US2809822A - Heat treating muffle furnace - Google Patents

Description

Oct. 15, 1957 .1. R. GIER, JR

HEAT TREATING MUFFLE FURNAC 2 Sheets-Sheet 1 Filed May 8, 1953 INVENTOR.

By Mu Oct. 15, 1957 J. R. GIER, JR

HEAT TREATING MUFFLE FURNACE Filed May 8, 1953 2 Sheets-Sheet 2 United States Patent Ofiice 2,809,822 Patented Oct. 15, 1957 2,809,822 HEAT TREATING MUFFLE FURNACE John R. Gier, Jr., Cleveland, Ohio Application May *8, 1953, Serial No. 353,789 6 Claims. ;(Cl. 263-41) This invention relates to heat treating furnaces and is directed particularly to a furnace in which the heat treating atmosphere is controlled more effectively, and in which a new and improved mufifle, constructed and arranged so that it is not subjected to severe thermal shocks, is provided.

For the purposes of illustration, that part of the invention directed to the structure by virtue of which the mulfie is protected from thermal shocks is described in detail hereinafter as applied also to a batch type furnace, its general application to a .continuous furnace being readily apparent therefrom and from the description herein of a preferred embodiment of a continuous furnace.

Heretofore, in heat treating furnaces, the practice has been to provide muffies in the form of heavy metal cases reinforced by suitable external ribs and the like, or configured by differences in thickness of the walls at different portions, to resist cracking, twisting and deformation of the mufile Walls'as a result of thermal shocks due to sudden reductions of temperature occasioned by insertion of cold charges of metal into the muffle.

In accordance with the prior practices, the metal case or mufile directly accommodates the charge of metal to be treated and, in turn, is accommodated in a furnace heating compartment in which it is heated by the application of heat to its exterior surface from a source within the furnace compartment.

Generally in such furnaces, the heating chamber has walls composed of, or lined interiorly with, insulating material'wh'ich defines a central heating cavity within which a source of heat is disposed exteriorly of the mufile. The insulating material is porous and cellular so as to assure high thermal insulating efliciency.

The muflle generally is metal of limited thickness-for example, less than one inch-and consequently cannot of itself absorb and store any large quantity of heat. Since the heat is applied to the outside of the mufile and only a very limited quantity of it can be stored in the heated mufile walls, a heavy charge inserted into the muffle reduces the temperature rapidly, almost instantly, to such a degree as to cause extreme differentials in temperature between the inner and outer wall surfaces of the muffle, between those parts of the mufile near the charge and those partsmore remote from the charge, and between the end portions of the muffie and the intermediate portions in which latter the ratio of wall to charge is much smaller than at the end portions.

Quite often, the introduction of a large cold charge of metal articles to be treated causes the temperature within the mufile to drop quickly or more. Thus, since the temperature in the furnace compartment exteriorly of the mufiie and in the muffle itself, often is 1800 F. and that in the muflle is reduced suddenly to 1650 F., it is apparent that the muflie is subjected to very severe thermal shocks.

Under these conditions, the highest quality muflle now 'nace, instead of providing obtainable has a useful life of about 4 to 6 months at most.

It is impractical to dispense with the use of some type of mufile in such controlled atmosphere heat treating furnaces even if the furnace compartment itself is constructed with refractory material so as to directly receive the charge.

In the first place, there is difliculty in sealing the joints between the elements of refractory insulating material so as to make them gas tight.

Furthermore, for efficiency, the refractory insulating material itself must be cellular and include myriad minute voids and these render it pervious to gases. Accordingly, a large volume of gases is absorbed by the insulation when the chamber is opened to the atmosphere for an appreciable interval. Consequently, after removal of a treated charge and before a fresh charge can be placed directly in such a chamber for heat treating under controlled atmosphere, the insulation as well as the chamber must be scavenged from all foreign gaseous media that might have been introduced at one time or another during loading or unloading. Often it requires many minutes to scavenge these foreign gases from the insulation and chamber sufliciently so that a subsequent charge can be introduced with assurance that the controlled atmosphere will be uncontaminated.

Not only is the muflle subjected to sudden and severe thermal shocks due to the introduction of cold charges but also to longer sustained thermal stresses. The latter stresses result from the maintenance of high temperatures on the outside of the muffle and much lower temperatures on the inside as a large charge is brought up to term perature.

