US3197346A

US3197346A - Heat treatment of ferrous metals with fluidized particles

Info

Publication number: US3197346A
Application number: US176006A
Authority: US
Inventors: John C Munday
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1953-11-27
Filing date: 1962-02-27
Publication date: 1965-07-27
Anticipated expiration: 1982-07-27

Description

July 27, 1965 J. c. MUNDAY 3,197,346

HEAT TREATMENT OF FERROUS METALS WITH FLUIDIZED PARTICLES original Filed Nov. 27, 195s 2 Sheets'sheet 1 9 .2955..6'2 25 )is flo "1 2| 4 Il 2 r" W s f @S A. F

u, v, l u, I u, u, u, u, I??? u il) o 0.o o o o o o I" '3 FIG. l

John C. Mundoy Inventor By j Po'fentAf'rorney July 27, 1965 J. c. MUNDAY 3,197,346

HEAT TREATMENT OF FERROUS METALS WITH FLUIDIZED PARTICLES original Filed Nov. 2?,v 1953 2 Sheets-Sheet 2 0m hm Inventor PoTenf Aforney John C. Munduy By QJ/ZKM7 United States Patent O lohn C. Monday, Cranford, NJ., assigner to Esso Re- Search and Engineering Company, a corporation of Delaware @riginal application Nov. 27, 1953, Ser. No. 394,747, now Patent No. 3,053,794, dated Sept. 1l, i952. Divided and this application Fell. 27, 1962, Ser. No. 176,606

i3 Claims. (Cl. 148-13) This application is a division of application Serial No. 394,747, filed November 27, 1963, now Patent No. 3,053,704, granted September l1, 1962, which in turn is a continuation-impart of prior application Serial No. 174,636, filed luly 19, 1950, now abandoned.

The present invention relates to a method and apparatus for heat treating metal objects, wherein the treatment involves heating and/or cooling steps applied primarily for the purpose of conserving in or imparting .to the metal objects certain improved physical properties. The metal objects contemplated may be parts, castings, forgings or the like which customarily are subjected to some form of heat treatment during or subsequent to manufacture of the objects, and these objects may be of ferrous or non-ferrous metals and their alloys. The heat treatments contemplated include annealing, hardening, tempering, and quenching in their various forms, and combinations of such treatments, and particularly such treating methods wherein the heating and cooling steps involved produce changes in the physical characteristics -of the metals. Under such circumstances it is Well known that control of the rates of heating and coolin(r of the objects during treatment is of critical importance in obtaining the desired results. ln the prior art, however, close and uniform control has been difficult to obtain when employing conventional heat treating furnaces and liquid treating baths, ln addition to the problems of process control set forth above, conventional systems according to the prior art are subject to defects produced oy the heating and cooling media employed therein, and the metal treated may suffer from cracking, oxidation, scaling or other undesirable surface injuries.

It is an object of the present invention to provide a method for accomplishing the heat treatment of metals in which the defects of the methods and systems are overcome to provide close and reproducible control of temperatures in all stages of the treating operation. lt is another object of the invention to avoid or substantially reduce any undesirable surface impairment of the metal objects as a result of oxidation, scaling and other comparable conditions normal to the processes and methods of the prior art.

The present invention also contemplates a system, including a method and apparatus, which is useful in heat treating ferrous metals as employed for the purpose of carburizing and nitriding such metals, and in the treatment of metals at high temperatures with high melting cyanides, such as sodium cyanide and potassium cyanide. Still another object of the present invention is to provide an improved method and apparatus for heat treatment of metal objects to form thereon an alloy coating of another metal, for example for the purpose of coating metal objects with other metals such as aluminum, Zinc, chromium, cadmium, vanadium, cobalt, titanium, silicon, Zirconium, tungsten, molybdenum, boron, manganese, and beryllium.

A particular object or" the invention is to provide a method and apparatus wherein metal objects are subjected to heat treatment in the presence of and by immersion in and in contact with a body of finely divided or powdered solid materials maintained in a iluidzed condition in a confined treating zone, and wherein lluidization of the solid materials imparts substantially constant motion to the individual particles of the solid materials such as to produce impinging Contact of such particles against the metal object when immersed therein. As contemplated by the present invention treatment of the metal objects may be accomplished under optimum conditions such as permit heat transfer between such objects and the powdered materials at more closely controlled rates and while avoiding undesirable side effects. Also as contemplated by the present invention the physical properties of the metal objects may be modified and determined within closer limits and with greater reproducible uniformity than has been previously known in the art.

The invention and its objects may be more fully understood from the following specification when it is read in conjunction with the accompanying drawings in which:

FIG. 1 is a view in vertical section through one form of apparatus according to the present invention;

FlG. 2 is a similar view of another embodiment of the invention taken along line 2-2 of FIG. 3; and

FIG. 3 is a cross-sectional view of the apparatus according to FIG. l, taken along the line 3 3 thereof.

Referring to the drawings in greater detail, in FlG, l, the numeral 1 designates a :treating vessel which contains a bed of finely divided or powdered solid material indicated by the numeral 2. Disposed within the vessel, in the bottom thereof one or more fluid conduits 3 are provided for the injection of a gaseous medium upwardly into the bottom portion of the bed of solid materials, to tluidize the bed. The discharge conduits 3 are counected to a manifold conduit d, which extends outwardly through la wall of the vessel into connection with a supply conduit S. rlhe supply conduit 5 is provided with a control valve 6.

