US2223603A - Equipment for heat treating - Google Patents

Description

Dec. 3, 1940. w. A. DARRAH EQUIPMENT FOR HEAT TREATING 4 Sheets-Sheet 1 Original Filed July 15, 1938 v M mm r hm Inventor Dec. 3, 1940. w DARRAH 2,223,63

EQUIPMENT FOR HEAT TREATING Ori iled July 15, 1958 4 Sheets-Shep 2 In van for Dec. 3, 1940. w. A. DARRAH 1 2,223,603

EQUIPMENT FORj-IEAT TREATING Original Filed July 15, 1938 4 Sheets-Sheet 3 [22 van t or 7 Dec. 3,1940. w. A. DARRAH EQUIPMENT FOR HEAT TREATING Original Filed July 15, 1938 4 Sheets-Sheet 4 In veni'or Patented Dec. 3, 1940 UNITED STATES PATENT OFFICE I I EQUIPMENT TREATING -riginal application July 15, 1938, Serial No.

219,332. Divided and this application November 10, 1938, Serial No. 239,860

6 Claims. (01. 266-4) This application is a division of application Serial No. 219,332 filed July 15, 1938, and issued Dec. 26, 1939, as Patent No. 2,184,972.-

This invention relates to apparatus used for treating metals and particularly the ferrous metals including steels of various types. One important application of the apparatus which forms the subject of invention is in connection with the so called carburizing of steel or various o materials which consists in forming a case of highly carburized steel on the outer surfaceof an article.

This apparatus may be utilized to form other compounds or gases on a metallic base as for ex- 16 ample a nitride case and a mixture of carbide and nitride c'ases. Other compounds such as volatile metals as for example zinc may be added to the surface of the metal being treated utilizing some phases of my invention.

20 In order to simplify the disclosure I will con-' fine the description of the apparatus largely to 30 Another object of my invention is to provide equipment which will utilize natural gas, city gas or the so-called liquid fuel gases such as butane and propane for carburizing operations 35 without the production of excessive soot or coke on the metal being treated.

Another object of my invention is to provide means whereby various relatively rich carbon I bearing gases may be utilized for carburizing.

Another object of my invention is to provide .simple, automatic and economical 'means for completely carburizing and heat treating steel articles the operations including carburizing, reconditioning grain size, quenching, drawing, washing, etc., so that the steel article when it leaves is completely treated.

Other objects of my invention will be apparent from the drawings, description and claims appended hereto. 50 Referring to the drawings Figure 1 shows a typical plan view of one form of my invention.

Figure 2 shows a side elevation of the carburizing zone of this furnace partly broken away to indicate the interior construction. Figure 2 55 is a partial section taker: along the line -II-II as indicated in Figure 1 and as further indicated in Figure 3.

Figure 3 shows a typical cross section through the furnace portion of my invention taken along line III-III of Figure 2.

Figure 4 shows a side elevation partially broken away indicating quenching devices, discharge door and other details of my invention.

Figure 5 shows a side elevation partially broken away indicating a general arrangement of a draw oven. Figure 6 indicates a typical tray or fixture loaded with material to be treated.

Referring to the drawings I indicates a carburizing furnace provided with loading vestibule 2 and loading table '3.

4 indicates a cooling zone attached to the discharge end of the carburizing furnace and 5 indicates a reheating furnace arranged to reheat the material preparatory to quenching after the material has been cooled in zone 4 or otherwise.

6 indicates a quenching housing or vestibule connected to spray washing station I and spray rinse zone 8 adjacent thereto. A recirculating draw oven B'is shown located at the end of the rinsing zone 8. f

Similar numbers refer to similar parts on the various drawings and figures.

The loading platform 3 consists preferably of a structural steel frame work although obvious-. ly any suitable material may be employed.

A series of fixtures or trays [0 are loaded while resting on platform 3 and then pushed either manually or by the means of operating cylinder I l into vestibule 2 in the position shown in dotted lines and indicated bynumeral l2..

