US 2691515 A
Description (OCR text may contain errors)
Oct. 12, 1954 J. D. NESBITT 2,691,515
FORGE FURNACE CONTROL Filed May 19, 1949 INVENTOR.
Patented Oct. 12, 1954 FORGE FURNACE CONTROL John D. Nesbitt, Sylvania, Ohio, assignor to Surface Combustion Corporation, Toledo, Ohio, a
corporation of Ohio Application May 19, 1949, Serial No. 94,216
8 Claims. 3%.
The present invention relates to method and apparatus for controlling the application of heat to work pieces that are being advanced through a furnace chamber for heating to elevated temperature preparatory to hot working and the general object of the invention is to provide for the application of heat to the work in a manner to reduce formation of scale on the work to a uniform minimum.
For a consideration of what I consider to be novel and my invention, attention is directed to the following specification and the claims appended thereto.
In the accompanying drawings, forming part of the specification:
Fig. 1 is a schematic representation of the present invention.
Fig. 2 is a longitudinal vertical sectional view of the type of furnace schematically shown in Fig. 1.
Fig. 3 is a transverse vertical section of Fig. 2.
Fig. 4 is a view similar to Fig. 3 with some modifications.
In its preferred form, the furnace chamber is cylindrical in transverse cross section and is defined by a structure 5 embodying an inner annular refractory lining 6, the structure being supported on a pair of longitudinally extending girders l in a manner to permit free thermal expansion and contraction of said structure thereon, as indicated by rollers 23, with reference to the anchored end iii of said structure. The charge or work entering end of the furnace is at the left as viewed in Fig. 2. In Fig. 1, the furnace is diagrammatically indicated by the three alined rectangular figures A, B and C, respectively, to indicate three successive heat zones in the furnace chamber. In Fig. l, the charge end of the furnace is the left hand end ll of zone A and the discharge end is at the right hand end [2 of zone C.
If the entire work piece 23 (see Figs. 2 and 3) must be heated to forging temperature, the pieces will ordinarily be pushed through the furnace on elevated skid rails M. On the other hand if the work pieces are elongated bodies 45 (see Fig. 4) of which only one end requires to be heated, said pieces will project into the furnace through a longitudinally extending slot in the sidewall thereof from any preferred supporting and conveying means 16 outisde of the furnace chamber.
Heat is produced in the furnace chamber by longitudinally extending rows of burners 2B arranged to fire tangentially thereinto so that the fiame of combustion may flow along the curved surface of the refractory lining 6 whereby to conconstitute said lining a source of radiant heat for heating the work pieces advancing through the furnace. Two such rows of burners will ordinarily suffice. Where the work pieces extend into the furnace through a slot as in Fig. 4, the burners are preferably arranged at opposite sides of said slot so that one set of burners fires clockwise and the other counterclockwise into the furnace chamber; otherwise, as shown in Fig. 3, all of the burners fire in the same general direction of rotation.
The burners are manifolded into groups 2|, 22 and 23 corresponding to the heating zones A, B and C. The respective manifolds for said groups are indicated at 24, 25 and 26, respectively. To minimize explosion hazards, each manifold has its own air and fuel mixing device '21, 28 and 29, respectively. The main gas supply line is indicated at 3| and the main air supply line for air under pressure is indicated at 32. Branch fuel gas lines 33, 34 and 35 conduct the fuel to the several mixing devices and branch air lines 36, 37 and 38 conduct the air thereto. Each branch air line is provided with a control valve 42, 43 and 44, respectively. The relative proportions of air and gas to each mixing device are automatically maintained by a gas governor in each branch gas line, the governor being responsive to changes in pressure in the associated branch air line through a connecting pipe 4| in a manner to vary the gas flow in accordance as the air flow is varied. Ratio control means of this type are well known in the art.
