US2290551A - Heat treating furnace - Google Patents

Description

July 2l, 1942. J, R, GlER, JR .2,290,551

HEAT TREATING FURNAGE Filed sept. 26, .19:59 s sneek-sheet 2 In( 68 /64 r11' ATTORNEY July 21 1942 l J. R. GIER, JR 2,290,551

HEAT TREATING FUR'NAcE A Filed sept. 26. 1939 .s sheetssheet 3 06 9,' I la@ T l i i 4 /M f '98 log,

i lo? l v /oa l /ofl 'n' l 116/ H4 94 I of O O O L/ZO @ya Fig 9 fig /a O O O O O O O O O O O WITNESSES: l INVENTOR a Z john/6. /eff.

ATTORNEY Patented July 2l, 1942 HEAT TREATING FURNACE John R. Gier, Jr., Wilkinsburg, Pah., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania.

Application September Z6, 1939, Serial N0. 296,586

6 Claims.

My invention relates generally to electric furnaces, and more particularly to electric furnaces especially useful in the heat treatment of tool steels in the form of tools, dies, and the like, while enveloped by an inert or protective atmosphere for the purpose of uniformly hardening the article heat-treated.

One of the primary purposes of my invention is to provide a furnace in which tool steel articles, including those of high-speed tool steels, may be hardened free from any of the types of damage heretofore usually experienced, the more important of which are oxidation, decarburization, distortion, and cracking.

Oxidation and decarburization are caused by chemical reaction of elements in the steel with certain elements in the furnace atmosphere at high temperatures. Oxidation produces an undesirable surface scale, and decarburization produces a soft surface layer, the metal thereof being deficient in carbon. Through the use of certain protective reducing gas atmospheres, steels are now com monly prevented from scaling, but these atmospheres do not ordinarily prevent decarburization.

To prevent both oxidation and changes in surface carbon content on all types of tool steel during heat treatment requires a highly reducing atmosphere, such as a mixture ofhydrogen and nitrogen, although if carburization is desired the mixture may have a carburizing constituent added which may contain CH; up to 2% and not more than traces of CO2 and H2O. Ordinary controlled atmosphere hardening furnaces are incapable of maintaining a protective 'atmosphere of the necessary purity and exactness of composition.

It is a purpose of this invention to provide a furnace in which protective atmospheres of high `purity and accurately controlled composition may be maintained to prevent both oxidation and decarburization of all tool steels during heat treatment. In the widely used and more expensive tool steels which must be heated up to temperatures in the range of 1700 F. to 2300 F., more or less, for hardening, decarburization can be extremely rapid due more to the high temperatures required for carbide solution than to any direct effect of alloy content.' Heretofore, it has been the practice to allow for surface damage, as well as for distortion, by leaving extra stock on the tool which had to be removed after hardening by one or more finishing operations, such contours, all of which add greatly to the cost of the tools and dies.

It is accordingly an object of my invention to provide a furnace for the heat treatment of tool steels, especially at elevated temperatures in the range of 1700 F. and higher, from which oxidizing agents can be effectively purged and kept out of contact with the charge, whether such agents come from the air, or are occluded gases contained in the charge, or in the firebrick of the furnace, or come from other sources.

Damage `by distortion includes the irregular or unpredictable changes in dimensions or contour of the tool in excess of the normal and determinable increase in specific volume which always accompanies the'-v hardening of steel with the production of martensite. Both distortion and cracking can be easily avoided by cooling the tool or die in air for hardening, or, following my procedure, by cooling in an appropriate controlled gaseous atmosphere. So l cooling for hardening, of course, restricts the number of steels which can be treated in a furnace having provisions for hardening the tool steels which include the step of cooling in air (gas). This restriction, however, is not at all serious or objectionable since it parallels the previous requirement, namely, that the steels considered are those requiring temperatures in the neighborhood of 1700 F., or even higher, for the hardening process. Such steels, in practically all cases, are capable of, and preferably are, hardened while enveloped by a protective gas, and can be successfully hardened without damage in block sizes up to 2 inches thick, and even more, and in lengths up to the capacity of the furnace.

