US3053704A

US3053704A - Heat treating metals

Info

Publication number: US3053704A
Application number: US394747A
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: 1953-11-27
Publication date: 1962-09-11
Anticipated expiration: 1979-09-11

Description

Sept. 11, 1962 J, c. AY

HEAT TREATING mz'ms Filed Nov. 27, 1953 2 Sheets-$heet 1 Sept. 11 1962 ,J. c, MUNDAY HEAT TREATING METALS 2 Sheets-Sheet 2 Filed Nov. 27, 1953 o E M W E g m I o R d a z, M-zuo G w a m E 2 JOHN C.MUNDAY INVENTQR lav 79 M momuzv United tat 3,055,704 HEAT TREATING METALS John C. Munday, Cranford, Ni, assignor to Esso Research and Engineering Qompany, a corporation of Delaware Filed Nov. 27, 1953, Ser. No. 394,747 14 Claims. ((31. 1432tl.3)

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 cooling of the objects during treatment is of critical importance in obtaining the desired results. In the prior art, however, close and uniform control has been difficult to obtain when employing conventional heat treating furnaces and liquid treating baths. In addition to the problems of process control set forth above, conventional systems according to the prior art are subject to defects produced by 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. It 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 of 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 fluidized condition in a confined treating zone, and wherein fluidization 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 Patented Sept. 11, 1962 conditions such as permit heat transfer between such objects and the powdered materials at more closely controlled rates and while avoiding undesirable side eifeots. 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.

This application is a continuation-in-part of a prior application Serial No. 174,636, filed July 19, 1950, now abandoned.

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;

FIG. 2 is a similar view of another embodiment of the invention; and

FIG. 3 is a cross-sectional view of the apparatus according to FIG. 2, taken along the line III-III thereof.

Referring to the drawings in greater detail, in FIG. 1, 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 intothe bottom portion of the bed of solid materials, to fiuid'ize the bed. The discharge conduits 3 are connected to a manifold conduit 4, which extends outwardly through a wall of the vessel into connection with a supply conduit 5. The 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 7. In the vessel as shown the distance between this upper level 7 and the upper end of the vessel 1 is preferably such that the finer particles of the solid materials 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 fluid heat exchange media. Within the vessel 1 are provided a series of heat exchange conduits 8, 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 9 extending into the vessel through a side Wall thereof. Exteriorly of the vessel 1 is a separate vessel 10 adapted to contain a supplementary body 11 of the finely divided solid material forming the bed 2. in vessel 1. When fluidized, this body of the material has an upper surface level as at 12. The :bottom of vessel 10 is connected to the bottom portion of vessel 1 as by means of a transfer conduit 13, provided with a control valve 1 4. The vessel lit is also connected to the vessel 1 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 10 provides for heating or cooling of the finely 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 18 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 10, there is provided a suitable type of separator element, such as a cyclone separator 20 having a dipleg 21 extending downwardly below the level 12 of the body of material 11 in the vessel 10. An outlet conduit for the fluid heat exchange medium introduced by way of the conduit 17, or for corn bustion gases where a burner is substituted, is designated by the numeral 22. A valve 23 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 fluidized 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 FIG- URES 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 baflle 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 bafiie 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 38, 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 baflle 34, the vessel provides an enclosed treating chamber which is designated in the drawings by the numeral 49. In the apparatus as illustrated by FIGURE 2, the chamber 49 is divided longitudinally by means of a vertical baflie member 41 disposed in laterally spaced substantially parallel relation to the side walls of the vessel, and in spaced relation at each end to the baffle 34 and the partition 37 respectively. Preferably the baffle is mounted on the floor 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 40 on each side of the baffle which paths are in communication at each end. As shown, the spacing of the baffle 41 from the partition 37 is preferably greater than from the transverse baffle 34.

The chamber 39 and the chamber 40 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 longitudinally thereof. Each

passageway

42 and 43 is provided with an adjustable valve-like closure plate element, such as the

elements

44 and 45 shown. These elements may be mounted as on rotatable shaft supports 46 and 47, respectively, extending through the vessel side walls and provided for operation as by suitable handles or valve wheels as indicated in FIGURE 3 by the numeral 48.

