US2049250A - Process of treating metal and apparatus therefor - Google Patents

Description

July 28, 1936. w. A. DARRAH PROCESS OF TREATING METAL AND APPARATUS THEREFOR Filed March 23, 1933 3 Sheets-Sheet 1 J y 1936- w. A. DARRAH ,250,

PROCESS OF TREATING METAL AND APPARATUS THEREFOR Filed March 25, 1955 s Sheets-Sheet 2 July 28, 1936. w. A. DARRAH PROCESS OF TREATING METAL AND APPARATUS THEREFOR s Sheets-Sheet 3 Filed Ma rch 25, 1933 1 6, A, VA 1 H 0",

Patented July 28,- 1936 UNITED STATES PATENT OFFICE PROCESS OF TREATING METAL AND APPARATUS THEREFOR William A. Darrah, Chicago, Ill.

Application March 23, 1933, Serial No. 663.378

Claims.

This invention relates to equipment, methods and processes in connection with operations which require heating metal materially above room temperature. My process and apparatus may be applied to annealing, drawing,hardening, or surface treating such as carburizing, nitriding, etc., various metals and alloys My invention is particularly applicable to the heat treatment of steel, although it may be applied with equal facility' to such metals as brass,'copper, aluminum, malleable iron, cast iron and various alloys.

Some of the objects of my invention are to provide economical, simple and eflicient means for handling metals during the, heat treating operation and also a process and the apparatus, therefor, which permits heating metals through substantially any desired temperature range without subjecting the metal to oxidation or only to a controlled amount of oxidation.

"In order to clearly describe my invention I will, for purposes of illustration, use the specific case of annealing steel, but it should be understood that I have. merely chosen this operation and material for purposes of illustration as obviously.

many other processes or materials may be applied without departing from the spirit of this invention.

My apparatus permits an economical apparatus and process having a relatively large production per unit of time and giving a low maintenance. My process and apparatus also permits convenient, safe, and economical handling of the materials throughout the cycle.

Other objects ofthis invention will be apparent from the specification, claims and drawings attached hereto.

In order to illustrate my invention most effectively, I have shown one type of structure on the enclosed drawings and it will be obvious that many departures may be made therefrom and still remain within the scope of this invention.

It is well recognized in the metallurgical industry that there are many cases where it is necessary to heat various materials and allow them manufacturing operationsit is necessary to remove the scale or surface oxidation before proceeding with'the remaining manufacturing operation. One of the objects of my invention is to overcome this difliculty which will be evident from the following description- Figure l shows a schematic arrangement or I0, surrounds the insulation and a heat trans general view of apparatusfor carrying out the process of my invention,

Figure 2 shows in more detail and partly in, cross section a vertical elevation of one form of my device, 5

Figure 3 shows a plan view of one form of my device.

Figure 4 shows in partial side elevation one form of container mounted on a truck for handling the materials being treated.

Referring to the drawings, I indicates a furnace housing or shell mounted on a foundation 2, in such a manner that the upper portion of furnace housing is approximately on the same level with building floor 3. A pit 4, houses a hydraulic lift a 5, placed below furnace -I, and so arranged that the upper end of plunger 6, carries a platform I, preferably of heat resisting material. Furnace I, may to advantage be constructed with the outer layer of insulation 8, surrounding a refractory wall 9. For purposes of convenience, I have formed the furnace in a circular shape which is a marked advantage, but. not necessary to the carrying out of my invention; A metal housing mitting refractory wall II, is placed within the refractory wall 9, and spaced therefrom so as to provide a combustion space I-I, between said heat conducting refractory I I, and refractory supporting wall 9. It will be noted that in the form of my invention shown, the combustion space is arranged in the form of an annular ring between theouter refractory wall 9, and the inner heat conducting refractory wall II. Wall I I may to advantage be constructed of such materials as silicon carbide, chrome iron alloys, etc. I con- .sider that it also comes within the scope of my invention to substitute for the combustion cham-- ber I2 and theheat conducting wall II, any heat radiating means such as an electrical resistance element or other source-of heat.

