US1999757A - Method of producing diffusion alloy cases - Google Patents

Description

April 30, 1935. J. w. HARSCH 1,999,757

METHOD OF PRODUCING DIFFUSION ALLOY CASES Filed Jan. 50, 1929 2 Sheets-Sheet 1 4-3 Jo l April 30,1935. J. w. HARSCH 1,999,757

METHOD OF PRQDUCING DIFFUSION ALLOY CASES Filed Jan. 30, 1929 '2 Sheets-Sheet 2 Patented Apr. 30, 1935 METHOD or PRODUCING DIFFUSION ALLOY CASES John W. Hal-sch, Ambler, Pa., assignor to Leeds &

Northrup Company, Philadelphia, Pa., a corporation of- Pennsylvania Application January 30,

16 Claims.

My invention relates to a method of treating one or more metal objects, including machine parts, to produce a case thereon; and more particularly to a method of case-formation involving concurrent heat transfer and chemical treatment by the forcible convection of a fluid, as a gas.

In accordance with my invention, there is formed on oneor more metal objects an improved diffusion alloy case, as a nitrided or carburized case, having uniform surfacecharacteristics and uniform gradient of characteristics in the subsurface, by substantially uniformly concentrating throughout the surface of the metal the reactive or case-forming component of a heated gaseous fluid medium by forcibly stirring it in contact with the heated metal, and concurrently by the forcible stirring of the fluid medium maintaining the metal throughout its said surface at substantially uniform case-forming reaction temperature. Besides producing the aforesaid characteristics, which in diffusion alloy cases are of great commercial and practical importance, my method is characterized by increase in the rate of the caseforming reaction, by material reduction in the duration and cost of treatment, and by regularly and repeatedly procuring the same uniform results.

Further and more particularly in accordance with my invention, a reactive gas or vapor, as ammonia for nitriding, is circulated into heat transfer relation with a source of heat where it is dissociated to a greater or less extent and utilized as a heat vehicle; and more particularly the aforesaid gas or vapor in its wholly or partially dissociated state is subsequently passed into direct contact with the metal for case-forming reaction therewith concurrent with absorption of heat by said metal from the reacting gas or vapor.

My invention resides in the methods of a caseformation hereinafter described and claimed.

In case-forming processes involving both application of heat and a chemical agent for effecting desired changes, either chemical or physical, or

I, both, in the properties of the metal, it has pre- 1929, Serial No. 336,0 5 (01. 148-16) phere be non-uniform, since two independently controlled factors are involved.

My invention comprises heating the metal and chemically treating the same by a single fluid medium or heat vehicle, as a gas, concurrently and in one process. The gas, which may be of whatever composition is required for the particular needs of the treatment, therefore serves a two-fold purpose in that it is the principal medium or vehicle of heat transfer to the metal for effecting heating or equalization of the reaction temperature thereof, and at the same time reacts chemically with the metal itself to produce desired properties. v

In practice, the chemically active gas is introduced under pressure into a heating chamber, where it is passed into heat-transfer relation with a source of heat, whereupon it is utilized as a heat vehicle and subsequently passed into heattransfer relation with metal to effect uniform chemical treatment thereof, concurrent with uniform heating of the metal to reaction temperature by absorption of heat from the gas. Accordingly, it is possible to insure uniformity of both the heat and chemical treatments, since the gaseous heat vehicle in flowing around the metal must necessarily chemically react therewith at the same time as the gas gives up part of its heat to the metal. Uniformity of treatment due to the fact that the chemically active gas is also the heating gas, is furthermore insured by the fact that the temperature of the metal is a determining factor in the reaction between the gas and metal. In other words, assuming that the metal is heated independently of the treating gas, the situation might arise wherein different portions of the batch of metal under treatment are at different temperatures, and further assuming uniform application of the treating gas, it is apparent that, notwithstanding the uniform application, the chemical treatment'of the metal will be non-uniform as well as the heat treatment thereof. The method of obtaining uniformity of heat treatment by circulation of a heated gas into heat-transfer relation with a metal has been practiced heretofore, and in itself does not constitute a part of my invention. By utilizing a chemically active gas,

. however, in lieu of the usual heating medium, uniform chemical treatment is assured due to the even temperature of the batch of metal, and furthermore due .to the fact that the heating gas is caused to flow evenly under substantially constant pressure throughout all parts of the batch.