The vessel 31 contains a body of finely divided or powdered solid material designated in FIGURE 2 by the numeral 49. The material is fiuidizable by a gaseous fluidizing medium, and when so fluidized will fill the vessel, including well 36, chamber 39, and chamber 40 to a depth and a level such as indicated by the

numerals

50, 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 FIGURE 1, and as may be described below. In any event, it is intended that the level attained in chamber 44 at all times will be above the lower end of the transverse baffle 34, 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.

Fiuidization of the body of material in the chamber 40 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 ir; FIGURE 3, and of which each nozzle manifold is con nected to a common supply conduit 54 as by means of branch lines 55 substantially in the manner illustrated in FIGURE 2. Valves such as indicated in the branch lines 55 may be employed to control the injection of the fiuidizing medium in any desired fashion. The supply conduit 54 is in turn connected to a source of a gaseous fluidizing medium as indicated by conduit 56. The conduit 56 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 40 is provided with suitable means for venting the gaseous fluidizing medium therefrom. This, as shown, includes a conduit connection 60 opening from chamber 49 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 56 ahead of pump 57 as by means of a conduit connection 64. Valves 65 and 66 in

lines

63 and 64 respectively permit selective disposition of the gaseous medium as vented from the chamber 40.

In the apparatus as illustrated in FIGURES 2 and 3, the chamber 39 is provided with means for heating the finely 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 63 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 flue gases or other gases at high temperatures may be fed through conduit 68 from an exterior source. In either event, the solid material in chamber 39 is fluidized 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 69 opening from the chamber 39 into a cycylone separator 70. A clip leg conduit 71 opens from the bottom of separator 69 to a level below the surface level 57 of the fluidized solids in chamber 39, to return solids separated in cyclone from the gaseous medium therein. A vent line 72 from the separator 70 discharges the gaseous medium passed through the separator from chamber 39.

The conveyor as illustrated in FIGURES 2 and 3 is designated by the numeral 73. In the form of the apparatus shown, the conveyor enters and leaves the vessel through the well 36. Entering 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 baflle 34, under the baflie 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 im prove 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 charactertistics in the structure of the metal composing the object, and in accordance with certain well know 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 or fluidized finely divided or powdered material, such as the bed 2 of FIGURE 1, or the bed 49 of FIGURES 2 and 3. The bed is fluidized to a degree within a range of not substantially less than the level of incipient fluidity, at which level it is a quiescent fluidized bed, such as has been defined in Industrial and Engineering Chemistry," vol. 41, p. 1249, June 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 fluidizing medium. These levels of fluidization, of course, are governed by several factors, including actual density of the solid material, particle size, and the linear velocity of the gaseous fluidizing 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 200400 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 fluidization 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 FIGURES 1, 2, and 3, is fluidized by the injection of a gaseous, fluidizing medium upwardly through the mass of materials as by way of the supply conduit 4, manifold conduit 5, and discharge conduits 3 as shown in FIGURE 1, or, as in FIGURES 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 fluidizing 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 57 of FIG-

URES

2 and 3.

Further, the metal object is immersed in the bed of fluidized material and conveyed therethrough in a series of treating stages, which may or may not be sharply defined. In each stage heat is added or abstracted from the metal object by contact with and by the individual solid particles. By fluidization of the bed, the particles are maintained in substantially constant motion, and with the metal object immersed in the bed, the individual articles impinge upon the surface of the object at a rate determined by the degree of fluidization 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 the rate of heat transfer is very low, and the bed of solid material has a substantial insulating effect, while with high fluidity 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 fluidization 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 conductivity 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 coeflicient of 46 when fluidized 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 31 at 1.2 ft/sec. With carbon powder of 20-48 mesh, the heat transfer coefflcient was about 6 when unfluidized by gas at 0.5 ft./sec., 16.5 when fluidized at 1.35 ft./sec. and 30 when fluidized 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 fluidity and turbulence of fluidized 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 and 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 fluidized solid material. This is accomplished according to the present invention in one or more of several ways. For example, in the operation, as carried out in the apparatus of FIGURE 1, the solid material is circulated from vessel 1 to vessel 10 by way of the transfer conduit 13.

In conduit 13 a fluid 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 10. Further, the injected gases are employed to increase the fluidization of the mass of particles in the conduit and in the vessel 10, so as to produce a lower density in the mass of material in vessel 10 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 10, 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 differential temperatures between the metal object and the solid material also may be established and maintained, as shown in FIGURE 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 1. In addition, by heating or cooling the gaseous fluidizing medium introduced through

conduits

5, 4 and 3, the desired control of differential temperatures may be further supplemented.