When fluidized, the bed or body of powdered materials is maintained with an upper surface level as indicated by the numeral 'i'. In the vessel' as shown the distance between this upper level 7 and the upper end Iof the vessel 1 is preferably such that the fine-r particles of the solid m-aterials in the bed which may be entrained by the gaseous fluidizing medium passing through the surface of the bed substantially are not carried beyond the upper end of the vessel under operating conditions. This distance is determinable by one skilled in the art on the basis of the physical characteristics of the powdered material, including 'the composition thereof, the particle size and the density of the material, and the velocity at which the gaseous fluidizing medium passes upwardly through the bed. The depth of the bed below the surface level 7 will be such as to permit complete immersion therein of a metal object to be treated in the vessel.

Also as shown by FIG. 1, means are provided for heating and cooling the bed of solid material, by direct or indirect heat exchange contact with iluid heat eX- change media. Within the vessel l are provided a series of heat exchange conduits arranged as a coil circumferentially of the vessel, but spaced from the wall thereof. in the arrangement illustrated, the conduits 8 are connected to a supply conduit 9 for a liquid medium, the conduit g extending into the vessel through a side wall thereof. Exteriorly of the vessel 1 is a separate vessel lil adapted to contain a supplementary body 11 ofthe finely divided solid material forming the bed Z in vessel l. When llnidized, this body of the material has an upper surface level as at l2. The bottom of vessel lll is connected to the bottom portion of vessel 1 as by means of a transfer conduit 1.3, provided with a control valve le. The vessel lll is also connected to the vessel l by means of a second transfer conduit 15 opening from an intermediate or upper level therein, above the upper level of bed 2 in vessel 1, into the upper portion of vessel 1 below the upper level of the bed 2. A control valve 16 is provided in the second transfer conduit 15. The vessel l@ provides for heating or cooling of the iinely divided solid material employed in the system by direct heat exchange between said material and a gaseous heat exchange medium. As shown, a supply conduit 17 for such medium is extended into the transfer conduit 13, having an outlet 1S therein opening in the direction of the vessel 10. If desired a burner device may be substituted for the outlet 18, to be supplied with a combustible mixture of fuel and air through means such as the supply conduit 17 shown. A control valve 19 is provided in the conduit 17.

In the upper end of the vessel 1%, there is provided a suitable type of separator element, such as a cyclone separator 23 having a dip leg 21 extending downwardly below the level 12 of the body of material 11 in the vessel 1?. An outlet conduit for the iiuid heat exchange medium introduced by way of the conduit 17,`or for combustion gases where a burner is substituted, is designated by the numeral 22. A valve Z3 is provided in the outlet conduit 22.

A conveyor 24 is provided for carrying a metal object, such as a block of metal indicated by the numeral 25, into and through the treating zone. As shown, the conveyor 24 is equipped with dependent carriers such as the arms 26 pivotally mounted thereon. The conveyor is arranged in such fashion that as it is moved over the body of tiuidized solid materials, the carriers will immerse the metal object supported thereby in the body of solids during the treating operation, and then Withdraw it from the bath and the vessel.

Referring now to the apparatus as illustrated in FIGS. 2 and 3, the numeral 31 designates another form of 'treating vessel. The vessel 31 is an elongated structure having a roof portion 32 terminating at one end in spaced relation to one end wall 33 of the vessel. A vertical baffle 34 is secured to the terminal end of the portion 32., transversely of the vessel, so as to'extend upwardly above said portion, and downwardly to depend therefrom into vertically spaced relation to the bottom wall of the vessel. The end wall 33 and the upwardly extended portion of the baffle 34, along with conforming side wall portions of the vessel, indicated by the numeral 35, define a well 36 open at its upper end, and in direct communication with the vessel at its lower end.

At the-opposite end of the vessel 31, a transverse partition 37, with the end wall indicated at 3S, and adjoining side wall portions of the vessel define an enclosed chamber 39 with the roof portion 32 extended thereover. Between the partition 37 and the baille 34, the vessel provides an enclosed treating chamber which is designated in the drawings by the numeral di?. In the apparatus as illustrated by FIG. 2, the chamber d@ is divided longitudinally bymeans of a vertical baffle member t1 disposed in laterally spaced substantially parallel relation to the side walls of the vessel, and in spaced relation at each end to the battle 34 and the partition 37, respectively. Preferably the bafie is mounted on the oor or bottom of the vessel extending upwardly into vertically spaced relation to the roof portion 32, and provides a pair of laterally defined substantially continuous travel paths through the chamber 413 on each side of the bafde which pathsl are in communication at each end. As shown, the spacing of the baille 41 from the partition 37 is preferably greater than from the transverse baliie 34.

The chamber 39 and the chamber 4t) are provided for direct communication with each other, as by means of a pair of parallel, vertically spaced, slot-like passageways 42 and 43 through the partition 37, and extending vlongitudinally thereof. Each passageway 42 and d3 is provided with an adjustable valve-like closure plate element, such as the elements d and 45 shown. These elements may be mounted as on rotatable shaft supports ze and 47, respectively, extending through the vessel side walls and provided for operation as by suitable handles or valve wheels as indicated in FIG. 3 by the numeral The vessel 31 contains a body of finely divided or powdered solid material designated in FIG. 2 by the numeral 49. The material is iluidizable by a gaseous uidizing medium, and when so tluidized will till the vessel, including well 36, chamber 39, and chamber to a depth and a level such as indicated by the numerals Si?, 51, and 52 respectively. The actual depth and therefore the actual level which may be attained will be determined substantially in the same manner and for the same purposes referred to in connection with the apparatus as shown in FG. 1, and as may be described below. in any event, it is intended that the level attained in chamber ed at all times will be above the lower end of the transverse baille 3ft, and such as to provide a free space between the body of solid material and the roof portion. Likewise, a free space will be maintained above the level 51 in chamber 39.