For purpose of convenience heat conducting refractory wall II is formed of separate individual panels I3, supported by refractory columns II, in such a way as to be free to expand and still remain substantially tight thus reducing to a minum leakage between combustion chamber I2 and interior chamber it, which contains the work being heated. Dotted line I6; within chamber I5, indicates diagrammatically the position normally occupied by-material being treated while the dotted position of plunger s and platform I, indicates its extreme positions of travel.

The door II mounted on four wheels, of which I9 and III indicate two, is provided to slide on rails above the top of the furnace housing I, when moved by cables or chain members 2| and 22 which may be pulled by means of rotation of winches II and 24.

A'housing a entirelysurrounds door u, when of" door l8, when closed. Housing 25, also extends around the top of 'furnace I, preventing leakage of gases out of furnace l, or of air into,

the furnace. Housing 25, extended over the top of chamber l5, carries attached to it in a substantially tight manner sealing member 26, which for purposes of illustration, may be called a sand seal and formed from a ring shaped channel filled with fine sand. Obviouslywater or other sealing means may be employed as desired. A shell or container member 21 is provided with-a downwardly extending ring 28 which is formed to engage with sealing member 26, forming therewith a substantially gas proof joint. Container 21 is also provided with a valved inlet 29 for permitting inert gas to be introduced into said container and also a valved outlet 30' for permitting a continuous flow of gases out of said container. In a similar way a valved inlet 3|, provides for the introduction of inert gas into the bottom of furnace I. The point of introduction of inert gas in furnace I is preferably made thru a tightly sealed bottom 32. In order to prevent an uncontrolled circulation of air or gases into or out of chamber IS, a sealing member 33 is attached to plunger 6, in such a position that when the plunger is in its lower position sealing member 33 will engage with a permanent sealing member 34, forminga tight joint therewith. For purposes of description, this joint may be called a sand seal although as stated above, other sealing means may be employed. A substantially tight ring or collar 36 extends downwardly from the bottom of furnace I, into a pool of water or other sealing medium 31, which is shown held in place by a dike or dam 3B. Plunger 6, is raised or lowered by the application of fluid medium to valve inlet 39, and the entire plunger may, therefore, be immersed in water if desired. In order to prevent excessive heating of plunger 6, in case it accidentally remains in the furnace for too long a period, it is provided with inlet and outlet pipes 40 and 4| through which water or other cooling medium may be circulated.

Referring specifically to Figure 1, 42 indicates a gas producer provided with a charging hopper 43, and a combustion air blower 44. The outlet of producer 43 is shown by pipe 45 containing damper air valve and head 46. Pipe 45 conducts the gases from producer 42 into'cooling device 41, which consists of a cooling coil 48, in a tank or container 49. A valved outlet 50, is provided near the lower .portionof coil 48 for the draining or removal of condensed products therefrom. Gases leaving cooler 41 pass through outlet pipe 5l, into the intake of blower or compressor member 52. Discharge outlet 53 from compressor 52' connects to the lower portion of scrubber member 54, which may be of the conventional type provided with a series of shells 55, 56, containing crushed materials such as lime stone and a water spray 51, which distributes a continuous flow of cooling and cleaning water over the crushed lime rock. An excelsior chamber 58 containers whether being cooled, heated orbeing Y prepared for heating.

