US3738312A

US3738312A - Molten metal bath level maintenance system

Info

Publication number: US3738312A
Application number: US00213144A
Authority: US
Inventors: G Padjen; R Helman; J Brugger
Original assignee: Bethlehem Steel Corp
Current assignee: Bethlehem Steel Corp
Priority date: 1971-12-28
Filing date: 1971-12-28
Publication date: 1973-06-12
Anticipated expiration: 1990-06-12

Abstract

The level of molten metal in a molten metal coating furnace is controlled by the use of a counterweighted hollow ceramic displacement block having thin walls not subject to spalling. The hollow ceramic immersion block is associated with a bath level control system having a fail safe feature.

Description

United States Patent [1 1 Padjen et al.

[ June 12, 1973 MOLTEN METAL BATH LEVEL MAINTENANCE SYSTEM [75] Inventors: George Padjen; Robert W. Helman; Joseph A. Brugger, all of Bethlehem, Pa.

[73] Assignee: Bethlehem Steel Corporation,

Bethlehem, Pa.

[22] Filed: Dec. 28, 1971 [21] Appl. No.: 213,144

[52] US. Cl. 118/2, 73/3225, 118/5, 118/8,118/12,118/420,118/421 [51] Int. Cl. B05c ll/l0 [58] Field of Search 118/7, 4, 8, 9, 10,

118/12, 5, 2, 423, 425, 419, DIG. 18, 420,

DIG. 19, 421, 429; 117/114 A, 114 B, 114 C,

[56] References Cited UNITED STATES PATENTS 418,801 1/1890 Lorimer 118/429 12/1919 Lutz 118/405 X 1,926,122 9/1933 Stenhouse 65/164 X 2,914,423 11/1959 Knapp 118/125 X 3,060,055 l0/1962 Bixler 118/405 X 3,079,889 3/1963 Jacobs et al. 118/8 3,227,577 l/l966 Baessler et al.... 1 18/405 X 3,368,525 2/1968 Sacre 118/7 X 3,510,345 5/1970 Marchant... 118/7 X 3,522,836 8/1970 King ll8/7 X 3,599,600 8/1971 Carleton et a1. 118/8 Primary Examiner-Morris Kaplan Attorney-Joseph J. OKeefe [57] ABSTRACT The level of molten metal in a molten metal coating furnace is controlled by the use of a counterweighted hollow ceramic displacement block having thin walls not subject to spalling. The hollow ceramic immersion block is associated with a bath level control system having a fail safe feature.

7 Claims, 2 Drawing Figures MOLTEN METAL BATH LEVEL MAINTENANCE SYSTEM BACKGROUND OF THE INVENTION The present invention relates to the coating of linear material with molten metal coatings and particularly to precise and effective control of the bath level of a molten metal coating bath.

In stepped types of molten metal coating pots, and also on occasion in other molten metal coating apparatus, it is frequently desired to precisely and quickly control the bath level in the coating section of the pot. Various means have in the past been used to control the bath level such as molten metal pumps and other devices. One means for controlling the bath level as' well as for detecting the bath level of a molten metal bath has been by the use of ceramic blocks immersed within the bath. When used for controlling the bath level such blocks are customarily referred to as displacement blocks. By driving the displacement block into the bath the level of the bath will be raised as the block displaces the metal. Conversely by withdrawing the displacement block from the molten metal the level of the bath may be lowered. Such displacement blocks have usually consisted of a solid block of a heat resistant ceramic material such as a high alumina refractory or graphite. The block is normally quite sturdy because of its solid construction. Nevertheless extreme difficulty has been experienced with cracking and spalling of such blocks over a long period of use. Pieces tend to spall off such blocks contaminating the molten metal bath and at times threatening to stop up any orifices in the coating pot under the surface of the bath such as the die orifices or, even more detrimentally, induction heating passageways in the bottom or sides of the molten metal pot thus interfering with the uniform heating of the molten metal. In addition the temperature of the molten bath has often been detrimentally affected by the use of the solid ceramic block to control the level of the molten bath. As the ceramic block is driven into the molten bath the mass of the block often acts as a heat sink to extract heat from the bath. Thus the molten bath will often suffer a sudden small but significant cooling as its level is increased. This has led to difficulties in maintaining a uniform bath temperature. Similar difficulties have not usually been encountered where a ceramic block or float has been used as a bath level indicator rather than a bath level controller because the float in this case maintains a substantially constant level in the molten bath.