Many attempts have been made to improve such mufiles both metallurgically and structurally for withstanding these thermal shocks and stresses but none has met with pronounced success.

For .some reason, efforts heretofore made appear to have been directed toward the rugged enough to withstand severe thermal shocks rather than toward the elimination of the causes of the thermal shocks and sustained thermal stresses.

I have found that these thermal shocks can be-eliminated or greatly reduced if, in a muffle type heat treating fura muffle within a thermally insulating heating compartment in which there is a source of heat which applied the heat to the exterior of the muflle, a metal muflle with an inner wall lining or facing of strong, dense, refractory material of high thermal conductivity and high heat absorptive capacity, and with a source of heat in its interior, preferably ator near the interior surface of the inner refractory lining is provided and installed in a furnace compartmentwhich is thermally insulated.

The refractory lining is relatively thick so that a large amount of heat relative to that required to heat the average charge for which the mufile is designed can be stored in the lining. Also the lining is of high thermal conductivity so that when, the charge is inserted in the heated muffle, heat can flow quickly from any part of the lining to that point which is lowest in temperature and thus bring about a quick and effective balance or uniformity of heat throughout the interior of the mutlle and in the charge.

With the mass of the lining charge, the temperature of the provision of a metal muffie large in proportion to the charge can be during charging and discharging of the muflle, it cannot.

brought up. rapidly to the temperature of the mufile without causing, great temperature differentials between the inner and outer' absorb an appreciable volume of foreign gases. This is advantageous in that, as a result, the interior of the muffie, including the lining and any spaces between its sections, can be scavenged of foreign gases readily within a -very short interval.

In fact, since the lining does not absorb any appreciable volume of foreign gases or air, the high temperature of the muffie itself is usually sufiicient to purge the absorbed gases.

The metal of the muffle is preferably nickel chrome steel. The Wall thickness is about of an inch.

The gas used for the controlled atmosphere is dependent upon the heat treatment to be given. For bright heat treating of stainless steels or other high chromium content alloys, hydrogen gas or disassociated ammonia gas is preferred. The gaseous media in the muflie is maintained at about one pound per square inch above outside atmospheric pressure and at a dew point of minus 40 F. or lower. For economy, the treating gas may be cleaned and recirculated, if desired.

The source of heat used is preferably a group of electric resistance elements as these lend themselves well to control. When such are used the refractory lining chosen for the muffle is one having high dielectric properties in addition to the properties hereinbefore mentioned so that the heating elements can be supported safely directly by the lining.

Various other advantages and objects of the invention will become apparent from the following description wherein reference is made to the drawing, in which Figure 1 is a vertical longitudinal sectional view of a batch type furnace embodying the mufile of the present invention and is taken by the line 11 of Figure 2;

Figure 2 is a vertical cross-sectional view of the furnace illustrated in Figure l and is taken on the line 2-2 of Figure 1;

Figure 3 is a top plan view of a continuous type furnace embodying the principles of the present invention;

Figure 4 is a side elevation of the furnace illustrated in Figure 3; and

Figure 5 is a fragmentary sectional view of the furnace illustrated in Figures 3 and 4, and is taken on the line 5--5 of Figure 3.

Referring first to Figures 1 and 2, there is illustrated a batch type furnace, indicated generally at 1, which comprises a suitable housing 2 lined with a thick lining of refractory heat insulating material 3 so as to define a central cavity 4. The material 3 may be in the form of preformed sections of the usual relatively light weight, high porosity, and cellular structure.

Within the cavity 4 is a muflie 5 which, in the form illustrated, comprises a metallic box 6 lined interiorly with lining 7 of dense refractory heat absorbing and conducting material, which may be preformed interfitting elements, as indicated. The lining defines the heat treating chamber 8 of the muffle.

The box 6 is sealed circumferentially and at one end and at the other end it and the chamber 8 are-provided with an entrance passage 9 which may be closed by a suitable door 10. The refractory lining 7 is one which is very dense and absorbs heat in large quantities very rapidly and which conducts it very rapidly.

