US3294596A

US3294596A - Process and apparatus for annealing thin steel plates

Info

Publication number: US3294596A
Application number: US448033A
Authority: US
Inventors: Daubersy Jean
Original assignee: Individual
Current assignee: Individual
Priority date: 1961-02-24
Filing date: 1965-04-14
Publication date: 1966-12-27
Anticipated expiration: 1983-12-27
Also published as: AT242727B; GB1011562A; LU41089A1; BE600643A

Description

J. DAUBERSY Dec. 27, 1966 PROCESS AND APPARATUS FOR ANNEALING THIN STEEL PLATES Filed April 14, 1965 United States Patent 2 Claims. (on. 148-16) This application is a continuation-in-part of applicants co-pending application Serial No. 174,887 filed February 21, 1962, now abandoned.

The process according to the invention relates to the fabrication of thin steel plates or sheets of mild steel which are specially provided for stamping; this quality is obtained by wet hydrogen by extracting during an annealing operation carbon and nitrogen which render steel aging.

The possibility of extruding carbon and nitrogen from steel sheet by annealing in moist hydrogen is well known. It has likewise been known for a long time that it is possible in extruding by this treatment, the last traces of carbon and nitrogen from a sheet of rimming steel to render this sheet insensitive to aging. Numerous theoretical and experimental studies have been made on this subject, and more specially a very complete study was made by S. W. Low and H. Gensamer (Metals Technology, December 1943) as well as in U.S. Patent No. 2,360,768 by Gensamer. However, in spite of the great technical interest of such a process for the drawing industry, it has not been possible to realize any practical application in this field, the cost of such an annealing operation being prohibitive as compared with the market value of a sheet of good drawing quality. In addition to the great expense for hydrogen necessary for the realization of the decarburization and denitrification one had to reckon with the very slight output or productive capacity of the continuous annealing furnaces which had to be installed instead of bell furnaces in order to permit the action of the furnace filling gases or atmosphere gases on the metal sheet. This slight output is of course due to the extreme slowness of the reactions necessitating periods of contact between the sheet and the atmosphere of the order of several hours.

More recently decarbonizing-denitrifying annealing has again aroused the interest of the metal sheet industries owing to the realization of the open-coil bell furnace which allows usage of a bell furnace in place of a continuous one (see US. Patent No. 639,939, LeeWilson and E. A. Corns). Indeed it has become possible to Wind coils of metal sheets in unjoined spirals with a precision such that the gas circulating in the bell annealing furnace can pass between the spirals of the coil and act with a sufficient regularity over the entire surface of the sheet so wound.

The technique now recommended by Wilson and Corns consists in using annealing furnaces very similarv to the bell furnaces traditionally employed for non-expanded coils and which comprise like these traditional furnaces a hearth provided with a means for internally circulating of the atmosphere gases (fan). (See also Arnold Open Coils Process Shows Way to Gas Alloying1ron and Steel Engineer-August 1960-p. 91-111.)

These furnaces for expanded coils differ practically from the traditional furnaces only by their dimensions and by some details relative to the packings or tight joints. 7

Their use for a decarburizing-denitrifying annealing operation comprises in principle a period of heating under a neutral protective gas (such as the known HNX. gas which is a dry mixture of nitrogen and3 to 10% in volume of hydrogen), a period of soaking (maintenance at constant temperature) in an atmosphere of moist or wet H gas 3,294,596 Patented Dec. 27, 1966 and a period of cooling under a protective gas such as said dry HNX gas.

'80 used, these furnaces give after annealing, non aging steels. However, this result is obtained only by an excessive prolongation of the time of soaking under an at mosphere of moist hydrogen. Denitrifying even when it is a question of steels particularly poor in nitrogen requires a duration of the order of 10 to 20 hours. This disadvantage considerably reduces the output or capacity of production and involves a greater consumption of hydrogen and heating calories.

These results are all the more deceptive because it is possible in the laboratory to render non-aging the same steel sheets by a treatment with moist hydrogen the duration of which does not exceed 3 hours (as also stated by Gensamer in the above-mentioned patent).

Decarburization-denitrification tests on a semi-industrial furnace and detailed study of the circulation of the gases realized in the bellfurnaces now used for annealing expanded coils have shown to the applicant that these difliculties arose from a bad circulation of the moist hydrogen atmosphere between the spirals of the coils. The known means for circulating the gases used in bell furnacesfor coils as 'describedby Wilson,.Corns, Arnold, comprise a fan of the centrifugal type intended to insure the circulationof the gases.