Atthe proper time of the cycle, either controlled by the operator or automatically by an electric timer, operating pushing cylinder l3 pushes tray [2 into carburizing chamber I through operating door 14. The electric timer may be any of the conventional types now available on the market. Of course loading vestibule 2 is closed by manually operating door I5 to prevent leakage of air in or gases out during 'the cycle. This vestibule is also supplied with purging gases, such as a mixture of products of complete combustion through pipe Hi, from gas generator [6A, which also supplies gas to the cooling chamber and quench vestibule.

The carburizing furnace itself may be connstructed in a variety of ways but I prefer the general type of construction shown herewith in which heat is supplied by a series vof radiant members such as tubes ll which may be located either above or below the work to be he ted and which are provided with a burner l8, burner block l9 and exhaust flue 20 which leads to a duct 2! located beneath the furnace or at some other convenient point. Radiant member I! naturally expands and contracts with changes of temperature and, therefore, is provided with a flexible seal or joint 22 which is filled with sand or other granular material. This construction provides for flexibility without permitting gases to leak into pr out of the furnace chamber. The object of the construction is to provide a leak proof arrangement such that products of combustion or air within the combustion chamber cannot enter into the furnace chamber and conversely the atmosphere within the furnace chamber cannot enter into the combustion chamber. metal there would be very substantial expansion which would cause cracking or breaking. The construction indicated provides a certain amount of flexibility by permitting the radiant tube member H to move in the cup member 22 by displacing the granular material. Referring to Figure 3 it will be noted that the radiant tube member H is sealed to the furnace shell I in'an-air tight manner to prevent leakage. Radiant member I! it will be noted extends transversely across the heating chamber and curves downwardly dipping into the sand filled seal of member 22.

Member 22 extends downwardly as indicated and is tightly attached to the bottom plate of the vfurnace.

Since sealed member 22 is attached to the bottom plate of the furnace and radiant member H is attached to the vertical side walls of the furmace and both are sealed to their respective plates it will be evident that we have a continuous metal contact except for the expandable joint within the sand seal.

The housing or carburizing furnace which may to advantage be constructed of steel plate properly reinforced as indicated is made so that all joints are tight. The housing should then be insulated with heat resisting insulation as indicated by the layers 24 and the filling material 25 above the arch of the furnace.

The interior of the furnace should be constructed of refractory capable of withstanding high temperatures continuously in the presence of a strongly reducing atmosphere as this is ordinarily used to obtain the carburizing results desired. I have found that a porous refractory is of decided advantage for this service providing the outer face is well plastered with a refractory cement. All refractory material within the heating zone of the furnace should preferably be extremely low in iron oxide and similar com-' pounds which may be readily reduced by temperatures ranging from 1600 to 1800? -F., in the presence of hydrogen. carbon monoxide or other strongly reducing gases such as methane, and ethane. I have designated this layer of refractory material by the numeral 26.

It is to be noted that the refractory material Since if all joints were bolted metal to to crumble away. This results partly from a change in volume of ironas against iron oxide and partly because of the different nature of the material.

Of course if iron oxide is reduced by such gases as methane and ethane, as mentioned above, free tween the radiating members. These piers should be made of strong materials and are used to support rails 28, 29 and 30, etc., on which the trays or other fixtures carrying the materials to be treated slide. The whole furnace is supported on a foundation indicated by 3|.

At the discharge end of furnace chamber I, a pusher 32 is located, arranged when actuated, to push the tray out of carburizing furnace I into cooling zone 4. In this zone the loaded trays are allowed to cool to temperatures ranging from 700 to 1 200 or even over wider limits depending are pushed out of chamber I they will slide on I doorway 36 into reheating furnace 5, where the temperature is raised preferably to around 1400" or higher to pass the critical temperature of the steel. When the tray reaches the end of reheat furnace 5 and assumes the position shown by the dotted tray 31; cylinder 38 pushes the tray into quenching chamber '6 as described below. Quenching chamber 6 is provided with 'an elevator member 39 raised and lowered by cylinder 40. In the bottom portion of chamber 6 quench liquid such as oil or, if desired water is maintained. Cylinder 40 lowers elevator 39 into the quench liquid 4| thus quickly cooling the metal and producing the' desired metallurgical characteristics.