The heating source, contrary to the usual practice, is disposed within the muffle itself and preferably in, or near, to the inner surface of the refractory lining 7. The heating source may comprise an electric resistance which may be in the form of a plurality of electric resistance elements 11 arranged to extend lengthwise of the muffle, each from one end to the other of the chamber 8 therein. For protecting the elements 11, they are located in suitable channels in the refractory lining 7 which, as illustrated, are open into the chamber 8 along one side. The heating elements 11 are connected, at one end of the chamber 8, to a common bus bar 12 which in turn is connected to one side of the line, and are connected at the opposite end to a common bus bar 13 connected in turn to the opposite side of the line.

In the form illustrated, a metal tube 14 is connected at one end to the box 5 and extends outwardly therefrom through the insulating lining 3 and outer housing 2 of the furnace. A suitable inner tube 15 extends through the tube 14 to the outside of the housing 2 at one end and at its other end extends into a suitable bore 16 in the refractory lining 7 of the muffle. This tube 15 carries electrical leads which are connected to the bus bars 12 and 13, respectively.

The tube 15 is of less diameter than the tube 14 and the bore 16 so as to provide radial clearance between it and the inner wall of the tube 14 and the bore 16.

Gaseous media for maintaining a controlled heat treating atmosphere in the mufile is supplied in controlled amounts through the clearance space between the tube 14, on the one hand, and the tube 5 and bore 16 on the other. This gaseous media passes readily through the joints between the refractory elements of the lining 7 into the interior of the muflle chamber 8. However, such gas may be admitted in some other convenient manner. The admission of gas is controlled by conventional control valves, not shown. The gaseous media fed into the chamber 8 is maintained slightly above atmospheric pressure so as to maintain a continuous outward seepage of gas from the mutfie.

Connected to the furnace at the inlet end is a feeding tunnel 17, one end of which is connected to the inlet passage 9 of the mufile and the other end of which is closed by a suitable door 18. Suitable power driven fans 19 are provided in this tunnel 17 for the purpose of circulating the gaseous atmosphere about charges of material which are being withdrawn from the muflie so as to cool them more rapidly.

In feeding the materials into the furnace, the mufile door 10 is closed and the mufile scavenged with the proper gases. While this operation is proceeding trays or other containers of the articles to be treated are introduced within the feeding tunnel 17 after which the door 18 is closed. Treating gases are fed into the muffle and leak into the tunnel 17. These gases are relatively hot, having escaped from the muffle, and are circulated about the materials in the tunnel 17 by means of the fans 19, thus causing substantially all of foreign gaseous media to be bled out of the tunnel. After the tunnel 17 has been scavenged of foreign gases, the muflile door 10 is open and the charge is pushed into the muflie whereupon the door 10 is again closed.

The tunnel 17 may be surrounded by a suitable water jacket in the form of spaced circumferential walls, as illustrated between which water or other cooling media is circulated. After the heat treatment, the articles are pulled from the mufilc into the tunnel 17, the door 10 then being closed and the door 18 being closed. The

fans are then started to circulate the gaseous media in the tunnel so as to lower the temperature of all of the articles in the tunnel 17. After they have reached the proper temperature, the door 18 is then opened and they are removed and a new charge is then placed in the tunnel 17 and the cycle repeated.

As heretofore mentioned, contrary to the usual practice the heating source is within the muffle and the lining 7 of the muffle is capable of absorbing and conducting rapidly large amounts of heat. As a result, when a cold charge is placed within the muffle, heat is drawn rapidly from the heat source and also withdrawn from that amount stored within the dense refractory lining 7. Accordingly, the temperature of the charge is brought up to that required by absorption of the heat concurrently both from the source and that stored in the refractory lining. Furthermore, since the lining is of high thermal conductivity, the heat can flow to any cold spots which might.

\ develop due to the non-uniform disposition of the bulk of the charge or the proximity of part of the charge to the endwalls as well as the side walls of the mufile; Since a large proportion of heat is supplied promptly from that accumulated in the lining, the thermal shock of the mufile which ordinarily would occur due to the introduction of the cold: charge is eliminated or sogreatly reduced as not to seriously affect the metal walls of the muflle box 6. Sudden and rapid changes in the temperature of the'lining 7 are not seriousas the lining is made in sections of a-low expansionceramic which can expand and contract independently and does not crack in service. Fluctuations in the temperature of the metal. box 6 of the mufile 5. do occur but these are quite gradual and much less pronounced thanin those instances where the metal wall of the casingisdirectly exposed to the cold charge and consequently donot cause severe shocks.