For an illustration of apparatus capable of 55 carrying out my method, which is claimed in my divisional application Ser. No. 511,694, reference is had to the accompanying drawings, in which:

Fig. 1 is an elevational view, partly in section, of a furnace.

Fig. 2 is a plan sectional view of the apparatus illustrated in Fig. 1 taken along the line 22.

Fig. 3 is a detailed view of apparatus associated with the furnace.

Referring to Fig. 1, there is illustrated a furnace or heating structure comprising an outer cylindrical shell I having its lower portion closed by a bottom plate 2 suitably secured to the lower edge of shell l, as by bolts 3 to flange 4. To the bottom plate 2 is secured the furnace-supporting structure 5. The upper edge of shell I is united to a similar shell 6 having an inwardly turned flange 1 whose inner edge is secured and sealed, as by weld 8, to the upper edge of a shell 9, disposed concentrically within, and spaced from shell I by insulating material ID of suitable characteristics. Shell 9, which comprises the inner lining of the furnace, is open at its upper end, and is closed at its lower end by members and I2, members 9, H and I2 being secured to each other by the hermetically sealed welded joints l3. Member l2, comprising the bottom portion of the furnace chamber, is secured and spaced with respect to plate 2 by members M which are also welded to member 9 for purposes of sealing. Upright supports 5, comprising angular members, are mounted upon the bottom of the furnace chamber by means of studs Ma, and support a plurality of heating elements, as electrical resistors l6, by means of annular supporting bands Ilia: and insulators IGy. Mounted also upon supports I 5 is a cylindrical shell-like member open at its top and bottompand Within which a work container I8 is concentrically disposed. The work container l8 comprises a cylindrical shell-like wall having an open grille work or spider l9 secured to the lower edge thereof, and an annular supporting flange 20 secured to the upper edge thereof adapted to seat upon the upper edge of cylinder l1, and thereby closing the intervening space from circulation of gases. Supporting flange 2|] is provided with eyes or handles 2| for facilitating withdrawal of the work container from the furnace.

Suspended beneath the furnace by means of structure 22 and through-bolts 22a is a motor M having the axis of its rotor shaft disposed in a vertical plane. Rotor shaft 23 extends through the bottom plates 2 and I2, and is substantially sealed with respect to the furnace walls by means of a packing gland 24 and a sleeve or collar 25 forming a part of said gland and welded to the bottom member l2 at 26. Shaft 23 terminates beyond a conical flow-deflecting member 21, and has secured to the end thereof a fan or impeller 28.

Resistors l6 are connected to an external source of electro-motive forceby means of lead members Mia and ||ib suitably mounted and protected within conduit or housing member |6c extending through the wall of the furnace. The inner end of conduit IE0 is welded at |6d to the co-acting edge of shell 9, and is provided with an electrical insulating bushing 29. Conduit I60 has mounted at its outer end a member 30 closed by a plug 3| at its lower end and supporting a sleeve-like member 32 for supporting the lead conductor I612. The conducting members 16a and "lb are suitably spaced and insulated with respect to their supporting members, as by an electrical insulating bushing 33, asbestos packing 34 and an insulating cement 35 which also serves to insure sealing of the conduits from the exterior against gas leakage. An electrical insulating bushing 36 and a lead terminal 31 are disposed at the end of sleeve 32. Although but one outlet supply conduit has been described and illustrated, it will be noted, referring to Fig. 2, that a proper number of them are supported by and extend through the furnace wall, for supplying current to the resistors, depending upon the power characteristics desired.