Accordingly, it is a further object of my invention to provide a furnace for the heat treatment of tool steels at temperatures in the neighborhood of 1700o F. and above, in which the charge can be completely enveloped by a non-oxidizing, non-decarburizing gas, whichfurnace has provisions for permitting the cooling of the charge in the same or another controlled, protective atmosphere in a manner to prevent distortion and cracking.

The furnace of my invention has several basic features and innovations which enabled me to harden tool steels of the character aforesaid without surface contamination, with dimensional changes confined to these which are predictable, and Without decarburization. In other words, I have devised a furnace in which I can harden tool steels with clean and fully hardened surfaces as grinding and hand stoning for fine interior having the wanted dimensions and 'free from have provisions for heating and cooling the work I within a controlled atmosphere, and for moving the work through the different heat-treatment zones of the furnace without harmful contamination of that atmosphere, as might occur, for example, with the opening of outside doors. In use, a satisfactory atmosphere must be delivered to the furnace, and particularly to its main heating chamber, and must remain innocuous to the hot work while in contact with it.

Additionally, the work must be so supported so as to heat and cool uniformly and freely, but without distortion from sagging under its own Weight. And since the furnace is for use in comniercial fields, it must be low in initial cost and long of life, low in upkeep cost, easy of manipulation, and simple in operation. Broadly, therefore, it is among the objects of my invention to devise a simple means, in the form of a commercially-acceptable furnace, for heating and cooling tool steels in a special gas so that the steen will be hardened without damage,

It is an object of my invention to provide a furnace in which the widely used and more expensive' tool steels can be heat-treated whereby the necessity for machining, grinding, or stoning after the-heat-treatment is kept to a minimum, and, in most cases, entirely eliminated.

In one embodiment of my invention for the purpose, I provide a metallic mufiie for the heating chamber 'of the furnace, the niuiile being capable of withstanding the high temperatures specied for long periods without appreciable deterioration, and further having the requisite strength at the temperatures involved. I prefer `to make the muiile in the high temperature zone of commercially-pure nickel or, in some cases, nickel alloyed with a non-carburizing metal, such as copper. The interior of the mule is in contact with a non-oxidizing protective gas, and will, therefore, be protected against oxidation. To further protect the muille on the outside, and also particular kinds of electric heating units about the muiile, I may encase the furnace in a metallic shell into which is introduced the same or a similar protective gas.

I specify commercially-pure nickel as the material from. which I prefer to construct that part of the muilie exposed to the high temperatures, for I have found by/ experiment, in which the deections of cantilever bars of identical size vbut of different materials were measured, thatone of nickel deected considerably less than chromium-alloyed nickel at temperatures about 2100 F., and while enveloped by. a non-oxidizing atmosphere, more particularly, pure, dried, hydrogen. Additionally,vexperiments in a carburizing atmosphere, specifically, dried, dissociated ammonia containing about 1% of methane, indicated-that nickel and copper-alloyednickel did not embrittle when heated for four hours at the aforesaid temperature, While various chromenickel alloys, with and without iron, apparently absorbed carbon from the carburizing gas to the extent of becoming brittle or having their meltaandoet use in a non-oxidizing atmosphere which may or may not include carbon-containing gases, these experiments indicate that a nickel muflle or nickel-copper alloy muille should be used.

' The mufiie itself extends beyond the end of the refractorybrickwork of the furnace to provide a chamber or section in which the charge may be initially placed. Protective gases are vadmitted to thisy section so that it may be purged be employed as a charging and purging chamber,"

and the other section as a coolin'g and discharging chamber, it being understood, of course, that the various sections of the muille, including the s ection in the heating zone, will be separated by manipulatable doors inside the lmuiile.

In building a furnace in accordance with my invention,v I necessarily have in mind that the' furnace must be as efficient as possible in order` tov be commercially attractive. Accordingly, I incorporate into my furnace certain features which not only increase its eiiiciency with respect to consumption of power, but also enable the heattreating process to be carried out more effectively. Among these features is the confining of the heating zone to definite limits substantially determined by a tray which carries the work. This tray is specially constructed for supporting the work and, in addition, has end shields which iit the roof of the muiile in contour so that the section between the end shields is in effect the heating chamber for the Work, the end shields further acting to confine the radiant energy between them.

