GB1571661A

GB1571661A - Method of repairing ingot moulds or mould stools

Info

Publication number: GB1571661A
Application number: GB4281/77A
Authority: GB
Original assignee: USS Engineers and Consultants Inc
Current assignee: USS Engineers and Consultants Inc
Priority date: 1976-03-08
Filing date: 1977-02-02
Publication date: 1980-07-16
Also published as: ZA77653B; ATA148477A; DE2705834A1; IT1082471B; NO770780L; AR215881A1; BR7701333A; AU2199377A; FR2343539A1; ES456590A1; USRE29646E; FR2343539B1; SE7702258L; SE432728B; AU503783B2; IN147462B; AT352309B; MX144530A

Description

(54) METHOD FOR REPAIRING INGOT MOLDS OR MOLD STOOLS (71) We, USS ENGINEERS AND CONSUL TANTS, INC., a corporation organised and exting under the laws of the State of Delaware, United States of America, of 600 Grant Street, Pittsburgh, State of Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method for repairing ingot molds and stools and closedbottom ingot molds by metal-producing exothermic reaction mixtures, e.g. by aluminothermic reduction (ATR) bulk metal disposition.

A practical method for the repair of ingot molds and stools damaged in service has been in great demand by steel-makers because mold and stool performance relates directly to the cost of steel production.

Among the major reasons for the scrapping or reiecting of molds and stools, which are usually made of cast iron, are cracking of mold walls and stool seats, and erosion of mold bottoms and stool seats. Conventional bulk welding methods for the repair of mold and stools are costly, time-consuming, and usually unsatisfactory as the weldments often crack and fall out. In an attempt to improve the welding method and to look in the weldment, V- or U-shaped notches or grooves or dove-tailed grooves are machined into the surface and the notions are filled with weld metal. This procedure has been generally successful, but remains quite costly. Attempts to fill cracks and craters with ceramic material have failed. Steel plates, spiked into place over cracks and holes on the outside of molds, have been used to contain and prevent cracks in molds from propagating further, but again this type of repair is not completely satisfactory as molten metal can still enter the cracks and' holes making ingot stripping extremely difficult.

In U.S. Patent No. 3,629,928 a repair method is described in which a groove is first cut along the damaged area in the mold wall. A plurality of nails or bolts are anchored in the damaged area such that the heads thereof are exposed in the groove.

The groove is filled with weld metal such that a breakout of the weld metal is minimized. Although successful, the method is very expensive.

Aluminothermic reduction (ATR) techniques have been used in the past to repair castings such as ingot molds and mold stools.

In those processes, it is common practice to perform an appreciable amount of surface conditioning before the metal is deposited.

For example, surface scale is usually removed from the casting and may have thought it essential to undercut the casting surface so that the deposited metal is "keyed" in, i.e. locked in place so that it will not fall out even if it is not bonded to the casting. In addition, it has been thought necessary that the casting be preheated in order to assume a good weld bond of the deposited metal. Since a given volume of ATR powder will yield only about a fifth as much deposited metal, by volume, it has always been necessary to use a containment perimeter to confine the ATR powder and ATR reaction products to the site repaired.

That it to say, in order to fill a crater of given volume with a like volume of ATR deposited metal. the volume of ATR powder used must be five times greater. Hence, a refractory perimeter system is necessary to confine the ATR powder directly over the cavity to be filled. In addition, the perimeter system becomes necessary to similarly confine the molten reaction products which also have a volume greater than the cavity.

That is, to yield a volume of metal equal to the volume of the cavity, an even greater volume of slag is necessarily produced.

After the reaction is effected and while the products are still in the molten condition; the heavier metal product will settle to the bottom filling the cavity, while the slag portion accumulates at the top within the volume defined by hte refractory perimeter system.

After the reaction products have solidified, the perimeter system and overlaying slag are removed, leaving the deposited metal in the cavity.

This invention concerns a method for repairing ingot molds and stools, particularly a method of filling erosion craters therein, which employs an exothermic reduction reaction, such as an aluminothermic reduction reaction, and which does not require the above described complicated procedures.

This method does not require any surface conditioning, any preheating, or the use of a perimeter system, and is described below in terms of the use of ATR mixtures.