In the apparatus of FIGURES 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 fluidizing gases through the conduit system including conduit 56, pump 57, and

conduit connections

54, 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 40 is produced, as in the apparatus of FIGURE 1, by increased fluidization 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 31 of FIGURE 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 manner, in combination with the dependent baflle 34, a seal or trap is established at the combined entrance and exit of chamber 40, such that solid particles which may be entrained by the fluidizing 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 fluidizing medium is injected into the well 36 at a rate to maintain fiuidization 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- URE 1.

As previously indicated, the gaseous fluidizing medium Which is passed through the solid materials contained in chamber 40 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 fluidizing medium are separated and returned to the main body by way of dipleg 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 fluidized solid materials is shown as being substantially below the indicated

levels

50 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 fluidization 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 fluidizing 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 ditficulties 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 fluidizing gas may include such gases as hydrogen cyanide or ammonium cyanide. It is a characteristic of the present invention, that by compan'son with the requirements of the prior art, the volume of gas required for operation is relatively small, 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 of 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 heattreating medium a material that will cause a change in the surface of the 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 fluidized 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 fluidized solid may be noncarbonaceous 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 heat ing, careful temperature control within i10 F. and avoidance of contact with air or furnace gases are easily met with the fluidized 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 powder or iron microspheres and employing a nitriding gas such as HCN, NH CN, 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 fluidized solid. The use of other fluidized metals such as aluminum, zinc, chromium, cadmium, tungsten, vanadium, cobalt, titanium, silicon, zirconium, molybdenum, tantalum, boron, manganese, and beryllium, at cementation temperatures which may range from about 650 F. to about 2550 F., together with relatively inert fluidizing gas such as nitrogen, hydrogen, helium, etc., results in the formation of quite even and tenacious alloy coatings.

The apparatus as described with reference to FIG. 1 is particularly adapted to accomplish an operation of the nature wherein a single metal object, or a group of such objects are subjected to a specific heat treatment of the nature of annealing, hardening, quenching, tempering or the like, as set forth according to the following examples.

Example I.Annealing Steel stock consisting of 3" x 12 bars having a carbon content of 0.33% is transported by conveyor 24 into a vessel such as designated by the numeral 1 in FIG. 1. The vessel 1 contains a bed of finely divided solid material such as fluidized foundry sand having a particle size of about -100 mesh and having a temperature of about 800 F. The steel stock is immersed in the fluidized solids below the level shown at 7. Flue gas is employed as a fluidizing gas for the solids, being introduced through line 5 at a rate such that the upward superficial gas velocity in vessel 1 is 1.5 ft./sec. and to produce turbulence 9 in the bed. The heat transfer coefficient under these con ditions is about 85 B.t.u./hr.) (sq. ft.) F.).

The temperature of the solids in vessel 1 is increased by introducing hot solids from heating vessel via line 15. Injection of hot flue gases through conduit 17 eifects circulation of solids from vessel 1 to heating vessel 10. The gas velocity in heating vessel 10 will normally be greater than that in vessel 1, and therefore in vessel 10 the density will be less and the level will be higher than in vessel 1. Under these conditions, fluidized solids will circulate from vessel 1, through line 13, into heating vessel 10 and thence through line into vessel 1. The rate of solids flow is controlled by the valves 14 and 16.

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 of soaking time being allowed for each inch of cross-section of the stock. During the heat soaking period, it is desirable to maintain the steel stock at the heat soaking temperature, and in order to reduce the loss of heat from the steel stock the fluidizing gas rate in vessel 1 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 coeflicient is about 3-5 B.t.u./(hr.) (sq. ft.) F.).

At the end of the soaking period, the temperature of the fluidized solids in vessel 1 and of the steel stock immersed therein is reduced slowly over a period of 4-6 hours. The temperature is decreased by passing a cooling medium such as water through cooling coils 8 in vessel 1. During the cooling period, the fluidizing gas velocity and thereby turbulence in the bed is increased to about 0.5 ft./sec., giving a heat transfer coeflicient of about 50 B.t.u./ (hr.) (sq. ft.) F), in order to increase the rate of heat transfer from the steel stock to the fluidized solids and from the fluidized solids to the cooling coils 8. When the temperature reaches 800 F. the annealed steel stock is removed from the fluidized bed by conveyor 24 and is allowed to cool further in air.