Cover structure for the furnace comprises a cylindrical shell-like member 38 of somewhat larger diameter than member 6, having the dished plate- like members 39 and 40 suitably secured thereto, member 39 being welded at its outer periphery to member 38 for purposes of hermetically sealing the junction of these members. Members 39 and 40 are spaced from each other forming an enclosure for insulating mate-- rial of suitable characteristics, and have mounted therein a thermo-responsive device 4| comprising a supporting tube 4|a welded on its outer surface at 4|b to member 39 and sealed by suitable cement or equivalent at Me. The thermoresponsive device 4|, as a thermo-couple, and a fluid conduit 42, extend through the cover structure to communicate with the furnace interior. Conduit 42, welded at 42b to member 33, is connected to a flexible tube or conduit 43 having interposed therein a measuring device, as-a flow meter 44 and a flow controlling device, as a needle valve 45 for controlling the flow from a supply conduit 46.

The lower portion of shell 38 comprises a sleeve-like structure surrounding the outer wall of the furnace and co-acting with a liquid seal, as an oil seal 41 for hermetically sealing the furnace chamber with respect to the atmosphere. The seal 41 comprises an annular wall structure 48 having its lower portion sealed at 43a, as by welding, to the outer side of member 6, and forming therewith an annular receptacle for contain-.

ing a sealing liquid such as oil, for example. As illustrated in Fig. 1, the lower portion of shell 38 extends into the oil to a suitable depth, thereby efi'ectively sealing the interior of the furnace from the atmosphere. Suitable heat' resisting and resilient material, such as asbestos rope 49, is disposed along the upper edge of the furnace wall to provide a seat for the above described cover structure. The cover structure, including the thermo-responsive device 4| and the conduit 42, may be lifted as a unit from the furnace by hoisting means connected to the lifting hook or eye 50.

The furnace is provided with an outlet or exhaust conduit 5|, welded at 5|a to the furnace lining, having seated at its outer end a gasketed closure member or plug 52 adapted to be unseated to open the outer end of the conduit by a member 53, pivoted at 54, and connected to the closure member through a pivoted connection 55. The closure member 52 may be maintained in snug and sealing engagement with the end of conduit 5| by means of the screw clamp 56 pivotally mounted at 51.

Referring more particularly to Fig. 3, conduit 5| communicates with a conduit 58 terminating in a liquid seal-comprising a container 59, a sealing liquid 60 and an outlet or exhaust conduit 6|. During closure of member 52, exhaust from the furnace chamber may take place through conduit 5|, conduit 58 and the liquid seal above described, provided of course that sufficient pressure exists in the furnace chamber.

The material of the exposed parts of the in'- terior furnace structure, such as the lining 9 and shell H, which serves to shield the work within container 18 for direct radiant heat. from the resistors, and other structure subject to contact by the particular gaseous medium utilized in the furnace chamber, comprises a metal or alloy thereof resistant to corrosive action of the heat and gaseous medium, such as for example, high chromium iron, or chromium-nickel iron alloys when ammonia is utilized as the gaseous medium. The electrical resistors l6 which are also exposed to the action of the furnace gas or gases are also composed of a corrosive-resisting material, and by way of example a nickel-chrome alloy is used in an atmosphere of ammonia.

The liquid seal at the exterior of the furnace is substantially at room temperature and is not appreciably affected by the high temperatures existing within the furnace, thereby preventing rapid deterioration or evaporation of the sealing fluid.

' The operation of the system is as follows;

Assuming the furnace to be in condition for recharging, the cover structure is removed by suitable lifting or hoisting mechanism secured to member 50, after which work container I8 is lifted from the furnace by means of eyes 2|. A work container filled with material to be treated is subsequently lowered into the furnace to the position shown in Fig. 1, after which the cover structure is lowered into its sealing position. In order that the furnace chamber may be entirely free from any gas other than that used in the treating process, the furnace is flushed for an appreciable length of time by the treating gas. To this end, needle valve 45 is opened, permitting flow of the treating gas under pressure through conduits 46, 43 and 42 to the interior of the furnace. Motor M is thereupon energized to rotate fan 28, which circulates the treating gas, depending on the direction of rotation of the fan, between the inner work containing chamber and the outer annular heating chamber containing resistors I6. In this manner, the treating gas is caused to circulate throughout all parts of the furnace interior to effectively flush out all gases such as air or other oxygen-containing gases, and to effect discharge of the same through exhaust conduit 5| and the liquid seal 60. During the above described flushing process, resistors l6 are deenergized and generate no heat, so that the new batch of material to be treated remains comparatively cold and is not acted upon to any appreciable extent by the flushing treating gas. Since the treating gas is generally introduced into the interior of the furnace at comparatively low pressure, the pressure therein during normal operation will be somewhat above atmospheric, so that there is a continuous flow of gas through the exhaust conduit and seal, thereby insuring a continuous supply of fresh treating gas.