The cylinders such as '32, 34, 38, and 42 maybe operated sequentially by means of conventional interlocks (not shown) as is well known. After a proper hesitation beneath the liquid to permit cooling of the steel to the desired temperature cylinder 40 lifts the elevator and tray to the operating level at which time pusher 42 is actuated and the tray is pushed into the washing zone This washing zoneconsists of a series of sprays 16A supplied with washing liquid which is usually water with cleaning compounds dissolved therein. A pump .43 rotated by a motor 44' delivers the liquid to the spray nozzles at I high pressures and large volumes as required. This type of equipment iswell known.

The 1- trays are pushed successively, the length of one tray passing gradually through the'yva' hing zone and the rinsing-zone, allowingam ,rle time to remove oll; gre'ase,-,carbon, dirt coke and other foreignmaterials. a

The rinsing zone is provided with a motor 45 and pump 46 which serves to'supply clean water for removing all traces of washing compound. If this is not done the compound is likely to dry on the steel parts causing an unsightly appearance. I prefer to use washing liquids strongly heated preferably close to 200 F.. This, however, is an operating detail which may be varied to suit working conditions.

After leaving therinsing zone the trays are pushed successively and intermittently through a draw oven as indicated in Figure 5.

In this oven the material is reheated gradually and uniformly to temperatures ranging from 300 to 700 or 800 depending again on the nature of the steel and the results to be obtained. It should be understood that wherever temperatures are specified they are given with a view of obtaining great toughness and hardness of the outer layer of carburized steel and toughness and strength in the inner layer of steel. Other results may be obtained by this invention and are not to be excluded. Other results may require different operating temperatures-and times.

The draw oven 9 shown in Figure 5, is supplied with circulating hot gases from duct 50 indicated in Figure 1. Duct 50 is arranged to receive the products of combustion from both the carburizing furnace I and the reheat furnace 5. These products of combustion pass from the radiant combustion chambers I! through the flexible joint 22 into down-comer 20 and finally connect to longitudinal flues 2| which run the entire length of the furnaces. Duct 5| connects with the longitudinal duct beneath the carburizing furnace and duct 52 connects with the duct beneath the reheat furnace. Obviously these ducts may be put above the floor or beside the furnace if it is found more convenient to do so. Duct 5| is provided with a draft control damper-53 and duct 52 has a similar draft control damper 54. The two ducts connect to riser 55 which in turn leads the combined products of circulating combustion into duct 50 and then into the intake of fan 56 which delivers the hot gases into draw oven 9 at a point 51 located beneath the rails 58 on which the trays 59 are carried. Since the trays are of opened or latticed construction the hot gases passing into the oven in space 51 travel upward, around and through the interstices of the trays and leave by means of spaces 6|] between plate Bl which form the upper portion of the draw oven. The circulating gases thus enter plenum chamber 62 from which they pass to duct 63 which leads them back again to the recirculating .fan. A control damper 54 in duct 50 may be operated by a thermostatically controlled motor in any conventional manner and thus draws more or less products of. combustion into the circulating fan to maintainthe temperature of the circulating gases constant. z

A thermostatic device 65 such as a sensitive couple placed in the circulating air stream in passage 51 serves to open or close damper 64 as required.

