appended claims taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF DRAWINGS
A preferred structural embodiment of this invention is disclosed in
the accompanying drawings, in which:
FIG. 1 is a schematic elevation section view of the major
components of a plasma arc remelting furnace and system to which
the present invention applies;
FIG. 2 is a front elevation diagrammatic sketch of a plasma arc
remelting system;
FIG. 3 is a plan schematic view of the positioning of the plasma
arc torches in the installation of FIG. 2 and includes a schematic
3 phase AC power circuit;
FIG. 4 is an enlarged vertical cross-section view of the ball and
socket connection providing projection of each torch into the
furnace chamber, and shows details of the correlated torch angle
positioning control members, the plasma arc torch not being
sectioned;
FIG. 5 is a section view taken on line 5--5 of FIG. 4 showing the
sliding collar and gimbal connection between the operating
mechanism and the torch mounting sleeve;
FIG. 6 is an enlarged vertical section through the nozzle end of a
plasma arc torch showing an intermediate aspect in development of
the embodiment illustrated in FIG. 8;
FIG. 7 is a bottom view of the nozzle shown in FIG. 6;
FIGS. 8 and 9 are views similar to FIGS. 6 and 7 but illustrating a
plasma arc torch nozzle structure according to the present
invention;
FIG. 10 is a vertical section of the upper end support bell end
operating mechanism for enabling introduction, feed and oscillation
of the blank;
FIG. 11 is a section view taken on line 11--11 of FIG. 10 showing
details of the blank oscillating structure and operator mounted in
the upper end bell housing;
FIG. 12 is a schematic circuit illustrating one way of obtaining
and connecting DC power to one or more plasmatrons;
FIG. 13 is a schematic circuit illustrating a starter or ignition
circuit by which a plasma arc torch can be started;
FIG. 14 is another schematic circuit illustrating an AC circuit
which can be used to power plural torches in multiples of
three;
FIGS. 15 and 16 are diagrams, respectively of three and six torch
plasma arc remelting installations with the electric connections to
each torch labelled to correlate with the torch connections shown
in FIG. 14;
FIG. 17 is another schematic circuit illustrating an AC circuit
enabling powering for plural torches in multiples of four,
particularly useful in remelting metal into molds of square or
other polygonal cross-section;
FIGS. 18 and 19 are diagrams, respectively of four and eight torch
plasma arc remelting installations with square cross section molds
and with the electric connections to each torch labelled to
correlate with the torch connections shown in FIG. 17; and
FIG. 20 is a table showing the heat conductivity coefficient and
melting temperatures for several materials considered for making
heat sink inserts .
GENERAL DESCRIPTION
With reference to FIG. 1, a brief general description of the plasma
arc remelting installation to which this invention pertains is
shown with a hermetically sealable chamber 10 having an opening 12
at the top portion thereof for accommodating a vertically
positioned metal blank 14 and an opening 16 in the lower portion
thereof for accommodating a solidified ingot 18. The chamber
includes an outwardly downwardly inclined roof portion 20 which
supports a plurality of plasma torch devices 22 which extend
through the chamber roof into the vicinity of the lower end portion
of the metal blank 14. A hollow wall fluid cooled mold 24 is
positioned into the lower end of the chamber with its upper mouth
26 spaced closely adjacent the nozzle ends 28 of the plasma arc
torches 22. A coolant system 30 is provided for supplying cooling
fluid through coolant lines 32 and 34 to the mold 24 and also for
providing cooling fluid for plasma arc torches 22 through dual
lines 36 and 38. The walls of the chamber 10 may also be made
hollow and the cooling system 30 may also be connected with the
chamber 10 to provide cooling in its sidewalls thereof.
In general, the operation of a plasma arc remelting installation
such as depicted in FIG. 1 is as follows. Plasma arc torches 22 are
connected with a conventional source of electrical power AC or DC
(not shown) which is also connected directly to the ingot 18 and
mold 24. A metal blank 14 to be melted is suspended from the top of
chamber 12 to a location within the upper portion of the chamber 12
in substantially the position shown in FIG. 1. The chamber is
sealed and all air within the chamber is exhausted to a vacuum in
the order of 10.sup.-.sup.2 mm. Hg and the chamber is scavaged with
an inert gas such as argon. The plasma arc torches are then
sequentially ignited by the use of a direct current pulse between
the torch body, e.g., nozzle, and the cathode from the power supply
source. A convenient method of establishing ignition or initiating
DC pulse is to employ a common oscillator connected between the
cathode and torch body of the torch means to be started and the
mold 24. Once the plurality of torch means have been started the
metal in blank 14 is progressively melted and drops into the mold
24, forming a molten metal bath on top of ingot 18. Metal blank 14
is progressively lowered as it is melted off and ingot 18 is
progressively extracted from the bottom portion of chamber 16 as
the molten metal pool progressively solidifies by reason of the
heat extracted by the fluid cooled mold 24. Metal blank 14 is
axially lowered into the remelting zone defined by the plasma arcs
issuing from the plasma arc torches 22. It can also be oscillated
or rotated as shown by the rotation arrow while being lowered into
the remelting zone. By carefully controlling the remelting
parameters and adjusting the positions of the plasma arc torches 22
with respect to the melting end of metal blank 14, the liquid level
of the molten metal pool in the top of mold 24 can be maintained at
a constant position within the mold to attain balanced
thermodynamic conditions within the remelting zone and the molten
metal bath and to produce a high quality metal ingot which is
substantially free from nonmetallic inclusions, stringers, and gas
bubbles. The grain pattern shown by the ingots thus produced is of
the ideal herringbone formation wherein the grain patterns are
directed from the center of the ingot upwardly and outwardly toward
the exterior surfaces thereof.