After the flushing process has beencompleted, resistors iii are energized from a source of power (not shown), and the furnace interior accordingly increases in temperature. Assuming now that fan 28 rotates in such direction that the furnace gas is caused to flow upwardly through the open grille or spider IQ of the work container I8, through the batch of work therein, the gas will return to fan 28 by way of the annular heating chamber, and flow around and contact with the highly heated resistors during the downward a furnace effects uniformity of heating of the work therein, and is fully described and claimed in my Patent 1,518,027, March 23, 1926.

After the furnace interior has been brought up to the desired temperature, it may be maintained substantially constant by well-known control means associated with the thermo-responsive'device 4|. During the treatment, the treating gas is caused to flow continuously through conduit 42 to replenish the gases which are exhausted during the treatment, and to force out of the furnace through the exhaust seal such exhausted gases. In general, the temperature of the work itself is a determining factor in governing the rate and/ or extent of reaction of the treating gas with the work. By utilizing the treating gas itself as the principal heat vehicle between the source and the work, the cylindrical shield I! together with container I8, both of which are un-perforated. effectively preventing appreciable transmission of radiant heat to the work, it is possible to effect gradual and uniform heating of the work while at the same time subjecting it to the even and uniform flow of the treating gas or gases, whereby the gas and the metal or material to be treated are concurrently in heat-transfer and chemicallyreacting relations.

Although no definite rate of circulation of the combined heat vehicle and treating gas is contemplated, the fan or impeller 28 should rotate at such speed that the circulating medium effectively removes or wipes off stagnant films on the surfaces of the work under treatment, in order that the rate of treatment may be materially increased, instead of being reduced by the existence of stagnant films which tend to prevent or retard the desired reaction between the treating gas and the work.

When the work within the furnace has been subjected to a predetermined duration and extent of treatment, the electrical resistors are deepergized and the fan motor shut off.

Before the removal of the furnace cover, however, the exhaust conduit 5| is opened to atmosphere so that removal of the cover structure will not create a vacuum within the furnace interior and so cause the liquid within seals 41 and 69 to be drawn into the furnace. To this end, scaling plug 52 is released by unloosening clampingv nut 56, after which lever 53 is rotated in a counter-clockwise direction to move plug 52 out of sealing engagement with the end of conduit 5!. Accordingly, the interior of the furnace is now directly in communication with atmosphere and hoisting of the cover structure cannot therefore create a vacuum within the furnace to break and disrupt the aforesaid liquid seals. It is essential that the sealing plug 52 be open only while the cover structure is being lifted from the furnace to permit removal of the work, since replacement of the cover would only tend to force excess air through the exhaust seal.

An example of the use of a chemically active gas for effecting concurrent heat and chemical treatment of a metal lies in the use of ammonia, part of which is dissociated into nitrogen and hydrogen for the nitriding of steel. As is well known in the art, a nitrided steel or suitable alloy thereof has valuable wear-resisting characteristies, the nitrided material having a very hard outer surface or shell highly resistant to wear or abrasion. An example of a practical use to which a nitrided steel may be put, lies in its application to shafts or equivalent members incorporated in high speed machinery, such as in automotive engines.