Example 1I.Hardening and Quenching Steel gears are heated to 1450 F. in a vessel such as vessel 1 of FIG. 1, 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 of 50'l00 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. superficial linear velocity. Under these conditions the bed is in a substantially turbulent condition, and the heat transfer coefficient 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 l560 minutes. At this point, the fluidized gas rate is descreased 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 coeflicient is about 330 B.t.u./(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.

Example IlI.-Tempering The steel gears hardened as in Example II are transported by a conveyor to another vessel such as shown in FIG. 1. The gears are immersed below the level of fluidized solids contained in vessel 1, the solids being 80- 400 mesh spent silica-alumina cracking catalyst obtained as a by-product in the petroleum industry. The temtaining a diluent gas such as carbon monoxide.

perature of the fluidized solids in vessel 1 is about 200 F., heat being supplied by hot solids circulated from the heating vessel 10 as was described in Example I. Flue gas relatively free of acidic gases and containing a relatively high proportion of carbon monoxide is supplied as fluidizing 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 Btu/(hr.) (sq. ft.) F). When the temperature of the gears approaches 800 F., generally in /2-2 hours depending on the size of the gears, circulation of hot solids from heater 10 is stopped and the fluidizing gas rate is decreased 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 coeflicient is in the range of about 2 to 10 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 fluidized solids and the steel gears immersed therein are slowly cooled to about 200 F., whereupon the tempered gears are removed from the fluidized solids.

Example I V.Carr'1 uriz in g An alloy steel part to be case hardened is introduced into the vessel 1 as shown in FIG. 1. Vessel 1 contains a 100-300 mesh fluidized solid carburizing agent, comprising hardwood charcoal, petroleurn coke and barium carbonate, at a temperature of about 200 F. The solids are fluidized by gas introduced through line 5. The fluidizing gasis a hydrocarbon gas, such as propane, con- 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 1700 F. by circulating through the vessel 10, with heat being supplied to the solids in heater 10 by hot flue gas of a reducing nature. When the temperature of the steel stock ap proaches 1700 F., the fluidizing gas rate is decreased to a value in the range of about 0.1 to 0.3 ft./sec. and circulation from vessel 10 is stopped. The steel stock is then beat soaked at 1700 F. for about 4-6 hours, which gives a carburized case depth of from about 0.045 to about 0.060 inch. Cooling fluid is then passed through coils 8 and the fluidizing gas rate is increased to a value in the range of about 0.75 to 1.5 ft./sec. When the temperature of the steel stock reaches 1450 F., it is removed from the carburizing agent and is subjected to a quenching operation as described in Example II.

Example V.Sh erardizing Sheet iron plates to be coated with zinc by sherardizing are immersed in a fluidized mixture comprising equal parts by weight of zinc powder and zinc oxide powder having a particle size in the range of about to 300 mesh. The powder is contained in vessel 1 as shown in FIG. 1 and is fluidized 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 flue gas of a reducing nature. When the powder in vessel 1 reaches a temperature of 700 F., powder circulation is stopped and the fluidizing gas rate is decreased to substantially zero. After the sheet iron plates have undergone a heat soaking period of about three hours the 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 plates is reduced to ambient temperature. The coated plates are then removed from vessel 1, and a fresh batch of plates is introduced thereto.

The apparatus as shown in FIG. 2, is particularly adapted to employment where the 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 which 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 a 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 bafile 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, th sand is heated and fluidized 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 40 through the well 36 by means of conveyor 73. As the steel stock is passed through the travel path along one side of bafile 41 from baffle 34 to partition 37, flue gas is introduced as a fluidizing medium by way of the conduit 54, branch lines 55 and the 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 coeflicient of about 85 B.t.u./(hr.) (sq. ft.) P.) Then as the metal stock progresses along the travel path and reaches a temperature of about 1500 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 battle 41, and continuing injection at such 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 fluidizing medium, the heat transfer coefiicient of the solids is reduced to from about 3 to B.t.u./(hr.) (sq. ft.) F.). By suitable regulation of the conveyor speed, this may be acr complished as the stock enters the return travel path, and in this area the iluidizing gas velocity is increased to about 0.5 ft./sec., increasing the heat transfer coefficient of the sand in this area to about 50 B.t.u./(hr.) (sq. ft.) F.). Preferably, in the area immediately adjacent the bafile 34, the rate of injection of the fiuidizing gas is again increased, as to substantially the rate of injection in the initial stage. At this point, the spacing of bafile 41 from the baffle 34 permits circulation of the solid material around the baffie 41 such as to aid in heat recovery and heat equalization at the combined inlet and outlet. The stock is then removed from the chamber 40 through the well 36 by further progress of the conveyor. In the well 36, the fluidizing 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 12 or other undesirable surface deposits and coatings. Such surface conditioning is accomplished substantially as a result of the metal objects scouring action of the finely divided solid particles when they are set in motion by iluidization and impinge against the metal objects during treatment thereof.

