US2233474A - Method of heat treating metal - Google Patents

Description

March4, 1941. H. A. DREFFEIN METHOD 0F HEAT TREATTNG METAL.

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H. A. DREFFEIN METHOD 0F HEAT TREATING METAL Filed June 3, 1958 5 Sheets-Sheet 2 March 4, 1941. H. A. DREFFEIN METHOD OF HEAT TREATING IIETAL Filed June 3, 1938 5 Sheets-Sheet 3 @5MM/fd@ March 4, 1941.` H. A. DREFFEIN IETHOD 0F HEAT TREATI'NG METAL Filed June 5, 1938 5 Sheets-Sheet 4 March 4, 1941. H. A. DREFFEIN IIETHOD 0F HEAT TREATING IETAL Filed June 3*, 1958 5 Sheets-Sheet 5 www | T ---lill m o o ooo oo wur// l .""Ullll-Ihl n l... 'UL Il. QQ WQ N Q\ NQ @www NW\ H Nuvi, vla'. MS u ,l mmv NQ Nw QQ fl -Ilh www Patented Main-4, 1941 I UNITED STATES PATENT oFFlcE METHOD OF HEAT TREATING METAL Henry A. Dreffein, Chicago, Ill.

Application June 3, 1938, Serial No. 211,481

8 Claims. (Cl. 263-52) This invention relates to improvements in 'Ihese and other objects will be more fully exmethods of heat treating metal and has for an plained and set forth in the following specincaobject annealing, normalizing, or malleablizing tion and illustrated in the accompanying drawof metal in the form of cold rolled strip, sheets, ings in which:

or the like, and castings. Fig. 1 is a vertical longitudinal section through 5 Heretofore, in the normalizing of cold rolled a portion of my improved apparatus, taken along strip, for example, it has been the practice in one the line I-I of Fig. 2; method to encase the coiled strip in a species Fig. la is a similar section of the remainder of single or double walled retort and to apply taken along the line Ia-Ia of Fig. 2a;

heat either to the exterior of said retort or be- Fig. 2 is a horizontal section taken along the 10 tween the walls. This is expensive and requires irregular line 2--2 of Fig. 1; considerable time. In another practice, electrical Fig. 2a is a similar section takenalong the heating means has been utilized for direct heat irregular line 2a-2'a of Fig. 1a; ing. Also, endeavors have been made to utilize Fig. 3 is a vertical transverse section taken l5 indirect heating means by burning gas in approalong the line 3 3 of Fig. 1; 15 priately arranged tubes to render them radiant. Fig. 4 is a similar section taken along the line In this last process the heat to which such tubes 4-4 of Fig.` 1'; must necessarily be subjected to obtain the de- Fig. 5 is a fragmentary section of journal sealsired heating of the strip or sheets is destructive ing means;

0f the tubes, Fig. 6 is a horizontal section of a modified form 20 In my process I first preheat the cold rolled of the apparatus;

strip as it is fed through a preheating chamber, Fig. 7 is a vertical longitudinal section of the after being uncoiled and flattened. It is next structure illustrated in Fig. 6; passed through a main heating chamber to which Fig. 8 is a vertical transverse section taken is supplied partially burned fuel gas mixture abalong the line 8-8 of Fig. 6; 25 normally rich in gas content so as to create a rich Fig. 9 is a horizontal section through a modied reducing atmosphere to preclude oxidation or form of apparatus particularly adapted for use tarnishing of the strip, at the same time eiliin malleablizing castings or the like; and ciently and rapidly bringing it to the desired Fig. 10 is a vertical section through the structemperature and maintaining it at that point for ture of Fig.,9. 30 the peilod of time requisite to effect the desired My apparatus generally is constructed, in the normalizing. After that a partial cooling is efusual manner, of refractory materials, properly fected during passage of the strip through a reinforced and supported, such details forming chamber for that purpose, such partial cooling no specific part of my invention other thanvas being preferably effected by air moving through they enter into the general combination. 35 tubes, the air thus being at the same time pre- In the structure illustrated in Figs. 1 to 4, at heated for subsequent use in combustion of the the charging end, designated by numeral I0, is gases supplied to the main and other heating provided a pair of feeding rolls Il extending chambers and units. Thereafter the strip is fed across the charging opening and arranged to grip through a succession of cellular subdivisions of the end of a strip or a sheet positively to feed it` 40 the main cooling chamber in its passage through into and through the apparatus, there being a which the strip is subjected to decreasing temseries of transverse, spaced, intermediate rolls l2, peratures. A reducing atmosphere is maintained rotated at such speed as to make the peripheral in these cells by the deliveryof condensed and linear velocity identical with the feeding rate, so