Before the nitriding treatment is actually started, ammonia gas or vapor is caused to circulate, in the manner previously described, by the fan for an appreciable length of time, generally about an hour, through the treating chamber and other parts of the furnace to the exterior through the exhaust seal, in order that air or any other undesirable gas, as an oxygen-containing gas for example, shall be completely forced out of the furnace. During this gas flushing period, the batch of metal to be treated is not acted upon by the ammonia, since the electrical resistors are deenergized. After the flushing period has been completed, the circuit through the resistors is closed, and the furnace interior accordingly increases in temperature. As the fresh ammonia gas is introduced under pressure into the upper portion of the furnace, it is caused to flow downwardly either through the treating chamber or the heating chamber, depending on the direction of rotation of the fan, and to circulate between the source of heat and metal. As the gas flows past the highly heated electrical resistors, it is dissociated to a greater or less extent by the heat absorbed from the resistors, and accordingly is adapted to react with the metal under treatment when the same. has been heated sufficiently by the heat-carrying gas to maintain the desired chemical action. As the ammonia gas, which comprises the heat vehicle, brings the metal up to the desired temperature, further dissociation of the ammonia occurs during its contact with the heated metal. However, since the source of heat, namely the resistors, is at a higher temperature than that of the metal under treatment, it will be apparent that dissociation of the ammonia occurs to a greater extent when it is passed into heat-transfer relation with the source, and so is in a chemically active state before coming into contact with the metal.

As an individual charge of ammonia would become exhausted within a comparatively short period of treatment, a continuous supply, controllable by the furnace operator, is admitted to the furnace as the weakened or exhausted gas is discharged from the furnace through the yielding exhaust seal to atmosphere, or to gas recovery apparatus.

My invention comprises numerous important commercial advantages, not the least important of which is the rapidity of treatment attained by the aforesaid method. By way of example, the actual time of an individual nitriding treatment has been reduced by my invention to approximately one-third of the time formerly required by other methods.

A carbonaceous gas may also be used in the above described manner as the principal me dium of heat transfer for the purpose of gas carburizing the metal to give also a greater degree of hardness thereto. The carbonaceous gas may obviously vary somewhat in composition, depending upon the degree of hardening required.

The forcible stirring of the gaseous atmosphere, having a case-forming reactive component, while in contact with the work ensures throughout the exposed surface thereof both that the concentration of the reactive component of the gas is maintained substantially uniform, and that a substantially uniform reaction temperature is maintained. As a result, the case produced upon the one or more objects under treatment is of substantially uniform surface concentration and of substantially uniform subsurface characteristics; more specifically in nitriding, the nitrid case is of substantially uniform surface hardness and the hardness gradient from the surface is substantially uniform throughout the work; and in carburizing, the concentration of carbon and the reaction temperature at and near the surface are substantially uniform throughout the surface of the work, and the case eventually resulting from those conditions is of substantially uniform surface hardness and of substantially uniform hardness gradient inwardly from the surface.

With all conditions maintained the same, excepting only omission of the forcible stirring of the fluid medium, the reaction products formed frequently differ from those formed when the fluid medium is forcibly stirred, and whatever the nature of the reaction products their occurrence in the surface and subsurface is of a character which fails to effect a diffusion alloy case of uniform surface characteristics and uniform gradient of subsurface characteristics.

It is also within the scope of my invention to utilize a gaseous mixture comprising two or more gases, or a vaporized liquid, as the principal medium of heat transfer to effect concurrently a plurality of treatments. It will therefore be understood that the term gas as used in the above specification and appended claims is not intended to be limited to a single gas, or vaporized liquid, but is intended to include a gaseous mixture of two or more gases in whatever proportions may be found desirable.

It will furthermore be understood that my method of utilizing a gas both as the principal medium of heat transfer to or temperature equalization of a metal, and to effect formation of a case thereon is not limited to the specific type of furnace herein disclosed, but is applicable to other types of furnaces wherein heating medium is forcibly circulated either in continuous or reverse directions, and wherein heat is transmitted to the heat-receiving substance by both radiation and convection. Furthermore, the heat may be developed by any suitable method, as by gas firing, electrical resistors or other means. In general, it is essential that the furnace interior be hermetically sealed with respect to the at mosphere in order that the treating gas, which may be irritating or obnoxious to the attendant, be entirely confined within the furnace, and that air may also be prevented from leaking into the furnace and producing undesirable results.

I am aware it has been suggested for carburizing to burn fuel to produce the furnace heat and by fan to pass the products of combustion over the work in contact therewith; such method is ineffective for the purposes of my invention because of the presence in the gas in contact with the work of a substantial proportion of carbon dioxide, resulting from the combustion, which is a decarburizing agent. I disclaim herefrom any method of carburizing in which the gaseous atmosphere in contact with the work to substantial extent comprises products of combustion.