This circulation internal to the furnace comprises the successive flowing of the gases through the turbine, along the heated walls of the bell and through the spirals of the coils, according to a cycle beginning again endlessly with a rate of flow resulting from the motive power due to the fan and the internal'resistance of the circuit. On this internal circulation there is superposed an external circulation comprising the injection in the furnace of a certain quantity of fresh gases and the extraction of an approximately equal volume of gas contaminated by the impurities coming from the chemical reactions which take place in the furnace.

The fan has without doubt an appreciable effect on the time of heating in a relatively dense atmosphere of HNX gas (more than ofN but when the gases become heated and especially when the protective atmosphere is replaced by moist hydrogen considerably lighter and carried to temperatures of the order of 700 C. the rate of flow shuts down considerably. In fact, it has been noted that the rate of flow of the gas circulating in the coils when measured in normal cubic meters (that is in cubic meters (m3) .at a pressure of 760 mm. of Hg and rat a temperature of 0 C.), was in the period of soaking with moist hydrogen reduced to less than of the rate of'flow of gas realized at the beginning of the heating under HNX gas.

The solution for having a good circulation of the gases in an open coil was suggested solely by the results of an experimental and theoretical study of the circulation of the gases.

It is a fact that the circulation of the gases takes place between the spirals, that is to say in a narrow space delimited by quite parallel walls. That has for a result, at least for hydrogen, that the circulation is of the laminar type even for very high flows (if reference is had to a more scientific notion it will be noted that the number of Reynolds is very slight). The reality of this type of laminar circulation has, furthermore, been checked experimentally by the applicant.

Now the type of laminar circulation has this in particular, that the loss of charge is independent of-the density of the gas but is proportional to the absolute viscosity. Ina circuitwith turbulent flow, the loss of charge is on the contrary proportional to the. density and independent of the viscosity. The motive pressure created by the ven- 3 tilator is, on the other hand, proportional to the density of the fluid.

It follows that the speeds of the fluids in circulation in a circuit comprising a ventilator and open-coils, react very fan, in inverse direction to the normal direction of circulation.

On the other hand, when the fan delivers on the internal circuit, the fresh gas injected must necessarily be differently from the speeds realized in a circut comprising 5 mixed with the contaminated gas in circulation and the a ventilator and charges of any kind (turbulent flow) high degrees of purity necessary for denitrification are when one passes from HNX gas (heavy and cold) to H only very slowly attained. To obtain a non-ageing steel gas (very light and hot). the levels of C and N in the steel must be so low as In the second case (any charge) the speeds of circula- 0.003% in weight for the C and 0.0005% in weight for tion do not vary for the drop in motive pressure due to the N the diminution of the density is compensated by the fact As stated above, all the furnaces used up to the present that the loss in charge is itself reduced in proportion to to house open coils always have a double circulation systhe density. Only the mass in circulation diminishes in tem. proportion to the density. This is the case with the furnace of the USA. Patent In the first case (open coils) the speed diminishes more 2,360,768, Lee Wilson or Corn, and described by J. Arnold than proportionally to the density for not only there is no in Open Coils Process Shows Way to Gas Alloying, compensation but there is even a supplementary reduction Iron and Steel Engineer, August 1960, pages 91 to 111, due to an appreciable increase in the viscosity with the or with the furnace of the Corn patent, Canada, 630,352, temperature. As for the mass in circulation, it diminishes or again with the furnace of the Lee Wilson USA. Patent more than proportionally to the square of the density. 3,109,877. All these furnaces have on the one hand in- In the following there is a formula of circulation estabternal circulation by fan and on the other hand external lished experimentally and a table giving the rates of flow circulation comprising a supplying with fresh gas and the which are established in an open coil, said rates of flow withdrawal of an equivalent volume of used gas. being calculated on the basis of this formula. The object of the present invention is a method of over- Observation on the laminar flow between two plates Commg the above'described dlsadyantages' h separated from one another by a distance E. The meihod l the Invention charaqtensed m at the two circulations are malntained during the period The 1055 of Charge P unit of length undergone by of heating, whereas during the period of soaking, which a fluid Circulating in laminar fashion at a means Speed U is the period of the reactions of decarbonisation and of between two Plates distant y E ff 0m 0116 another is given denitration by humid hydrogen, the internal circulation y the formula: is stopped and at the same time the external circulation U is considerably increased. The reactive gases then pass 11:18 HE- through the furnace and its change under the eifect only of the pressure existing in the feed system, and pass only in which is the absolute viscosity of the fluid and K once through the spirals of the coil thus carrying off a cofifiicient of proportionality This cotifhcieht, after I111- directly the impurities occurring through the reactions; merous measurements, was determined experimentally by among these impurities which retard the reactions, amthe applicant and found to be equal to 12. The formula mania gas is by far the most injurious. above thus reduced 4 According to the invention, it is adequate to have a U 40 system of hydrogen under sufiicient pressure and to pro- -EE vide, for the bells of the annealing furnaces, hermetic joints capable of bearing the pressure used. By way perfumed drawmg up the following table of example and in no restrictive sense, and to show Circulation calculated for-2.2 height of coilsclearly the effectiveness of the invention, it is possible between turns of 1 mm.Diameter: 2,800 mm. to feed with hydrogen an annealing furnace by means Specific Pressure Loss of Charge Caleu- Rates of Fluid Gravity, Ventilator, In Open coil, lated flow in kg/m. mm. water 11ml. water/m. speed, in 9 N/h cmJsec.