Fludization of the body of material in the chamber du and well 36 is accomplished by means for injecting a gaseous medium such as provided by a series of manifold injection nozzles 53, arranged substantially as shown in FIG. 3, and of which each nozzle manifold is connected to a common supply conduit 54 as by means of branch lines 55 substantially in the manner illustrated in FIG. 2. Valves such as indicated in the branch lines S5 may bc employed to control the injection of the uidizing medium in any desired fashion. The supply conduit 54 is in turn connected to a source of a gaseous uidizing medium as indicated by conduit E6. The conduit 55 is provided with a pump 57. Flow through conduit 56 may be controlled as by means of a valve 58 preceding the pump, and by operation of the pump itself.

The chamber fr@ is provided with suitable means for venting the gaseous liuidizing medium therefrom. This, as shown, includes a conduit connection 6) opening from chamber d@ into a cyclone separator 61. The separator 61 is provided with a dip leg return 62 for solids entrained by the gaseous medium and a vent line 63 for the gaseous medium. The line 63 is also connected to the conduit 55 ahead of pump 57 as by means of a conduit connection 64.

Valves

65 and 65 in lines 63 and 6d respectively permit selective disposition of the gaseous medium as vented from the chamber 4t).

In the apparatus as illustrated in FIGS. 2 and 3, the chamber 39 is provided with means for heating the iinely divided solids contained in the vessel. As shown, chamber 39 is provided with a` plurality of fuel burner elements 67 disposed in the lower portion thereof. Conduits such as conduit 68 connect the burners 67 with a source of a combustible mixture of fuel and air for burning within -the chamber 39. Alternately the burners 67 may be eliminated and hot ilue gases or other gases at high temperatures may be fed through conduit 68 from an exterior source. In either event, the solid material in charnber 39 is liuidized by the gases thus formed or introduced.

The chamber 39 is also provided with means for venting the gaseous medium passed into heat exchange relation with the solid materials therein, as by a conduit connection 6i) opening from the chamber 39 into a cy- .clone separator 7d. A dip leg conduit 71 opens from .the bottom of separator 69 to a level below the surface ,level S1 of the tluidized solids in chamber 39, to return solids separated in cyclone 7@ from the gaseous medium therein. A vent line 72 from the separator 7) discharges the gaseous medium passed through the separator from y chamber 39.

,shown, the conveyor' enters and leaves the vessel through the well 35. Enteiing the vessel downwardly through well 36, the conveyor extends under the baffle 34, through a first travel path toward the partition 37, substantially across the face of the partition, and thence extends through the second travel path toward the baie 34, under the bafiie and then upwardly and out through the well 36. A metal object `such as indicated by the numeral 74 is supported on the conveyor in any conventional fashion to be transported through the bed of solids.

In general, the method according to the present invention concerns a process for treating metal objects to improve their physical properties. The treating steps contemplated involve the transfer of heat to or from such objects in a controlled manner in order to obtain or to modify certain characteristics in the structure of the metal composing the object, and in accordance with certain well known basic standards for such treatment. According to this method, a metal object to be treated is immersed in a bath as provided by a bed of fluidized finely divided or powdered material, such as the bed 2 of FiG. 1, or the bed 49 of FGS. 2 and 3. The hed is iiuidized to a degree within a range of not substantially less Ithan the level of incipient fluidity, at which level it is a quiescent fiuidized bed, such as has been defined in Industrial and Engineering Chemistry, Vol. 41, p. 1249, .lune 1949, and not substantially more than required to produce a turbulent bed as therein defined, and below that level at which a bed of such material no longer retains a discernible upper surface and the whole mass of finely divided solid material becomes a dispersed suspension in the fiuidizing medium. These levels of fiuidization, of course, are governed by several factors, including actual density of the solid material, particle size, and the linear velocity of the gaseous uidizing medium as injected into the mass or bed of solid material. The factors may be readily determined and correlated, however, for the purpose of this invention, by any person skilled in the art. For example, in the case of relatively fine particles such as those in the G-400 mesh range, the point of incipient fluidity may be as low as 0.01 ft./sec. superficial linear Velocity (i.e., calculated on the basis of an empty vessel) of fluidizing gas passing upward .through the powder, while coarser particles such as 6-12 mesh may require as much as 1.0 or 2.0 ft./sec. The upper limit of fiuidization contemplated will be at a linear velocity of about 5.0 feet per second. Density has a similar effect, heavy materials such as iron, nickel, etc., requiring higher fluidization gas velocities than light materials such as carbon, silica, magnesium, aluminum, etc.

The bed of solid materials, as shown in FIGS. 1, 2, and 3, is fiuidized by the injection of a gaseous, fiuidizing medium upwardly through the mass of materials as by way of the supply conduit 4, manifold conduit 5, and discharge conduits 3 as yshown in FIG. 1, or, as in FIGS. 2 and 3, by way of the pump 57, supply conduit 54, branch lines 55 and the manifold injection nozzles 53. Control of the rate of injection of the iiuidizing medium is obtained by suitable means such as the valves shown in the several supply and branch line conduits, and also by operation of the pump S7 of FIGS. 2 and 3.