As distinguished from such results as the effects of oxidizing or reducing gases, the case referred to herein and in the appended claims is a true case, a diffusion alloy case, comprising the metal, on which the case is formed, alloyed at and beneath the surface with compounds of the metal formed by reaction therewith of the case-forming reagent; and I disclaim hercfrom and from the appended claims all methods except those producing 01' forming diffusion alloy cases.

What I claim is:

1. The method of producing upon one or more definitely formed metal objects a diffusion alloy case of substantially uniform surface concentration and of substantially uniform sub-surface characteristics, which comprises forcibly stirring a hot gas, having a component of character to react with the metal to form the diffusion alloy case, in contact with the metal to effect substantially uniform concentration of said reactive component throughout the surface of the metal while utilizing said gas to maintain said surface throughout at substantially uniform reaction temperature.

2. The method of imparting to one or more definitely formed metal objects a nitrid case substantially uniform in surface hardness and of substantially uniform sub-surface characteristics,

' which comprises forcibly stirring at hot gas, containing a nitriding component, in contact with the metal to effect substantially uniform concentration of said nitriding component throughout the surface of the metal while utilizing said gas to maintain said surface throughout at substan- ,tially uniform nitriding temperature.

3. The method of imparting to one or more definitely formed metal objects a diffusion alloy case substantially uniform in surface carbon concentration and of substantially uniform sub-surface characteristics, which comprises forcibly stirring a hot gas, containing a carburizing component. in contact with the metal to effect substantially uniform concentration of said component throughout the surface of the metal while utilizing said gas to maintain said surface throughout at substantially uniform carburizing temperature.

4. The method of nitride hardening a metal article which comprises heating the article to a nitriding temperature by circulating a heated gas about it in a nitriding zone closed to the atmosphere and thereafter causing ammonia to pass in cyclic circulation between an activating zone and said nitriding zone, said ammonia being subjected in said activating zone to the dissociating action of a catalyzer and heated sufficiently to maintain a nitriding temperature in said nitriding zone.

5. The method of producing a diffusion alloy case upon one or more pieces of metal which comprises effecting generation of heat formaintaining the metal at case-forming reaction temperature, into one zone introducing a case-forming reagent produced independently of said generation of heat and effecting partial dissociation thereof, passing the partially dissociated agent into a second zone containing the metal, and forcibly stirring the treating agent in contact with the metal, to effect further dissociation of the treating agent, to remove film from the surface of the metal, to ensure uniform reaction between the metal and the dissociation product, and substantially to enhance the speed of reaction between the metal and the dissociation product.

6. The method of producing a diffusion alloy case upon one or more pieces of metal by carburization thereof, which comprises effecting generation of heat passing into a zone a treating agent produced independently of said generation of heat, having a carburizing eomponent,-in said zone effecting partial dissociation thereof, passing the partially dissociated agent into a second zone containing the metal, and forcibly stirring the treating agent in contact with the metal, to effect further dissociation of the treating agent to facilitate carburization of the metal, and to remove film from the surface of the metal.

7. The method of producing a diffusion alloy case upon one or more pieces of metal, which comprises heating the metal to case-forming reaction temperature in .a reaction zone closed to the atmosphere, causing gas having a case-forming component to pass in repeated cyclic circulation between an activating zone and said reaction zone, said gas being subjected to partial disso- 'ciation in said activating zone, and forcibly stiruniformity and speed of formation of the case which comprises effecting generation of heat, forcibly stirring in contact with the heated metal a gas. having a component of character to react with the metal to form the diffusion alloy case, to

effect substantially uniform conwntration of said reactive component throughout the surface of the metal and to effect convective transfer of heat between the gas and the metal to maintain its surface at substantially uniform reaction temperature, and independently, of said stirring supplying reagent, produced independently of said generation of heat, to replenish the case-forming component of said gas.

