US2872176A - Rotary furnaces - Google Patents

Description

" Fem; 1955 JiBORGNON.

ROTARY FURNACES Filed Oct. 25, 1955 INVENTOR BY 0 AT TORN EY States ROTARY FURNACES Jean Borgnon, Paris, France, 'assignor to Diffusion Alloys Limited, London, England, a British company Application October 25, 1955, Serial No. 542,723

1 Claim. (Cl. 263-34) the finished treatment.

Furnaces of this kind generally comprise an externally heated cylindrical retort rotated about its axis, which is horizontal or slightly inclined, at a speed so chosen that temperature regularity is obtained and the heated objects are deformed to the least possible extent. Usually the retort is closed at one end by a fixed base and at the other end by a movablecover, but is occasionally open at both ends and forms a kind of tunnel through which pass the objects being treated, the said objects flowing through the tunnel either through the agency of a screw conveyor or merely due to the effect of gravity owing to the inclination of the retort.

When a closed retort is used, a furance of this kind should be adapted to permit of all heat treatments being carried out in any kind of atmosphere, but in most cases (white annealing, chromising, etc.) the desired result is not obtained because the hot objects discharged from the retort come into contact with ambient air which, by oxidation, modifies the final conditions of the treatment. In addition, with furnaces of this kind it is almost impossible to cool the treated objects at a controlled speed and in a controlled atmosphere unless the furnace is stopped at each operation, with the result that considerable heat has to be expended to set the furnace going again.

This invention-has an object to develop the advantages of furnaces of this kind and to adapt the same to effect substantially continuously controlled atmosphere treatments comprising cooling at a controlled speed.

An improved rotary furnace according to the invention comprises a rotary retort closed at one end, open at the other end and mounted on a fixed pivot and a removable retort which is a good heat conductor and the external dimensions of which are such as to permit the said removable retort to be engaged axially in the rotary retort, the said removable retort being closed at one end by a fixed base and at the other end by a cover with the interposition of fluid-tight packing.

The removable retort forms a fluid-tight container rotating with the rotary retort, and the temperature inside the said removable retort is more evenly distributed than in the rotary retort because the walls of the removable retort are good heat conductors and because the rotary retort acts as a distributing screen, being intermediate the flames or similar heating means and the removable retort.

Since the removable retort is fluid-tight, it'is possible to maintain therein any desired atmosphere before, during and after treatment and, because of its mobility, to cool the removable retort and the contents thereof acatent cording to any desired law of temperature decrease without the treated objects coming into contact with air. For this purpose, the removable retort ismerely withdrawnfrom the furnace and placed in a suitable enclosed space, while the furnace can immediately receive a new charge of objects contained in another removable retort. This method makes it possible to obviate the considerable heat loss unavoidable heretofore when the furance had to be left to cool with the objects therein.

In one advantageous embodiment of the invention, the removable retort is of a length sufficient to permit of the cover and fluid-tight packing being situated relatively far away from the very hot zone of the furnace in order to prevent any deformation likely to be detrimental to fluid-tightness, and the effective inner space can be limited to the heated zone in regular fashion by a false bottom on the cover.

Preferably the cover is provided with a gas outlet tube and with a cock which is kept open for the entire heating period to permit evacuation of the gas contained in the removable retort, which cock is closed as soon as it is required to carry out a treatment necessitating a flow of gases inside the retort, the cover can also comprise an intake tube which extends right down to the bottom of the retort so as to ensure that the entire inner space is scavenged by the flowing gases. A similar tube can be provided if cooling is to be carried out in a neutral or reducing atmosphere, the last-mentioned tube then being connected to an appropriate external gas source. The fixedbase of the removable retort may or may not comprise a closed inner tube forming a recess in which engages a similar tube which is fast with the rotary retort and which can accommodate temperature controllingrneans.

When the. objects to be treated have been placed in a removable retort according to the invention, the same is introduced into the rotary retort of the furnace which, if required, is previously heated. Upon terminationof the heat cycle to be effected, the removable retort is withdrawn from the furnace either by tilting of the latter or by the removable retort being drawn on a carriage or slide, the removable retort then being cooled outside the furnace under the conditions desired.

In all cases, and merely by operating one or two cocks, cooling of the treated objects can be carried out under optimum surface physico-chemical conditions;

A removable retort according to the invention can be made of any desired material, but it should be noted that from the viewpoint of thermal efficiency the heat inertia of the material should be high. Thus, the retort accord ing to the invention is preferably made of metal with thin walls which follow as intimately as possible the internal shape of the rotary retort, but it is obvious that the invention is not limited to this kind of wall and includes retorts of the kind hereinbefore described and made of other materials such, for example, as refractory materials, more particularly silicon carbide which has a satisfactory heat transfer coefficient at high temperatures.

The invention will now be further described by way of example with reference to the accompanying drawing, which is a view in axial diagrammatic section of a furnace according to the invention.