Further, the metal object is immersed in the bed of fiuidized material and conveyed therethrough in a series of treating stages, which may or may not be sharply defined. n each stage heat is added or abstracted from the metal object by contact with and by the individual solid particles. By liuidization of the bed, the particles are maintained in substantially constant motion, and with the metal object immersed in the bed, the individual particles impinge upon the surface of the object at a rate determined by the degree of fiuidization of .the bed and the degree of turbulence imparted thereby. Inasmuch as the rate of heat transfer has been found to depend upon the rate or frequency of particle impact, at zero or incipient fluidity lthe rate of heat transfer is very low,

and the bed of solid material has a substantial insulating effect, while with high fiuidity and the individual particles of the bed in turbulent motion, the heat transfer rate and also the thermal conductivity of the mass of solid material are increased markedly,

As an example of the change in heat transfer between powder and a metal Wall as fiuidization is varied, a metal powder having a density of about 9.0 and a particle size in the range of about 200 to 400 mesh had a thermal con-v ductivity constant of 0.19 B.t.u./(hr.)(sq.ft.)( F./ft.) when unfluidized, a heat transfer coeflicient of 20 B.t.u./ (hr.) (sq. ft.) F.) when fluidized at a superficial linear gas velocity of 0.2. ft./sec., and a heat transfer coefiicient of 46 when fiuidized at a velocity of 2.0 ft./sec. With carbon powder of the same particle size range, the heat transfer coefficient was about 12 at 0.05 ft./sec. and about 3l at 1.2 fL/sec. With carbon powder of 20-48 mesh, the heat transfer coefiicient was about 6 when unuidized by gas at 0.5 ft./sec., 16.5 when fluidized at 1.35 ft./sec. and 30 when ffuidized at 2.4 ft./sec. By increasing the turbulence of these powders Still further it would be possible to increase the heat transfer coefficient to in the neighborhood of or even higher. This method of varying the heat transfer rate by varying the fiuidity and turbulence of fiuidized solids is utilized in the present invention to obtain a degree of flexibility and a degree of control in the heat treating of metals that was not obtainable in the prior art. In the method now contemplated, these heat transfer characteristics are employed to control the rate at which the metal objects are heated or cooled while immersed in the bed of solid materials provided in either of the vessels 1 or 31.

In order to obtain the desired transfer of heat to or from the metal object it is, of course, essential to maintain a temperature differential between the object and the fiuidized solid material. This is accomplished according tothe present invention in one or more of several ways. For example, in the operation, as carried out in the apparatus of FIG. l, the solid material Vis circulatedl from vessel 1 to vessel l@ by way of the transfer conduit 13. In conduit 13 a fiuid heat exchange medium, such as a hot or cold gaseous medium, is injected into direct heat exchange relationship to the solid material. By injecting such gases in the desired direction of flow, the stream of gas acts to move the solid particles in the direction of vessel l0. Further, the injected gases are employed to increase the fluidization ofthe mass of particles in the conduit and in the vessel lil, so as to produce a lower density in the mass of material in vessel lil than in the bed of material in vessel 1, and thereby create gravity circulation between the two vessels. The solids circulated through line 13 are maintained in contact with the injected heat exchange medium during passage therethrough, and through the vessel l0, receiving or giving up heat therein. By suitable control of the rate of circulation any desired temperature differential may be established and maintained between the metal object being treated and the bath of finely divided solids in vessel 1.

These dierential temperatures between the metal object and the solid material also may be established and maintained, as shown in FIG. 1, by circulation of a liquid heat exchange medium through the conduits 8 into indirect heat exchange relation with the bed of solid materials in vessel l. In addition, by heating or cooling the gaseous tluidizing medium introduced through conduits 5, 4 and 3, the desired control of differential temperatures may be further supplemented.

In the apparatus of FIGS. 2 and 3, the desired temperature differentials between the metal object and the solid material may be established and maintained both by circulation of solids through the chamber 39, and by introducing heated or cooled ffuidizing gases through the conduit system including conduit 56, pump 57, and conduit connections 5.4, 55, and 53. In chamber 39, heat may be added to the solid material circulated therethrough by direct heat exchange either with hot combustion gases produced by burning fuel and air in burners 67 or otherwise, as previously set forth. In any event, circulation through chambers 39 and i0 is produced, as in the apparatus of FIG. 1, by increased iiuidization of the solid materials in chamber 39, as compared with that in chamber 40. Circulation may be controlled positively by suitable adjustment of the plate elements 44 and 45. In fluidizing the materials contained in vessel 3l of FIG. 2, the gaseous fluidizing medium is supplied and injected into the well 36 at a somewhat lower velocity than into the chamber 40, so as to maintain the mass of material at a higher density in the Well. In this marmer, in combination with the dependent baflie 34, a seal or trap is established at the combined entrance and exit of chamber 4t), such that solid particles which may be entrained by the iiuidizing medium at the higher injection velocities which may exist in chamber 40 during certain portions of the process, may be prevented from escaping from the vessel, and may be substantially recovered by separation as in the cyclone separator 61. The gaseous iiuidizing medium is injected into the well 36 at a rate to maintain iiuidization at the minimum carryover or loss through the open upper end of the well substantially as described with reference to the open upper end of the vessel 1 in FIG. 1.

As previously indicated, the gaseous uidizing medium which is passed through the solid materials contained in chamber 4t) of vessel 31 in FIGS. 2 and 3, is vented from the chamber by way of the cyclone separator 61, wherein solid particles carried over with the uidizing Vmedium are separated and returned to the main body by way of the dip leg 62. The gaseous medium itself may be exhausted through the line 63 with valve 65 open and valve 66 in conduit connection 64 closed. Alternately, valve 65 may be closed, and valve 66 opened to provide for recirculation of the vented gas by way of conduit 64 and the pump 57. This mode of operation is particularly contemplated when the gaseous medium may be rare or expensive, such as hydrogen and dissociated ammonia,

In FIG. 2, the level 52 of the body of iiuidized solid materials is shown as being substantially below the indicated levels 5G and 51 of the solid materials in Well 36 and chamber 39, respectively. These differences in levels are exaggerated to some extent to illustrate the tendency of pressure drop through the separator 61 to produce a positive gas pressure in chamber 40, and thereby to depress the level 52 below that which may exist in the well 36. Also as indicated above, the density of the mass of material in the chamber 39 will be somewhat less than that existing in chamber 40. The body of material in chamber 39 will thus be expanded by the greater degree of uidization induced therein, with a consequent elevation of the surface level. In actual operation, these levels may not vary, one from another, to such an exaggerated extent as shown.