The axial moving of metal blank 14 and the rotation thereof can be
selectively used when needed, and particularly rotation of the
ingot need not be used in all cases since the plasma arc torches
are adjustably mounted within chamber 10.
In the schematic diagram of FIG. 1, the plasma arc torches are
positioned within chamber 10 in such a manner that their
longitudinal axes define an angle A with a plane defined by the
horizontal top of mold 24. This angle can be varied by virtue of
the adjustable feature of the plasma arc torches by reason of ball
joints 40 and operating mechanisms (FIGS. 2, 3 and 4) which are
secured in and to the roof portion 20 of the plasma arc remelting
installation. The position of the ball joints with respect to the
plasma arc torches 22 define a fixed point along the axis about
which variable angular positioning of the torch is possible. It is
also possible to adjust the longitudinal axial position of the
torches 22, which is usually fixed for a specific furnace system,
by moving them inwardly and outwardly along their axis in order to
bring the plasma arc torches closer to and further away from the
mouth portion 26 of mold 24. This adjustable feature of the plasma
arc torches allows an existing plasma arc installation as described
herein to produce a variety of cross sections of ingots 18 by
inserting different molds 24 into the plasma arc chamber 10. For
example, a circular cross section mold can be removed and replaced
with a square or rectangular cross section mold whereupon the
plasma torches will be adjusted as needed to accommodate the
different size and shaped molds and hence the ingots being made. As
shown the mold 24 is removable retained within the body of chamber
20. The top portion, the chamber, the mold and the lower portion
are secured by interconnected flanges enabling separability of the
furnace components.
The schematic diagram of FIG. 2 shows a plasma arc installation 50
equipped with operational supporting subsystems therefor. Although
not depicted, the furnace components can be separable as described
for FIG. 1. The hermetically sealable chamber 52 is provided with a
fluid cooled mold 54 which is shown integrally constructed with
respect to the remaining portions of chamber 52 and is arranged to
form an ingot 56. A plurality of plasma arc torches 58 are
positioned so as to direct the plasma arcs toward the upper mouth
portion 60 of mold 54. The plasma arc torches are adjustably
mounted and in the position shown have their axes in vertical
planes which are directed at acute angles to the vertical axis of
the mold 2. These adjustable torches can either extend in a
vertical diametric plane of the mold or their yaw angle can be
adjusted so the plasma arc has a direction with a component
extending tangentially which will rotate the bath of molten metal
in the mold 54. An arrangement of the torches 58 having a
tangential component is diagrammatically illustrated in FIG. 3.
The mold 54 has a movable bottom or carriage 62 which is connected
via rod 64 to an ingot extracting mechanism 66.
Metal blank 68 is supported to project down through the upper
sleeve extension 69 on neck portion 70 the roof of chamber 52 by a
flanged end bell housing 72 and blank feeding and oscillating
mechanism 74. The power driven mechanism 74 operates to feed and to
rotate or oscillate the blank during remelting. A side hatch 76 can
be provided in sleeve extension 69 and/or upper neck portion 70 in
order to provide for insertion of the metal blank 68 and for
withdrawing of the spent stub of the blank following the melting
operation, however the entire bell housing 72 and the operating
mechanism can be disconnected and lifted as a unit from the chamber
52 to insert the blanks and to withdraw the spent stub.
Plasma arc torches 58 are positioned within the upper roof portion
78 of chamber 52 and are adjustably mounted therein in the manner
as will be hereinafter described relative to FIG. 4. In order to
melt a metal charge in installation 50 it is not necessary to
employ a solid metal blank 68. It is also possible to add the
charge by feeding metal particles, such as scrap, into the upper
mold mouth 60 from a hopper 80 connected with the interior of the
chamber through a feed trough 82.
Other parts of the support system include a vacuum pump 84
connected through a pipe line 86 with the crystallizer chamber 52
which, along with plasma arc torches 58, is supplied with plasma
forming gas or gas mixtures from a gas supply tank 88 which is
connected to each plasma arc torch through a gas distribution
device 90 (one shown) designed to simultaneously control the gas
consumption as well as the ratio of gases in the plasma gas
mixture.
To assure constant pressure in the crystallizer chamber 52 during
the melting cycle, provision is made for a gas recirculating
circuit comprising a diaphragm compressor 92 connected to the
distribution device 90 and through a system of gas filters 94,
which include a chemical purification system containing absorber
devices, to pipe lines 86.
The plasma arc remelting installation can be supplied with either a
DC or AC power supply from an electrical power source through a
control unit 96. In a conventional manner, the cathodes of the
plasma arc torches 58, in a DC mode, are connected to the negative
terminal of the supply source 96 while the mold 54 is connected to
the positive terminal. In such case the positive terminal of the DC
source may connect directly to the mold or to the ingot through the
ingot withdrawing mechanism 66 or to both the mold and the
mechanism 66.
The electrical power source portion of unit 96 can be a three-phase
alternating transformer having its secondary windings arranged in
start form 97 (FIG. 3) with each leg connected to a respective
plasma arc torch 58 and with the star neutral point or center
connector insulated from ground and from the mold 54. In this AC
circuit connection the mold 54 can be and in the normal
installation is usually grounded.