The chemical reaction in ATR can be represented as: 3Me0 + 2A1 > 3 Me + Al203 + heat where MeO represents the oxide of the metal to be deposited, such as hermatite (Fe < O3) Al is the aluminum fuel and Al203 the oxide of aluminum, which is a major constituent of the resulting slag. One novel and critical step in our method is the proper disposition of the ATR charge when the ATR reaction is initiated. The ATR charge consisting of a stoichiometric mixture of aluminum and iron oxide, is placed within the defective area such as an erosion pit. Since the reaction takes place and the superheated metal is generated in intimate contact with the substrate surface instead of in a crucible as is normally practiced, the heat of reaction is efficiently utilized, thereby enhancing the bonding of the deposited metal to the steel substrate.

The present invention provides a method of repairing an erosion cavity in an ingot mold stool or the bottom of a closed-bottom metal mold comprising placing in the cavity a metal-producing exothermic reaction mixture including a metal oxide and having a volume no greater than one and a half times the volume of the cavity igniting the mixture to form a superheated melt contained entirely within the cavity and comprising a metal phase and a slag phase, mnintaining the melt in the cavity for a time sufficient to allow the melt to separate so that the metal phase is at the bottom and the slag phase thereover, and permitting the melt to solidify with the metal phase securelv bonded to the bottom of the cavitv and the slag phase securelv attached to ssid metal phase to fonn the exposed surface of the repair. It also provides a method of repairing an erosion cavity in the bottom of a closedbottom ingot mold comprising placing in the bottom of the mold a metal-producing exothermic reaction mixture including a metal oxide and having a volume greater than 1.5 times the cavity volume but less than 5 times the cavity volume, igniting the mixture to form a superheated melt com prising a metal phase and a slag phase, maintaining the melt at the bottom of the mold for a time sufficient to allow it to separate into a lower metal phase entirely contained within the cavity and a slag phase thereover and covering the entire bottom of the mold, and permitting the melt to soldify with the metal phase securely bonded to the bottom of the cavity and the slag phase securely attached to the metal phase to provide the exposed surface of the repair extending over the entire bottom of the mold.

The exothermic reaction mixture is suitably an ATR mixture i.e. one containing aluminum powder, the latter suitably being of particle size of - 100 mesh and +400 mesh. The exothermic reaction mixture preferably contains Fe2O3, which is most preferably at least as fine as -35 mesh. The exothermic reaction mixture is most preferably a substantially stoichiometric mixture of Fe < 03 and aluminum powders.

The method according to the invention can be quick economical and can be performed without elaborate or expensive eqiupment. It can be applied to repairing mold stools without removing the stools from the ingot cars on which they are mounted. The method can form a high-melting, abrasion-resistant ceramic covering over the deposited metal.

The invention is illustrated by the following detailed specification and the attached drawings in which: Figure 1 is a transverse cross-section of an eroded mold stool and the necessary material to accomplish our method.

Figure 2 is a transverse cross-section of a mold stool repaired by the method of the invention.

Figure 3 is a transverse cross-section of a big-end-up (BEU) ingot mold set-up according to another embodiment of our method of depositing an abrasion-resistant monolithic ceramic liner in the bottom thereof.

Figure 4 is a transverse cross-section of the ingot mold of Figure 3 after deposition of a ceramic bottom liner.

As shown in Figure 1, a mold stool 10 has been eroded in service to such an extent that a crater 12 exists in the surface of the stool. We repair this stool while it remains on the ingot car, bv placing an exothermic reaction mixture 14 inside the crater 12. The mixture 14 is deposited in an amount sufficient to overfill the crater 12 so that a mound 16 of mixture 14 is formed.

Ideally, up to a 50% excess of a mixture 14 is desired. As shown, the 50% excess is domed over the crater 12. In the mixture 14 is preferably an aluminothermic reduction (ATR) reaction mixture which suitably consists of above three parts powdered iron oxide, which is preferably Foe203 and not finer than + 200 mesh, and preferably having a size at least as fine as -35 mesh, ,and one part aluminum powder preferably having a size between about - 100 mesh and +400 mesh.