Other details, features, elements, and combinations of my invention will be apparent from the following description thereof, taken in connection with the drawings, in which:

Figure l is a vertical view, mostly in longitudinal section and partially in elevation, of a furnace built in accordance with the teachings- Figs. 6 and 7 are elevational and end views, re-

spectively, of the tray of Fig. 5;

Fig. 8 is an elevational view of one of the shields secured to the end of the tray of Fig. 5;

Figs. 9 and 10 are enlarged elevational views of end and intermediate baffles, respectively, which depend from the roof of one embodiment of a 'mulile and which have lower contours substantially following the lines of the upper contour of the shields of the work-carrying tray; and

Figa/11 is an enlarged, sectional view of a modifled form of muiile and heating means. p

The furnace of my invention is of the so-called muiile type and comprises a muiile, indicated in its entirety by the reference numeral 2, of sumcient length to provide a plurality of different heat-treatment zones or sections therein, and projecting from both sides of the furnace proper 3. 'Ihe furnace propercomprlses the usual wall structure, such as refractory tile 4, backed. by heat-insulating rebrick 6` or the equivalent. The refractory tile and nrebrick are suitably assembled in the furnace to provide a central opening or heating space 8 in which the muille 2 is lo.

cated. The entire furnace proper is encased in a metallic shell I except, of course, for such openings as are necessary to accommodate the muille and pipes and other elements to be later described, all of which are secured gas-tight to the shell so that it is, in effect, completely lgastight. The furnace may be supported on any suitable under-structure, such as framework II,

and may have a removable top I2, bolted to the I8. As shown more particularly in Fig. 1, a backing ring 20, welded to the shell, reinforces the shell at points opposite the flange I6, and the parts are suitably apertured so that the bolt and nut means may extend-therethrough.

A second section 22 of the muffle, which is the cooling and discharging section, is also provided with a flange 24 mating the flange I6 of the central tube of the muffle. By means of the bolt and nut devices I3, the flanges I6 and 24 are clamped to the shell and backing ring 20, with gaskets 26 and 28 properly interposed to assure a gas-tight joint.

'Ihe muflle also comprises a third section 30,

which is the charging chamber in which charges to b-e heat-treated are placed preliminary to their entrance into the furnace proper. This section also serves as a purging chamber for purging the undesirable gases which might nd their way into the mulile with the admission of a charge.

In this particular embodiment, the mufe is substantially rectangular in cross-section, and the central section I4 is preferably made of nickel for reasons aforesaid. As already stated, it has been found that in processes that require oarburizing atmospheres for enveloping thecharge, such as, for example, the hardening of high-speed tool steels, a nickel muie does not absorb carbon, and consequently does not embrittle, nor is its melting point lowered, so that its life is considerably longer than muliles which might contain chromium'or iron or both. For an all-purpose muiile, I have found that a central section of commercially-pure nickel has exceptionally long life, and does not affect the heating process.

The outer or cooling chamber 22 of the muiile is exposed to the outside atmosphere and insulated from the heat of the furnace proper so that a cheaper metal such as hot-rolled steel may be employed in the fabrication of this part of the muille. The same'holds true for the third section 30 of the muflle which is welded, as indicated at 32, to the nickel muille. The three sections together comprise the mullle 2.

In one very satisfactory embodiment of my invention, I have used a mullie having a base width of approximately lOl/2 inches, and a height of approximately 61/2 inches with the central heat- Y 3 ing section having a-length of approximately 65 inches and-the outer sections having lengths' in y the neighborhood of 32 inches.

As the heating means for the furnace, I prolvide anon-metallic refractory tube orshell 34 slightly larger than the central tube I4 of the muille, and also disposed in the central opening 8 of the furnace. This shell 34 is preferably formed of alumina with a suitable clay binder, and is employed because of its availability at low prices, its low coefficient of thermal expan-A sion, and its highly refractory character. Additionally, it is a relatively good conductor of heat while being an electrical insulator, and is chemically inert tov electrical high temperature resistors.