purified partial gases of combustion. The strip, that the metal will not be marred or scratched. 45

after reduction of its temperature to the point It will be understood that the cold rolled strip, if where it is not readily oxidized, is discharged coiled, will be unrolled and flattened before defrom the apparatus. livery into the furnace. At the discharge end I3 Another object consists in the utilization of a is provided a similar pair of pinch rolls, as they generally similar process `except as to temperaare termed, designated by numeral I4. 'I'hese 50 ture ranges, to sheets or other metal articles for also assist in effecting the movement of the met-al annealing the same. by gripping it and drawing it through the furnace.

Another object-consists in the utilization of a lAll of the rolls, for convenience, are driven by a generally similar process for the malleablizing of common drive chain I6 and sprockets I1, one of Castmgs. v v the latter being provided for each of the inter- 55 mediate and feeding rollers, as illustrated in Figs. 2 and 2a, for example. This chain I6 may be conveniently actuated by an electric motor and speed reducer F, diagrammatically illustrated, for example, in Fig. 2a.

At the charging end of the furnace is provided what I term a preheating chamber A. Extending from one end of that is themain heatingchamber B, there being a bridge wall or partition I8 between the two chambers. At the opposite end of the main heating chamber is a partial cooling chamber C from which in turn extends Ythe nal or main cooling chamber generally indicated by D. A bridge wall I9 partially separates chambers B and C and a similar wall 20 is located at the juncture of chambers C and D.

At one side of the main heating chamber B is provided a combustion chamber E, a wall 22 being located between B and E. At the top, this wall is provided with a plurality of discharge ports 23. The end walls of the combustion chamber extend outwardly from the body of the structure and in one of them is provided a burner generally indicated by numeral 25 arranged to discharge the fuel mixture longitudinally of the chamber E. A small gap'26 is provided in the wall 22 adjacent the burner 25, this construction being generally similar to the combustion chamber, etc., of my earlier Patent No. 1,938,814 of December l2, 1933, although it should be understood that the gap 26 may be eliminatedv if so desired. Gas is supplied to the burner 25 through the supply pipe 21 suitably controlled, and an air supply pipe 28 is also provided and equipped with suitable regulating means not illustrated, so that the mixture of fuel, such as gas and air, may be regulated to obtain the desired results.

At the forward end of the floor or hearth of chamber B is provided an outlet port 30 from which extends a ue or duct 3| (see Fig. 1). This in turn communicates with top and bottom longitudinal flues 32 and 33 in one side wall of the preheating chamber A, suitable vertical ducts being provided to connect the ues 32 and 33 (see Fig. 1, for example).

Extending across. between the side walls of chamber A, near the top and bottom thereof, and suitably spaced, are what I term radiant tubes 35. One end of each of these tubes terminates in communication with a flue 32 or 33 to receive a supply of fuel gas, While the other end projects beyond the opposite side wall of the chamber A and is upwardly turned at 35a for discharge of gases of combustion, as illustrated in Fig. v3. Associated with each of these is a plug or other suitable type of valve 31, shown as frusto-conical in form. I have illustrated the valves for each pair of corresponding top and bottom tubes 35 inter-connected through levers 38 and connecting rod 39, these mechanisms being provided in the form shown, with an operating handle 40 so that the valves may be simultaneously regulated for control of the pressure and flow in each pair of tubes simultaneously. At the opposite end of each tube is provided a pipe 42 extending through the adjacent side wall of the chamber A and connected through a branch 42a with an air supply pipe 43, and throughanother branch 42h with an auxiliary gas supply pipe 44. Suitable valves are provided in each of these branches for controlling air or gas or both when thereby supplied to the tubes 35.