Briefly stated, to start a plasma arc remelting operation, the
metal blank 68 is inserted into the upper neck portion 70. Air in
chamber 52 is pumped out by vacuum pump 84 until a satisfactory low
pressure is reached such as 10.sup.-.sup.2 mm Hg. Chamber 52 is
then filled and scavaged with a neutral gas which is then
recirculated with the gas recirculation equipment 90, 92 and 94.
Plasma arc torches 58 are then initiated in a manner described
above and the positions thereof are adjusted in order to
accommodate the particular type of cross-section of the ingot 56
being solidified within installation 50. When a molten metal bath
is obtained in the upper part of mold 54 as the result of melting
of the lower end portion of metal blank 68, the metal blank feeding
mechanism 72 and the ingot extracting mechanism 66 are
initiated.
Removal of nonmetal and gaseous admixtures in the molten metal
bath, as well as the change of shape of the end of the metal blank
68 as a result of the fusion action and control over the drip
forming process on the lower end of the metal blank can be
conveniently varied within the installation 50 by adjustment of the
axial positions thereof as above described without the necessity of
replacing and rebuilding the entire installation 50 for each new
position of the torches which is desired.
When the plasma torches 58 are positioned with a tangential
component as described above with reference to FIG. 3, the molten
metal bath under the action of the plasma arcs will rotate and
speed of rotation can be regulated by varying the angle B (see FIG.
3) which the plasma torches form with the radial planes passing
through the axis of the mold 54. Also, the force of the plasma arcs
and intensity of heat radiation can be additionally controlled by
varying the volume of the inert gas being pumped through the
torches and also by varying the current applied to the torches.
The forming of molten metal drops on the lowermost end of metal
blank 68 can be additionally regulated by varying the current being
supplied by electric power supply means 96 by employing modulated
pulses of current having an optimal pulse shape and duration.
Pulsing of the gas being delivered from the torches can also be
employed.
If during the melting process a molten slag cover 100 for the
molten metal bath is required for refining of the metal, slag can
be added through the trough of hopper 80 as desired during the
process.
The cooled mold 54 is designed to extract up to 80 percent of the
heat released by the solidifying ingot through the cooling system
therefor.
The molten metal pool 101 supported by the solidified ingot 56
forms a flat shallow bath in the installation 50 which has a
maximum depth dimension to maximum cross-sectional dimension
(diameter for FIG. 3) of the mold of from one-fifth to one-tenth.
Within this molten metal bath the temperature gradient can be up to
about 200.degree. C per centimeter by employing currents of from
500 to 5,000 amps. and voltages of from 40 to 200 volts. The
operation power level of the installation 50 is from 150 to 3,000
kilowatts in order to produce ingots having weights varying from
50kg. to 5,000 kg.
TORCH ADJUSTMENT
The manner in which the plasmatrons are adjustably mounted in the
walls of the crystallizer will now be described with general
reference to FIG. 2 and reference to FIGS. 4 and 5 for details.
Each torch 58 is an elongate, essentially tubular member, normally
made from a material such as copper which has extremely good heat
conductivity. The main torch body 58, not herein described in
detail, is made with hollow walls (FIG. 4) and has provision for
circulating a fluid cooling medium such as water, from an inlet at
the exterior of chamber 52 down through the torch body to and
around the nozzle end and then back up through the body to an
outlet, as has been described in connection with FIG. 1. Some
torches also have the central electrode holder cooled by cooling
fluid. Surrounding the electrode is an annular passage for
introduction of inert plasma gas such as argon.
In accord with the present invention, the roof portion 78 of
crystallizer chamber 52 includes a plurality of flanged apertures
100 accommodating the sealed mounting of a desired number of
torches 58 through associated adjustment assemblies 99. Each torch
58 projects through its own adjusting assembly 99 into the
crystallizer and each assembly 99 has a ball and socket unit 102, a
torch to ball sleeve gland arrangement 104 and a torch swivelling
adjustment mechanism 106.
Torch 58 projects down into the crystallizer chamber 52 through a
seal in the sleeve gland arrangement 104 which passes through and
is secured and sealed to the ball 108, which is secured and sealed
in its socket.
The socket of the ball and socket arrangement 102 consists of
several parts including a base 110 having a flange 112 by which the
base is secured by suitable bolts to an associated upstanding
flanged aperture 100 in the crystallizer cover. The connection
between the socket base flange and the aperture flange is sealed by
a spigotted construction 114 and a heat resistant seal ring 116.
Ball 108 seats in a parallel spherical seat 118 machined in the
socket base 110 and multiple contoured ring seals 120, placed in an
annular recess 122 in the base and secured by a ring nut clamp 124
socket base, provide a friction tight fluid seal between the socket
base and the surface of ball 108. A ring-shaped, partially
spherical ball cap 126 is disposed to extend down within the seal
ring unit 124 and fit against the ball 108, being held in its
position by a threaded, seat cap clamping ring 128 screw threaded
over the socket base 110. Clamping ring 128 is tightened an amount
sufficient to provide a proper seating relationship yet still
enable the desired swivelling fit between the ball 108 and seat
surfaces. Ring nut 124 is used to tighten the seals 120 to provide
a gas tight seal between the ball and the socket.
Ball 108 is apertured with a through bore, partially threaded at
134 below an enlarged annular recess 136 within which is received a
group of heat resistant seal rings 138. A sleeve 140, having a
close sliding fit over the cylindrical body of torch 58, is part of
the sleeve gland assembly 104, and has several external stepped
portions. The lowermost terminal portion 142 of the sleeve 140
projects into the crystallizer chamber 52 and is threaded. A heat
shield ring 144, disposed with a sliding fit over the lower end of
the torch, is threaded on the inner terminal end of sleeve 140 and
in assembly is positioned just within the crystallizer chamber.