It will be evident that the hot circulating gases which utilize waste heat from the furnaces serve to give a closely controlled draw to the steel parts which have been carburized, cooled, reheated and quenched.-

This oven also serves as a drying oven to remove the water from the washing, and rinsing process and may be so used exclusively, if it is not desired to draw and temper in-the steel pot. In most cases the drawing operation serves to toughten and soften the inner core of steel thus increasing the strength and durability of the product. Since the carbon content of the carburized shell on the outer surface of the steelis higher than the carbon content of the core, temperatures which willdraw the core satisfactorily will not soften the shell appreciably.

After the trays have been pushed successively through the draw oven occupying in turn each of the positions indicated they finally come to the positions shown as 66 in Figure 5, after which they are pulled or pushed into vestibule 61. Vestibule 61 serves as a seal or hood to prevent excessive leakage in or out of draw oven 9. Sueceeding position 68 provides a place for the trays to cool before unloading and reloading.

It will be apparent that the trays have thus made the complete cycle of the apparatus having been submitted consecutively to each of the operations. Since the timing may be controlled by an automatic timer or electric clock all times in carburizing, cooling, reheating and drawing may be accurately controlled. Furthermore, since temperatures andatmospheres are closely controlled the entire process gives a uniform product having predetermined characteristics.

Referring to Figure 6, tray 10 may be taken as a typical unit used in my process but I do not wish to be restricted to this particular design as any commercial trays properly suited for this process may be employed.

The tray 10 consists of a frame 1| which is sufliciently rugged to permit satisfactory pushing on thetray to suit the loading conditions ref quired. A series of ,fixtures 14 may be provided to slip into the hollow portion of hubs similar to 13 and these fixtures 14 support gears or other parts indicated by I5. It' is understood that the trayswill be loaded as desired and a typical installation may represent a load of 800 or 1000 pounds of steel to each tray which may weigh, by way of example, 200 to 300 pounds. Naturally these ratios and arrangements will vary widely with the nature, size and type of articles'being treated and the kind of treatment given.

It will be apparent that in the apparatus here described products of combustion are excluded from the chamber in which the steel articles are 7 heated so that it is possible to absolutely 'control the nature of the atmosphere in contact with I am aware that carburizing has been performedby heating steel articles in a closed chamber in contact with carburizi'ng gases- This process, however, has not met all requirements for numerous reasons among which are slowness of forming a desired case, difliculty in controlling the type and depth of case and the presence of large amounts of soot or coke on the surface of the steel.

Various expedients have been proposed to over- I 60 v the steel being treated. This is an essential feature of this invention in that proper control of the chemical and physical condition of theat-- mosphere is essential for obtaining. the results In my invention I introduce carburizing gases or four feet apart for purposes of control. These.

inlets may be connected to a suitable source of carburizing gas. I prefer to use either natural gas, propane or city gasdirect without dilution oradditionwhen it is desired to produce a carbon case on the surface of thework. In thepast such a procedure wouldapoat-the surface of, the steel. heavily with soot or coke. This coating has several cbviousobjections;, ln, the first place, it is diffi ult toremove and; requires costly andexpensive handling or polishing to entirely remove it as it is strongly, adherent. If it is not fully removed it naturally causes serious wear in the devices utilizing, the gears or other equipment treated. I I 2 A further objection tothe sootis the fact that it apparently delays and retards the penetration of carbon into the steel.

In my apparatus I have found that while I provide a number of inlets for the natural gas 30 both above and below the work, his preferable to introduce the gas principally at the discharge end of the carburizing furnace at a point where the articles are about to leave the treatment. The carburizing gas will flow backward toward the 35 loading-end and promptly fill the entire chamber with atmosphere. 7 1

If. gas is introduced the full length of the chamber sootingis likely to occur in'the early stages with consequent slowing up of the car- 40 burizing reaction or a reduction of the percentage of carbon in the case.