In the present invention, the final heat zone C is not a soaking zone where the temperature of the work pieces is equalized but is a heating zone wherein the thermal head is maintained substantially higher, say 400 deg. F. higher, than the discharge temperature, say 2,300 deg. F., of the work pieces. When the demand for heated work is at a maximum, the heat input to the zones A, B and C" is at a maximum. When the production demand for heated work is reduced, the rate of advance of the work pieces through the furnace must be correspondingly reduced. This presents the problem of how best to control the furnace to avoid overheating of the work and to prevent excessive scale formation on the work due to longer time in the furnace. In the present invention this problem is solved not by reducing the thermal head of the final heat zone C but by controlling the heat input to the first heat zone A and then to the next heat zone B to such degree as may be necessary to insure that by the time the work piece has passed through the final high temperature heat zone C, it will not have been at scaling temperature substantially longer than when the work was being advanced through the furnace at maximum production rates with maximum heat input to zone A as well as the other zones B and C. The means for effecting such heat input control to the first zone A comprises a heat sensing element 45 mounted to be responsive to changes in temperature within the final heat zone C and more especially to the temperature of the work pieces in said zone. The said heat sensing element 45 operates a control instrument 46 which in turn operates a valve adjusting means 41 associated with the heat input control valve 42 for zone A to reduce the heat input to zone A as operating conditions may require. If still further reduction of heat input is required a valve adjusting means 48 associated with the control valve 43 for zone B is next operated on by the instrument 46 as will now be readily understood. It is preferred to separate zone A from the next succeeding zone B by means of a transversely extending heat shield 49 so that radiant heat in zone A is localized to that zone. The heat input to zone C to maintain the desired elevated temperature therein is determined by the setting of the control valve 44 for said zone. Hand setting of said valve 44 will ordinarily suffice but automatic control means may be employed.
In conventional forge heating furnaces there are two major types of heating equipment, one of which is characterized by heating the wor to the temperature at which the forging will be done and soaking the pieces uniformly to that temperature, and the other method is to heat the pieces in a high termal head furnace to the nominal forging temperature and removing the pieces immediately without soaking to a uniform temperature. The first method has the advantages of uniformity of temperature throughout each piece and from piece to piece, but the disadvantages attendant on soaking at temperature, namely, of large grain growth and scaling effects. The second method has the advantage of minimizing grain growth and scaling effects and is capable of producing within a work piece desirable temperature differentials but is not flexible enough to maintain these conditions at varying production rates from the furnace because of the variations in heating rate which result therefrom.
The present invention combines the advantages of both systems by maintaining the rapid heating with its attendant reduced scaling and grain growth effects and controls thermal gradients within the piece and also maintains the uniformity from piece to piece. This is done as disclosed by maintaining in the discharge zone a maximum heating rate prior to discharge and supplying in a prior zone the remainder of the heat necessary to bring the work to temperature.
A comparison of the two prior methods of heating, that is to say the rapid heating and the soaking at temperature, may be shown as follows:
In heating a billet 4" x 4" x 7" in a furnace whose wall temperature is 2700 deg. F., in ten minutes the surface of the billet will reach a temperature of 2300 deg., at which time the core temperature will be 2130 deg. F. Somewhat less than one minute after leaving the furnace the surface and core temperatures will both be 2200 deg. R, which is ideal for forging. Heating the same billet to 2200 deg. F. and soaking it at that .1000 pieces.
temperature prior to forging, the surface of the billet will necessarily cool somewhat on removing from the furnace and before forging. On the other hand, in a rapid heating furnace advantage may be taken of thermal gradients within the piece by forging at the proper time after heating. It will thus be seen that rapid heating furnaces are capable of producing desirable temperature differentials as illustrated but it is necessary to maintain these effects uniformly in every piece of work, irrespective of the production rate of the furnace. In the present invention this is accomplished by maintaining the heating rate in the discharge zone substantially constant, and controlling the final work temperature by control of heat input in a prior heating zone.
In a furnace with three zones whose wall temperature is 2700 deg. F. if a billet 1%" x 3 x 23" is heated to a nominal temperature of 2250 deg. F. while it is advancing at maximum rate, that is to say, at the full capacity of the furnace it will have about .003" of scale. If the same piece is heated in the same furnace at of maximum production rate and the furnace temperature in the last zone is reduced to make a holding zone for control of the final temperature of the work, the scale will be about .050" and die life on a given forging will be about If the same piece were heated in the same furnace, throttling the heat input to the entire furnace as a unit, the scale would be .017" and the die life would be about 12,000 pieces. Heating the same piece in the same furnace but controlled according to the present invention by maintaining the discharge zone at a maximum heating rate and throttling the charge zone, the scaleprocluced would then be .005" and the die life about 18,000 pieces.