Cold air 1. 33 75 Cold nitrogen". 1. 25 70. 5 20. 3 15s 13, 000 Nitrogen at 700 C 0.35 19. 8 49 18 415 H cold 0. 09 5.1 10. 5 20 H at 700 C 0.025 1. 4 24 2. 6 H sat. .50 0-700 0.. 0. 04.4 2. 5 24 4. 7 10s The exploitation of industrial furnaces and numerous 60 of a duct connected to a gas container filled with gas pressure and flow measurements carried out at different under a pressure of 300 mm. of water. It is thus pospoints of furnaces provided with internal ventilator have sible to have in the annealing furnace an operating presconfirmed the reality at the industrial level of the combisure of more than 200 mm. of water, whilst in the case nation deduced from the calculation. NB. The losses of the use of a turbine for putting the hot hydrogen in charge were determined by linear meter and for aspeed in movement, operating pressures of the order of 2.5 of 1 meter per second. mm. of water only were obtained, and this when using Furthermore, and this is without doubt still more a turbine of 1 meter in diameter rotating at 750 rpm. serious, the presence of a fan is an impediment to the A preferred variation of the device of the invention establishment of a sufficient and rational circulation. consists in using a heat exchanger in the gas circuit be- In view of the slight motive pressure produced by the tween the outlet and the inlet of the furnaces. It is thus fan, it is indeed impossible to create a considerable expossible to recover a large part of the heat of the amternal circulation and it is evident that it is not possible bient gases. to produce a rate of flow of fresh gas exceeding that of In accordance with another advantageous variation of the internal circulation for the excess will not pass into the the invention the fan of the furnace is also blocked up spirals of the coils but will be short-circuited through the and use is made of the pressure of an independent source of protection gas, such as the known gas HNX or any other gas, in the course of the cooling period. This variation has the advantage of speeding up the heating and cooling speeds considerably.

A very important and unexpected advantage arises from this variation of the annealing method of the invention. It-need actually only be observed that the time necessary for the cooling of the coils in accordance with the customary method was very long. In the furnaces at present used it is of the order of 24 hours for 60 tons and up to the present there has been no indication of accelerating this cooling without due thought, as industrial experience has shown indisputably the interest that exists in cooling the open coils slowly to obtain good stamping properties.

The inventor has now stated that the steels made nonageing by treatment with humid hydrogen keeptheir suitability for stamping very high cooling speeds, even up to oil tempering speeds.

It is thus possible in accordance with the invention to obtain very high cooling speeds,-since after the blocking of the fan or the turbine of the furnace, use is made of the supply of a productive gas such as the gas HNX, dry and under pressure, to force thisgas through the coils in course of cooling; this heated gas is collected at the outlet of the'furnace and caused to recycle in the system of the protective-gas by'means of a blower, after it has been cooled in a large capacity cooler. The cooling speed of the coils is thus a function of the rate of flow of the protective gas and is no longer restricted to the sole possibilities of cooling of the furnace bell. a period of the order of hours by means of a circulation of HNX gas at a rate of the order of 3,000 rnfi/hour.