Carbon of course first collects in the outer layer of steel and gradually penetrates to the interiorof the steel. The concentration of carbon 45 15 naturally higher in the outer layers initially and lower in the inner layers. Most articles require for maximum hardness and durability a layerof hyper eutectoid in the outer layer, just below this a strata of eutectoid and below this 50 a final layer of hypo-eutectoid. If soot deposits occur they may'retard the addition of carbon tothe outer layer but of course they do not retard the diffusion of carbon from the outer layer into the interior of the steel. Soot' deposits, therefore, 55 may be objectionable by causing a lower percentage of carbon in the case distributed over the entire band.

By my apparatus the carburizing gas, as for example natural gas, may be partly above .the no material being treated and partly below. The gas as added eventually comes in contact with the highly heated radiant chambers made of alloy or other heat resisting material. These chambers may range from 1800 to 2000 or over and 65 serve to crack the methane and other hydrocarbons in the gas. The degree of cracking depends the lower portion of the furnace violently circulating atmosphere streams are produced, and the v atmosphere within the carburizing chamber is kept well mixed and in rapid motion. 75 The breaking down of methane an other hyburizing gas.

drocarbons naturally deposits some free carbon around the combustion chamber and the adjacent refractory walls and produces large amounts of hydrogen. The hydrogen serves as a dilutant for the incoming new quantities of methane in 5 the natural gas and therefore protects it against rapid decomposition by the well known laws of mass action.

As a result of this condition it is only necessary to provide a proper ratio of hydrogen and inert l0 gases to the amount of methane and hydrocarbon gases so that the deposit of carbon on the steel is reduced to a rate approximately equal to that at which it will be absorbed by the steel.

Further the hot cracked gases appear to be in 15 a nascent or active condition and a greatly ac- I celerated rate of carburization of the steel results.

It should be understood that while I have described my apparatus as applied to steel it may also be used with tungsten, chromium and other 20 metals. 1

It should be understood that the statements made in this specification relating to reasons for obtaining the specially advantageous results which are secured, represent my theory as to 0p crating principles and are not necessarily actual facts.

It is apparent that there is a rapid and violent circulation throughout the zone of the furnace occupied by the steel being carburized. It isalso evident that the temperature of the gases doing the carburizing is higher than the temperature of the steel since the gases pass over the intensely hot radiating chambers at the bottom of the fur-' nace and travel upward around the work being I carburized. These two points I believe are of great importance in reducing the time required for carburizing and increasing the depth of case and the amount of carbon absorbed.

These factors also make it possible to carburize with less additional atmosphere than is required in the-case of present day muiile type furnaces in which the carburizing chamber of necessity maintained at a relatively uniform temperature at the top and bottom of the chamber. The present carburizing chambers or muflles being made of alloy and, therefore, very costly are kept as small as possible. It is necessary to keep the present carburizing furnaces small in order to reduce the expansion and warping which is, an extremely serious item in the case of a large muflie. In the case of the furnace which I. have invented there is no large muflie but merely a brick wall which is'ordinarily'formed of insulating refractoryplastered with a refractory cement I to seal the pores against the entrance of the car- As a result of my construction many of the limitations of the present furnaces are removed and I can build a furnace of my design in any desired size in any dimension, using commercial materials now available. This is not practical with present day carburizing mullies, owing to the limitations of making and main taining large alloy boxes or containers which of course must be kept air tight.

In the case of equipment of my design, making a longer furnace merely means building the steel container of greater length and lining it with firebrick as shown. More of the small individual radiating units are then added as required. Obviously the same principle applies to increasing the width of my furnace.

By introducing the new carburizing atmosphere which may be methane, propane; butaneor various other hydrocarbon gases or amixture of these,

or hydrocarbon containing .00, nitrogen, etc., at the upper portion of my heating zone, I obtain one set of conditions and I obtain a different set of conditions by introducing the carburizing gases steel, which is not possible in present day carburlzing units.