From the foregoing it will now be apparent that the present invention provides an improved heating system for continuously heating work pieces to forging temperatures which maintains the desirable characteristics of full capacity production rates in high speed heating over a wide range of production rates.
What I claim as new and desire to secure by Letter Patent is:
1. In a furnace for heating work to elevated temperature the combination which comprises wall structure defining a furnace chamber where through work may be advanced for heating to said temperature, means for maintaining in a zone next adjacent the work-leaving-end of said chamber a temperature substantially constant and higher than the desired temperature of the work leaving said chamber, and means responsive to the temperature of the work leaving said chamber for controlling the input of heat to a zone in advance of said zone.
2. The method of controlling the application of heat to work which is being advanced along a path at a substantially uniform rate which comprises applying heat by first and second heating means in the order named, said second heating means applying heat at a substantially constant heating rate and said first heating means applying heat in quantity controlled to maintain a desired work temperature after heating by said second heating means.
3. In a furnace for heating work to elevated temperature, the combination which comprises structure defining an elongate furnace chamber wherethrough the work is advanced for heating, first burner means for adding heat to the r work in a first zone next adjacent the Work entering end of said furnace, second burner means for adding heat to the work in a second zone next adjacent the work leaving end of said furnace, wall means for dividing said first and second zones to substantially reduce loss of heat from said second zone to said first'zone, control means for controlling said second burner means to maintain in the second zone a substantially constant heating rate, means responsive to the temperature of work leaving said second zone for controlling said first burner means in a manner to maintain said work temperature substantially constant, and means forming an exhaust flue individual to said second zone for discharging products of combustion therefrom whereby the heating of the work in the first zone may be varied to substantially zero while maintaining a maximum and substantially constant rate of final heating of the work in said second zone.
4. In a furnace having cylindrical walls defining a furnace chamber wherethrough work may be advanced for rapid heating to working temperature, combination, first burner means adapted to fire tangentially into said furnace chamber transversely of the axis thereof to heat a substantial portion of said walls next adjacent the entering end of said chamber and to form thereby a first heating zone, second burner means adapted to fire tangentially into said chamber transversely of the axis thereof to heat a substantial portion of said walls next adjacent the leaving end of said chamber and to form thereby a second heating zone, control means for controlling said second burner means to maintain in said second heating zone a substantially constant heating rate, and control means responsive to temperature of work heated in the furnace for varying the firing rate of said first burner means whereby the rate of heating the work in said second heating zone may be maintained at a maximum while the rate of advancing the work is varied from the maximum rate at which said first and second zones together may heat the work to the working temperature to the maximum rate at which said second zone alone may heat the work to the working temperature.
5. In a furnace according to claim 4, wall means separating said first zone and said second zone, said wall means having an aperture wherethrough work may pass, and means, individual to said second zone for removing products of combustion therefrom to avoid preheating by said products of the work in said first zone, whereby to maintain said maximum rate of heating of the work in the second zone over a wide range of rates of advance of said Work.
6. In apparatus for rapidly heating advancing work to working temperature, in combination, a series of at least two heating means adapted to consecutively heat the work as it advances, and control means responsive to the temperature of work leaving the last heating means of said series for reducing the heating effect of the series as a whole by successively reducing from maximum to minimum the heating rate of each successive heating means starting from the first heating means whereby to maintain in the remaining heating means next adjacent the work leaving end of said apparatus a maximum rate of heating the work.
7. In a furnace for heating work to elevated temperature, the combination which comprises structure defining an elongate furnace chamber wherethrough the work is advanced for heating, first burner means for adding heat to the work in a first zone next adjacent the work entering end of said furnace, second burner means for adding heat to the work in a second zone next adjacent the work leaving end of the furnace, wall means for dividing said first and second zones to substantially reduce loss of heat from said second zone to said first zone, control means for controlling said second burner means to maintain in the second zone a substantially constant heating rate, and means responsive to the temperature of work leaving said second zone for controlling said first burner means in a manner to maintain said work temperature substantially constant.
8. In a furance according to claim 3, the improvement wherein said control means for controlling said second burner means comprises means for supplying fuel to the second burner means at a constant rate while said first burner means is controlled responsive to the temperature of work leaving said second zone.
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