The blocking means for the fan or of the turbine can, in accordance with the invention, be placed optionally over the entry or outlet orifice of the turbine, and can be operated externally by a device passing through the base of the bell.

The present invention also aims at a device for carrying outthe method described above. The deviceforming the object of the present invention is essentially characterised in that the open coil annealing furnace has a device making it possible to block the entry or the outlet of the turbine, and also the forced circulation of gases through the spirals of the coils using only the feed pressure. Preferably, use is made of a heat exchanger between the entry and the outlet of the gases.

In accordance with a preferred variation of the device of the invention, device can also have a circuit for the recovery of the protective gas used during the cooling period with means for. recycling theseagases after having cooled' them.

The figure at the attached drawing sheet ShOWs by way of example and in no restrictive sense an apparatus by which it is possible to carry out the method of the invention.

The decarbonising, 'denitrifying and annealing of the method of the invention is carried out as follows:

The open coils use-d weigh 10 to 20 tons and have spirals separated by a distance of 2.5 mm. by twisted metal wires.

A first open coil 5a is disposed on the base 6 of the furnace 1, with its axis 1a vertical, and care being taken that the circular crown formed by the coil coincides with the perforated crown of the base.

If the height of the furnace permits, on the first coil 5a a second coil 50 is placed. The two coils being separated by a wire netting 5b placed above the first coil. It is even possible, the case arising, to arrange a third coil in the same way. The spirals 10 of the coils are not in contact.

When the coils have been put in position, the sealing bell 3 of the furnace is put on, then the furnace hell 1 itself, and the sealing joint 4 is filled with oil. The blocking means 12 of the fan 7 is raised by the device 13 and,

6 as protective atmosphere, there is introduced into the bell a gas-composed of 94% nitrogen and 6% hydrogen and known under thename of gas HNX, the dew point of which is between 20 C. and 40 C.

The content in hydrogen in this gas HNX may be from 5 to 10% and that of nitrogen from 95 to 90%.

This HNX gas, accumulated in the tank 18 under a pressure of at least 200 mm. water column, is introduced at the rate of 50 m. /hour by opening the valve 15 on the conduit 15a and the valve 17 on the conduit 9; the

valves

23 and 16 on the conduits 15b and 16a remain closed. The fan 7 is then set in motion and the furnace heating is set in operation by known means not shown, the flow of the heating flames being shown by the arrows 2 of the'exterior of the bell 3.

The gases surrounding the coils, located in the sealing bell 3, arethus put in circulation by the fan 7 and contribute to accelerate theheating of the coils. The gases coming from the fan 7 ascend along the wall of the sealing bell 3, becoming heated, and descend by means of the fan through the spaces between the non-contacting spirals 10, cooling on contact with these. As Well as this internal circulation there is imposed the external circulation created bythe supply at 50 mfi/hours of cold HNX gas through the conduit 9 and the outlet of an equivalent amountof hot gases through the outlet pipe 11 and the exchanger 14. At thebeginning of the heating, these gases comprise oil vapours coming from the distilling of the rolling oil covering the coils charged in the furnace.

As soon as the issue of these oil vapours ceases, the

valve 16 is opened and the valve 17 closed, which makes it possible to recover a part of the sensible heat of the gases 14 and heat the gas coming from the tank 18 and passing via the

conduits

15a, 16a, 16b and 9 into the furnace. The

conduits

9 and 11 have a diameter of approximately 80 mm. in the case where the furnace has a capacity of 50 tons i.e. it is advantageous to have a section of approximately 1.5 cm per ton of coil treated. The exchanger 14 has a heat exchanger surface between 25 and m Then the heating operation proceeds until the coil is at approximately 700 C.

At the moment, without stopping the turbine 7 which would bring the risk of imposing on the latter unbalanced thermal dilatations,the' valve '12 is closed, by actuating the device 13, which interrupts the internal circulation. The supply of HNX gas isstopped, by closing the valve 15, and hydrogen H is introduced from the tank 19 by opening the valve 23 on the conduit 1512. In the tank 19 the hydrogen is under a pressure of approximately 300 mm. water column; the rate of flowof'hydrogen is controlled preferably, by means of the valve 16, at 10 mfi/ hour per ton of sheet treated.