Sleeve 140 is made from insulating material. Shield ring 144 is
made of a heat resistant material, having a high melting point, and
includes an inverted, frusto-conical skirt 146 which by deflecting
and blocking the high temperature radiation from the plasma arc
flames shields the sealed ball joint zone at the associated chamber
aperture 100 from the intense heat.
A mesial threaded portion 148 of sleeve 140 screws into the
internal threads 134 of the ball 108 so that a shoulder 150 abuts
and compresses the seal rings 138 between the sleeve and the ball.
A portion 152 of sleeve 140 external of the chamber is cylindrical
and terminates in a threaded gland cup 154 which holds packing
rings 156 compressed by a gland nut 158. By loosening nut 158, the
torch 58 can be axially shifted through the gland sleeve so the
torch nozzle end can be positioned the desired distance from either
or both of the blank and the molten metal pool or slag bath at the
upper part of the mold. When the desired axial disposition of the
torch is obtained, the torch if rigidly secured to the sleeve by
tightening gland nut 158, and the torch will normally be maintained
in such position throughout the remelting operation.
With the construction shown in FIG. 4, torch 58, sleeve 140 and
ball 108 can be swivelled 15.degree. in any direction from a
coaxial axis through the associated chamber aperture 100. The
angular disposition of torch 58 can be selectively set via the
adjustment mechanism 106 which connects to the torch assembly
through a gimbal device 160 slidably embracing the heavy
cylindrical outer portion 152 of gland sleeve 140.
The gimbal device is shiftable in two directions normal to each
other, which for convenience can be designated as movements having
components in X and Y axes, by adjustable control members 162 and
164, a part of mechanism 106. Gimbal device 160 has an inner ring
166 slipped over the heavy outer portion 152 of gland sleeve 140
with a free sliding fit. The ring 166 has diametrical trunnions 168
and 169 (FIG. 5) by which it is journalled to the outer gimbal ring
170 which in turn includes an integral projecting boss 172 through
which adjustment control movements in both the X and Y axes are
transmitted to the gimbal device. Gimbal boss 172 has an axial bore
and includes, fixed within the bore, a threaded nut 174 for
purposes which will presently be described.
Adjustment mechanism 106 is secured on a bracket 176 mounted as by
screws or welding to the associated socket flange. A base member
178 secured to the bracket 176 has spaced bearing blocks 180 which
journal a rock shaft 182 on an axis through the center of ball 108.
Rock shaft has a keyed connection 182 to and carries both a rocking
arm assembly 186 and a worm wheel sector 188. A black plate 190
integral with base member 178 provides an axially fixed bearing
connection for the shaft of a worm gear 192 meshed with the worm
wheel sector 188. Both ends of the worm gear shaft will be
journalled in rigid supports although only the back support plate
190 is shown. Control member 164 is a manual operating knob secured
to the worm shaft so that rotation of the control member will
rotate the worm and rock the rocking arm assembly 186 about an axis
extending through the center of the ball 108.
The other control member 162 is carried at the upper end of the
rocking arm assembly 186 in a sleeve-like construction 192 parallel
to the rocking axis and into one end of which the elongate boss 172
of the gimbal device is telescoped. The control member knob 162 is
fixed to one end of a threaded shaft 196 which is rotatably
journalled in an axially fixed disposition within the sleeve
construction 194 via bearings 198. The threaded end of shaft 196 is
threaded into the nut 174 in the boss of the gimbal device 160,
whereby rotation of control knob 162 will move the gimbal device
160 toward and away from the rocking lever along an axis parallel
to the rocking axis, providing a swivel adjustment to the torch in
one direction of the aforenoted X-Y adjustment. Rocking of the
rocking arm 186, by means of control knob 164, rocks the gimbal
device in the second direction of the X-Y adjustment.
Coordinated adjustment of the two control knobs 162 and 164 will
enable selective positioning of the torch in any direction up to an
angle of 15.degree. so the plasma arc path can be directed, for
example, along a diametral plane through the vertical axis of the
blank to impinge closer to the blank or to an greater degree down
into the molten metal bath and closer to the side walls. In
conjunction with such adjustment the torch can be swivelled to
provide a tangential component of the plasma arc path around the
vertical axis of the mold to cause a swirl of the molten metal
bath. Control members are manual knobs but power actuators,
selected from many available types could, if desired, be
substituted for the manually operated control knobs.
More accurate control of the positioning of the torches as well as
ability to selectively control the torch angles during operation
enables better and more even distribution of the melting
temperatures relative to the blank and the molten pool at the upper
end of the mold. This becomes extremely important as higher powered
longer life torches are made possible as a result of the following
advantageous improvements in the torch itself.
TORCH NOZZLE CONSTRUCTION
Operational experience has taught that the weak link in a PAR
system is the torch nozzle. The electrode terminal end is disposed
within the reduced diameter nozzle orifice. The torch start-up arc
occurs between electrode and the nozzle surface while the running
or operating arc occurs from the electrode to the melt or ingot
being made. During operation an undesirable cross-arcing from
electrode to nozzle occurs due to non-stable flow conditions. This
undesirable cross-arcing, something which has been essentially
impossible to avoid, is called intermittent arcing and causes
ablation of the electrode and the nozzle and often results in
burn-through at the nozzle aperture. The highest tempratures, i.e.,
the hottest plasma exists at the torch tip. The prior art torches
including nozzle tip structure are water cooled and are
conventionally made from copper. The prior art nozzle structures
normally include hollow walls to provide the annular chamber for
fluid cooling. Nozzle constructions, known prior to this invention,
due to ablation and burn-through, had an extremely short operating
life, not more than several hours and often not long enough to
permit making a complete ingot.