I prefer to exhaust a small amount of the gases at each end of the carburizing furnace partly to provide space for introducing new gas and partly,

to provide a visual indication of the nature of the gas within the furnace. By lighting the stream of gas which leaves .the furnace at thev ends, I am able to produce a flame which has distinctive characteristics depending on the composition of the gas. Thus an extremely rich gas will burn with a deep red or reddish yellow flame, producing considerable smoke, while a leaner gas will produce-a lighter colored flame and a gas of hydrogen and CO will produce a flame which is still more transparent. It has been the experience of operators using my equipment that as long as the nature of the flame. is held constant the gas condition within the furnace is substantiallyconlation of the gases due to the thermal difference I caused by an intensely hot radiating member in the bottom of the furnace and partly to the fact that the gases passing over the steel or other material being treated are hotter than the steel itself. I

It will be understood that my invention includes a process of carburizing and also the equipment for accomplishing this result. My invention includes also a cycle and process as well as the associated equipment, in which the waste heat from the high temperature operation is utilized in the low temperature operation.

My invention also includes various other features as set forth in the specifications and the attached claims. 7

It will be understood I do not wish to be restricted to any special set of temperature, time or gas conditions as these are ordinarily varied to suit the requirements of the material being treated: I have found, however, that my proc- 'ess and equipment permits substantially shortening .the time, lowering the temperature and increasing the depth and carbon content'of the case. The relative degree to which these different advantages exist aresubject to control depending on conditions to be obtained.

, characteristics ordinarily associated with the treatment of steel in a molten bath of sodium cyanide.

As indicated above, I may use as atmosphere straight natural gas without adding any from of -of ,air into the furnace chamber.

diluting material. Under these conditions the cracking of the gas liberates hydrogen, a portion.

' of the carbon being absorbed by the steel and a portion of the carbon being precipitated as coke orgraphite, some on the walls of the furnace and some on the heat radiating member or other parts. The presence of this accumulation of carbon appears to ultimately-retard the cracking of the methane in the natural gas thus preserving more of the gas for use in forming the carburizing case on the steel. This is my theory as to one of the reasons why I am able to carburize a given quantity of steel to a give depth with much less carburizing gas than is ordinarily used in present. day conventional furnaces. I

It will be apparent,.therefore, that the atmosphere within my furnace will ordinarily contain a considerable amount of free hydrogen which has proved to be of no objection in the case of any commercial application so far tested.

In the opening of the doors for loading and discharging of my furnace there is naturally a slight leakage of gas outward from the doors and This has proved to cause no difflculty but accounts for the presence of a small amount of nitrogen and oxides of carbon within the furnace chamber. 0iv course vestibules are provided as indicated to reduce this condition to a minimum but my experience has been that quite substantial amounts causing difliculties. I

Water vapor may be present in a small degree but itis desirable to exclude water vapor as far as possible when a highcarbon content is deing carburized or otherwise treated is located in I another portion of the chamber, usually the, upper portion. As a result of this condition, the

atmosphere within the carburizing chamber.

comes in contact with intensely hot surfaces,

so of air may enter the furnace chamber without acquires a portion of this intense heat andthen, travel upward by thermal circulation, coming in contact with the cooler steel or other articles being carburized. As a result of this condition, the hydrocarbon gases are hotter than the steel which is being carburized, even when the steel is up to its maximum temperature. This condition causes the soot, which is precipitated on the steel in other competitive processes now in use. to deposit in those portions of the furnace which are hot rather than on the steel. This condition leaves the steel relatively'clean and free from deposits. of coke and soot, which are characteristic of other methods of carburizing now in use. The deposit of coke or soot on the surface of the steel is a serious disadvantage from many standpoints. It retards the rate of carburization because it forms a coating or shield and keeps the carburizing gases from having intimate contact with the steel. Naturally a heavy coating of soot or coke requires a cleaning and in the case of present commercial process this cleaning is quite difficult and costly, v

It will be noted that in present commercial apparatus an alloy muille forms the containing chamber in which the steel being treated is enclosed, together with the carburizing gas. It is necessary to heat the muilie throughout its cross section to a fairly uniform temperature to avoid excessive warping, cracking and deterioraand the gas containing the carburizing material.. It appears that. the .desired results. can