Through the conduit 20 on which is mounted the valve 21, there is then introduced superheated steam at approximately 200" C., coming from a source which is not shown; the amount of steam is usually between 50 and 75 gr. per m. of dry hydrogen. This amount of steam is not absolutely essential. It sufiices if a dew point of 70 C. is not exceeded in the mixture. During this soaking phase, the humid hydrogen does not pass through the spirals 10 of the coils and the gases only issue from the furnace through the conduit 11 by the action of pressure in the tank 19, as the blocker 12 prevents the setting up of an internal circulation by the fan 7.

Decarbonisation is thus very quick and, taking into account the thickness of the sheet, usually takes less than 3 hours. It can be seen by the analysis of the CO present in the gas issuing from the furnace through the conduit 11, the content in CO then falling to below 0.2% in volume.

.The total soaking time necessary to obtain the sheet contents of carbon and of nitrogen such that the steel, is no longer ageing (0.003% C and 0.005% N is under these conditions less than the time necessitated by the annealing itself, i.e. for obtaining a grain size of 6 ASTM. In actual fact, this soaking time is generally less than 10 hours. It is, moreover, possible by analysis of the content in NH of the issuing gas to determine the moment when there is no longer denitrification.

When the soaking is judged to be adequate, one proceeds to the cooling by cutting off the heating of the furnace 1 which is removed. Then the valves and -17 are opened and the

valves

23 and 16 closed; in this way the supply of hydrogen is stopped, whilst the flow of HNX gas is set up, further, the exchanger 14 is put out of circuit. The cooling is accelerated by passing very large amounts of cold HNX gas through the furnace for instance, there can be a rate of flow of HNX of 3000 m. /hour. The internal circulation circuit may be reestablished during the cooling period.

Further, it is advantage to cool the issuing HNX gas. To do this, these gases issuing through the conduit 11 pass through an exchanger 26 where they lose a part of their heat, then they are sent back to the gasometer 18 by means of the blower 24 mounted on the conduit 25 connected at 25a, re-entering the tank 18.

The various values above relating in particular to pressures, rates of flow of gas, temperatures, periods, are

given only by way of examples, since in actual industrial practice these values can vary considerably. Thus, it has been found that in a 50 ton furnace the rate of flow of HNX gas in the heating can be from 30 to 100 m. /hour, the soaking period can be between 6 and 12 hours with a flow of hydrogen of 300 to 1,000 m. /hour, having an amount of steam such that the dew point is between C. and 70 C. whilst the cooling can be carried out by the HNX gas at a flow rate reaching 3,000 mfi/hour.

The advantages of the method result from the decrease in the operating time and in the outlay in hydrogen necessary for annealing a ton of sheet. While keeping unchanged the time necessary for the heating, the time necessary for the decarbonisation and denitrification is reduced by more than half, because larger amounts of humid hydrogen can be passed between the spirals of the open coils and because the reactions are facilitated by the rapid removal of the reaction'products. The cooling times are also reduced by half. Concerning the hydrogen its consumption drops from 300 in. per ton, necessary in the open coil furnaces known up to the present,

' to 50-100 m. /ton in the method of the invention.

While the foregoing description contains specific details of my invention, it is to be understood that these details have only been presented with a view to enable those skilled in the art to take full advantage of my invention, but the monopoly which I seek in the United States is to be derived from the gist and spirit of the following patent claims.

I claim as my invention:

1. Process of annealing steel plates in open coils as deep drawing plates of mild steel combined With a treatment of decarburizing and denitriding utilizing a bell furnace provided with a ventilator for internal circulation of an atmosphere of gases, comprising the steps of annealing the plates in a bell furnace at a temperature of about 700 C. during which there is admitted and extracted therefrom a relatively slight flow of protective gas having a base of dry nitrogen admixed with 5 to 10% of hydrogen, the said flows being established between 0.6 and 2 m. per ton of steel treated, there being established both an external circulation and an internal circulation during this step of heating and thereafter soaking said steel at about 700 C. for about 6 to 12 hours to ensure a course grain structure, during this period of soaking, at the time when the temperature ceases to rise the protective gas is replaced by hydrogen gas the flow of which will be established between 6 and 20 m. /h. per ton of steel treated, and the internal circulation is stopped by the manipulation of a valve to provide a direct passage of the hydrogen gas through the furnace and the coils without any internal recycling thereof, water vapor is admixed with the hydrogen gas in a quantity such that the dew point of the mixture will be between 25 to C., as soon as the protective gas has been expelled and replaced by the hydrogen gas, the said hydrogen gas being introduced hot as a result of a temperature exchange with the outcoming gas after the end of the soaking period, after the desired soaking period the furnace is shut down, the supply of hydrogen gas is stopped, internal circulation is once again effected and the said protective gas is fed into the furnace to effect cooling of the coil.