In accord with the present invention some conditions causing
intermediate arcing are negated by providing a smoother gas flow
pattern. Also by providing a heat sink structure at the nozzle,
i.e., or controlling the location of the intermediate arcing by
affording it a directed path from electrode to nozzle, the
possibility of burn-through by the intermittent arcing is almost
completely eliminated. To accomplish this desired function it was
decided to shield the prior art hollow nozzle 200 tip with
cylindrical metal inserts 204 as is shown in FIGS. 6 and 7 having
good heat conductivity and high melting temperatures. Tungsten,
being one material having a good heat conducting coefficient of
0.38 through 0.47 and a high melting temperature of 3,200.degree.
C., was used in the form of small cylindrical rod 204 spaced around
the inner periphery of the torch nozzle orifice 202. While a
problem with tungsten is difficulty of machining, it has been
determined that ground tungsten bars (cylindrical inserts) provide
satisfactory heat sink inserts.
The first attempts at use of such inserts as shown in FIG. 6, were
satisfactory to increase life at lower powers, however they did not
provide much increased life at the desired higher powers due to
thermal shock destruction of the brittle tungsten inserts, occuring
because of intermittent arcing now being confined to the inserts.
Proceeding from those attempts at using inserts, the nozzle of
torch 218 has been further improved, in the manner shown in FIGS. 8
and 9, in several important respects, each of which contributes to
the materially increased life of the nozzle tip and negates
burn-off ablation or destruction of nozzle, electrode and heat sink
insert material which, besides shortening the torch life,
contaminated the ingot being made.
Referring specifically to FIGS. 8 and 9, orifice 222 of the nozzle
220 is constructed with a smoother profile curvature of the side
wall configuration so flow of plasma gas adjacent the walls remains
laminar rather than turbulent. Smooth nozzle curves can be
calculated from known techniques but the advantages of acquiring
and using such nozzle profiles in plasma arc torches has not been
previously known, used, or appreciated. Laminar flow of the plasma
forming gas is essentially necessary for stability of the plasma
arc in higher powered torches which use a higher velocity gas flow
and provide a long plasma flame. Note: non-laminar flow which has
turbulent zones can be tolerated in low powered torches, i.e.,
those which produce short length plasma flame.
Nozzle exit diameter can vary from 10 mm to 30 mm. It has been
found through experience that high power PAR torches should have a
preferred nozzle exit diameter of 25 mm while lower powered torches
have been found to perform satisfactorily using a 20 mm exit
diameter.
Together with the changed nozzle orifice profile, the torch tip
structure 224 (FIG. 8) is changed so that the annular cooling
chamber as in the attached hollow tip 206 of FIG. 6 is omitted. In
tip 224, the fluid passages 226 in the hollow torch walls do not
extend down into the tip body around the reduced diameter nozzle
222 which surrounds the portion 230 of the terminal end portion of
the electrode 228 which is disposed laterally adjacent the heat
sink inserts 232. In other words, the nozzle tip 224 where the
inserts are placed is solid metal (copper being preferred). Leaving
out the water cooling at the nozzle tip avoids the intense thermal
shock, resulting from inadvertent intermittent arcing from the
electrode to the nozzle structure. In prior art nozzles, the
intermittent arcing directly impinged on and resulted in rapid
burn-through of the copper nozzles which were water cooled. Thermal
shock of such arcing could be accommodated by the ductile property
of the copper nozzles, however when tungsten inserts were added,
the water cooled areas maintained the tungsten at a temperature
sufficiently cool so that the thermal shock of an intermittent
arcing under the high powers resulted in fracturing the brittle
tungsten inserts.
Together with the above structure it has been found that by
projecting the electrode terminal tip 230 slightly below the
terminal end of the nozzle tip per se, as shown by distance a in
FIG. 8, laminar flow of the gas is enhanced, it will help maintain
continuity and stability of the plasma arc flame path and it
reduces wear of the electrode tip. An electrode projection distance
a of from 2 to 3 mm below the torch tip plane has given highly
satisfactory results at high powers. While the heat sink inserts
204 (FIG. 6) were mounted wholly within the nozzle tip, they were
extended down past the tip end of electrode 210 which did increase
useful life by shifting the location of the arcing as well as by
shielding the nozzle tip from direct arcing action. The inserts 232
(FIG. 8) in accord with further aspects of the present invention
are mounted slightly lower in the nozzle tip 224 and project from a
transverse plane at the nozzle tip a distance b which is greater
than the projection of the relocated electrode 228. Desired
projection dimensions b of the inserts have been found to be from
10 to 15 mm. The resulting structural relationship between the
solid nozzle tip, the smoothly curved laminar flow profile of the
nozzle orifice, the relocated projecting electrode terminal end and
the projecting heat sink inserts each are improvements which in and
of themselves increase the useful life of the torch and when all of
these improvements are used in combination the resulting plasmatron
is capable of a very high powered operation with an extended
length, stable plasma flame has been accommodated for over a 1,000
hour life period before repair or replacement of component torch
parts is necessary. It is believed that a very important aspect of
this startling torch life improvement in high power operation is
due to the essentially smooth walled profile 222 of the nozzle
orifice and the projection of both the electrode tip 230 and the
ends of inserts 232 beyond the terminal plane of the annular nozzle
body per se, resulting in an annular laminar flow nozzle path whose
internal wall formed by the electrode surface does not terminate
prior to the termination of the outer peripheral confining wall
nozzle orifice of the annular zone, and results in the controlled
relocation of the zone of intermittent arcing to a position near or
outside of the terminal edge of the nozzle.