Extending across the chamber C, near the top thereof, is` a plurality of air pipes 45, each of which communicates at one end with a manifoldl 4s and at the other end with a d envery pipe n which is connected to the air supply pipe 28.

vThe manifold 46 is connected through pipe 50 with a motor driven blower generally indicated by numeral I Also leading from the interior of chamber C is an exhaust pipe 53 communicating with a cooler or condenser 54. This, in turn, is connected through the pipe 55 with a purifier 56,- the primary purpose of which is to desulphurize gases passing therethrough. This latter element may be of any suitable type such as is now used by gas manufacturers and others which employs iron oxide to desulphurize gases. Communicating with the purifier 56 through the pipe 58 is van exhauster 59 operable to vwithdraw gases from the chamber C through the condenser 54 and purifier 56. Such gases are then delivered through the pipe 6| to a manifold 62, from which valve controlled branches 63 lead to the interior of the chamber D. An extension leads from said manifold to the interior of chamber A, there bef ing a suitable valve 64a interposed in the line.

The main cooling chamber D is divided into -a plurality of cells by means of swinging flaps or gates 66 depending from the top of the chamber above each of the intermediate rolls l2. Coacting with these suspending flaps 66 are pivoted upwardly inclined aprons 61 arranged to lie loosely against the surface of each of the rolls |2'and in conjunction with the corresponding vertical flap to form a cellular subdivision of the chamber, flap 66 being pivoted to lie loosely against or close to the upper surface of the rolls l2 so as to permit the passage of a sheet or strip without obstruction.

One of the branch pipes 63 communicates with each of the cells, as illustrated in Fig. 1a..

Extending across the cells thus formed, near the top and bottom thereof, are transverse water tubes through which water is circulated to assist in cooling the strip or sheets passing therethrough. It is desirable to obvlate as far as possible the escape of gases from the apparatus, and, conversely. to prevent the entrance of air from the outside, and with that end in view I construct my rollers with tapered ends |2a from which project shaft portions I2b, the refractory lining and walls of the apparatus being apertured to conform toand closely surround the ends of the rollers while permitting protrusion of the shaft portions which are mounted in suitable bearings, generally indicated by numerals 10. It is desirable to provide side plates 12 on the outside of the furnace walls and it is necessary to aperture these to accommodate the extended shaft portions I2b of the rollers. These apertures 12a in my construction are made larger than the shaft portions I 2b to permit relative expansion or ymovement withoutv binding upon or cutting the shaft portions. To prevent the escape of gases or the entrance of air through these apertures 12a, I provide glands, generally indicated by numerals 13 and of conventional construction, around lwhich are mounted short tubular members` 15 of flexible tubing, such members at their other end being seated upon bosses 12b projecting from side plates 12. 'Ihese flexible tubes permit relative contraction and expansion of the rollers and side plate as welll n heating and main heating chambers, also in the partial cooling chamber, but in the main cooling chamber I use ordinary steel rollers centrally apertured for the passage of cooling water from a suitable source, not shown.

In normalizing cold roller strip, for example in the apparatus described above, fuel gas in admixture with air under pressure is delivered to the burner and ignited'and the relative proportions of air and gas are so adjusted that only partial combustion of the fuel gas results. I desire to maintain an abnormally rich mixture of partially burned gases in the main heating chamber to obvlate the possibility of oxidation of the strip or other metal passing therethrough. I contemplate maintaining a gas mixture of in excess of fifteen per cent of unconsumed fuel gas and can increase the richness to a point where there is approximately fifty per cent of unconsumed gas in the mixture.

In order to utilize this unburned fuel gas, I deliver a substantial portion thereof through the v port 3D and the4 associated ducts and flues.

delivered through the pipesv 42, the amount of` such delivery being :regulated by valves to complete the combustion of f the unburned fuel mixture. Proper regulation of thevalves 31 is made so as to control the heating effect as well. as the flow and pressure .of the gases through and in the tubes 35. y

It is my desire to maintain a reducing atmosphere throughout the entire apparatus which entails the maintenance of a pressure upon the partially burned fuel gases at least equal to and preferably greater than the external atmospheric pressure. 'Ihe operation of the burner is controlled to obtain such pressure which also requires a consideration of and overbalancing of the suction eiect of the induction air from the pipes 42. In the event that additional fuel gas is required for the heating of and through the tubes 35, an auxiliary supply'of gas may be obtained through the pipes 42h leading from the auxiliary gas supply M (see Fig. 3). It will be apparent that a portion of the partially burned fuel gases will iiow beneath the bridge wall I8 into the chamber A if the pressure be properly regulated, and this Will maintain the necessary pressure in such chamber, at the same time providing the desired reducing condition to prevent oxidation of the strip or sheets.