2. Process of annealing steel plates according to claim 1, in which in the cooling step the protective gas is introduced without reestablishing the internal circulation circuit, the protective gas being introduced cold without passing through a heat exchanger between inlet and outlet of the gases.

References Cited by the Examiner UNITED STATES PATENTS 2,360,868 10/1944 Gensamer 14816 2,746,742 5/ 1956 Comley 2665 2,752,148 6/1956 Kincaid et al. 2665 2,998,303 8/1961 Huebler 14816 3,105,780 10/1963 Low l4816 3,109,877 11/1963 Wilson 2665 3,127,289 3/1964 Beall 148-16 FOREIGN PATENTS 585,921 2/ 1947 Great Britain.

OTHER REFERENCES Arnold: Open Coil Process, Iron and Steel Engineer, August 1960, pp. 91-111.

DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, C. N. LOVELL, Assistant Examiners.

Claims

1. PROCESS OF ANNEALING STEEL PLATES IN OPEN COILS AS DEEP DRAWING PLATES OF MILD STEEL COMBINED WITH A TREATMENT OF DECARBURIZING AND DENITRIDING UTILIZING A BELL FURNACE PROVIDED WITH A VENTILATOR FOR INTERNAL CIRCULATION OF AN ATMOSPHERE OF GASES, COMPRISING THE STEPS OF ANNEALING THE PLATES IN A BELL FURNACE AT A TEMPERATURE OF ABOUT 700*C. DURING WHICH THERE IS ADMITTED AND EXTRACTED THEREFROM A RELATIVELY SLIGHT FLOW OF PROTECTIVE GAS HAVING A BASE OF DRY NITROGEN ADMIXED WITH 5 O 10% OF HYDROGEN, THE SAID FLOWS BEING ESTABLISHED BETWEEN 0.6 AND 2 M3 PER TON OF STEEL TREADED, THERE BEING ESTABLISHED BOTH AN EXTERNAL CIRCULATION AND AN INTERNAL CIRCULATION DURING THIS STEP OF HEATING AND THEREAFTER SOAKING SAID STEEL AT ABOUT 700*C. FOR ABOUT 6 TO 12 HOURS TO ENSURE A COURSE GRAIN STRUCTURE, DURING THIS PERIOD OF SOAKING AT THE TIME WHEN THE TEMPERATURE CEASES TO RISE THE PROTECTIVE GAS IS REPLACED BY HYDROGEN GAS THE FLOW OF WHICH WILL BE ESTABLISHED BETWEEN 6 AND 20 M3/H. PER TON OF STEEL TREATED, AND THE INTERNAL CIRCULATION IS STOPPED BY THE MANIPULATION OF A VALVE TO PROVIDE A DIRECT PASSAGE OF THE HYDROGEN GAS THROUGH THE FURNACE AND THE COILS WITHOUT ANY INTERNAL RECYCLING THEREOF, WATER VAPOR IS ADMIXED WITH THE HYDROGEN GAS IN A QUANTITY SUCH THAT THE DEW POINT OF THE MIXTURE WILL BE BETWEEN 25* TO 70* C., AS SOON AS THE PROTECTIVE GAS HAS BEEN EXPELLED AND REPLACED BY THE HYDROGEN GAS, THE SAID HYDROGEN GAS BEING INTRODUCED HOT AS A RESULT OF A TEMPRATURE EXCHANGE WITH THE OUTCOMING GAS AFTER THE END OF THE SOAKING PERIOD, AFTER THE DESIRED SOAKING PERIOD THE FURNACE IS SHUT DOWN, THE SUPPLY OF HYDROGEN GAS IS STOPPED, INTERNAL CIRCULATION IS ONCE AGAIN EFFECTED AND THE SAID PROTECTIVE GAS IS FED INTO THE FURNACE TO EFFECT COOLING OF THE COIL.