US2359157A - Gas-carburizing furnace - Google Patents

Description

Mannini,

f UNiTED STATES PATENT O'FHCE l l I Gas-carismi; roam-ica f l t winne auch, wimmbm, n., ...im .tu wang' inghouse Electric Manufacturing Comnnu. Ent

'anla Pittsbllrlh EL, a 1m'poratlor1j of Application May 22, 1943, Serial No. 488,047

l vl Claims. This invention relates to electrically heated industrial furnaces: oi' a type provided with a heat-treatment chamber in which processes are carried out involving the heat-treatment. of

charges, -usually metallic, while such charges are enveloped by a' carbonaceous gas-atmosphere Vwhich permeates the heat-treatment chamber.

Heat-treating processes of this kind are s known, and generally involve a carbonaceous gas-atmosphere which reacts with a charge in Y, order to change its physical or chemical characteristics.

heated to temperatures more commonly up to about 1700 F.; but in the use of my invention the temperatures also may be substantially higher. In general, the relation between the carbon pressures of the gas-atmosphere and the charge depend on the manner in which the charge is to be heat-treated. However, such atmospheres, and especially gas-atmospheres having relatively high carbon content for gas-carburizing, tend to deposit free carbon on highly heated parts, perhaps by dissociation of the carbon-containing gas-constituents, such as hyy One such process is case-hardeur. ing in which the charge to be case-hardened is charge, so

temperature gradients along i its length because of the heating-up or the cooling-down of the that the mufiie may quickly deterio rate. i

My invention has for its primary object the provision of an electrically heated gas-carburizing' furnace lacking the aforesaid and other difdrocarbons and oirides of carbon. Such deposits may be relatively heavy,.reaching thicknesses of several inchesA or more after only relatively short operation of the industrial furnace. This characteristic of carbon-deposit has made it im` practicable to permit the gas-atmosphere toenvelope the electrical heating means of the furnace because the deposited carbon short-circuits;

or heat insulates the parts of the heating elements with' which it is in contact, resulting in dimculty in the control of the heat-treating process and sometimes in the destruction of the heating elements.

Accordingly, it has been a practice of the prior art to provide a gas-impervious charge-holding munie which constituted the heat-treatment chamber; the muiiie being permeated by the carbonaceous gas-atmosphere and the electrical heating elements being disposed outside .the

inuiiie.`

Such muille-furnaces have certain disadvantages. For one thing, the charge is indirectly heated by heat transmitted through the muiile, so that the electrical heating means must operate at a slightly higher temperature in order to compensate for heat-travel through the chargeholding muille. Another disadvantage is the cost of the muille and the cost of maintaining it in satisfactory operating condition. The muiile is fculties and disadvantages.

It is an object of my invention to provide an electrically heated refractory-wall muiileless industrial furnace in which suitable charges can be heat-treated to high temperatures in a carbonaceous gas-atmosphere. L Another object of my invention is to provide a mutileless gas-carburizing furnace which may be electrically heated without rapid deterioration of the electrical heating elements exposed to the gas-atmosphere.

Still another object of my invention i-s the provision of an electrically heated muiiieless gascarburizing furnace which will be capable of being operated continuously for long periods oi time, without interruptions for cleaning carbon from the electric heating elements,'or for the replacement or repair of such elements.

The above and other objects of my'invention`A will be discernible from the following description thereof, which is to be taken in conjunction with i 1 the accompanying drawing. In this drawing, the` various figures more or less diagrammatically show the essential parts of my invention on different scales.

Figure 1 is a view diagrammatically showing, in

transverse section, a furnace embodying my invention, but with some parts omitted for clarity.

Fig. 2 is a longitudinal sectional view for showing a detail'of a part of the furnace; and

Fig. 3 is asectional broken view showing, in greater detail, a heating element and associated furnace structure involved in a preferred embodiment of my invention.

The furnace' I comprises, as is customary in furnaces of this kind, side walls 2 and 3, a bottom wall 4, -top wall 5, and a pair of end walls one or both of which may be provided with a suitable door-closed opening through which a charge may beinserted into or withdrawn from a heat-treatment chamber 6 which isdefined by the walls. The walls may be constructed in any customary manner as,l for example, by heat-insulating wool and brick and non-metallic refractory brick bounding and deilning the chamber 8. 'I'he furnace is provided with a gas-impervious sheet metal shell 1 which encases the outside of subjected to repeated temperature shocks or the walls and doors, and which is provided with l 2 A Y. time suitable expansion and contraction Within the heat-treatment chamber), a pluralityoi' relatively small, spaced brick refractory tiers I support a charge-receiving hearth I comprising a plurality oi spaced rails Il onA which the charge is moved into and fromlthe heattreatment chamber. A gas inlet pipe II passes through the shell l and top wall l for introducing a suitable carbonaceous gas into theJ heattreatment chamber C from a gas-generator I2 which supplies such carbonaceous gas.