Referring next to Figures 3 and 4, the continuous furnaceemploying the mufile of the character described is illustrated. This furnace includes a furnace heating compartment which is essentially the same as the furnace compartment heretofore described and comprises an outer casing or housingZl lined with insulating material, as indicated at 22, so as to define a central cavity in which a muffle 23-is closely confined. The mufile 23 comprises a metallic casingllwithin which is the heating source and which is lined with the refractory material having high heat absorbing and conducting properties, the'mufile being essentially the same as that described in connection with Figures 1 and 2 except that it is open at both ends so as to render it suitable for a continuous furnace.

The muffie has an inlet opening 24a'nd an outlet opening 25 Which-are connected, respectively, to the inlet portion 26a and outlet portion 26b of a tunnel indicated generally at 26, part of which is formed'by the passage through the mufile itself. Suitable conveyor means such as a motor driven conveyor belt 27 are provided for transporting materials continuously from the inlet end 28 of the tunnel to the outlet end 29 thereof.

Beyond the muflie 23 in the direction of travel through the furnace are water jacketed cooling chambers 30 and 31 in the latter of which is a bank of suitable motor driven fans 32, such as described in connection with the tunnel 17.

The muffle is heated internally in any suitable manner and preferably by electric heating elements such as described in connection with Figures 1 and 2 and gaseous media for proper heat treatment may be, if desired, introduced directly into the muffle in controlled amounts. However, it is desirable that the proper metal treating atmosphere be maintained throughout the entire length of the tunnel and, in addition, that outside atmosphere tending to enter the inlet end 28 of the tunnel along with the charge be scavenged and removed. For reducing the natural influx of outside air, the ends of the portions of the tunnel, as indicated at 26c and 26d are arranged to oppose the influx of outside air. For example, the inlet portion 260 has its top wall 32 disposed below the normal level of the bottom wall 33 of the tunnel and has its side walls 34 arranged to extend below the level of the bottom wall 33. Thus, the inlet opening 28 is below the level of the passage through the major portion of the tunnel inwardly from the inlet end. In the form illustrated, this relation is obtained as a result of the tubular tunnel portion 26a sloping downwardly toward the inlet end at an angle of about 10 to the horizontal, this sloping being continued for a sufficient extent to dispose the inlet below the plane of the bottom wall 33.

The inlet end of the tunnel is also provided with a branch 35 which is a continuation of the horizontal portion of the tunnel and which is closed at its outer end with an observation door 36. This door may be removed, if desired, for feeding extremely long articles into the furnace, especially articles of such a shape and length that they cannot negotiate the curved portion 34. When used as a feeding entry, suitable packing material through which the. articlesarepushed is provided at theentrance so as to restrain the entry of outside air.

If, of course, the tunnel'is to be left open at the bottom, thenthe top wall portion 32 and the side walls 34 at the end portion 26c must extend downwardly a considerable distance below the level of the path along which the articles travelin'passing throughthe tunnel so that air cannot rise and contact the articles and the treating atmosphere can escape from the tunnel only when an amount in excess of the capacity of the tunnel is introduced thereinto. However, a circumferentially closed tunnel is preferred.

While the furnace described is satisfactory so long as considerable drafts do not exist at the work site, nevertheless, the balance of internal and external atmosphere is quite delicate and'if any substantial drafts occur in the outside atmosphere, there is a tendency for a flow of the treating atmosphere to become established from one end of the tunnel toward the other with the result that outside air can enter the former end.

In order to maintain an equalized flow of gases outwardly from the respective ends of the tunnel so as to prevent the entrance of outside atmosphere, the main supply of treating gases isfed into the tunnel between-its end. For this purpose in the form illustrated, there is provided just beyond the outlet end of the mufile a main treating gas supply or feed pipe 37 which discharges into and generally directly across the tunnel so as not to cause the flow of gases toward one end to the other to predominate. At each side of the pipe 37 are auxiliary supply pipes38 and 39; respectively. Each of the pipes 38 and 39 isprovidedwith a restricted'nozzleand each is connected to the source of treating gas.