In the operations contemplated, a variety of gaseous fiuidizing media may be employed. The most common of these will be flue gas. Preferably the flue gas employed will be rich in carbon monoxide or unburned hydrocarbons in order to avoid diiiiculties from scaling of the metals where high concentrations of carbon dioxide, free oxygen or air, sulfur dioxide, and water vapor may be present. In certain treatments, such as where a reducing atmosphere is specifically indicated, hydrogen or 'dissociated ammonia may be employed alone or in combination with other gases. In other treatments, as in nitriding ferrous metals, the iiuidizing gas may include such gases as hydrogen cyanide or ammonium cyanide. It is a characteristic of the present invention, that by comparison with the requirements of the prior art, the volume of gas required for operation is relatively small,

.8 and therefore the use of rare and more expensive gaseous media becomes economically practical.

The finely divided and powdered solid materials suitable for use as the heat transfer medium according to the present invention may include finely divided sand, Zirconia, ferro-silicon, silica gel, alumina, bauxite, carbon, coke, brick dust, iron oxide, clay, ground porcelain, powdered or microspherical metals, used powdered silica alumina cracking catalyst, or any other inert or reactive solid material according to the treatment contemplated. The particular material which is employed depends somewhat on the particular heat treating process, since some materials are relatively inert at low temperature but may cause changes in the surface 0f the metal being treated at high temperatures. An especially desirable solid material for heat treating is finely divided metal, particularly metal having approximately the same composition as the metal stock being heat treated. For example, in the heat treating of cast steel, steel powder having about the same carbon content may be employed to advantage.

In other cases it is advantageous to employ as the heat treating medium a material that will cause a change in the surface of ythe metal being treated. For example, the present invention is eminently suitable for the surface carburizing and nitriding of ferrous metals, and for the cementation of various metals. In carburizing, the metal stock is heat treated at a temperature in the range from about 1600 F. to about 1750 F. in the presence of a iluidized carburizing material such as hardwood charcoal, petroleum coke, metallic carbides such as iron carbide, charred bone, and bituminous coal, to which may be added activators such as barium carbonate, calcium carbonate and sodium carbonate. The fluidization gas may be a neutral gas, for example nitrogen, but preferably it is a carburizing gas such as a petroleum gas. In the latter case the tluidized solid may be non-carbonaceous if desired. The advantages of the invention as applied to carburizing will be evident from a consideration of the prior art stationary process, wherein it was necessary to place small metal parts in small treating pots because of heat gradients, wherein it was necessary to pack the parts uniformly separated according to a pattern which varied with size and shape, and wherein it was necessary 'to seal the pots carefully against the advent of furnace gases. The requirements for successful carburizing of oven heating, careful temperature control within il0 F.

and avoidance of contact with air or furnace gases are easily met with the iiuidized solid process of the present invention.

Similarly, the present invention is useful in nitriding ferrous metals, for example, by immersing the metal in a fluidized solid such as iron power or iron microspheres and employing a nitriding gas such as HCN, NH4CN, etc., which may be diluted with other gases if desired. High melting cyanides, such as sodium cyanide or potassium cyanide, can also be employed as the tluidized solid. The

use of other tiuidized metalsrsuch as aluminum, zinc,

chromium, cadmium, tungsten, vanadium, cobalt, titanium, silicon, zirconium, molybdenum, tantallum, boron, manganese,` and beryllium, at cementation temperatures which may range from about 650 F. to about 2550o F.,

together -with relatively inert fiuidizing gas such. as nitrogen, hydrogen, helium, etc., results in the formation `of quite even and tenacious alloy coatings.

Example I-Annealz'ng Steel stock consisting of 3 x l2 bars having a carbon content of 0.33% is transported by conveyor 24 into a vessel such as designated by the numeral l in FIG. l.

, 9 Y The vessel 1 contains a bed of nely divided solid material such as luidized foundry sand having a particle size of about 80-100 mesh and having a temperature of about 800 F. The steel stock is immersed in the luidized solids below the level shown at 7. Flue gas is employed as a lluidizing gas for the solids, being introduced through line at a rate such that the upward superficial gas velocity in vessel 1 is 1.5 ft./sec. and to produce turbulence in the bed. The heat transfer coeliicient under these conditions is about 85 B.t.u./(hr.)(sq. ft.)( E). The temperature of the solids in vessel l is increased by introducing hot solids from heating vessel via line 15. Injection of hot hue gases through conduit 17 etlects circulation of solids from vessel l to heating vessel l0. The gas velocity in heating vessel l0 will normally be greater than that in vessel 1, and therefore in vessel 1! the density will be less and the level will be higher than in vessel 1. Under these conditions, luidized solids will circulate from vessel 1, through line 13, into heating vessel 10 and thence through line 15 into vessel 1. The rate of solids ow is controlled by the valves 1d and lr6.