It has also at times been a problem in molten metal heating furnaces or pots where the material to be coated enters the bath through a die in the bottom of the pot, or under the surface of the bath, when the linear material such as wire or the like suddenly breaks. In such case the wire or the like may pull out of the die and the molten metal may flow through the die out of the coating pot causing damage not only to the die but also to the surroundings.

SUMMARY OF THE INVENTION The foregoing disadvantages and problems of the prior art practices have now been obviated by the herein described discovery of the present applicants. The present applicants have discovered that if a hollow ceramic displacement block having relatively thin outer walls is immersed in a molten metal coating bath it will have a greatly decreased tendency to crack and spall in comparison with a solid ceramic displacement block and will also not tend to distort the heat balance within the molten bath as it moves during adjustment of the bath level. The hollow ceramic displacement block, because of its much decreased mass, does not provide the large heat sink of a solid block which extracts heat from the bath and the thin walls of the hollow block can easily adjust to changes in temperature and quickly come to a uniform temperature without cracking and spalling of the ceramic. A solid ceramic block on the other hand takes a significant period of time to reach an equilibrium temperature after every change of the level of the block in the molten bath and the cooler internal and upper portions of the block are subject to significant temperature differences and temperature induced physical stress in relation to the outer and lower portions of the block. In other words as a solid block begins to heat up, the outer sections of the block tend to expand and attempt to pull away from the cooler inner core of the block while as the block begins to cool the outer portions of the block tend to shrink with relation to the hotter core and the core of the block thus has a tendency to split the outer portions. All this temperature induced stress tends to cause spalling and cracking of a solid immersion block.

The hollow ceramic displacement block of the present invention on the other hand has only a small mass so that it does not significantly affect the heat balance of the molten bath and its thin walls are not subjected to high differential rates of cooling and heating and thus are not subject to cracking and spalling.

The present inventors have also, using their durable immersion block devised a failsafe type bath level control system which will lower the bath level of a molten coating bath quickly if longitudinal material passing through the bath should break. Thus molten metal is prevented from escaping from the bath through the die orifice if the wire escapes from the die. The buoyancy of the hollow ceramic block aids in quick withdrawal of the block from the molten coating bath.

Also if the coating equipment is shut down and the operator should through oversight neglect to lower the bath level the failsafe feature will serve to lower the bath level instantly thus avoiding needless erosion of the wire by the hot bath and the necessity for restringing the wire before restarting.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic elevation of a molten metal coating bath apparatus incorporating the improvements of the present invention.

FIG. 2 is an enlarged broken away section of the hollow immersion block shown in FIG. 1 showing the construction of the block.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 there is shown a molten metal coating pot 11 having a lining of refractory brick 13. Induction heating tubes 15 positioned in the bottom of the bath are surrounded by an induction coil 17. On one side of the coating pot 11 there is a shelf 19 which provides a shallow section 21 of a bath 23 of a molten metal such as aluminum, zine, copper or like. In the bottom of the shelf 19 or shallow section 21 of the bath 23 there is positioned a housing 25 containing a die 27 through which a wire 29 enters the coating pot 11. A screw cap 31 secures the die 27 within the housing 25. Preferably the die 27 will be composed of a metal or ceramic material non-wettable by the molten metal which composes the bath 23. Within the coating pot 11 there is supported from piston rod 33 and a wire cable 35 a hollow ceramic displacement block 37. The block 37 will desirably have vents 38 in its upper surface. The piston rod 33 is attached to a piston 39 contained in a large pneumatic or, if desired, hydraulic cylinder 41 which serves to drive the displacement block 37 into the molten bath 23 when it is desired to raise the level of the bath from its lowest level, shown by the dotted line 43, to a higher level sufficient to submerge the die 27 beneath the surface of the bath, for instance, at the level shown by the line 45. In some cases it may be found that a hydraulic cylinder will provide more precise control of the bath level system. The cable 35 secured at one end to the hollow displacement block 37 passes over the sheaves 47 and 49 and is attached at its opposite end to a counterweight 51 heavy enough to lift the hollow ceramic displacement block 37 entirely from the bath 23 when not opposed by the action of the pneumatic or hydraulic piston and