A series of burners 60, 6|, etc. serve to deliver combustible medium to the combustion chamber I2. I find it convenient to circulate the products of combustion tangentially around the combustion chamber introducing the bulk of the heat at the botttom of the furnace and removing the products 'Of combustion from points near the top of the 5 furnace. 62 indicates a connection from the top of the furnace to the stack and it is apparently quite essential to actually control the amounts of draft on combustion chamber l2, at all times. This may be done by automatic damper 63, which may be operated by any of the usual commercial methods'as for example by regulator 11. I have found that it is quite essential to maintain the pressure in combustion chamber l2, substantially equal to the atmosphere or perhaps 0.001 or 0.002 of an inch of water below the atmosphere, while it appears quite desirable to maintain the pressure within the muffle l5, slightly above the atmosphere or 0.001 to 0.002 of an inch positive pressure. In the same way it is desirable to carry a very slight positive pressure at all .times in the bells while they are cooling or while they are being prepared for loading.

In operating this equipment I first adjust the performance of the gas generator so as to de- 25 liver-a gas having the desired analysis for the temperature and product in question.

I have found that in order to keep bright surfaces of metal without discoloration of any kind using gas containing substantally nitrogen, carbon monoxide, carbon dioxide, hydrogen and water vapor, that there are two ratios which are of great importance. The first important ratio is the percentage of carbon monoxide to the percentage of carbon dioxide and the second important ratio is the percentage of hydrogen to the percentage of water vapor. In referring to these percentages, I am of course referring to volume measurement.

Table I, given below, shows the variation of the carbon monoxide-carbon dioxide ratio with temperature, while Table II shows the variation of the hydrogen-water vapor ratio with temperature. Table III shows the ratio of carbon monoxide to carbon dioxide in the presence of free carbon at the various temperatures given in Tables Iand II.

Table I TemperatureC 700 800 l00O mm 50 Percent carbon monoxide.-. Percent carbon dioxide..." Ratio CO to CO2 55 Table II Temperature o 900 1000 1100 Percent hydrogen Percent water vapor Ratio H to [I10 c Table III Temperature "C -Q 1100 Percent carbon monoxide. Percent carbon dioxide... Ratio C0.to C02; l

In order to'obtain clean bright steel, free from discoloration, it is essential to maintain the ratio ;of carbon monoxide to carbon dioxide and of hydrogen to water vapor substantially that shown i Tables I and n. It will be noted that the ratio varies with the temperature and the figures given should be understood to be approximate sired ratio for any temperature may be interpolated. It should of course be understood that more carbon monoxide or more hydrogen is permissible than specified in the ratio, but not less carbon monoxide or less hydrogen. If the percentage of carbon monoxide is materially increased above that demanded by the given tem .perature there will be a tendency for the steel to absorb carbon. If the percentage of carbon dioxide or the percentage of water vapor is greater than specified the metal will be discolored or scaled depending on the relative amount of the difference. I have found, however, that it is possible to correct-for a deficiency inone ratio by an. excess in the other ratio. This I explain by the formula'CO'+HzO=COa+Hz. In other words an amount of water vapor greater than would be permissible in case water vapor and hydrogen were the only active ingredients of the gas may be compensated for by an excess of carbon monoxide, which in contact with the steel and the elevated temperatures will liberate suflicient hydrogen to balance the ratio, producing in turn a greater percentage of carbon dioxide. It will be-apparent, therefore, for example that work at a temperature of 800 C. with ag'as containing nitrogen, carbon monoxide and carbon dioxide only, the ratio of carbon monoxide to carbon dioxide should be approximately 1.78. On the other hand, in an atmosphere containing primarily only nitrogen, hydrogen and water vapor at 800' the ratio of hydrogen to water vapor should be approximately 1.44. If the percentage 'of water vapor is in excess of the figure given an increase in the percentage of carbon-monoxide may'be utilized to counteract the'eifect of the water vapor, but it will be evident that the carbon dioxide produced by such action must in turn be balanced by a corresponding further increase in the carbon monoxide. In other words, for each one volume of carbon monoxide utilized at 800 C. to compensate for one volume of water vapor,- there must be available in the gas beyond the specified ratio 2.78 volumes of carbon monoxide (1+1.,78). A similar calculation' can obviously be-made for any of the usual operating temperatures. As a matter of theory I am inclined to believe that the reaction specified occurs by first causing the excess of water vapor to pro-- duce iron oxide and then the excess of carbon monoxide reduces the thin film of iron oxide thus produced, liberating carbon dioxide in the course of the action. Another catalytic reaction which appears to take place is the liberation of a limited amount of free carbon in the form of soot which" deposits in light flakes around or near the surface of the steel. The presence of this free carbon or soot is an indication that the reaction is pro-' gressing satisfactorily.