When the temperature of the solids and of the steel stock in vessel 1 reaches 1500 F., the circulation of solids from heating vessel 10 to vessel 1 is stopped. The steel stock is then subjected to heat soaking at 1500 F. for about 3 hours, about one hour or" soaking time being allowed for each inch of cross section yof the stock. During the heat soaking period, it is desirable to -l aintain the steel stock at the heat soaking temperature, and in order to reduce the loss of heat from the steel stock the uidizing gas rate in Vessel l is reduced to from about 0.05 to 0.1 ft./sec. superficial linear velocity, producing a less turbulent condition in the bed than in the initial stage of treatment. Under these conditions, the heat transfer coecient is about 3-5 B.t.u./ (hr.) (sq. ft.) F. At the end of the soaking period, the temperature of the uidized solids in vessel 1 and of the steel stock irnmersed therein is reduced slowly over a period of 4-5 hours. The temperature is decreased by passmg a cooling medium such as water through cooling coils 3 in Vessel i. During the cooling period, the uidizing gas velocity and thereby turbulence in the bed is increased to about 0.5 ft./sec., giving a heat transfer coeliicient or about 50 B.t.u./(hr.)(sq. ft.)( E), in order to increase the rate of heat transfer from the steel stock to the iluidized solids and from the uidized solids to the cooling coils ti. When the temperature reaches 800 F. the annealed steel stock is removed from the tluidized bed by conveyor 2dand is allowed to cool further in air.

Example II-Hardenz'ng and Quel/:cking Steel gears are heated to 1450 F. in a vessel such as vessel 1 of FIG. l, substantially as described for the heating period of Example I. They are then removed from the heating vessel as by the mechanical conveyor 24, and are transported to a second similar vessel which contains a bed of fluidized iron powder 50-100 mesh at a temperature of about 100 F. The gears are there immersed below the level of the fluidized iron. Flue gas is used to fluidize the iron powder, the gas rate being about 1.3 ft./sec. superiicial linear velocity. Under these conditions the bed is in a substantially turbulent condition, and the heat transfer coeticient is about 80 B.t.u./ (hr.) (sq. ft.) F.) and the gears are rapidly quenched to a temperature of about 750 F. Depending on the dimensions and volume of the gears being treated, the time of quenching may vary from less than a minute to -60 minutes. At this point, the uidizing gas rate is decreased sharply to about 0.05 to 0.35 ft./sec. in order to decrease the cooling rate and allow the hardening transformation to take place. Under these conditions, the heat transfer coetiicient is about 3-30 Btu/(hr.) (sq. ft. F.). When the gears have reached a temperature of about 150 F., they are removed from the fluidized solids by the conveyor and can be tempered immediately.

sasso l0 VExample III--Tempering The steel gears hardened as in Example II are transported by a conveyor to another vessel such as shown in FIG. l. The gears are immersed below the level of iiuidized solids contained in vessel 1, the solids being -400 mesh spent silica-alumina cracking catalyst obtained as a by-product in the petroleum industry. The temperature of the luidized solids in vessel 1 is about 200 F., heat being supplied by het solids circulated from heating vessel l0 as was described in Example I. Flue gas relatively free of acidic gases Vand containing a relatively high proportion of carbon monoxide is supplied as iiuidizing gas through line 5 at a rate equivalent to a superficial gas velocity of about 0.30 ft./sec., giving a heat transfer coeflicient of about 40-60 B .t.u./ (hr.) (sq. ft.) l). When the temperature of the gears approaches 800 F., generally in 1/2-2 hours depending on the size of the gears, circulation of hot solids from heater l0 is stopped and the fiuidizing gas rate is decreasd to a low level in order to decrease heat transfer and provide a soaking period of about two hours. The gas rate during the soaking period is preferably at or near the minimum fluidizing gas velocity, for example, in the range of about 0.01 to 0.05 ft./sec., where the heat transfer coefcient is in the range of about 2 to l0 B.t.u./ (hr.) (sq. ft.) F). At the end of the soaking period, the gas rate is increased to about 0.15 to 0.25 ft./sec. in order to increase turbulence and thereby the heat transfer rate. Water is passed through cooling coils 8, and the uidized solids and the steel gears immersed therein arel slowly cooled to about 200 F., whereupon the tempered gears are removed from the iiuidized solids.

Example lV-Carburzz'ng An alloy steel part to be case hardened is introduced into the vessel l as shown in FIG. l. Vessel 1 contains a 10G-300 mesh iluidized solid carburizing agent, com' prising hardwood charcoal, petroleum coke and barium carbonate, at a temperature of about 200 F. The solids are luidized by gas introduced through line 5. The iluidizing gas is a hydrocarbon gas, such as propane, containing a diluent gas such as carbon monoxide. The gas velocity is in the range of about 0.75 to 1.5 ft./ sec. After the steel stock is introduced below level 7, the temperature of the solids is increased to about l700 F. by circulating through the vessel l0, with heat being supplied to the solids in heater l0 by hot ue gas of a reducing nature. When the temperature of the steel stock approaches 1700 F., the liuidizing gas rate is decreased to a value in the range of about 0.1 to 0.3 ft./sec. and circulation from vessel t0 is stopped. The steel stock is then heat soaked at 1700 F. for about 4 6 hours, which gives a carburized case depth of from about 0.045 to about 0.060 inches. Cooling huid is then passed through coils S and the uidizing gas rate is increased to a value in the range of about 0.75 to l.5 ft./sec. stock reaches l450 F., it is removed from the carburizing agent and is subjected to a quenching operation as described in Example il.

Example V-Sherrzrczing Sheet iron plates to be coated with zinc by sherardizing are immersed in a tiuidized mixture comprising equal parts by weight of zinc powder and zinc oxide powder having a particle size in the range of about 80 to 300 mesh. The powder is contained in vessel )l as shown in FIG. 1 and is tluidized by nitrogen passing therethrough at a superficial Velocity of 1.2 ft./sec. The powder in vessel 1 is then heated to a temperature of 700 F. by circulating the powder through heating vessel 10 in heat exchange relation to hot ue gas of a reducing nature. When the powder in vessel i reaches a temperature of 700 F., powder circulation is stopped and the uidizing gas rate is decreased to substantially zero. After the sheet iron plates have undergone a heat soaking period of about three hours the When the temperature of the steel Y combined inlet and outlet.

fluidizing gas rate is restored to its former level, `cooling water is admitted to coils 8, and the temperature of the powder and of the steel platesis reduced to ambient temperature. The coated plates are then removed from vessel l, and a fresh batch of plates is introduced thereto.