(only be obtained in the best measure by utilizing the thermal circulation which this process and type of construction provides. Forced circulation by fans does not give the results obtainable by this process, possibly because the difference in temperature betweenthe circulating gases and the material being. carburized isnot as great as-in the. case of the thermal circulation which I utilize. q I

. It is notable that articles treated by my process come out clean and free from coke and contain no visible soot. While I am familiar with many of the present day carburizing processes I have found none which give thiscondition. It should be noted that this condition is obtained without the addition of foreign gases of any kind and no oxidizing agents or oxidizing gases are essential to my process although they may be present to a limiteddegree without interfering with the operation of my process.

A substantial portion of my invention, therefore, is the means of obtaining clean and umformly carburized articles in a minimum time and with a lower temperature than is customary today.

Owing to the rapid thermal circulation of the hot carburizing gases and the fact that they are at a higher temperature than the metal being carburized, carbon is added with great uniformity which is not the case with muflie furnaces now in use.

Bythe term "material support used in the claims I intend to 'bring'out the structure which supports the material being treated within the furnace. Itis obvious that the supports must extend .to the floor or foundation by some intermediate means and it is intended that the heating means be below: the material being carburized and not necessarily below the entire portion of any support.

I claim:

1. A metal treating apparatus of the type described consisting of a carburizing chamber, a cooling chamber attached thereto, a reheat chamber attached to said cooling chamber, a quench chamber attached to said reheat chamber, a washer attached to said quenching chamber and a draw oven attached to said washer, and a mechanism for moving material to be treated through said chambers, the whole forming an integral unit for successively and continuously treating materials.

2. A metal treating apparatus of the type described consisting of a refractory carburizing chamber, a metallic vestibule sealed thereto in a. tight manner, closure means for said vestibule to prevent entrance of air into said chamber during loading, a quench chambersealed to said carburizing chamber to prevent entrance of outside air, a washer attached to said quenching chamber, and a draw oven attached to said washer and a mechanism for moving the material to be treated through said .-chambers, the whole forming an integral unit for successively and continuously treating metals.

3."'An apparatus for carburizing metals which consists of a refractory carburizing chamber resistantto disintegration by carburizing atmosphere, support means for the material to be treated in said chamber, a combustion chamber located within said carburizing chamber below said material being treated, sealed from communication with said carburizing chamber to prevent products of combustion from entering said carburizing chamber, an inletfor carburizing gas into said carburizing chamber located adjacent the end of said carburizing chamber and near said radiant chamber, arranged to lead the carburizing gas toward said combustion arranged to lead the carburizing gas into contact with said radiant member so that said heating member may preheat said carburizing gas after passing through said inlet.

5. An apparatus for carburizing metals consisting of a carburizing chamber with refractory walls, a support for material beingcarburized, a radiant heating member in the lower portion of said chamber, said radiant chamber extend ing transversely across said carburizing chamber and being provided with a sand seal near the point at which said radiant member leaves said carburizing chamber, and an inlet for carburizing gas adjacent said heating member, said-inlet located and arranged to lead said carburizing gas into contact with said radiant heating member after passing through said inlet.

6. An apparatus for carburizing metals consis'ting of a carburizing chamber with refractory walls, a support for the material being carburized, a series of tubular combustion chambers arranged below the material-being carburized and arranged transversely across said carburizing chamber, a separate burner arranged to deliver combustible materials into the interior of each combustion chamber, each combustion chamber being sealed to prevent entrance of air into said I carburizing chamber, and gas tight means connecting each of said combustion chambers with an exhaust duct arranged to carry products of combustion from said apparatus, and an inlet for carburizing gas. adjacent one of said combustion chambers and located and arranged to lead said carburizing gas into contact with said combustion chamber.

WILLIAM A. DARRAH.

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