The flow of gas through all three of the pipes is controlled by valves 40, 41 and 42, respectively. The nozzle 38a of the pipe 38 is directed toward the inlet end of the furnace and the nozzle 39:: of the pipe 39 is directed to the outlet end of the furnace.

The volume of gas supplied through pipes 38 and 39 is only a small fraction of that supplied by the pipe 37. The supply from the pipes 38 and 39 is primarily for inducing and controlling the flow of gas longitudinally of the tunnel and out of the ends thereof.

Thus, if there is any tendency of outside air to flow into the discharge end of the furnace, it can be counterbalanced and eliminated or controlled by directing a jet of gas in the required volume into the furnace through the pipe 39 of whch the nozzle tends to induce a flow toward the outlet 3. On the other hand, any tendency of air to enter the inlet 28 can be controlled or eliminated inlike manner by introducing gas through the pipe 38.

Generally, it is desirable to maintain a larger flow of gas out of the inlet end 28 than out of the discharge of outlet end 29. This is because a certain amount of outside gases are carried into the inlet end in the charge of materials and an excess flow of heat treating gases toward and out through the inlet and scavenges these foreign gases by carrying them out of the inlet. On the other hand, only sufficient flow toward the outlet end is maintained to assure that the outside gases do not enter the outlet end. Theoretically, it would be desirable that there be a direct balance in pressure between the gas at the outlet end and the outside atmosphere so that no heat treating gas would be lost and no outside air would be admitted. This is impractical, however, and enough excess gas is supplied in the furnace so that a light scavenging flow can be maintained at the outlet also for preventing the entry of outside atmosphere.

It is apparent from the foregoing that in the furnace structure provided, the mufiie has much longer useful life and the heat treating atmosphere desired is more effectively maintained.

Having thus described my invention, I claim:

1. In a heat treating mufile furnace, an enclosing structure providing a compartment enclosed by walls of high thermal insulating properties, an impervious walled muffie in said compartment and having therein a heat treating chamber enclosed by a dense refractory lining of high thermal absorbing capacity and conductivity, said lining being thick relative to the impervious wall of the muflle, means for maintaining in said chamber a controlled heat treating atmosphere, and a heat source within said muffie for heating the chamber.

2. A heat treating muffie furnace according to claim 1 characterized in that said muffle comprises a metal casing and its said chamber enclosing walls line the inner wall surface of the metal casing.

3. A heat treating muffle furnace according to claim 2 characterized in that the heat source is adjacent the inner surface of the chamber walls.

4. A heat treating muffle furnace according to claim 2 characterized in that the heat source is an electrical resistance heating element and said element is exposed to the interior of the chamber.

5. In a heat treating furnace, an elongated generally horizontal tunnel having at least one end opening directly into the atmosphere, means for conveying articles through the tunnel along a predetermined path, said open end being below the level of said path for opposing the flow of gases out of said tunnel through said end, means for heating the interior of the tunnel, a first gas inlet means for supplying heat treating gas into the tunnel at a location remote from said end, an additional gas inlet means operable for introducing additional heat treating vgas in the form of a jet into the tunnel at a location spaced 8 from said end and having its discharge opening facing toward for directing the additional gas as a jet generally toward, said end, and means to control the introduction of gas by said additional gas inlet means independently of the introduction of gas by the first gas inlet means.

6. A structure according to claim 5 characterized in that said end is the inlet end, said tunnel has an outlet end, said outlet end is below the level of said path for yieldably opposing the flow of heat treating gas out of said outlet end, a third gas inlet means is provided and is operable for introducing additional heat treating gas in the form of a jet into the tunnel between the outlet end and the said additional gas inlet means and has its discharge opening facing toward, for directing, as a jet, the gas it introduces generally toward, the outlet end, and means are provided for controlling the admission of gas by said third gas inlet means independently of the admission of gas by said additional gas inlet means and said first gas inlet means.

References Cited in the file of this patent UNITED STATES PATENTS 383,147 Rau May 22, 1888 1,057,745 Kohn Apr. 1, 1913 1,133,647 Kammerer Mar. 30, 1915 2,175,922 Scott u Oct. 10, 1939 2,404,059 Hall July 16, 1946 2,701,712 Gilbert Feb. 8, 1955 FOREIGN PATENTS 496,497 Great Britain Dec. 1, 1938 502,020 Great Britain Mar. 6, 1939

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