The shell 34 is relatively thin and has wound about it electrical resistor ribbons in helical form to which energy may be supplied for raising the temperature of the furnace.

In the embodiment shown in Fig. l, the heatv ing resistors are divided into two zones arranged so that more energycan be supplied at one zone .than the other zone. By this construction, I arrange within the central mullle section I4 two heat zones, one of which may be designated as the preheat zone, which is employed to bring the, charges up slowly to a temperature of about 1300 F., and the other as the high heat zone, in which the charges are soaked at higher temperatures. The preheat zone comprises the re-v sistor unit 3B wound on the alumina shell 34 at one side of the section I4, and the high heat zone comprises the resistors 38 vsimilarly wound on the shell 34 at the other side of the section. Suitable gas-tight terminals 40 for each of the resistors convey their ends outside of the furnace for connection to an electrical-source of power, preferably through control means under the control of temperature-responsive means 42 for the preheat zone, and 44 for the high heat zone. The temperature-responsive means may include thermocouples and suitably control, through wellknown apparatus, the application of the power to the different resistors in the furnace so that its temperature may be regulated as desired.

The muiile structure is completed'by suitable appurtenances which are preferably removably mounted thereon. 'Ihese appurtenances include a door 4B separating the mulile into a charging and purging chamber comprising the section 3u and a central heating chamber comprising the section I4, and. a door 4B separating the heating chamber of the muliie from the cooling chamber. Additional doors 5u and 52 close the ends of the charging and cooling chambers, respectively. The cooling chamber is further provided with a water jacket 54 having a water inlet 56 and a water outlet 58 so that the' rate of cooling of any charge disposed in the cooling section 22 may l be suitably controlled.

The doors 46 and 48 are preferably top-hinged inside the mu'ie, the shafts upon which they rotate extending through one side of the muie through-gas-tight glands, and can be manually operated by levers 6I).

The exterior door structures and operating mechanism therefor are more particularly shown in Figs. 3 and 4, and each comprises a-bar 62 se- The bars 64 andt are spaced somewhat from the exterior sides of the muifle, and in each of these spaces is disposed a tension spring l2 having one end hooked to the bar 62 and the other end hooked to the door. Aldoor frame or jamb. having a suitable opening iltting the interior of the mule is secured to the muilie and includes a at upwardly-projecting portion it: As shown in Fig. l, the bars 6i and 6d extend somewhat downwardly from their pivot point to about the center of the door, so lthat a toggle arrangement is provided for the door. In its lower closing position, the springs maintain the door against the door frame, while in the upper portion the tension of the springs I2maintains it `by friction in raised position, although a latch may be added if desired. A door rest 18 at the bottom of the door frame determines the disposition of the door in closing position and supports it.

.phere of dissociated ammonia which has been dried to at least-a dew point in the neighborhood of 35 F. or even lower; and for high-speed `tool steels a small amount of carburizing gas may be added, in which case the dissociated ammonia need not be dried so thoroughly.

Since it is also desirable to envelope with a suitable protective gas the nickel muille of the heating chamber, a protective gas is fed into the furnacev proper through a gas inlet 86 and discharged from a gas outlet pipe 88.v Inasmuch asl ordinary rebrick and refractory arel porous to some extent, it is manifest that the gas will .permeate through these and ow into the central opening 8 of the furnace enveloping the muiile and resistor ribbons, finally flowing out through the gas outlet pipe 88. It should, of course, be understood that the expansion joints customarily provided in the assembly of the refractory and insulating lining of `the furnace further facilitate the flow of protective atmosphere there,-

through.

It may be observed that the gas outlet pipe 88 is bent downwardly to have its discharge end somewhat below the :bottom of the furnace |proper, and by igniting the out-flowing gas the flame is an indication of the satisfactory operation of the gas supply means. In this connection, a gas outlet pipe may also be provided for the muille.