9. The method of producing a uniform diffusion alloy ease upon metal objects, which comprises raising the temperature of a fluid, having a component of character to react with the metal to form the diffusion alloy case, by convective absorption of heat generated independently of the supply of said fluid, and forcibly stirring the heated fluid into contact with the metal to effect substantially uniform concentration of said reactive component throughout the surface of the metal while effecting convective transfer of heat from the fluid to the metal to maintain its surface throughout at substantially uniform reaction temperature.

10. The method of forming a diffusion alloy case upon metal objects, which comprises effecting generation of heat for heating the objects to the temperature of dissociation of a treating fluid, having a component of character to react with the metal to form the diffusion alloy case, forcibly circulating the fluid into contact with said heated objects to effect substantially uniform concentration of said reactive component throughout the surface of the metal while utilizing the fluid to effect by convection equalization of temperature of the metal to maintain its surface throughout at substantially uniform reaction temperature, and supplying reagent, produced independently of said generation of heat, to replenish the caseforming component of said fluid.

11. In a system for producing a diffusion alloy case upon metal objects comprising a furnace 'having a work chamber containing the metal, the method which comprises forcibly stirring in said chamber a fluid, having a component of character to react with the metal to form thediifusion alloy case, concurrently to effect substantially uniform concentration of said reactive component throughout the surface of the metal, and interchange of heat between the fluid and metal, without interruption of said stirring of the fluid adding fresh fluid to and removing fluid from said chamber in quantities small as compared with the quantity of fluid in the furnace, and effecting generation of the heat, for maintaining the reaction temperature, independently of the supply of said fluid.

12. The method of producing on one or more pieces of metal a substantially uniform diffusion alloy case, which comprises forcibly stirring in contact with the metal a hot gaseous fluid, having a component of character to react with the metal to form the diffusion alloy case, to clear the surface of the metalfor facilitating the case-forming reaction, and to effect substantially uniform and enhanced concentration of said reactive component throughout the surface of the metal while utilizing said gaseous fluid to insure uniformity of reaction temperature throughout the surface of the metal and effecting generation of the heat, for maintaining the reaction temperature, independently of the supply of said fluid.

13. The method of producing upon one or more metal objects a diffusion alloy case of substantially uniform surface concentration and of substantially uniform subsurface characteristics, which comprises effecting generation of heat,

' forcibly stirring a hot gas, having a component of character to react with the metal to form the diffusion alloy case, in contact with the metal to effect substantially uniform concentration of said reactive component throughout the surface of the metal while utilizing said gas to maintain said surface throughout at substantially uniform reaction temperature, and supplying material, produced independently of said generation of heat, to replenish the case-forming component of said gas.

14. The method of imparting to one or more metal objects a diffusion alloy case substantially uniform in surface carbon concentration and of substantially uniform subsurface characteristics, which comprises effecting generation of heat for maintaining the work at case-forming reaction temperature, forcibly stirring a hot gas, having a carburizing component produced independently of said generation of heat, in contact with the metal to effect substantially uniform concentration of said component throughout the surface of the metal while utilizing said gas to maintain said surface throughout at substantially uniform carburizing temperature, and supplying reagent to replenish the carburizing component.

15. The method of producing upon one or more metal objects a diffusion alloy case of substantially uniform surface concentration and of substantially uniform subsurface characteristics, which comprises subjecting the metal to a gaseous medium having a component of character to react with the metal at elevated temperature to form the diffusion alloy case, electrically generating heat to provide the reaction temperature, substantially uniformly concentrating said reactive component throughout the surface of the metal by forcibly stirring the heated medium while in contact with the heated metal, and concurrently by said forcible stirring of said medium maintaining the metal throughout its said surface at substantially uniform reaction temperature.

16. The method of producing upon one or more metal objects a carburized diffusion alloy case of substantially uniform surface carbon concentration and of substantially uniform subsurface characteristics, which comprises subjecting the metal to a gaseous medium having a carburizing component of character to react with the metal at elevated temperature to form the diffusion alloy case, electrically generating heat to provide the reaction temperature, substantially uniformly concentrating said carburizing component throughout the surface of the metal by forcibly stirring the heated medium while in contact .with the heated metal, and concurrently by said forcible stirring of said medium maintaining the metal throughout its said surface at substantially uniform reaction temperature.

JOHN W. HARSCH.

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