Having now obtained from the gas generator 42, a gas containing the desired ratios, it is passed through cooler 49 where its temperature is sufflciently lowered to make it possible to safely handle the gas by the compressor 52. At this point an appreciable amount of moisture, oil and tar is condensed and removed. Compressor-52' then delivers the gas to scrubber 54, where it is passed in close contact with a stream of clean cold water, flowing over crushed lime stone. The alkaline action of the lime stone which is slightly soluble in the water together with the pressure carried in the scrubber causes the absorption of miscellaneous harmful gases, such as sulphur dioxide, hydrogen, sulphide, etc. At this point an appreciable amount of carbon dioxide is also ab- 5 sorbed so that the ratio of carbon dioxide to carbon monoxide is slightly improved in the scrubber. I would prefer to carry a slight excess of pressure in the-scrubber and I find 5 to 6 pounds is an economical ,condition.

In order to operate my equipment the furnace is now brought up to temperature by lighting the burners or heating the electrical elements as the case may be. After the furnace has reached a temperature of approximately 1000 F. or in excess thereof the gas specified above is introduced in the bottom of the furnace through inlet 3| and the muflle l5 fllled with the gas maintained at a slight positive pressure. The dampers that control the outward flow of gas from the combustion chamber are now adjusted so that the pressure within the combustion chamber is as nearly as possible balanced with the pressure within the muiiie, it being desired to maintain a slightly less pressure within the combustion chamber than within the muiiie, so that the travel of gas is from the muiile into the combustion chamber rather than in the reverse direction.

The seals around the muiile and combustion chamber are held tight and the door is closed so that no gas can escape from the mutile.

, A load of work is then placed on plate 10, which is carried on car or truck H. The space indicated by 12 and enclosed by the dotted line indicates the work to be heated. Bell or container 21 is nowplaced over the work and so arranged that the sand seal 13, on plates 10 prevents the entrance of air or the outward leakage of gas. When this is arranged gas is then passed into inlet 29, of bell 21 for a sufliciently long time to displace the air which would otherwise be within container 21. When this condition has been reached container 21 is locked to plate 10 by inserting wedges indicated by 14 and 15 and the bell with the plate and load lifted from truck H and placed in position shown in Figure 2, above the furnace with the sand seal 26 on the top of the furnace engaging with skirt 28 to prevent leakage.

The door ill of the furnace is now opened and plunger 6, carrying platform I is lifted by applying oil pressure to the. bottom of the cylinder through inlet 39. When the platform 1 has risen to the position shown in dotted lines in Figure 2 so as to carry the load on plate 10, the'wedge's -14, 15, etc. are removed and plunger 6 allowed to return into piston 5 thereby lowering platform 1 into the position at the bottom of the furnace as shown in the solid line in Figure 2. The door i8 is then closed and the temperature of the furnace raised to the desired degree.

It will be noted that during this period the bell 21, rests above the furnace and is separated from the hot chamber at the furnace by the door l8, which is thoroughly insulated. This system, therefore, permits heating a charge of work from room temperature to .any desired heat treating temperature without at any time exposing it to the air or to an oxidizing atmosphere.

when the heat treatment is completed the door I8 is again opened, plunger 6 raised and the hot metal resting on plate I0, is returned again to hell 2! all without being exposed to the outer air. It will be noted that bell 21 in the meantime has been kept filled with the inert gas and sealed 75 from the'outslde atmosphere by the sand seals as described.