In the operation of the furnace just described, a charge is iirst placed in the charging section 30 of the mulile where it remains until-the air is flushed out by the iiow of the pure dissociated ammonia. After this chamber has been thoroughly purged and flushed, the inner door 4B between it and the heating chamber may be opened and the work moved into the preheat zone without the admission of air. through the medium of a rod inserted through suitably-located openings 90 in the end doors The work is pushed mosphere through small openings and crevices rather than. vice versa. It is only when the outer doors are open that any appreciable amount of air can nnd its way into the mniile. In such case, the intermediate doors iii and it aid in Ipreventing air flowing into the heating zones.

After the charge has been treated by'preheating in the preheat zone, it may be pushed into the high heat zone where it is permitted to soak for a required length of time, and then it is drawn into the cooling section 22 of the mule for cooling treatment. Y

In order to convey ythe charge through the muille and at the same time further increase the efliciency of the furnace, i provide a special tray having many advantages for the particular use to which my furnace is put. One of these trays is shown more Iparticularly in Figs. 5, 6, 7, and 8, and comprises three relatively narrow longitudinal skid bars 92, 9.4, and 96 having a length slightly less than that of the helical resistor 36 and the helical resistor 3d, the lengths of both of the latter being substantially equal. Across the ends of the bars 82 and 99, and on top surfaces thereof, are Welded two transverse end bars 98 and imi, so that a rigid skeleton lunder structure is provided for the tray. By securing the end bars on top of the longitudinal bars, the tray can be readily slid along the bottom or the muflie, and additionally the raised end bars permit a pusher rod to readily engage them on one side, or a puller hook to engage an opposite side for pulling, rather than for pushing, the tray. In operation, a charge-carrying member y or means in the form of a wire mesh screen or v netting or even a `wire basket may be placed on the top of the skid bars and the particular articles being heat-treated suitably arranged on this mesh to be carried along with the tray.

The tray is completed by a plurality of shields at each of its ends. To secure these shields to the under-structure, small upright bars or angles |02, |04, and |06 are welded to the longitudinal bars 92, 94 :and 96, respectively, somewhat inward of the end bars 98 and |00. Bolts |08 are suitably secured in these upright bars, extending inwardly of the tray, and, suitablyapertured shields ||4 fit over the bolts. oi' these shielding plates preferably comprises a thinnickel sheet and four are used in this embodiment at each end of the tray, maintained in spaced relation by spacers H6 on the bolts |08,

and held in position by any suitable means, such as cotter pins and washers. It is to be understood, of course, that the dimensions of the spacers H6 land the thicknesses of the shielding plates ||4 are chosen so that the shields are suitably spaced and will be maintained in spaced relation during the heat-treating process. I have found that one-quarter to one-half inch spacing between the individual shields is quite satisfactory.

The shields are further provided with small aligned windows |20 in the event that it is desired to observe the work during the heat-treating process, a closed window |22 being provided The tray shields are preferably so di-mensioned as to occupy the tull available cross-sectional area within the muiiie. However, in this particular embodiment, I prefer to reinforce the mule Each rectly into the heating Section of -the muiiie. In

section in the heatingl zones by means of upper .bulles-secured at difIerent intervals along the top of -the 'mule and to the upper parts of its sides. In this instance, the end baffles |24 are formed solid, while the intermediate baiiies |26 are formed with a plurality of apertures so that l the heat will be better distributed through the different heating sources, although very satisrac-tory heat distribution obtains with sono intermediate baflles. The bellies are preferably nickel and occupy but a small fraction of the space within the muiiie. The bottom contours are curved for strength, and to increase the useable heating space in the furnace. It follows, therefore, that the upper ends of the shields are also curved in a line similar to the curvature of the lower periphery of the bailles so that the tray may be moved in and out of the muiiie without interference. Manifestly, the shields and baffles will cooperate to form a confined heating zone between the. ends of the tray.u y

To facilitate the repair of the heating means and to permit expansion and contraction of the muiile which is anchored to the shell by the flange I6, a gas-tight stuffing box is provided at the end of the muiiie away from the flanges I6 and 24, this closure comprising an angle frame welded to the shell, as' shown, and a second angle frame |30 bolted to a horizontal leg |32 of the angle frame |28. The leg |32 is spaced from the munie and an asbestos backing |34 may be compressed by. a movable plate frame |36 moved by means of bolts |38 threaded in suitable apertures in a depending leg |40 of the angle frame |30 in an obvious manner.