Other materials that might be used instead of aluminum are magnesium, calcium, silicon and calcium-silicon alloy or mixtures thereof. The ATR mixture 14 is ignited by a flame, flare or hot filament. The reaction causes the formation of a superheated melt comprising a metal phase 20 (Figure 2) and a slag phase 22. The more dense metal phase quickly separates from the melt and settles to the bottom where it becomes metallurgically bonded to the stool. Any oxide scale which may have existed on the surface of the mold stool is either chemically reduced or melted with the overlaying slag phase 22. It is believed that this "in situ" bulk disposition process uses the heat of reaction efficiently to provide a mechanism for cleaning and descaling the surfaces, thereby enhancing the formation of additional filler material which becomes welded to the stool. Upon cooling, a metal phase 20 has been weld bonded to the stool 10.

The overlaying slag phase 22 is firmly attached to the metal phase 20 therebeneath, and is left in place to provide additional, although temporary, protection from erosion.

In earlier efforts to deposit an ATR metal within such a cavity as shown in the above description, it was thought that the base metal casting had to be substantially preheated prior to application of the ATR mixture in order to assure sufficient heat at the solid-liquid interface to weld-bond ATR metal to the cavity wall. In the practice of this invention, however, such a preheat is not necessary, nor even desirable. In addition, prior to this invention it was also thought that the presence of the slag phase would be detrimental, and hence such slag phase was always removed. Since the slag phase was removed, it was necessary therefore to provide sufficient ATR mixture so that the entire crater was filled with the reaction metal. The overlaying slag phase was then broken-away and discarded. Since a considerably greater amount of deposited metal was necessary, a correspondingly greater amount of ATR mixture was necessary. As noted above, this then necessitated building or placing a refractory containment perimeter system around the cavity to contain the extra ATR mixture and reaction products so that the metal would enter the cavity. In practice, it was found necessary that the loose ATR mixture have a volume of approximately six times greater than the volume of the cavity. In the present practice, however, wherein both reaction products are to fill the cavity, i.e. the metal and slag phases, the ATR mixture should have a volume of one and a half times that of the crater in order to fill the crater. Accordingly, a refractory perimeter system is not necessary, and hence the repair can be effected without the need of any apparatus whatsoever.

In the above-described embodiment of this invention it was noted that in order to fill the cavity flush full with the metal and slag phases, the loose ATR mixture provided should have a volume up to about 50% greater than the volume of the cavity.

It should be noted, however, that it is not necessary that the metal and slag phases completely fill the cavity. Indeed, the upper surface of the slag phase 22 may be lower than the upper surface of the mold stool 10 without sacrificing any advantages. In some applications, it may even be desirable that the cavity be under-filled as the slag phase 22 tends to "lock" in place into the cavity walls. In addition to the above modifications, it is not necessary that all of the ATR mixture be applied in the cavity at one time.

For example, the cavity can be partially filled with ATR mixture, the mixture reacted, and then subsequently more ATR mixture may be added and then that mixture reacted. If the reaction products from the first added ATR mixture are still molten when the second ATR mixture is added, the two metal phases and the two slag phases will combine to yield just two phases substantially as shown in Figure 2. However, if the reaction products from the first added ATR mixture have solidified when the second ATR mixture is added, four distinct layers will be formed, i.e. metal and overlaying slag from the first ATR mixture and then second metal and slag layers thereover from the second ATR mixture.

The above described procedure for adding the ATR mixture at two different times does provide one advantage in that upper slag layer formed when the second ATR mixture is reacted tends to be more dense, i.e. less porous. It is believed that the first ATR reaction results in some slag porosity because the reaction is in contact with the cast iron mold stool, and carbon in the caet iron reacts with oxygen in the ATR mixture to form some CO2. However, the second ATR products are exposed to substantially less cast iron surface, and hence less CO is formed. It is interesting to note that this greater slag density results whether or not the first ATR reaction products have solidified when the second ATR mixture is added.

The above embodiment describes a method of filling a cavity in a mold stool 10.