The cylindrical heat sink inserts 232 provide a greater body
surface to shield the copper nozzle tip 224 which has a
substantially lower melting temperature than the inserts. One
refinement to the inserts 232 which helps provide longer life is to
chamfer 234, 236 both ends to avoid sharp corners. This feature is
of particular importance at the inner ends of the inserts because
it helps reduce the sharp structural break interference type of
flow path interference which creating Blasius turbulence which in
turn contributes to the intermittent arcing. To a lesser extent a
sharp terminal edge at the exit end of the inserts can also create
Blasius type of turbulence but by projecting the inserts beyond the
nozzle exit, any turbulence created by their terminal edges is at a
location in the flow path of the plasma flame which is outside of
the nozzle where it has essentially stabilized into relatively free
laminar flow so the effect of turbulence caused by the terminal end
of the insert tips is negligible.
As a result of successful operations of the new plasma torches,
several desired parameters have been found. Referring again to
FIGS. 8 and 9, the diameter D of the nozzle inner periphery at the
nozzle outlet 238 should preferably be from 10 to 30 mm; the
inserts 232 should use that nozzle diameter or a minutely larger
diameter as a common mounting circle for their center axes in order
to provide a cylindrical keyway 240 slightly greater than
180.degree. for an embracing keyed interfit between insert and the
keyway groove 240 into which they are pressed; the keyway interfit
c can be up to 10 mm in length; the electrode end 230 projects a
distance a of 2-3 mm beyond the nozzle terminal edge; the inserts
232 project a distance b of 10-15 mm; the distance e between
electrode end 230 and inserts 232 is from 2-7 mm; and the
circumferential spacing between inserts may be from "0" to 2 mm. Of
course a zero circumferential spacing between inserts is
technically impossible when a keyed groove insert mounting
arrangement is used but when other insert mounting techniques are
used the inserts can be placed to touch each other.
Solid electrode diameters d of 10-12 mm have been found
satisfactory for high power (up to 2,000 Amperes) operation. Above
2,000 Amperes, multiple strand or composite electrodes up to 25 mm
in diameter are satisfactory. For low power (up to 1,000 Amperes)
solid electrodes of 8 mm diameter are satisfactory.
The Table of Various Metals, Alloys and Metal Oxides in FIG. 20 of
the drawings shows the Heat Conductivity Coefficient and the
melting temperatures for the metals. Similar values can be found
for other metals, alloys and metal oxides. While various materials
have been tried for heat sink insert materials, tungsten and
Rhenium are presently found to be the best because of their
excellent coefficient of heat conductivity and its high melting
temperature. The insert material should be electrically conductive
and preferably should have a coefficient of heat conductivity of
0.3 and a melting temperature above 2,500.degree. C.
BLANK FEED AND OSCILLATING MECHANISM
This portion of the description will have general reference to
FIGS. 1 and 4 and specific reference to FIGS. 10 and 11. As briefly
described hereinbefore, the PAR furnace is arranged to accommodate
top feeding of a blank 68 downwardly along the vertical axis of the
furnace remelting chamber 52 as shown in FIG. 2, and feeding of the
blank is accomplished by a blank feed and oscillating mechanism 74
mounted coaxially on top of chamber 52. Mounting for mechanism 74
includes a furnace bell housing 72 which can be constructed for
direct installation over a flanged central opening in the remelting
chamber, as shown in FIG. 1, or as an upwardly directed sleeve
extension fastened on the upper end of a sleeve-like bell housing
extension 69 such as shown in FIG. 2.
Bell housing 72 constitutes a rigid heavy support structure for the
blank feed and oscillating assembly 74 which includes blank feeding
mechanism 250 as well as blank oscillating mechanism 252. Housing
72 is a vertical sleeve-like member having lower end flanges 254
which enable the bell housing to be secured as by bolts to a mating
flange on the top of the melting chamber or to the upper end of a
chamber top extension. A suitable heat and vacuum seal 256 may be
placed between the flanged connection. The upper end of housing 72
has a reduced diameter opening and incorporates a radial bearing
258 with a heat and vacuum seal arrangement 260 for a hollow
rotatable support shell 262 which projects coaxially down through
the bell housing 72. The upper end opening of housing 72 is
recessed at 263 to receive the seals 260 which are secured by a
gland ring 264. The gland ring can be threaded or otherwise
adjustable secured to press the seals tight between the recess
walls and the support shell 262. The inner periphery of the gland
ring and the bearing 258 below the seal recess 263 provides a
radial bearing and guide to maintain the support shell 262 coaxial
within the bell housing 72.
Support shell 262 carries the entire blank feed mechanism 250, is
rotatably mounted in the upper bearing end of the bell housing, and
also carries a massive large diameter spur gear 266 which serves
the dual purpose of transferring oscillatory drive power to the
shell and of locating and maintaining the shell 262 in fixed axial
disposition on the bell housing. Spur gear 266 is bolted to an
outer annular flange 268 on the shell 262 adjacent but spaced down
below the top end of the hollow support shell. In turn the spur
gear 266 is axially fixed in a rotatable fashion between three sets
of upper and lower support rollers, 272 and 274 respectively,
arranged in equiangular spacing on associated double bearing
support assemblies 276, the brackets of which are secured as by
welding to the outside of the upper end of the bell housing 72.