For heating the heat-treatment chamber 8 and "i the charge therein, heating means in the formo! a plurality of spaced heating-units I2 are provided along the top and bottom oi' the heat-treatment chamber. the furnace walls and outer shell beingrprovided with suitable holes for accommodating the units. Each such unit comprises a heater-rod I4. So called "Glo-bar rods comprising essentially silicon-carbide, or a suitable metal orany other appropriate material may be used for such heater-rods These rods Il extend across the width of the heat-treatment chamber 8, and partially into spaces in the side walls .2

and 3; and, in order to conduct electricity to them, each of their ends contacts electricity conducting terminfalrods I5. Terminals I8 of any suitable form may be provided for pressing the terminal rods against the associated heater-rod Il so that the last is eXpansively held in slight compression, the extent oi which may be varied. Conductors I1, connected to the terminals IB, convey electrical power to and from the heatingunits. In each heating-unit I3, the terminal rods IB and the heater-rod. I 4 are preferably solid; and electrically insulating sleeves I8 surround terminal rods I and' the end portions of the heater-rod Il, the holes in such sleeves being slightly larger than the rods to permit sliding movement of the heater-rod Il and terminal rods I5. 'I'he inner edges I9 of the sleeves Il terminate somewhat inside the side walls 2 'and 3, and the outer ends of the sleeves project outside the i'urnace.l i

Protective means are provided to prevent contact o1 the gas-atmosphere in the heat-treatment chamber 8 with the heater-rods and terminal rods. To this end, each heating-unit I3 comprises a hollow tube 20 which encompasses the heater and terminal rods and the insulating sleeves I8 of the unit. The internal diameter of the tube 20 is considerably greater than the diameter of the rods I4v and I5. One end of this tube 20, which may be o1' metal, has an annular ilange 2| gas-tightly welded thereto; the ange 2l, in turn, being gas-tightly secured to' the shell I in any suitabe manner. The outer end of the tube is slidably, .but gas-tightly, held in position with respect to the shell 1 by a packing gland 22 comprising two apertured plates 23 and 24 forming an annular cut-away V-groove which holds a heat-resistant compressible packing 26 against the tube 20 in such manner that tightenlng oi' the nuts 21 on bolts secured to and passing through the shell 1, forces the packing firmly against the outside of the far end 28 of the tube 20. A gasket 29 may be provided between the shell 'l and plate 24 for gas-tightly sealing the hole in the shell 1 through which the associated end of the heating-unit II 3 passes.

In operation, carbonaceous gas is admitted through one or more gas inlet pipes Il into the heat-treatment chamber 8, preferably at a pressure slightly above atmospheric pressure so that a plenum systeml results where the gas-atmosphere tends t0 leak out oi' the chamber 0 through any crevices in the shell or separate gasvent means 30 may be provided to encourage spent-gas leakage. 'I'he gas permeates the heattreatxnent chamber. lbeing in direct contact with the walls 2. l, l, l, the end walls oi' the furnace and the cylindrical outside of the tubes 20. and enveloping any charge on the hearth 9. By applyingelectrical energy to the conductors I1 oi' each of the heating-units Il, the heater-rods Il heat up, and heat is transmitted acrossthe emptyy space 3| between the heater-rods Il and the outer encasing tubes 20. 'Ihe tubes 20, in turn,

become hot and heat the chamber 6.

Carbonaceous gas around the 4hot tubes 20 breaks down and carbon accumulates on each oi' the tubes 20. Since these tubes 20 are electrically insulated from the power-carrying parts of the heating-units, by means of the space II and insulating sleeves I8, such deposited carbon will have no detrimental eilect on the electrical conducting properties oi.' the electrical heating-units.

However, in time, the carbon deposited may adversely ailect the heat-transmission of the heat from theftube 20 to the chamber B, so that my invention further contemplates a carbon-removing means for keeping the tubes' 20 fairly clean.