Purified and dehydrated gas may be admitted from pipe 64 in a desired amount into chamber A. I contemplate, when it seems desirable, to utilize this supply of cooled and purified gas to the preheatingchamber A to oppose the entry thereinto of the partially burned fuelgases drectly from the main heating chamber beneath the bridge wall while at the same time maintaining a desirable reducing atmosphere of rich gas.

After the strip moves out of the chamber B into and through the chamber C, it is partially cooled by the cooling pipes 45 and at the same time serves to preheat the air passing through said pipes to the burner 25. I nd it efficient and economical to use preheated air in some instances for combustion because it will increase the ltemperature `of the `resultant combustion and I may obtain the desired heating eii'ect With less combustion than will` otherwise be required, thereby enabling me to provide a richer mixture of partially burned gases to heat the strip, there- -by further reducing the probability of oxidation I or tarnishing of such metal.

As the strip moves -into the receiving end of chamber D, cooling thereof is continued, but by dividing'the chamber into a number of cells, 5 as described, convection currents through the entire chamber are practically eliminated and each cell may bemaintained at a marked lower temperature than its predecessor, so that cooling action is more rapid and more eiective than where a single cell or chamber is utilized. Also, Ibyi separate control of the cooling water and cooled purified gases delivered to each cell of the main cooling chamber, I am enabled to` control the cooling rate of the metal passing there'- through, as will be apparent.

Also, the supply of the cooled purified gases is preferably controlled to set up pressure in the A cells of the cooling chamber which will resist or prevent iiow of the hot fuel gases beneath the bridge Wall from the main heating chamber while at the same time maintaining the desired reducy ing atmosphere of rich partially burned gas.

In normalizing strip with my apparatus I regulate the speed with which such strip is fed through the apparatus so that it attains a temperature of between 1300 F. and 1400 F. when it reaches the bridge wall I8. After movement into the main heating chamber B this temperature is quickly raised to around 1700 F. (preferably 1650 F. 30 to 1800 F.) land remains at about that point dur` ing the remainder of the travel .through the chamber B, which therefore functions not only to bring the metal to the desired maximum temperature, but as a soaking chamber in which 35 such temperature is maintained until the proper action is had upon the metal. During the passage of the strip through the chamber C, cooling action is relatively slow and its temperature is lbroughtA to around 1400 F. at the time it 4 reaches the bridge wall 20. In passing through l the lcellular chamber D the drop in temperature is more rapid and the strip is preferably brought l I to at least 200 F. at the point of discharge from said chamber. This I term for convenience a non-oxidizing temperature because such strip is not susceptible to quick oxidation or tarnishing under normal conditions after that temperature is reached.

A generally similar process is practicedin the 50 normalizing of metal sheets other than cold rolled strip, it being apparent that. if hot rolledv sheets or the like yare fed directly into the preheating chamber at high temperature, the lradiant tubes will be called upon fora lesser heat contribution 55 than When the cold rolled strip is being heated, and, if desired, the preheating chamber may be utilized quickly to bring the hot sheets or metal closer to the desired maximum temperature before their movement into the main heating cham- 60 ber and, likewise, the heat contribution fromH the combustion chamber associated with the main heating chamber will be correspondingly less, as will be apparent. v

I may also utilize this apparatus for annealing strip or metal sheets, in which operation the lmetal is brought to a temperature of around In the process described I am able efliciently and economically to obtain the desired heat treatment of the strip or other metal by direct application of fuel gas mixture in a continuous op- 5 eration and in such a manner as substantially to obviate vthe formation of scale upon or the oxidation of the metal, and at the same time contribute to the economy of operation by salvaging the unburned fuel for preheating use and for the maintenance of a purified reduced atmosphere in the cooling chamber.