Oscillatory rotational movement of the support shell is
accomplished by means of a double acting hydraulic motor 278
mounted via its support bracket 280 on one side of the upper end of
the bell housing 72. The exemplary hydraulic power unit has a fixed
position 282 intermediate the ends of a piston rod 284 secured at
each end to an ear of bracket 280. The motor cylinder 286, is
mounted on the rod 284 and surrounds the piston 282 for
reciprocation between the support bracket ears. Hydraulic fluid
under pressure from a suitable source (not shown) and through a
suitable automatically reversing control system (not shown) is
supplied through one or the other of lines 288 and 290 which
connect through suitable drilled passages in the fixed piston rod
to respective internal orifices 292 and 294 on opposite sides of
the piston. As in well-known double acting hydraulic motors when
pressure is applied in line 288, line 290 is automatically
connected through valving to a drain or return conduit back to the
source and vice versa. Automatic control of hydraulic system
valving can be accomplished in any known manner, e.g., by electric
motor driven rotary valving (not shown).
The reciprocable cylinder 286 is prevented from rotary movement by
a suitable tracking device, e.g., a bar 296 fixed between the ears
of bracket 280. The bar 296 can slidably fit into notches 298 (see
FIG. 10) in the cylinder end plates. Rigidly fastened to or
integrally formed on the outer surface and extending between the
ends of cylinder 286 and parallel to its axis is a rack of gear
teeth 300 which mesh with the teeth of spur gear 266. Reciprocation
of the hydraulic cylinder 286 thus causes oscillation of the spur
gear 266 which in turn oscillates the blank support shell 262.
The top end of blank support shell has an end wall 302 and is
internally contoured by suitable means such as casting or machining
into a cable drum chamber 304, receiving a cable drum 306 fixed to
a support axle 308. The ends of axle 308 are journalled in heavy
radial bearings 310 and 312 received in bearing recesses 314 and
316, respectively, machined in diametrally opposed walls of the
drum chamber 304. The bearings and drum are maintained against
axial shift by a bearing end cap 318. One end 320 of the axle 308
projects to the exterior of the chamber through a heat and vacuum
seal arrangement 322 and carries a spur gear 324 which is suitably
drive connected to the axle as by a key and keyway or via a splined
fitting.
Mounted on top of the upper end of the blank support shell 262 is a
reversible electric motor 330 (or a suitable alternate kind of
reversible rotary motor) which connects through a reduction gear
box 332 to an output spur gear 334 meshed with the drum drive gear
324.
Dual wrought steel cables 336 and 338 are wound around and have one
of their respective ends secured to the cable drum 306. The other
ends of both cables depend in equally spaced apart diametral
arrangement relative to the axis of the blank support shell and are
secured in such diametral spacing to a vertically movable
cross-head 340. Cross-head 340 has diametral projecting guide ribs
342 and 344 which slidably ride in internal, vertical groove tracks
346 and 348 extending from adjacent the top of shell 262 to its
bottom end. Firmly secured to and depending from the underside of
cross-head 340 in the exemplary disclosure is a heavy hook 350 from
which is hung a blank 68 by means of eye 352 rigidly secured in the
blank and projected from the center of the blank end face. Because
the cross-head is keyed to the blank support sleeve and because the
cross-head hook 350 and blank eye 352 are rigidly secured to the
cross-head and blank respectively, oscillatory rotation of the
blank support sleeve 262 will cause oscillatory rotation of the
blank 68.
Furthermore, the blank feed mechanism 250 being wholly supported on
the top of the blank support sleeve will be rotated with the sleeve
so there is no interference between oscillatory movement and the
gearing of the blank feed mechanism, each can be selectively
operated independently of the other, oscillation does not effect
feed movement and feed movement does not effect oscillation.
The reversible electric motor 330 will of course be operated from a
suitable power source (not shown) and via suitable controls which
can be manual or automatic to gradually lower the blank 68 into the
furnace chamber 52 as the lower end of the blank is melting off
during the plasma arc remelting process.
The construction of the feeding and blank-revolving mechanisms, are
relatively simple and negate manufacture and servicing
problems.
From past experience in operation of plasma-arc furnaces, it is
known that a radial arrangement of plasmatrons around the
crystallizer provides better operation through control of the
heating of the bath because the peripheral distances between the
plasmatrons can be changed. Thus one can obtain in the same furnace
(by changing only the crystallizer and priming) round, square,
rectangular and other shaped ingots from the same blank, for
instance, of round cross section. Blanks need not be round, they
can be of round or square cross section, or they can be composed of
end and side scrap of sheet. In any event the novel nozzle
construction, the improved controls for torch manipulation and
positioning, and the simple rugged blank feed and turning
mechanism, as has been hereinbefore described, results in more
satisfactory operation as well as enabling operations not
previously possible. The new nozzle tip construction may be
utilized on existing torch bodies and will enable longer continuous
operation at higher powers and will provide longer and hotter
plasma flames. The torch positioning control (FIG. 4) permits
accurate torch manipulation and adjustment even during remelting.
At higher powers the blank is remelting at a faster rate so
reliable blank feeding is absolutely necessary in order to keep the
blank down in position between the plurality of radially disposed
torches in the furnace cover.
ROUND AND SQUARE MOLDS
Ingots made in a round mold 54, 54' and 54", such as shown in FIGS.