Referring to the drawing, the furnace comprises a refractory brick chamber 1 closed at its forward end by an appropriate front 2 through which extends the head of a rotary retort 3 which is of cylindrical form and which is adapted to rotate within the chamber 1 about a substantially horizontal axis. For this purpose the retort 3 rests upon three pairslof rollers 4 which are placed symmetrically in each pair with respect to the axial vertical plane of the retort 3. The latter extends rearwardly by a shaft 5 extending with clearance through that wall In of the furnace which is at the opposite end to the front 2. Keyed to the shaft 5 is a gearwheel 6 or any otherpequivalent means adapted to permit the retort 3 to 'be rotatedby anysuitable driving means.

retort 7 may be introduced into and removed from the retort 3by sliding axially. The space between the walls of the two retorts should be small to ensure efiicient heat transfer.

In the embodiment illustrated in the drawing, the shaft 5, which is fast with the rotary retort 3, penetrates some distance into the'sarne and is closed at its inner end. The shaft 5 therefore forms a housing or chamber 8 adapted to receive temperature-controlling means, the indications of which reveal, in conjunction with error tables, the exact temperature inside the removable retort 7. That end thereof which is inside the furnace is closed by a base 9 having in its centre a re-entrant part 9a which closely follows the shape of the end of the shaft 5. The opposite end of-the removable retort 7 has an outer radial flange 10. against which bears a closure cover 11 with the interposition of suitable fluid-tight packing 12 such as an asbestos gasket which is clamped between the flange 10 and cover 11 through the agency of fixing bolts 13 on the latter.

It will. be seen in the drawings that the cover 11 is at some distance from the front 2 so as to be removed as -far as possible from the heat of the furnace and so as to obviate deformations of the said cover, such deformations being possible ways in which air may enter the retort 7. In order that that part of the retort projecting beyond the furnace, inside which part the temperature is less elevated, may not be used for the treatments to be carried out, the cover 11 has in its interior a kind of box l l forming a false bottom, the length l of which is such that the wall 14a of the false bottom is located inside the chamber 1 and substantially at the same distance from the front 2 as is the base 9 from the rearward wall 1a of the said chamber.

A tube 15 is welded to the centre of the false bottom 14a and extends axially in fluid-tight manner through the cover 11. The outer end of the tube 15 bears a .cock 16 adapted to allow the entry of ambient air into the interior of the removable retort 7 or to isolate the same. nected through the medium of rotary packing 17 of any known type. A tube 18 of smaller diameter than the internal diameter of the tube 15 is welded axially to the centre of the re-entrant part 9a on the base The tube 18 is pierced with radial holes 19 in the vicinity of the part 9a, and is of such length that its free end penetrates a short distance inside the tube 15 when the cover 11 of the removable retort 7 is in position. A third tube 20 is fixed axially to the centre of the cover 11 and is of such diameter as to slide with very slight clearance in the tube 13, the free end of the tube 20 being slightly set back from the radial orifices 19 when the cover 11 is in position. The tube 29 is extended externally up to a rotating packing (not shown) coaxial with the retort 7, and a cock which can be the cock 16 if the same is of the multi-way type, permits the tube 20 to be connected to an appropriate gas source or enables such a connection to be broken.

4 As has already been stated, the retort 7 is made of a Preferably the tube 15 and cock 16 are intercon- I .4, wall of the retort 7 is substantially in contact over its entire periphery with the inner wall of the rotary retort 3, so that heat transfer is readily efiected from the hot gases flowing in the chamber 1 to the objects to be reated which are contained in the retort 7, the double screen which is formed by the conductive walls of the retorts 3 and 7 ensuring regular temperature distribution inside the retort 7. The rotation of the latter is effected merely by the two retorts rubbing together.

The furnace is operated as follows: The objects to be treated are charged with or without treatment material or insulating material, into the annular-space surrounding the tube 18 of the removable retort 7, charging being terminated when the level is reached corresponding to the position of the false bottoml4a of the cover. The latter is placed in position, the tube 20 first penetrating into the tube 18 which keeps a path free for the tube 20 through the centre of the objects to be treated, and the retort is hermetically sealed by tightening the nuts on the bolts 13 after the asbestos gasket 12 has been positioned. The cock 16 is opened so as to place the interior of the retort in communication with the ambient air, and the latter retort is so placed in the rotary retort through the agency of a suitable means that the end of the shaft 5 engages in the re-entrant part 9a of the bottom of the retort.