cylinder

39 and 41. Stops 52 and 53 within the cylinder 41 restrict the travel of the piston 39 so that the block 37 cannot be driven too deeply into the molten bath 23 and so that ports, referred to hereinafter, in the upper portion of the cylinder 41 are not closed off by upward movement of the piston 39. At one side of the coating pot 11 opposite the shelf 19 there is a recess 54 in the side of the pot 11, which recess 54 accommodates a ceramic float 55 which is buoyed up by the molten metal of the bath 23 and detects the level of the bath 23 when it is at its higher levels. The float 55 may be composed of either a hollow or a solid ceramic block. Since the level of the molten metal customarily remains at the same level on the float 55, it is not subject to great fluctuations in temperature and thus does not have a tendency to crack or spall due to differential rates of cooling and heating. The use of a solid ceramic block for the float 55 is thus not detrimental. The movement of the float 55 is transferred to a rod 57 and then to fulcrum arm 59 pivoted on support 60. Arm 59 has a contact and pointer 61 on the end which rides over the contacts of a potentiometer 63 attached to one pole of a source of current 65. The contact 61 taps off a potential from the potentiometer 63 and conducts it via flexible lead 66 to a suitable controller 67. The other pole of the current source 65 is also attached to the controller 67 through the lead 69. The controller 67 will have suitable internal circuits to apply a current through leads 71 and 73 to solenoid 75 to move a fluid control valve 77 to apply more or less pneumatic or other pressure from an air or other pressure source 79, usually comprising a standard compressed air line in the shop, through an inlet 81 to the cylinder 41 to control the position of the piston 39 in the cylinder and thus the position of the hollow displacement block 37. The valve 77 may either apply more or less pressure to the cylinder 41 from compressed fluid source 79 or may bleed off fluid from cylinder 41 through exhaust port 83 depending upon erted by a spring 85 which biases the solenoid in the opposite direction.

The wire 29 passes from some previous treating or other operation, not shown, passes over a guide sheave 87 and downwardly to a lower guide sheave 89 which changes the direction of the wire and passes it upwardly through the die 27 to guide sheave 91 which in turn directs the coated wire away from the coating pot 11 to a takeup block 92 or, alternatively, to some other treating operation, not shown. Between the guide sheaves 87 and 89 the wire 29 passes over a contact sheave 93 which steadies the wire against the thrust of a biasing sheave 95 which is mounted upon the end of a piston rod 97 secured to a piston 98 in a pneumatic or other fluid pressure cylinder 99. The biasing force applied by the biasing sheave 95 against wire 29 is provided by fluid pressure applied against the piston 98 from a pressure source 101. A pneumatic pressure source will usually be found to be very satisfactory for this purpose. The air or other pressure from the source 101 is regulated through the use of pressure regulator 103 by adjustment of knob 105 and is then applied via conduit 107 to junction 109 where the pressure is directed through two separate conduits 111 and 113 to cylinder 99 and to a control cylinder 115 respectively. The pressure applied to the piston 98 in cylinder 99 will bias sheave 95 against the wire 29 with a constant force which will be resisted by the tension in the wire. The tension exerted by the wire 29 against the sheave 95 serves to maintain the piston 98 above an exhaust port 117 in the side of the cylinder 99. The pneumatic pres sure applied to control cylinder 115 is applied to the surface of a piston 119 within the cylinder 115 and serves to force the piston 119 back against the force of a compression spring 121 until exhaust outlet 123 from cylinder 115 and exhaust outlet 125 from cylinder 41 to control cylinder 115 are blocked off or closed by the piston 119.

The desired bath level 45 to be maintained by the controller 67 may be indicated to the controller by a set point potentiometer 126. Aswitch 127 is included in the circuit of potentiometer 126 to activate or deactivate the circuit. A second switch 128 serves to activate the controller 67 from a power source 129 through the coils 131 of a transformer 133.

A second coil 135 of transformer 133 serves upon the closing of a switch 137 to activate the coils 17 of the induction heated pot 11 through a suitable controller 139. A third coil 141 of transformer 133 serves to apply power through a switch 143 and controller 145 to the wire takeup 92 through

leads

147 and 149.

A branch 151 from exhaust outlet 125 leads to a pressure safety release valve 153 which may be adjusted to open at various pressures by adjustment 155. If the pressure within the cylinder 41 should become excessive for some reason, possibly through the failure of the control 67, tending to drive the hollow block 37 too forcefully or too far into the molten bath the pressure safety release valve 153 will open until the excessive pressure is relieved.