It should be realized however, that the exact same procedure could be used for filling a cavity in the bottom of a closed-bottom ingot mold. Insofar as closed-bottom ingot molds are concerned, we have successfully utilized another embodiment of this invention wherein the entire bottom of the ingot mold is provided with a monolithic slag layer. Figures 3 and 4 illustrate this embodiment wherein a closed-bottom ingot mold 30, having an erosion cavity 32 in the bottom thereof is repaired. In this embodiment sufficient ATR mixture 34 is provided to cover the bottom of the mold to a depth no greater than about three inches, as sufficient to assure that the ATR reaction slag phase 38 not only fills the upper portion of cavity 32 but also forms a thin layer of slag completely covering the bottom of mold 30. The metal phase 36 will be securely bonded to the bottom of the cavity 32, while the slag phase 38 will be attached to the metal phase 36 and the bottom of the ingot mold 30. Ideally, the slag phase layer should not be more than about one or two inches thick, since thicker deposits may result in shorter ingots stripped therefrom, unless of course, special overly tall ingot molds are involved.

In the above described embodiment it is important that only the slag phase 38 be allowed to over-fill cavity 32 for the best results. If the metal phase 36 covers the entire bottom of the ingot mold 30 problems may be encountered. Specifically if the metal layer is thick, the slag layer will be even thicker and hence the depth of the mold will be appreciably reduced. This will result in appreciably shorter ingot cast therein. On the other hand, if such a metal phase layer is thin the heat therein is quickly dissipated to the mold bottom, and hence the metal does not form a good bond with mold bottom. For optimum results therefore, the amount of ATR mixture used in this embodiment should be such that the entire metal phase is contained within the cavity so that only the slag phase extends across the mold from wall to wall. This would require that the volume of ATR mixture used should be more than 1.5 times the volume of the cavity, i.e. that amount necessary to fill only the cavity with metal and slag; and should be less than 5 times the volume of the cavity, i.e. that amount which would completely fill the cavity with metal phase.

Since the overlaying slag deposit in each of the above embodiments is rather brittle, continued use of such a repaired stool or ingot mold will eventually cause the slag phase to spall and break away. For this reason it was originally thought that the slag phase should be removed before the repaired part was placed in service, as noted above. That is, it was feared that the slag phase would break away as steel was teemed into the mold and end up within the cast ingot. Contrary to these fears, however, we have utilized the above procedures to repair many stools and molds and have never encountered an ingot contaminated with slag inclusions from the ATR repair. Accordingly, not only does the slag deposit not cause ingot contamination, but it does provide the beneficial result of being less susceptible to melt away erosion during teeming.

Hence, the slag phase does provide extra protection from erosion and does extend the life of the repaired stool or mold.

WHAT WE CLAIM IS: 1. A method of repairing an erosion cavity in an ingot mold stool or the bottom of a closed-bottom metal mold comprising placing in the cavity a metal-producing exothermic reaction mixture including a metal oxide an having a volume no greater than one and a half times the volume of the cavity, igniting the mixture to form a superheated melt contained entirely within the cavity and comprising a metal phase and a slag phase, maintaining the melt in the cavity for a time sufficient to allow the melt to separate so that the metal phase is at the bottom and the slag phase thereover, and permitting the melt to solidify with the metal phase securely bonded to the bottom of the cavity and the slag phase securely attached to said metal phase to form the exposed surface of the repair.

2. A method of repairing an erosion cavity in the bottom of a closed-bottom ingot mold comprising placing in the bottom of the mold a metal-producing exothermic reaction mixture including a metal oxide and having a volume greater than 1.5 times the cavity volume but less than 5 times the cavity volume, igniting the mixture to form a superheated melt-comprising a metal phase and a slag phase, maintaining the melt at the bottom of the mold for a time sufficient to allow it to separate into a lower metal phase entirely contained within the cavity and a slag phase thereover and- cover- ing the entire bottom of the mold, and permitting the melt to solidify with the metal phase securely bonded to the bottom of the cavity and the slag phase securely attached to the metal phase to provide the exposed surface of the repair extending over the entire bottom of the mold.