What is claimed is:

1. In a process for improving the physical properties of individual metal objects, the steps which comprise maintaining in a substantially confined treating zone a bed of finely divided solid heat transfer particles substantially incombustible under the conditions of the treatment, passing a gaseous fluidizing medium upwardly through said treating zone to fluidize said finely divided solid heat transfer particles and to impart motion to the individual solid particles in said bed, introducing a metal object to be treated into said treating zone and substantially completely submerging said metal object in said bed of finely divided fluidized solid heat transfer particles in said treating zone, maintaining a predetermined heat exchange temperature relation between said heat transfer solid particles of said bed and the metal object to be treated, subjecting said metal object within said treating zone to direct impinging contact of individual particles of said finely divided fluidized solid heat transfer particles in motion and in said heat exchange relation thereto for a period of time sutficient to accomplish improvement of the physical properties of said metal object and then withdrawing said treated metal object from such contact and from said treating zone.

2. In a process according to claim 1 in which said treating zone is a heat treating zone, the steps which comprise positively conveying said metal object through said fluidized bed of solid particles in a portion of said zone which is maintained at successively increasing temperatures in the direction in which said metal object moves until said object reaches the maximum temperature desired, and then further conveying said object through another portion of said zone maintained at successively decreasing temperatures.

3. In a process according to claim 1 in which the treating zone is a heating zone and in which said metal object is to be annealed, the steps which comprise contacting said metal object with solid particles maintained at an increasing temperature gradient until said object reaches an annealing temperature, and thereafter maintaining the solids contacting said object at gradually decreasing temperature for gradual elimination of heat from said object.

4. In a process according to claim 1 wherein the metal object is to be annealed, the steps which comprise introducing a heated fluid exchange medium into heat exchange relation with said solid heat transfer particles, introducing said metal object into said treating zone at a temperature lower than that of said particles, maintaining said object in said treating zone for a time period sutlicient to raise the temperature of said object high enough for annealing, and thereafter introducing a cooler heat exchange medium in contact with said solid particles to gradually reduce their temperature and thereby reduce the temperature of said metal object.

5. Process according to claim 1 in which the metal object is gradually raised in temperature by gradually increasing the velocity of said gaseous fluidizing medium until the metal object reaches the desired temperature and then is gradually decreased in temperature by gradually lowering the velocity of said medium.

6. A process according to claim 1, in which the finely divided solid particles are non-reactant with the metal of the object submerged in said bed.

7. In a process for improving the physical properties of individual metal objects, the steps which comprise maintaining a bed of 'finely divided solid heat transfer material in a substantially confined treating zone, passing a gaseous fluidizing medium upwardly through said zone to fluidize said finely divided solid material and to impart motion to the individual solid particles in said bed, contacting at least a portion of said bed with a fluid heat exchange medium at a lower temperature than said solid particles whereby to establish and to maintain a predetermined heat exchange temperature relation between said bed and said metal object, introducing a metal object to be treated into said treating zone and substantially completely submerging said metal object in the bed of fluidized solid particles in said zone, said metal object being at a higher temperature than that of said finely divided solid particles in said bed, subjecting said metal object within said zone to direct impinging contact of individual particles of said fluidized solid material in motion and in heat exchange relation thereto for a period of time sufficient to accomplish improvement of the physical properties of the metal object and then withdrawing said treated metal object from such contact and from the treating zone.