Such a carbon-removing means may comprise a scraper plate 32 having a somewhat semi-circular opening 33 therein which is somewhat larger than the tube 20 sothat the plate rests loosely on the associated tube 2l, and can be moved without binding along the length of the'tube for scraping deposited carbon therefrom. A pair of rods 2l, secured to the plate 32, pass through guiding holes the sidewall 2 and shell 1, being of suilicient length, outside the furnace, to permit reciprocating the scraper plate the'iull widthof the heattreatment chamber i. Packing and packing glands may be used to limit heat and gas losses through the holes in the walls and shell through which the diilerent pairs of rods 34 pass.- I have found that, generally, the carbon deposit accumulates on the top oi' the tube 20 so that it is unnecessary to provide a scraping plate which completely surrounds the tube. f

The securing oi' one end only of the tube 2l flxedly to the shell 1 permits the tube to expand and contract with temperature chages'by the sliding of the other end oi' the tube in the gland 22. Any suitable metallic or non-metallic gasimpervious material having satisfactory` heat.- resistant qualities may be used-for the tube 20, such as a nickel-chromium-iron alloy or other heat-resistant composition. i

It is obvious that by utilizing my invention charges may be gas-carburized without the need oi' a muille which may be an expensive item, especially for a large furnace such as one actually embodying my invention, which had a heattreatment chamber with a length oi' ten feet, a width of about two and one-hall' feet. and a height oi' about three .and one-halt i'eet, and required about twenty spaced heating-units.

Any suitable means may be provided for maintaining the insulating sleeves loosely in position;l

.treating process.

vparts during heating and cooling steps' of a heatof my invention.

I claim as my invention:

1. A muleless industrial heat-treating furnace comprising: non-metallic refractory walls delining a heat-treatment chamber; charge-supporting means in said chamber for supporting a charge in said chamber in exposed relation to said walls; means comprising gas-inlet-means for admitting a carbonaceous gas-atmosphere to said chamber and causing the chamber to be permeated by said gas-atmosphere; electrical heating means comprising electrical heatingportions for heating said chamber; protective means for said heating means, comprising casing means exposed in said chamber for encasing said electrical heating-portions for protecting said electrical heating-portions from direct contact with carbonaceous gas-atmosphere permeating said chamber, said casing means being electrically insulated from said electrical heating-portions.

2. A mufileless industrial heat-treating furnace comprising: non-metallic refractory walls deiining a heat-treatment chamber; an outer substantially gas-impervious shell for said walls, charge-supporting means in said chamber for supporting a charge in said chamber in exposed relation to said walls, a plurality of rod-like heating elements for heating said-chamber; a plurality of hollow tube means, each associated with and around a said heating element, said tube means being exposed in said chamber; means comprising gas-inlet means for admitting a carbonaceous gas-atmosphere to said chamber for enveloping, at least to some extent, the exposed charge and said tube means; and means for said tube means for removing deposited carbon therefrom. 3. In a device of a class described, in combination, means defining a heat-treatment chamber, tube means comprising a heat-transmitting hollow gas-impervious tube-portion passing through said chamber, means for permeating Phel'e.

5. A mullieless industrial heat-treating furnace comprising: non-metallic refractory walls defining a heat-treatment chamber; an outer substantially gas-impervious shell for said walls; chargesupporting means in said chamber for supporting a charge in said chamber in exposed relation to said walls; a gas-impervious hollow tube passing through said chamber; means for movably holding said tube in gas-tight relation to said shell; means comprising gas-inlet means for admitting a carbonaceous gas-atmosphere to said chamber for enveloping the charge therein; said hollow tube being exposed to said gas-atmosphere; and electrical heating means comprising a heating conductor inside said tube in electrically insulated heat-transmitting relation thereto.

6. A muiileless industrial heat-treating furnace comprising: non-metallic refractory walls defining a heat-treatment chamber; an outer substantially gas-impervious shell for said walls; charge-supporting means in said chamber for supporting a charge in said chamber in exposed relation to said walls; heating means for electrically heating said chamber comprising a gas-impervious hollow tube passing through said chamber, means for holding said tube in gas-tight relation to said shell, said holding means-being such as to permit relative displacement of an end of said tube whereby to permit expansion and contraction of said tube with changes in tem-- perature.

7.- The furnace of claim 5 further comprising cleaning means operable for cleaning carbon deposited on said tube by said gas-atmosphere, said cleaning vmeans comprising a carbon-removing device inside said furnace, which is relatively movable with respect to the outside of said tube, said cleaning means comprising operating means therefor outside said shell.

8. The furnace of claim 6 further comprising cleaning means operable for cleaning carbon deposited on said tube by said gas-atmosphere, said cleaning means comprising a carbon-removing device inside said furnace, which is relatively movable with respect to the outside of said tube, said cleaning means comprising operating means therefor outside said shell.

9. IIhe furnace of claim 5 further comprising a scraper loosely about the top of said tube, said scraper being movable along said tube, and means comprising a rod passing through a guiding hole in oneof said Walls and said shell for moving 'said scraper back and forth along said tube.

10. The furnace of claim 6 further comprising a scraper loosely about the top of said tube, said scraper being movable along said tube, and means 4comprising a rod passing through a guiding hole in one of said walls and said shell for moving said scraper back and forth along said tube.

WILLARD ROTH.

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