3. A method according to claim 1 or 2 wherein the mixture contains aluminum powder.

4. A method according to claim 3 wherein the aluminum powder is of particle size of -100 mesh and +400 mesh.

5. A method according to any prece

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The above embodiment describes a method of filling a cavity in a mold stool 10. It should be realized however, that the exact same procedure could be used for filling a cavity in the bottom of a closed-bottom ingot mold. Insofar as closed-bottom ingot molds are concerned, we have successfully utilized another embodiment of this invention wherein the entire bottom of the ingot mold is provided with a monolithic slag layer. Figures 3 and 4 illustrate this embodiment wherein a closed-bottom ingot mold 30, having an erosion cavity 32 in the bottom thereof is repaired. In this embodiment sufficient ATR mixture 34 is provided to cover the bottom of the mold to a depth no greater than about three inches, as sufficient to assure that the ATR reaction slag phase 38 not only fills the upper portion of cavity 32 but also forms a thin layer of slag completely covering the bottom of mold 30. The metal phase 36 will be securely bonded to the bottom of the cavity 32, while the slag phase 38 will be attached to the metal phase 36 and the bottom of the ingot mold 30. Ideally, the slag phase layer should not be more than about one or two inches thick, since thicker deposits may result in shorter ingots stripped therefrom, unless of course, special overly tall ingot molds are involved. In the above described embodiment it is important that only the slag phase 38 be allowed to over-fill cavity 32 for the best results. If the metal phase 36 covers the entire bottom of the ingot mold 30 problems may be encountered. Specifically if the metal layer is thick, the slag layer will be even thicker and hence the depth of the mold will be appreciably reduced. This will result in appreciably shorter ingot cast therein. On the other hand, if such a metal phase layer is thin the heat therein is quickly dissipated to the mold bottom, and hence the metal does not form a good bond with mold bottom. For optimum results therefore, the amount of ATR mixture used in this embodiment should be such that the entire metal phase is contained within the cavity so that only the slag phase extends across the mold from wall to wall. This would require that the volume of ATR mixture used should be more than 1.5 times the volume of the cavity, i.e. that amount necessary to fill only the cavity with metal and slag; and should be less than 5 times the volume of the cavity, i.e. that amount which would completely fill the cavity with metal phase. Since the overlaying slag deposit in each of the above embodiments is rather brittle, continued use of such a repaired stool or ingot mold will eventually cause the slag phase to spall and break away. For this reason it was originally thought that the slag phase should be removed before the repaired part was placed in service, as noted above. That is, it was feared that the slag phase would break away as steel was teemed into the mold and end up within the cast ingot. Contrary to these fears, however, we have utilized the above procedures to repair many stools and molds and have never encountered an ingot contaminated with slag inclusions from the ATR repair. Accordingly, not only does the slag deposit not cause ingot contamination, but it does provide the beneficial result of being less susceptible to melt away erosion during teeming. Hence, the slag phase does provide extra protection from erosion and does extend the life of the repaired stool or mold. WHAT WE CLAIM IS:

1. A method of repairing an erosion cavity in an ingot mold stool or the bottom of a closed-bottom metal mold comprising placing in the cavity a metal-producing exothermic reaction mixture including a metal oxide an having a volume no greater than one and a half times the volume of the cavity, igniting the mixture to form a superheated melt contained entirely within the cavity and comprising a metal phase and a slag phase, maintaining the melt in the cavity for a time sufficient to allow the melt to separate so that the metal phase is at the bottom and the slag phase thereover, and permitting the melt to solidify with the metal phase securely bonded to the bottom of the cavity and the slag phase securely attached to said metal phase to form the exposed surface of the repair.

5. A method according to any prece

ding claim wherein the mixture contains Fe2O3..

6. A method according to claim 5 wherein the Fe2O3 is at least as fine as -35 mesh.

7. A method according to any preceding claim wherein the exothermic reaction mixture comprises a substantially stoichiometric mixture of Foe203 and aluminum powders.

8. A method according to any preceding claim wherein the stool or mold is cast iron.

9. A method according to claim 1 modified in that a portion of the mixture is placed in the cavity and ignited to form a first melt comprising a metal phase and a slag phase, and thereafter another portion of such mixture is placed in the cavity and ignited to form a second melt comprising a metal phase and a slag phase.

10. A method according to claim 9 in which the first melt is solidified before the other portion of mixture is placed in the cavity.

11. A method according to claim 9 in which the first melt is still molten when the other portion of mixture is placed in the cavity.

12. A method according to claim 9 in which the mixture is divided into more than two portions each of which is individually placed in the cavity and ignited.

13. A method according to claim 2 in which the slag phase is no thicker than 2 inches.

14. A method of repairing an erosion cavity substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.

15. A method of repairing an erosion cavity substantially as hereinbefore described with reference to Figs. 3 and 4 of the accompanying drawings.

16. An ingot stool or mold repaired by a method according to any of claims 1 to 15.