Wedge members 14, 15, etc. are now replaced locking the plate 10 to container 21. Plunger 6, is withdrawn into cylinder 5, door 18 closed and container 27 may be lifted off of the furnace and replaced on truck member II where it is allowed to cool, care being takenthat the inert gas is passed in continuously during the entire cooling cycle. ,In order to give a general idea of the periods involved, I have found that a heating cycle ranging from one hour to three hours will take care of loads ranging from two to three tons while a cooling cycle in case the work is allowed to cool normally in the open room, requires from six to eight hours. Naturally these time periods can be increased or decreased by various well-known expedients. Having unloaded one hot charge and placed it on truck H for cooling a second charge may be placed in the furnace following the cycle previously outlined.

By the method shown and described it is possible to get a hot charge of metal out of the furnace at frequent intervals as for example every one or two hours thereby making it possible to pass a very large production of work through a therefore, quite inexpensive.

of the well-known means.

small furnace. Since the cooling bells or housing are not put within the furnace and therefore are not heated, it is possible to use relatively light metal in their construction and they are, The first cost of the equipment, therefore, is very much less than the first cost of other types of competitive equiprnent now available on the market for the same output.

Since the furnace heats only the work. being treated and not the housing or container it will be evident that the time of heating is greatly reduced and the cost of heating is also correspondingly'reduccd. The equipment is, therefore, veryefficient and economical in operation.

It will be apparent that many modifications may be made in the arrangement and details of this device when departing from the scope of my It should be understood that many as pure nitrogen, pure hydrogen, pure carbon monoxide or various mixtures of these gases. The gas described has been chosen merely because it is an extremely cheap gas to produce and one which can beeasily handled. I, therefore, consider the gas described to be one of the best suited for this purpose, but I do not wish to limit my invention specifically to a gas having the composition specified.

It will also be apparent that other obvious: modifieations in mechanical arrangement and construction can be made. I

Inusingtheterm heat treating,, it is, desired to indicate the usual commercial operations requiring considerable changes in temperature for the purpose of softening, hardening, toughening or otherwise changing the characteristics or prop- 'erties of the material.

It is common to control the rate and time of heating and of cooling in operations of this class, but in many cases merely the upper temperature limit requires accurate control. a

It should be understood that the equipment I have invented may be supplied with heat in any In most cases fuel oil. gas.or coal are the preferred sources of heat,

- owing to the lower cost.- In these cases a muffle is required as shown 'in the drawing to separate the combustion chamber from theheatin: chamber. However, my invention may be carried out equally well in an electrically heated furnace in. which case the heating elements are within the heating chamber and the walls of the chamber itself form the enclosure or muflie. I have found from extensive experiments, that at the temperatures normally employed in this class of equipment, that is to say from 1300 F. up to 2000 F., there is a very marked tendency for leakage through very small openings. This probably re- 1 sults from variations in pressure due to temperature head at elevated temperatures and possibly also to the attenuated condition of the gases within the muflie at thesehigh temperatures.

It should benoted that .the equipment and 1 process which I have invented depend primarily for heat transfer on direct radiation both in connection with the heating and cooling of the metal being treated. It is true that minor portions of the heat are transferred by localized convection 2 currents, but the affect of these is negligible compared to the total heat transfer. The cooling of the metal in the bells or cooling chambers is also primarily a matter of radiation.

It will of course be understood that the figures 2 given in Tables I and II apply to iron, steel or ferrous metals. When treating copper, brass or non-ferrous metals a different set of ratios control the production of a bright surface. I do not wish to restrict my invention to ferrous metals 30 only as many other materials can be treated to advantage.