- In Figs. 6 to 8 I have illustrated a slightly modied form of apparatus in which the intermediate supporting rollers have been eliminated from the main heating and the partial cooling chambers. In lieu thereof I construct the bottom or hearth |00 of these chambers' as a continuous tank in which is maintained a pool of molten lead |0| (during the operation of the furnace) and the metal being treated is supported upon and moved over the surface of this molten pool. In this manner I am able to dispense with the use of expensive alloy rollers and to eliminate the objections of any rollers in these zones of greater heat which aresusceptibleto lthe deposit of scale or growth which causesexcrescences upon the surface of such rollers which has a tendency to scratch or mar the sheets supported and moving thereon. Various methods may be employed to engage and initiate the said strip or sheets being treated. For example, an alloy `sheet may be attached to each end of the strip and initially fed through the furnace, drawing after it .the strip. At the conclusion of the passage of the strip, the drive is` continued until the alloy sheet at the rear end is moved completely into the furnace in engagement with each pair of pinch rolls ready for initiating the move-n ment of another strip or sheet when so desired.

l Other methods may be employed, as, for example, the use of a chain or tongs inserted through ports or the like at intervals along the side Walls of the furnace.

In the form illustrated, in Figs. 6 to 8, ports |05 are provided in communication with the main duct |06 which in turn supplies the partially burned fuel gases through suitable iiues or ducts through the charging ends of the radiant tubes, as in the form of Figs. 1 to 4.

A slightly modied form of roller drive is also provided comprising a motor and speed reducer unit diagrammatically illustrated in Fig. 6 and generally designated by numeral H0. A drive shaft is driven by this last described mechanism and through suitable drive mechanism,

including chains, sprockets and bevel gears, actuates the drive chains ||2 at each end of the apparatus, the action and arrangement being otherwise .generally similar to that illustrated n0 in Figs. l to 4, the modification being due to the elimination of the intermediate rollers and. the main heating and partial cooling chambers.

In Figs. 9 and 10 I have illustrated a modied form of apparatus more particularly designed 05 for utilizing my process in the malleablizing of castings.

In this apparatus a charging vestibule |50 is provided at one end and a similar discharging vestibule |5| is located at the discharge end. 0 These are provided with suitable counter-balanced doors |52. A plurality of pairs of tracks |53 extend through and beyond the apparatus, being mounted upon suitable spaced partitions. The apparatus includes a preheating chamber 5H, a main heating chamber I, and a cooling chamber J, bridge walls I 54 defining these chambers, as illustrated in Fig. 10. A combustion chamber is located at each side of the main heating chamber, the construction being other- Wise generally similarv to that illustrated in Figs. 5 1 to 4. It should also be noted that I find it desirable to reverse the structural arrangement of the combustion chambers and the associated burners etc., i. e. one burner is facing toward the discharge end of the furnace and the other l0 toward the charging end.

A counter-balanced sliding door |62 is located at each end o f the structure, as illustrated in Fig.10. will Extending transversely across the preheating 15 chamber, near the top and bottom thereof, are radiant heating tubes |65. The supporting abutments or walls for the tracks |53 are cut away or apertured at |66 to accommodate the lower radiant heating tubes, as illustrated in 20 Fig. 9. These tubes are supplied with partially burned fuel from the heating chamber and with compressed air for combustion, as in the stmoture illustrated in Figs. 1 to 4, and are similarly controlled. 25

Extending transversely across the cooling chamber J, near the top thereof', are spaced air pipes |61 which are in communication with a blower or the like and serve the dual purpose of cooling the material passing therebeneath and 30 of preheating the air used for cooling so that it may be utilized for admixture with the fuel gases delivered to the burners. Similar air tubes |68 extend across the cooling chamber J near the front end thereof, the track walls being 35 apertured or cut away at |69 to accommodate such tubes. v

Also extending across the cooling chamber J, near the bottom thereof, are radiant heating tubes |10 similar in arrangement and operation 40 to the tubes |65, the track walls being apertured at to accommodate them.

In the practice of my method as applied to malleablizing with the apparatus just described,

a transverse row of cars charged with castings 45 is moved into the vestibule |50 and thereafter, the door |62 having been raised, these cars are moved into the preheating chamber H by a pusher indicated generally by numeral |12 and contact the trains already therein to move them a 50 step toward the discharge end. Simultaneously, the rear door |62 having been raised, a transverse row of cars and their contents will be discharged into the adjacent vestibule. In this manner a step by step movement through the entire apparatus is imparted to the cars and their contents.

The radiant tubes are controlled so as to bring the castings in their travel through the chamber so H to a temperature of approximately 1300 F. to 1400 F. and thereafter move into the heating chamber I Where hot partially burned gases of combustion function to bring them to a desired lmaximum temperature around 1700 F. in a o5 short interval of time. This temperature is maintained for a soaking action during the remainder of travel of such castings'through the chamber I.