In the assembly of the heating means, the end closure just described is to be assumed completely loose so that the' shell 34 may be slid into position.. until it abuts a suitable refractory brick |42 at the other end of the furnace proper. A second brick |44 may then be placed against the end of the shell 34 so that the full length of the furnace is substantially occupied by the two bricks |42 4and |44, and the shell 34. This structure not only positions the shell 34 and the heating means wound on it, but, by tightening the packing after the insertion of the shell, a gas-tight seal is obtained.

'It may thus be observed that I have provided a furnace having a charging chamber outside of the furnace proper and two heating sections which may be independently controlled Within the event' the end doors and frames nt quite tightly and do not permit a slow continuous flow of atmosphere between the crevices of their joints, adjustable vents may be provided in the ends of the muflle and the out-'flowing gas may be burned in a ame. The inner doors iit loose enough to allow the free iiow of gas between sections of the muiile, but prevent air getting into the heating' section of the muiiie during 4the short period when the end doors are opened, the air losses out of the end of the muiiie. Without the shields, which act as a heat-barrier or heat-cone iining means, the heating source would have to be considerably longer to get the same length of uniform temperature within the charge-treating zone and, consequently, the furnace would have to be longer. T

At extremely high temperatures, there may be a tendency for the trays to weld to the iloor of the muiile due to the'clean surfaces. Where such sticking might occur, and also to make the trays slide more freely, a thin coating of refrac-f tory sand, such as zircon sand, may be scattered on the muiile oor.

' In Fig. l1, I show a second form of muiile and heating means combined. In this case, the

' muiiie, rather than being squared in cross-section,

tective atmosphere into the outer shell of the furnace proper, the heating ribbons and the outside surface of the heating section of the muliie are protected from oxidation, and, consequently, I may use electric resistors capable of attaining higher temperatures than 'the customary alloys which must .be made resistant against oxidation. Thus, the resistors might be nickel or even molybdenum.

The protective gas is fed into the muiile at three places, although with sufficient pressure one only would be sufficient. In such case, it would be preferred to have the gas inlet admit the gas dias is the muftle of Fig. 2, follows the contour of that of the tray so that baffles need not be addedto the muffle, the arch curvature of the mulile serving as a strengthening means at high temperatures. 'I'he mufe of Fig. 1l is further different from the prior described device in that the heating means is made unitary therewith by first applying a layer of electrically-insulating refractory `cement outside of the muffle and then drying it.

The heating ribbon may then be wound directly on the resulting tube, and thereafter a second layer of the insulating' refractory cement may be applied so that the resistor is fully embedded in the unit. Manifestly, the tray of Figs. 5 to 8 can. be used with a mufile such as shown in Fig. 11.

While I have shown my invention -in a form which I now believe to be very suitable to my purposes and objects, it is obvious that many modifications may be made and other equivalents employed, especially in themanner by which the furnace is heated, and in a copending application Serial No. 296,587, filed currently herewith, entitled Heat-treating furnaces, and assigned to Westinghouse Electric & Manufacturing Company, I described another preferred form of my invention. In that application I also describe in greater detail the methods of operation, accomplishments and vresults that I can obtain in that embodiment. These can be duplicated in. this furnace, the essential differences being that in this furnace a separate preheat zone is-provided, and in this furnace continuous operation the heat-treatment, at elevated temperatures, and hardening of the higher grades of tool steels and the like while enveloped by a protective atmosphere, comprising a plurality of refractory and insulating Walls dening a heating space, a

metallic munie-section in said heating space, a metallic shell for gas tightly encasing said walls, said shell having an opening through which an end of said munie-section protrudes, said end having an outwardly-extending i peripheral ange, a second munie-section aligned with the first said munie-section, said second munie-section having a peripheral flange, and means for gas tightly securing said flanges together and to said shell, the lrstsaid munie-section being passable through said opening. Y