In FIG. 2 there is shown enlarged and broken away to illustrate a preferred construction, a hollow ceramic displacement block 157 similar to the displacement block 37 of FIG. 1. The hollow block has a steel or other metal or heat resistant structural material inner framework 159, which, as shown, may take the form of metal box 161 is preferably made of stainless steel because this alloy has a lower coefficient of expansion and is more resistant to the molten bath. The end of the piston rod 163 is secured to the box 161 through a structural web 165 welded to the inside of the box 161. A series of studs 167 are welded over the surface of the box 161 and to the end of the studs 167 there are welded transverse sections of metal lath 169 spaced about one quarter to one third of an inch from the surface of the box 161. The metal lath 169 is completely covered by a plastering of a high alumina or other suitable type of ceramic plaster 171 over and around the lathe to completelycover the surface of the hollow box 161. The ceramic plaster 171 may be any suitable proprietary refractory composition, preferably having a high alumina composition, particularly if the molten bath 23 is comprised of molten aluminum. Such a plaster composition is commonly used for patching or molding in and about ceramic heating furnaces. A typical anslysis would be 93.4 percent A1 0 5.8 percent P 0, (used as a binder) and less than 1 percent residual or contaminating elements. It is advantageous for the hollow ceramic block to be open at the upper end as shown in H6. 2 so that there may be a free transfer of heat to the atmosphere by convection from the inside of the block. An open end in addition allows a slightly freer expansion of the refractory plaster 171 along the major length dimension of the block.

In the operation of the bath level control of the coating apparatus shown in FIG. 1, after the activation of switches 127 and 128, the desired level 45 of the molten bath 23 is set on the reference potentiometer 126 causing the controller 67 to operate the solenoid 75 to adjust the pneumatic valve 77 until pneumatic pressure admitted to cylinder 41 from source 79 is sufficient to drive the hollow ceramic displacement block into the bath 23 sufficiently to bring the level 45 of the molten bath 23 to a point where the ceramic float 55 transmits a signal to controller 67 through the various mechanical linkages of the float 55 and the potentiometer 63 indicating that the correct level as determined by the setting on potentiometer 126 has been attained. The controller 67 will thereupon control the valve 77 to maintain sufficient pressure in cylinder to maintain the hollow ceramic block 37 at the correct level and to adjust the level of the bath by adjusting the level of the hollow displacement block if the level of the bath varies for any reason.

As the wire 29 passes over the guide sheaves 87 and 89 and contact sheave 93 the tension in the wire opposes the thrust of the contact sheave 95 occasioned by the piston 98 acted upon by the pneumatic pressure from the source 101 as adjusted by pressure regulator 105. The exhaust port 117 in cylinder 99 is thus closed off by the piston 98. If, however, the wire 29 should break so that it may pull out of the die 27 and possibly release the bath metal through the empty die orifice, the movement of contact sheave 95 is no longer opposed and the piston 98 will abruptly move downwardly exposing the exhaust port 117 and allowing the pressure in the conduit 111, junction 109 and conduit 113 to drop. Upon the drop in pressure in conduit 113 the pressure in cylinder 115 will also drop and the compression spring 121 will force-the cylinder 119 downwardly, opening the exhaust outlets 123 and 125 and allowing the pressure within the cylinder 41 to drop precipitously. As the pressure in the cylinder drops the action of the counterweight 51 combined with the buoyancy of the block quickly pulls the hollow displacement block from the molten bath 23 thereby quickly dropping the level of the bath below the level of the die 27 so that no molten metal from the bath 23 has a chance to escape from the coating pot 11. The buoyancy of the hollow ceramic displacement block considerably facilitates the speedy withdrawal of the block from the molten bath.

If the electrical or pneumatic fluid pressure systems in FIG. 1 should fail due to a failure, for instance, in power source 129, the compression springs on solenoid 75, or 121 in cylinder 115, will act respectively to open the exhaust ports 83 or to exhaust the pressure from cylinder 41 and allow the counterweight 51 to quickly raise the hollow ceramic displacement block 37 from the molten bath 23 to prevent the making of off gauge wire or the freezing of the wire 29 in the die 27 if the electrical power for the takeup block 92 pulling the wire through the bath should fail or be interrupted. Likewise if the power source 129 should fail cutting off the induction heating of the coating pot, the hollow ceramic block will be immediately withdrawn from the molten bath to prevent it from freezing into the bath.