I have found that under some conditions ferrous and other metals treated by my process and with my equipment have a tendency to corrode somewhat more rapidly than metals treated in the ordinary way and, therefore, coated with a layer of oxide or scale. In order to overcome this condition which presumably results from the unusually clean bright surface of metal passing through my process,'I place a. small container 16 made for example of cast iron or steel within the bell and holding a relatively small amount of some volatile protecting metal such as zinc or cadmium. I prefer to place the container at a point such that it is exposed to radiant'heat in such a manner that it reaches the maximum temperature'before the steel being treated. Under these conditions the zinc or cadmium volatilizes and has a tendency to recondense upon the cooler contents of the furnace which is of course the metal being heated. Under these conditions the zinc or cadmium will tend to form a thin surface alloy with the metal being treated. In many cases the appearance of the surface of the metal being treated is practically unchanged, but the resistance to corrosion is considerably increased.

It should be noted that I strictly avoid directcontact between the treating metal and the ma-- teri'al which is being treated, but fill the space surrounding the material being treated with a mixture of vapor from the treating metal and the inert gas. It should also be noted there is no special agitationof the metal being treated or 6 v I have found that instead of placing a container with melted metal within the heating chamber, that satisfactory results may be obtained by using 7 a metal surface coated with zinc or cadmium. .In

fact when containers are employedvit is not unusual for the innersurface to become more or less permanently coated with a layer of zinc or an alloy rich in zinc. 7

Having now fully described my invention, what I claim as new and wish to secure by Letters Patent in the United States is as follows:

1. A.furnace for heating materials in a controlled atmosphere consisting of a housing, a heating chamber within said housing, a combustion chamber for supplying heat to said heating chamber, burners for said combustion chamber,

a heat conducting wall separating said burners from said heating chamber, a movable cover for said heating chamber serving to close it from the outer air, an outlet for exhausing products of combustion from said combustion chamber, a

control damper regulating the fiowv of products of combustion out of said combustion chamber and a pressure controller connected to said combustion chamber and operating said damper, said damper and said regulator co-acting to maintain the pressure in said combustion space slightly below that in said heating chamber.

2. A furnace for heating materials in a controlled atmosphere comprising a closed heating chamber for said materials, a combustion chamber for burning fuel adjacent said heating chamber and separated therefrom .by a heat transmitting wall, an enclosure for said heating chamber and an outlet for products of combustion from said combustion chamber, a movable damper controlling said outlet and pressure regulating means actuated from said combustion chamber for moving said damper so as to maintain a lower pressure in said heating chamber than in said combustion chamber.

3. An apparatus for heat treating materials in a controlled atmosphere consisting of a closed heating chamber, a refractory muffle surrounding said chamber and serving to prevent products of combustion from entering said heating chamber, a combustion chamber outside said muffie, burner means for heating said muffle, means for introducing non-oxidizing gas into said heating chamber to protect material being heated from oxidation, an opening in the uppe portion of said chamber for loading and unloading said material, a tightly closing cover for said opening arranged to seal said gas in said chamber, a movable cooling chamber mounted above said heating chamber having a closed top and a removable normally sealed bottom and serving to receive said material after heating, means for moving material being treated from said heating chamber through said opening into said cooling chamber, sealing means for preventing leakage of air into said chamber, and sealing means for movably closing the bottom of said chamber, on outlet duct exhausting products of combustion from said combustion chamber, a control damper in said duct and means controlled by the pressure in said combustion chamber for moving said damper to continuously maintain the pressure in said combustion chamber'slightly below the pressure in said heating chamber.

4. In a furnace, a tight heating chamber for containing material to be heatedin a controlled atmosphere, a combustion chamber for uniformly heating said chamber, a burner for said combustion chamber, a loading opening in the top portion of said chamber, an elevating device operating in the lower portion of said chamber, said elevating device serving toraise andlower material to be' treated into and out of said heating chamber, a sealed door closing said loading opening, an air tight housing for enclosing said door, an outlet duct from said combustion chamber, a

control damper in said duct designed to vary the flow of gases therethrough and means operated by the pressure in said combustion chamber for moving said damper thus maintaining the pressure in said combustion chamber slightly below the pressure in said heating chamber.