8. In a process for improving the physical properties of individual metal objects, the steps which comprise maintaining a bed of finely divided solid heat transfer material in a substantially confined treating zone, passing a gaseous fluidizing medium upward-1y through said zone to fluidize said finely divided solid material and to impart motion to the individual solid particles in said bed, indirectly contacting at least a portion of said bed with a fluid heat exchange medium whereby to establish and to maintain a predetermined heat exchange temperature relation between said bed and said metal objects, introducing a metal object to be treated into said treating zone and substantially completely submerging said object in the bed of fluidized solid particles in said zone, subjecting said metal object within said zone to direct impinging contact of individual particles of said fluidized solid material in motion and in heat exchange relation thereto for a period of time suflicient to accomplish improvement of the physical properties of the metal object and then withdrawing said treated metal object from such contact and from the treating zone.

9. In a process for improving the physical properties of individual metal objects, the steps which comprise maintaining a bed of finely divided solid heat transfer material in a substantially confined treating zone, passing a gaseous fluidizing medium upwardly through said treating zone to fluidize said finely divided solid material and to impart motion to the individual solid particles in said bed and at a flow rate within the limits of not less than that required to produce incipient fluidity of said bed, nor substantially more than that required to produce and maintain a turbulent bed of said finely divided solid heat transfer material, said bed being in a pseudo-liquid condition with individual particles thereof in motion, contacting at least a portion of said bed with a fluid heat exchange medium whereby to establish and to maintain a predetermined heat exchange temperature relation between said bed and the metal objects, introducing a metal object to be treated into said treating zone and substantially completely submerging said metal object in said bed of fluidized solid particles, in said treating zone, subjecting said metal object within said treating zone to direct impinging contact of individual particles of said fluidized solid material in motion and in heat exchange relation thereto for a period of time suflicient to accomplish improvement of the physical properties of said metal object and then withdrawing said treated metal object from such contact and from said treating zone, establishing and controlling said predetermined heat exchange temperature relation between said bed and said metal object by increasing and decreasing the flow rate of said gaseous fluidizing medium within said flow rate limits to increase and decrease the frequency of impinging contact of said solid particles of said bed with said metal object thereby to increase and decrease the rate of heat transfer between said solid particles and said metal object as required to accomplish said predetermined heat exchange temperature relation.

10. In a process for improving the physical properties of individual metal objects, the steps which comprise maintaining a bed of finely divided solid heat transfer material in a substantially confined treating zone, passing a gaseous fluidizing medium upwardly through said treating zone to fluidize said finely divided solid material and to impart motion to the individual solid particles in said bed, contacting at least a portion of said bed with a fluid heat exchange medium whereby to establish and to maintain a predetermined heat exchange temperature relation between said bed and the metal o bject, introducing a metal object to be treated into said treating zone and substantially completely submerging said object in said bed of fluidized solid particles in said treating zone, subjecting said metal object within said treating zone to direct impinging contact of individual particles of said fluidized solid material in motion and in heat exchange relation thereto for a period of time suflicient to accomplish improvement of the physical properties of said metal object and then withdrawing said treated metal object from such contact and from said treating zone, maintaining said fluidized bed of finely divided solid material in a predetermined heat exchange relation to said metal object by continuously withdrawing portions of said finely divided solid material from the lower part of said treating zone, passing said withdrawn portions into the lower part of a substantially separate heat exchange zone, and into heat exchange relation with a fluid heat exchange medium therein, fluidizing said withdrawn portions in said heat exchange zone, maintaining said portions in said heat exchange zone at a lower density than said bed of finely divided solid material in said treating zone, and continuously returning said withdrawn portions to said bed of finely divided solid material in said treating zone and to an upper part thereof.

11. A process according to claim 10, in which said fluid heat exchange medium in the separate heat exchange zone is the combustion product obtained by burning fuel and air within said separate heat exchange zone.

12. A process according to claim 10, in which the fluid heat exchange medium is a gaseous material introduced directly into the withdrawn portions within said heat exchange zone, fluidizing said portion.

13. An apparatus for heat treating metal objects, comprising a first walled chamber adapted to contain a body of finely divided solid material, a second walled chamber adapted to contain a supplementary body of said material, means providing for controlled communication between said chambers from a lower portion of said first chamber to a lower portion of said second chamber and from an upper portion of said second chamber to an upper portion of said first chamber, means disposed in at least one of said chambers for passing a fluid heat exchange medium into heat exchange relation to a body of finely divided materials contained by said chambers, conduit means disposed in the bottom of said first chamber including fluid discharge means opening upwardly therefrom to inject a gaseous fluidizing medium into the body of finely divided material contained by said chamber, and a conveyor means associated with said first chamber adapted to carry a metal object through said first chamber while it is immersed in the body of finely divided solid material contained by said chamber.