If the treatment to be carried out does not need a special internal atmosphere (ordinary annealing) or if this atmosphere is created in the heart of the mass being treated, the cock 16 is kept open until the retort reaches the treatment temperature and is then closed for the duration of the treatment to prevent any entry of air. If the treatment needs a special atmosphere, for example a reducing, halogenated or other atmosphere, the internal tube 20 is connected to an appropriate gas source as soon as the retort 7 is positioned in the furnace and the cock 16 is placed (or the special cock is opened it a special cock is provided for gas inlet and outlet) in the position corresponding to the entry of a relatively rapid flow of gas which, upon issuing from the tube 20, passes through the orifices 19 in the base of the retort and expels the air contained therein. The air issues from the retort through the annular gap between the tubes 15 and 18, while the tube 15, rotating packing 17 andcock 16 place the interior of the retort in communicationeither with atmosphere or with an exhaust chimney or with an absorptive tank if the gas is a harmful one. Once scavenging has been eflected, the speed of gas flow can be reduced to the value required for the process being used.

When the treatment is terminated, the retort 7 is cut off from the atmosphere by suitable operation of the cock 16 or of the two cocks if need be, the retort 7 is withdrawn from the retort 3 and, according to the cooling cycle to be carried out, is placed in a heated chamber, in a medium which is a poor heat conductor or just in the open air. Depending upon requirements, the retort can be kept closed for the duration of cooling or a flow of appropriate gas maintaining the desired atmosphere in the heart of the retort can be caused to circulate therein.

It will be apparent from the foregoing that the improved rotary furnace according to the invention readily enables all treatments to be carried out on objects present in large quantities, for example, cementing followed by slow cooling permitting machining to be begun before tempering, white annealing in a controlled atmosphere of metals sensitive to rapid cooling, the nitridation of steel objects, chromising with a return to ambient temperature in the complete absence of oxygen, etc.

It is obvious that if no gas flow is necessary for carrying the intended treatment into effect, the gas inlet tube 20 can be omitted, as can also the sheath 18 thereof fixed to the base 9a of the retort, the objects to be treated then being positioned more readily.

By way of example of a treatment which can be carried out without difficulty with the aid of the improved furnace according to the invention, the results will be hereinafter given which were obtained during tests in a method of chromising steel objects.

Example The diffusion method used was that described in United Kingdom specification No. 646,637, using chromous iodide formed in situ fromits elements charged in solid form. Since no gas was to flow in the retort during this treatment, the retort did not comprise tubes 18 and 20 and a single-way tap 16 was used. The retort was charged with steel articles soft cemented to 0.12 to 0.13 mm. and with semi-hard steel articles with 0.3 to 0.35% carbon. These articles were immersed in a chromising mixture comprising 66 parts by weight of pulverised ferrochromium, 66 parts by weight of chromium, 33 parts by weight of alumina and 0.2 part by weight of ammonium iodide. After the removable retort had been charged and hermetically sealed through the agency of the cover 11 with the false bottom 14, the removable retort was inserted in a conventional rotary furnace previously heated to 900 C. in which the rotary retort revolved at a rate of three revolutions per minute. The cock 16, which was open at charging, was closed as soon as the air contained in the retort had been partly expelled by expansion, and was kept closed for the duration of heating and cooling. The furnace temperature was maintained at 900 C. for the whole operation, and after 4 hours heating the removable retort was withdrawn and cooled in the open air with the cock 16 closed, while another removable retort filled with articles to be treated was placed in the furnace. After cooling, the cock 16 was opened and the articles extracted from the retort had a silver-grey appearance but showed no trace of oxidation, thus demonstrating a complete absence of air in the retort. The diffused layer had a uniform thickness of about 0.007 mm., the mean hardness of the layer being in the region of 1600 Vickers. The uniform thickness of the difiused layer demonstrates the uniformity of the internal atmosphere and the equal distribution of temperature in the removable retort.

It is obvious that modifications can be made to the rotary furnace hereinbefore described, more particularly by the substitution of equivalent technical means, without for that reason departing from the scope of this invention. More particularly, the retort 7 could be positively driven by the retort 3, for example through the agency of the shaft 5. That end thereof located in the retort 3 could be of polygonal cross-section, in which case the re-entrant part 9a on the base of the retort 7 would be of matching cross-section.

1 claim:

A rotary furnace comprising a closed chamber, a first retort rotatably mounted therein and having an open end projecting beyond one end wall thereof, a second retort closely fitted in said first retort and insertable and removable through the open end of the first retort, said second retort having an open end projecting beyond the open end of the first retort, a cover closing the open end of the second retort, a fluid-tight packing between said cover and said second retort, a tube passing through said second retort, a cock in said tube for placing said second retort in communication with external air and for cutting off said second retort therefrom, and a second tube having radial apertures and extending through said second retort to a point close to the end thereof opposite said cover, a third tube in said second tube for connecting said second tube to a source of gas, and a cock for controlling gas flow through said third tube.

References Jilted in the file of this patent UNITED STATES PATENTS 528,016 Naef Oct. 23, 1894 1,177,218 Wharrad Mar. 28, 1916 1,414,614 August May 2, 1922 1,470,887 Sieck Oct. 16, 1923 1,789,149 Machlet Ian. 13, 1931 2,469,078 Robeson May 3, 1949 2,646,268 Jackson July 21, 1953

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