5. In a furnace a tight heating chamber for confining material to be heated away from the atmosphere, 2. heat conducting wall adjacent thereto, a combustion chamber for heating said wall, burners for supplying fuel to said combustion chamber, said wall and combustion chamber serving to supply heat uniformly to said heating chamber, a loading opening in the top portion of 'said chamber, an elevating device operating in the lower portion of said heating chamber, and carrying a load supporting platform, said elevating device serving for loading and unloading said heating chamber, a sealed door closing said loading opening and serving to maintain a controlled atmosphere within said heating chamnicating with said opening, said cooling chamber providing a sealed joint at its lower portion contacting with the upper portion of said heating chamber, and also with said load supporting platform, an outlet duct exhausting products of combustion from said burners in said combustion chamber, a damper in said duct for varying the passage of gases therethrough and a pressure regulator controlled by pressure in said combustion chamber for moving said damper so as to maintain the pressure in said combustion chamber slightly below the pressure in said heating chamber.

6. An apparatus for bright annealing, consisting of a closed heating chamber for containing materials to be heated in a controlled atmosphere, an adjacent combustion chamber, a heat transmitting wall therebetweenproviding uniform radiant heating means for said heating chamber, means for introducing inert gas into said chamber, an opening in the upper portion of said heat-;

ing chamber, a door closing said opening, a housing for said door and a movable cooling chamber;

,ing of a closed heating chamber, for containing the material to be annealed, an adjacent combustion chamber providing uniform radiant heating means therefor but isolated therefrom by a heat transmitting wall means for introducing inert gas into said heating chamber, an opening in the upper portion of said heating chamber, a door closing said opening, a housing for said door and a portable cooling chamber, having an open bottom, located above said heating chamber and communicating therewith, means for removably sealing said cooling chamber into contact with said heating chamber and separate means for closing and sealing the opening in the bottom of said cooling chamber when not in contact with said heating chamber an outlet from said combustion at all times less than the pressure in said heating chamber. g 8. An apparatus for heat treating metal consisting of an enclosed heating chamber, for containing the metal to be treated, an adjacent comform carried by said elevator and means for preventing leakage of air into or out of said chambers, said means including a control device for the flow of gases from said combustion chamber so that the pressure in said combustion chamber willat all times be less than the pressure in said heating chamber, and means for sealing said plat-' form to said open bottom portion of said cooling chamber in a manner to prevent leakage.

9. In a fuel burning heat treating device a muflled heating chamber, for containing the material to be heat treated, a combustion chamber surrounding and arranged to. heat said 'muflie, burners supplying fuel to said combustion chamber, a portable cooling chamber with open bottom located above said heating chamber, a mov able door separating said heating chamber and said cooling chamber and means for sealing said cooling chamber, said door and said heating chamber one to the other to prevent entrance of air and independent means for sealing the open bottom of said cooling chamber when same is not in contact with said heating chamber, an outlet from said combustion chamber, a movable damper closing said outlet and a pressure regulator actuated by the pressure within said combustion chamber for operating said damper so that the pressure within said combustion chamber is at. all times lower than the pressure within said heating chamber. a

10. A furnace for'heating materials in a controlled atmosphere, comprising a heating chamber for said materials, a door closing said heating chamber, a combustion chamber providing a space forburning fuel and supplying heat to said heating chamber, said combustion chamber being separated by a heat transmitting wall from said heating chamber, a housing enclosing said heating chamber and said combustion chamber, burners to deliver fuel to said combustion chamber, an outlet for products of combustion from said combustion chamber, a member regulating the flow of products of combustion from said combustion chamber, and a pressure actuated device controlling saidregulating member, so as to maintain a lower pressure in said combustion chamher than in said heating chamber.v

WILLIAM A. DARRAH.-

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