The apparatus, as shown in FIG. 2, is particularly adapted to employment wherethe metal object to be treated must be passed through a series of treating stages in succession. The apparatus is most suitable for an operation such as an annealing operation, wherein the metal object is required to pass through a heating stage in Awhich it is gradually raised to an annealing temperature, held at such temperature for a predetermined period, and then gradually cooled before removal from the treating vessel. In the apparatus as shown, it is possible simultaneously to accommodate metal objects in each of the treating stages indicated.

, In the apparatus as shown the treating stages are in substantially continuous sequence. This is made possible by suitable control of injection of the fiuidization medium into the body of solid material in the chamber 40, and by establishment of a suitable temperature gradient through the mass of solid material in the chamber, from the partition 37 to the bale 34.

In an annealing operation, for example, finely divided solids such as foundry sand of a particle size of about 80 to 100 mesh may be circulated from and to chamber 40 through the chamber 39 in the manner previously described. In chamber 39, the sand is heated and uidized by combustion gas produced by burning a fuel gas and air in burners 67. By suitable control of the heating and circulation of sand a temperature gradient may be established in the bed ranging from about 800 adjacent the baffle 34 to about 1500 adjacent the partition 37. With such temperatures, a metal object such as the steel bars of Example I above may be introduced into chamber i0 'through the .Well 36 by means of conveyor 73. As the steel stock is passed through the travel path along one side of bathe 41 from bale 34 to partition 37, ue gas is introduced as a fluidizing medium by way of the conduit 54, branch lines and 4the injection nozzles 53.

By means of the valves in branch lines 55, this gas is injected through the nozzles along the path so as initially to produce a superficial gas velocity in the initial stage in the neighborhood of about 1.5 ft./sec. At such rate of injection, the rate of heat transfer between the metal stock and the finely divided sand is high, the sand having a heat transfer coefficient of about 85 B.t.u./ (hr.) (sq. ft.) F.). Then as the metal stock progresses along the travel path and reaches a temperature of about l500 F., in that area the rate of injection of the fluidizing medium is reduced to from about 0.05 to about 0.1 ft./sec. during further progress of the metal stock as across the face of partition 37, and into the return travel path along the opposite side of baie 41and continuing injection at suchV rate for a period of about 3 hours to obtain the desired temperature throughout the stock. In this stage, and at such rates of injection of the iuidizing medium, the heat transfer coefficient of the solids is reduced to from about 3 to 5 B.t.u./ (hr.) (sq. ft.) E). By suitable regulation of the conveyor speed, this may be accomplished as the stock enters the return travel path, and in this area the liuidizing gas velocity is increased to about 0.5 ft./sec., increasing the heat transfer coeiiicient of the sand in this area to about 50 Blu/(hr.) (sq. ft.) R). Preferably, in the area immediately adjacent the bafe 34, the rate of injection of the uidizing gas is again increased, as to substantially the rate of injection in the initial stage. At this point, the spacing of baie 41 from the baiiie 34 permits Acirculation of the solid material around the baille 41 such as to aid in heat recovery and heat equalization at the The stock is then removed from the chamber 40 through the well 3f by further `progress o f the conveyor. In the well 36, the fiuidizing was l2 gas is injected at a rate below that which may exist in adjoining portions of the chamber 40.

Further in accordance with the method as has been set forth above, surface conditioning of the metal objects is also accomplished to remove accumulations of scale, rust or other undesirable surface deposits and coatings. Such surface conditioning is accomplished substantially as a result of the metal objects scouring action of the nely divided solid particles when they lare set in motion by fiuidization and impinge against the metal objects during treatment thereof.

What is claimed is:

l. The method of altering the surface of a ferrous metal object which comprises introducing a gas to a vessel containing a multiplicity of finely-divided carbonaceous solid particles substantially incombustible under the conditions of the treatment and reactive with the surface of said ferrous metal object at a rate suitable to uidize said solid particles, introducing a ferrous metal object into the said vessel and lsubmerging it in said tluidized solid particles at a region spaced from said vessel, maintaining the temperature of said fluidized solid particles at a temperature between about lai-50 F. and l750 F. and thereafter withdrawing said ferrous metal object from said vessel.

2. Process according to claim 1 in which the finely divided solid material is a metal cyanide.

3. A process for altering the surface of ferrous metal objects which comprises maintaining in a substantially confined treating zone a bed of nely divided carburizing solid particles, passing a gaseous fiuidizing medium containing petroleum gas upwardly through said treating zone at a controlled velocity to tiuidize said nely divided carbun'zing solid particles and to impart motion to the individual solid particles in said bed, introducing a ferrous metal object to be treated into said treating zone and substantially completely submerging said ferrous metal object in said bed of nely divided fluidized carburizing solid particles whereby said ferrous metal object within said treating zone is subjected to direct impinging Contact of individual particles of said finely divided uidized solid particles in motion, selecting an elevated temperature for said bed of solids and continuing the treatment for a period of time sufficient to accomplish the desired alteration in the surface of said ferrous metal object, heat soaking said ferrous metal object for an extended period of time in said fluidized solid particles and then cooling and withdrawing said treated ferrous metal object from said treating zone.