2. A bright-iinishing metal-treating electric furnace adapted for substantially damage-free.

.hardening heat-treatment in a controlled atmosphere of the higher grades of tool steels and the like, said furnace comprising non-metallic refractory walls defining a heating space, an elongated tube-like metallic muiiie including a rst munie-portion in said heating space and a cooling munie-portion, heating means in said heating `space outside of said first munie-portion for providing a heated zone therein for heating a charge, cooling means for said cooling muilie-portion for providing a cooling zone therein for cooling a charge, a metallic charge tray means movable in said mufile, each of said mufile-portions being of a length to accommodate said tray means, said tray means comprising a bottom for supporting articles to be heat-treated, a pair of metallic heat-conning means, one at each end of said charge-supporting bottom, each of said 3. A 4bright-hardening metal-treating muillefurnace of the type described, said furnace comprising an elongated metallic-refractory muille having means whereby the inside of the muilie may be permeated with a flowing controlled atmosphere, heating means for heating a first portion of said muilie for providing a heating zone, said flrst portion of said muie being in open communication with a second portion of said muifle, means for providing a treating zone in said second portion of said mufile, whichpis of `lower temperature characteristics than said heating zone, a metallic charge-tray mov-able inside said muiiie into said heating zone and said treating zone, said charge-tray comprising an article-carrying means, metal heat-coniining means carried by said charge-tray at each end of said article-carrying means for thermally delimiting, in conjunction with said muilie, the ends of a charge-treating zone, said heat-confining means being conducive touniform temperatures along said article-carrying means, and permitting said controlled atmosphere to flow between said heating zone and said treating zone.

4. A bright-nishing metal-treating furnace of the type described adapted for the hardening heat-treatment in a controlled atmosphere of the higher grades of tool steels andl the like, said furnace comprising an elongated tube-like metallic muiiie having means whereby its tube-like interior may be permeated with' a flowing controlled atmosphere, said muiile comprising two muiiie-portions in open communication and in -contiguous relation, means for aiecting the temperature in one of said muie-portions to provide a treating zone therein,` means for heating the other of said munie-portions to provide a heated zone therein of higher temperature characteristics than said treating zone, said heated zone and said treating zone being also in open communication, means disposable in said muiie between said heated zone and said treating zone for thermally separating the two zones, said thermal separating means comprising a plurality of adjacent relatively closely-spaced sheet-like metallic members loosely, but relatively closely, tting the interior of said muille, wherebycontrolled atmosphere may flow between edges of said plurality of sheet-like members and the munie, and closing means for the ends of said Inutile. y

5. A bright-finishingI metal-treating furnace adapted for substantially damage-free hardening heat-treatment in a controlled atmosphere of the higher grades of tool steels and the like, said furnace comprising non-metallic refractory and insulating walls deiining a heating space, an elongated tube-like metallic mufiie having means whereby the inside of the muiile may be permeated with a flowing controlled atmosphere, said muflie including a first munie-portion in said heating'space, and a second muiile-portion extending outwardly from said walls, heating means for said first munie-portion, cooling means for said second munie-portion, an articlesupporting tray means axially movable in said muille, each of said munie-portions having metaltreating zones of a length to accommodate said tray means, said tray means comprising a gaspervious article-supporting member and lower f supporting means for said article-supporting member, said supporting means being provided with spaces through which controlled atmosphere may flow to the lower side of said article-sup'- porting member, said tray means further comprising heat-confining means comprising a plurality of relatively closely-spaced upwardly-dil rected sheet-like metal members carried by said.

supporting members above the bottom of said muiile, said plurality of sheet-like metal members closely, but loosely, fitting the interior of said muiiie to permit the flow of controlled atmosphere between the munie and the edges of said plurality of sheet-like metal members.

6. As an articlev of manufacture, a heat-resistant tray for conveying articles to be heat-treated in a controlled atmosphere, comprising a gaspervious metallic article-carrying means, and a pair of spaced metallic heat-confining means carried by said article-carrying means, the spacing between said heat-confining means providing an article-receiving portion on said article-carrying means, each of said heat-conning means comprising a plurality of relatively closely-spaced adjacent metallic sheet-like members, said members extending for substantially the width of the said article-carrying means, and being upwardly disposed with respect to a horizontal position of said article-carrying means.

JoHN a. GIER, Ja.

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