3, 15 and 16 should use at least two torches, but can very
conveniently be made by using a crystallizer with one or more
torches of any number (furnaces with at least up to eight torches
are feasible and the number very probably could be higher) and the
torches such as A, B and C in FIG. 15 will be equally spaced around
the axis of the furnace. As shown in FIGS. 1 and 2, the blank 14 or
68 hangs down, along the furnace axis, so its lower end portion is
positioned between and is radiation screening each of the torches
from the plasma flames of the other torches. In circular molds, the
plural plasma jets, in cooperation with radiation screening by the
blank, provide a good distribution of heat over the top of the
molten metal bath so its melted face takes on a flat or only a very
slightly concave form and even heat disposition is easily
maintained around the cooled mold wall perimeter.
The torches A, B and C in FIG. 15 are radially disposed, in plan
view. FIG. 3, shows three torches 58 equally spaced around a
circular mold but having an angular disposition in plan view to
cause a rotation of the molten metal bath. Blank oscillation when
three torches are used should be at least in a 60.degree., back and
forth rotation to result in even melting off of the blank tip. When
more torches are used, as by adding torches A.sub.1, B.sub.1 and
C.sub.1 to the three torches A, B and C of FIG. 15, the six torch
pattern of FIG. 16 is obtained. Again, such torches can be radial
or inclined to obtain the desired plasma flame paths but with an
increased number of torches, a shorter arc of blank oscillation can
be used, e.g., a 30.degree. arc, to obtain even melting off of the
blank tip.
When square or cornered molds, such as molds 360 and 362 shown in
FIGS. 18 and 19, are used, it is found to be of importance to
provide at least a torch for each corner. Thus, for rectangular
(including square) molds at least four torches D, E, F and G are
required. The corner mounting and plasma flame direction against
the blank tip intentionally results in maximum radiation of heat
back into the corner areas of the molten metal bath, areas which
require more heat to maintain a proper molten bath thickness
because the cooled mold corner structures have disproportionate
cooling of the volume of metal adjacent the mold corner wall
surfaces in relation to the volume of metal adjacent the flat
intermediate wall surfaces.
In polygonal cross-section molds, the torches will be increased in
multiples of the number of corners of the mold, e.g., the square
molds 360 (FIG. 18) and 362 (FIG. 19) respectively have four
torches D, E, F, G or eight torches D, E, F, G and D.sub.1,
E.sub.1, F.sub.1, G.sub.1. In such installations the torches are
equally spaced apart. However it is understood that with irregular
shaped molds the torch positioning must of course be varied to
provide the best possible disposition of the heat radiation
correlated with the mold cooling wall shapes.
POWER CIRCUITS
Power for the furnaces can be either AC or DC and several exemplary
circuits which can be used have been shown. The power circuits are
essentially well known and will only be briefly described.
FIG. 12 is a simple circuit using DC for the torches T.sub.1,
T.sub.2, and T.sub.8. The circuit derives AC power from a three
phase input and the three phase lines connect in parallel to three
phase transformers K.sub.1, K.sub.2, K.sub.8. One or up to eight
individual transformers can be provided depending on the number of
torches used. The output of each transformer K connects to a
rectifier bank R.sub.1 , R.sub.2 or R.sub.8. The positive output
terminal A of all rectifier banks is connected to the furnace mold,
ingot and hence the molten bath indicated as BI and the negative
terminal of each rectifier R is connected to the electrode of an
associated torch T. Equalized power can be obtained by suitable
controls such as providing variable transformers K.sub.1, K.sub.2 .
. . , K.sub.8.
Excitation of a torch can be accomplished through circuits also
known previous to this invention, one such suitable circuit being
depicted in FIG. 13, where the starting arc is struck between the
torch electrode TE and nozzle body TN. The circuit is energized by
oscillator 370, and a parallel circuit consisting of a high
frequency induction coil 372 variable resistor 374, switch 376 and
condensor 378. The common ground is also connected to one side of
the main power circuit and the mold, ingot bath arrangement BI.
The in-turn excitation of plural plasmatrons can be accomplished by
using a common oscillator and suitable plural switching.
AC power circuits are shown in FIGS. 14 and 17. FIG. 14 illustrates
a circuit suitable for three torches or for multiples of three
torches and uses three phase inputs to the primary windings of a
three phase transformer KA for each set of three torches, A, B, C,
the respective electrodes of which are connected to individual
phase windings of the transformer secondarys.
FIG. 17 shows an AC circuit suitable for torches arranged in groups
of four. The right hand circuit is a duplicate of the left hand
circuit and enables powering of eight torches. Transformer KA.sub.4
is a special wound transformer having a three phase primary winding
connected to a three phase power source. Using suitable ratios of
secondary windings S.sub.1, S.sub.2, S.sub.3 and S.sub.4 to the
associated primary windings of transformer KA.sub.4 four different
secondary outputs of equal current valves can be obtained and all
four outputs will be out of phase with each other. Each output
S.sub.1, S.sub.2, S.sub.3 and S.sub.4 connected through a
respective variable inductance I.sub.1, I.sub.2, I.sub.3 and
I.sub.4 to the electrode of its respective torch D, E, F and G, and
the common lead from the secondary connects to a common ground with
the mold, ingot bath unit BI.
The invention may be embodied in other specific forms without
departing from the spirit or essential characteristics thereof. The
present embodiments are therefore to be considered in all respects
as illustrative and not restrictive, the scope of the invention
being indicated by the appended claims rather than by the foregoing
description, and all changes which come within the meaning and
range of equivalency of the claims are therefore intended to be
embraced therein.
* * * * *