After movement into the cooling chamber J this temperature is dropped with relative quickness to approximately 1400 F. and is reduced more slowly during the balance of travel through this cooling chamber J until it is reduced to at least 800 F. at the point of discharge. This 76 final cooling may be checked if too rapid by use of some or all of the radiant tubes |69. As indicated, it is preferable to eiect this nal cooling at a relatively slow rate, as it is in the nature of an annealing operation.

With the use of this apparatus I contemplate operation with a timev cycle of from fourteen to fifty hours (depending upon the temperature control, character and size of castings etc.), as contrasted with the time cycle of one hundred to two hundred hours now required in the intermittent .or periodic pot type of treatment.

Suflicient pressure is maintained on the partially burned rich gases in the heating chamber I to maintain the pressure throughout the entire apparatus at least as high as atmospheric, and preferably slightly above, to preclude entrance of outside air and there is sufficient flow in both directions from the heating chamber to maintain a reducing atmosphere in all of the chambers, the same rich mixture described in my normalizing method being desirable'in the malleablizing process.

It will be apparent that the specific method described hereinabove is susceptible of variation and modification, and I do not Wish to be restricted to such disclosure except as the claims, properly interpreted in View of the prior art, must lbe so limited.

What I claim as new and desire to cover by Letters Patent is: 1. A process of treating material comprising subjecting such material to a thermal sequence including heating of such material in the heating zone directly by rich partially burned fuel. gases, a portion of said partially burned gases being thereafter utilized for combustion in radiant tubes or the like for indirect heating of said material in another zone, and a portion of said partially burned fuel gases from said heating zone being withdrawn and vutilized to maintain a special atmosphere about said material in still another zone.

2. A process of treating material comprising subjecting such material to a thermal sequence includingheating 'of such material inthe heating zone directly by rich partially burned fuel gases, a portion of said partially burned gases being thereafter utilized for combustion in radiant tubes or the like for indirect heating of said material in another zone, and a portion of said partially burned fuel gases from said heating zone 'being Withdrawnfsubjected to additional treatment, and utilized to maintain a special atmosphere about said material in still another zone.

3. A process of treating material comprising subjecting such material to a thermal sequence including heating of said material in a heating zone directly by rich partially burned fuel gases, a portion of said gases being thereafter utilized for combustion in radiant tubes or the like for indirect heating of said material in another zone, and a portion of said partially Vburned fuel gases. from said heating zone being cooled and utilized for cooling said material in a cooling zone while maintaining a special atmosphere therein.

4. A process of treating material comprising subjecting such material to a thermal sequence including heating of said .material in a heating zone directly by rich partially burned fuel gases,

a portion of said gases being thereafter utilized for combustion in rafdiant tubes or' the like for indirect heating of said material in another zone, and a portion of saidpartiall,1 burned fuel gases being withdrawn from said heating zone, cooled, and dehydrated and, subsequently utilized for cooling said material in a cooling zone whilev maintaining aspecial atmosphere therein.

5. A process of treating material comprising subjecting such material to a thermal sequence including heating ofsaid material in a heating zone directly by rich partially burned fuel gases, a portion of said gases being thereafter utilized for combustion in radiant tubes or the like for indirect heating of said material in another zone, and a portion of said partially burned fuel gases from said heating zone being cooled and applied in subdivided amounts of differing temperatures to said material to progressively cool the same.

6. A process of treating material comprising subjecting such material to a thermal sequence including heating of said material in a heating zone directly by rich partially burned fuel gases, a portion of said gases being thereafter utilized for combustion in radiant tubes or the like for indirect heating of said material in another zone, and a portion. of said partially burned fuel gases from said heating zone being cooled and delivered to a plurality of cellular chambers through which said material is passed to cool the same, the temperatures insaid cellular chambers being successively and markedly lower in the direction of travel of said material.

7. A process of treating material comprising subjecting such material to a thermal sequence including heating of said material in a heating zone directly by rich partially burned fuel gases, a portion of said gases being thereafter utilized for combustion in radiant tubes or the like for indirect heating of said material in another zone, and air for admixture with and combustion of said fuel gases in said radiant tubes being preheated by passage through tubes or the like in proximity to said material to simultaneously partially cool said material in its passage from the heating zone to a cooling zone.

8. A process of treating metal comprising sequential treatment of such metal in a preheating zone, a heating zone, and a cooling zone, heating

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