14. In a process for heat treating metal objects wherein a heated metal object is quenched by introducing said metal object into a confined quenching zone, immersing said object in a body of finely divided solid material contained in said zone and conveying said object in a confined path through said zone while so immersed in the body of finely divided solid materials contained therein and in heat transfer relation thereto, wherein said body of finely divided solid material is maintained in a fluidized condition by the injection of a fluidized medium into and through said body of material, imparting motion to the individual particles and producing impinging contact thereof with the metal object when immersed therein, and

wherein said finely divided solid materials are in heat exchange relation with a relatively cool heat exchange fluid whereby the body of finely divided solid material is maintained at a predetermined diiferential temperature relationship to said metal object, the improvement which comprises the steps of initially injecting said gaseous fluid medium into said body of finely divided solid material at a linear velocity of from about 1 to about 5 feet per second thereby maintaining a relatively high frequency of impact of said individual particles of said solid material upon said metal object, and thereby producing a rapid rate of heat transfer between said metal object and the solid material particles and then, while substantially maintaining said temperature diiferential between said metal object and the finely divided solid material, reducing the linear velocity of said gaseous fluidizing medium to a level substantially below that at which said medium was initially injected, which level is above the level of incipient fluidity of said finely divided solid material and not substantially below a linear velocity of .05 foot per second, thereby substantially reducing the fluidity of said body of finely divided solid material and the frequency of impact of individual particles upon said metal object thereby reducing the rate of heat transfer between said metal object and the finely divided solid material, and then Withdrawing said metal object from said body of finely divided solid material and said zone.

References Cited in the file of this patent UNITED STATES PATENTS 414,122 Roberts Oct. 29, 1889 489,194 Mustin Jan. 3, 1893 1,974,541 Osterman Sept. 25, 1934 2,322,777 Prunell June 29, 1943 2,348,009 Johnson May 2, 1944 2,393,636 Johnson Jan. 29, 1946 2,459,836 Murphree Jan. 25, 1949 2,509,866 Hemmiuger May 30, 1950 2,516,516 Holden July 25, 1950 2,670,573 Sullivan Mar. 2, 1954 FOREIGN PATENTS 720,359 Great Britain Dec. 15, 1954

Claims

1. IN A PROCESS FOR IMPROVING THE PHYSICAL PROPERTIES OF INDIVIADUAL METAL OBJECTS, THE STEPS WHICH COMPRISES MAINTAINING IN A SUBSTANTIALLY CONFINES TREATING ZONE A BED OF FINELY DIVIDED SOLID HEAT TRANSFER PARTICLES SUBSTANTIALLY INCOMBUSTIBLE UNDER THE CONDITIONS OF THE TREATMENT, PASSING A GASEOUS FLUIDIZING MEDIUM UPWARDLY THROUGH SAID FER PARTICLES AND TO IMPART MOTION TO THE INDIVIDUAL SOLID PARTICLES IN SAID BED, INTRODUCING A METAL OBJECT TO BE TREATED INTO SAID TREATING ZONE AND SUBSTANTIALLY COMPLETELY SUBMERGING SAID METAL OBJECT IN SAID BED OF FINELY DIVIDED FLUIDIZED SOLID HEAT TRANSFER PARTICLES IN SAID TREATING ZONE, MAINTAINING A PREDETERMINED HEAT EXCHANGE TEMPERATURE RELATION BETWEEN SAID HEAT TRANSFER SOLID PARTICLES OF SAID BED AND THE METAL OBJECT TO BE TREATED, SUBJECTING SAID METAL OBJECT WITHIN SAID TREATING ZONE TO DIRECT IMPRINGING CONTACT OF INDIVIDUAL PARTICLES OF SAID FINELY DIVIDED FLUIDIZED SOLID HEAT TRANSFER PARTICLES IN MOTION AND IN SAID HEAT EXCHANGE RELATION THERETO FOR A PERIOD OF TIME SUFFICIENT TO ACCOMPLISH IMPROVEMENT OF THE PHYSICAL PROPERTIES OF SAID METAL OBJECT AND THEN WITHDRAWING SAID TREATED METAL OBJECT FROM SUCH CONTACT AND FROM SAID TREATING ZONE.