4. A process for altering the surface of individual metal objects which comprises maintaining in a substantially conned treating zone a bed of finely-divided solid particles chemically reactive with the surface of said metal objects under selected temperature conditions, passing a gaseous medium upwardly through said treating zone to iluidize said finely divided particles, introducing a ferrous metal object to be surface treated into said treating zone and submerging said ferrous metal object in said bed of finely divided uidized solid particles, selecting an elevated temperature for said tluidized bed of -solids and selecting the time of contact between said ferrous metal yobject and the tiuidized solid particles to effect chemical reaction between the surface of said ferrous object and said solid particles to kalter the surface of said ferrous metal object, and then withdrawing the surface treated ferrous metal object from said treating zone.

5. A process according to claim 4 wherein the surface of said ferrous metal object being treated is carburized and thercarbon content of the surface of said metal object is increased.

6. A process according to claim 4 wherein said surface treating of said ferrous metal object is effected in the presence of a cyanide compound.

7. A process according to claim 6 wherein nitriding of the.v surface Aof said ferrous. metal object is effected.

8. A method of treatingferrous metal objects to effect a change in the surface thereof which comprises submerging a ferrous metal object in a bed of finely divided solids, maintaining said finely divided solids in a fluidized condition by passing a carburizing gas upwardly through said bed of solids, selecting a temperature of above about 1600 F. for said uidized bed and selecting the time of contact between said ferrous metal object and said carburizing gas to effect chemical reaction between said ferrous metal object and said carburizing gas in the presence of said solid particles of said iiuidized bed and removing the surface treated ferrous metal object from said bed.

9. A process according to claim 8 wherein said carburizing gas comprises a hydrocarbon gas and said solids comprise non-carbonaceous solids.

10. A method for case hardening steel objects which comprises maintaining a bed of finely divided carburizing solids, passing a gaseous medium upwardly through said bed of solid particles at a velocity controlled to maintain said solid particles in a iiuidized state, submerging a steel object to be case hardened in said bed of tluidized solid particles heated to a temperature between about 1600 F. and l750 F. whereby said steel object is subjected to impinging contact with said fluidized solid carburizing particles, regulating the temperature of said uidized bed and the time of Contact between said steel object and said fluidized solid carburizing particles to effect reaction between said steel object and said carburizng solids to case harden said steel object, cooling said steel object and thereafter removing said case hardened steel object from said fluidized bed of solids.

11. A process for sherardizing ferrous metal objects which comprises maintaining a bed of finely divided solid particles comprising zinc powder and zinc oxide powder in a substantially conined treating zone, passing an inert gaseous medium upwardly through said bed of solid particles at a velocity controlled to maintain said solid particles in a liuidized state, immersing a ferrous metal object to be sherardized in said bed of liuidized solid particles whereby said ferrous metal object is subjected to impinging contact with said uidized particles, selecting a temperature of said uidized bed of solid particles and the time of contact between said ferrous metal object and fluidized particles to effect the desired extent of sherardizing and thereafter removing said treated ferrous metal object from said treating zone.

12. The process according to claim 11 wherein the selected temperature of said fluidized bed is about 700 F., said ferrous metal object is soaked in said confined treating zone for an extended period of time and thereafter cooled and removed from said confined treating zone.

13. A process for coating a ferrous metal object with another metal which comprises maintaining a bed of nely divided metal other than said ferrous metal object in a treating zone, passing a relatively inert non-oxidizing gaseous medium upwardly through said bed of metal particles at a velocity controlled to maintain said metal particles in a tiuidized state, immersing said ferrous metal object to be coated in said bed of tiuidized metal particles whereby/,said ferrous metal object is subjected to impinging contact with said fluidized metal particles, selecting a cementation temperature of said uidized bed of metal particles between about 650 F. and 2550 F. and regulating the time of contact between said ferrous metal ob-V ject and the fluidized metal particles to effect the desired coating of said other metal on said ferrous metal object to form a tenacious coating on said ferrous object and thereafter removing said coated ferrous metal object from said treating zone.

References Cited by the Examiner UNITED STATES PATENTS 414,122 10/89 Roberts 148-14 489,194 1/93 Mustin 148-14 2,393,909 1/ 46 Johnson. 2,45 9,83 6 1/ 49 Murphree. 2,509,866 5/50 Hemminger. 3,053,704 9/ 62 Munday 14S- 20.3

OTHER REFERENCES The Metals Handbook, 1948 Edition, American Society for Metals, Cleveland, Ohio, pages 677-702, and 712-716 relied on.

DAVID L. RECK, Primary Examiner. WINSTON A. DOUGLAS, Examiner.

Claims

1. THE METHOD OF ALTERING THE SURFACE OF A FERROUS METAL OBJECT WHICH COMPRISES INTRODUCING A GAS TO A VESSEL CONTAINING A MULTIPLICITY OF FINELY-DIVIDED CARBONA-CEOUS SOLID PARTICLES SUBSTANTIALLY INCOMBUSTIBLE UNDER THE CONDITIONS OF THE TREATMENT AND REACTIVE WITH THE SURFACE OF SAID FERROUS METAL OBJECT AT A RATE SUITABLE TO FLUIDIZE SAID SOLID PARTICLES, INTRODUCING A FERROUS METAL OBJECT INTO THE SAID VESSEL AND SUBMERGING IT IN SAID FLUIDIZED SOLID PARTICLES AT A REGION SPACED FROM SAID VESSEL, MAINTAINING THE TEMPERATURE OF SAID FLUIDIZED SOLID PARTICLES AT A TEMPERATURE BETWEEN ABOUT 1450*F.AND 1750*F. AND THEREAFTER WITHDRAWING SAID FERROUS METAL OBJECT FROM SAID VESSEL.