Likewise if the operator should shut down the control apparatus by operating switch 128 at the end of a run or for some other reason the hollow ceramic block will be automatically withdrawn from the coating bath. It will readily be recognized that switches 137 and 143 could be easily ganged together to operate as a unit so that opening of any of the switches will cause withdrawal of the block.

The sudden changes in the immersion of the hollow displacement block 37 within the molten metal do not cause any cracking and spalling of the block because while the thin built up walls of the block are exposed to sudden extremes in temperature they are not differentially heated and cooled across any significant thickness of material so that the material of the wall quickly comes to an equilibrium temperature no matter what its position within the hot molten bath may be.

The reinforcing of the thin walls of the displacement block by the metal bath is also a considerable aid in enabling the ceramic to withstand repeated heating and cooling cycles.

It will be understood that various means to detect breakage of the wire 29 or failure of the power to any of the parts of the apparatus, either electrical or pneumatic, could be used in place of the apparatus shown. For example, a gravity type electrical contact switch could with suitable circuits be used to detect breakage or loss of tension in the wire 29. Likewise various alternative means could be devised to exhaust the pressure from the cylinder4l to allow the block to be raised from the coating pot.

We claim: I

1. A bath level maintenance system for a molten metal coating apparatus comprising:

a. a ceramic pot for a molten metal coating bath havi. a reservoir section for the maintenance of a reservoir of molten coating metal, and

ii. a coating section having a wire entrance die spaced at an elevated position with respect to the bottom of said reservoir section,

b. a thin walled hollow ceramic surfaced displacement member arranged for adjustable positioning within said reservoir section to determine the level of the molten bath with respect to said wire entrance die,

c. a first control means to move said ceramic displacement means into and out-of said molten bath to maintain a desired level of molten metal over said wire entrance die, and

d. counterweight means secured to said ceramic displacement member to lift said member from said molten bath if said fluid control means fails to maintain control.

2. A bath level maintenance system for a molten metal coating apparatus according to claim 1 wherein said ceramic displacement member comprises a metal underframe with a ceramic outer coating molded over the surface thereof.

3. A bath level maintenance system for a molten metal coating apparatus according to claim 2 wherein said metal underframe comprises a box member having lath affixed to the surface thereof and an outer high alumina ceramic coating molded thereabout.

4. A bath level maintenance system for a molten metal coating apparatus according to claim 1 additionally comprising:

e. detector means to detect the tension in wire pass- 8. ing through said molten coating bath, and

f. second control means to activate said first control means of (c) to quickly raise said ceramic displacement member and lower said bath level below the orifice of said wire entrance die upon the detection by said detector means of (e) of a sudden loss of tension in the wire passing through said molten coating bath.

5. A bath level maintenance system for a molten metal coating apparatus according to claim 5 additionally comprising adjustable means to limit the force which can be exerted upon said ceramic displacement member by said first control means to limit the depth into the molten coating bath with respect to the surface thereof to which the ceramic displacement member can be driven.

6. A bath level maintenance system according to claim 5 additionally comprising means to activate said control means of (c) to initiate withdrawal of said block from said molten bath upon interruption of the power for heating said molten bath.

7. A bath level maintenance system according to claim 6, additionally comprising means to activate said control means of (c) to initiate withdrawal of said block from said molten bath upon interruption of the power for passing said wire through said bath.

Claims

1. A bath level maintenance system for a molten metal coating apparatus comprising: a. a ceramic pot for a molten metal coating bath having i. a reservoir section for the maintenance of a reservoir of molten coating metal, and ii. a coating section having a wire entrance die spaced at an elevated position with respect to the bottom of said reservoir section, b. a thin walled hollow ceramic surfaced displacement member arranged for adjustable positioning within said reservoir section to determine the level of the molten bath with respect to said wire entrance die, c. a first control means to move said ceramic displacement means into and out of said molten bath to maintain a desired level of molten metal over said wire entrance die, and d. counterweight means secured to said ceramic displacement member to lift said member from said molten bath if said fluid control means fails to maintain control.

4. A bath level maintenance system for a molten metal coating apparatus according to claim 1 additionally comprising: e. detector means to detect the tension in wire passing through said molten coating bath, and f. second control means to activate said first control means of (c) to quickly raise said ceramic displacement member and lower said bath level below the orifice of said wire entrance die upon the detection by said detector means of (e) of a sudden loss of tension in the wire passing through said molten coating bath.