US2867431A - Metal heating - Google Patents

Description

Jan. 6, 1959 F. o. HEss METAL HEATING Filed July i, 1955 uzoN OPEN:

NVENTOR. FREDERC O. HESS 12144347( lum ATTORNEY.

United States Patent METAL HEATING Frederic 0. Hess, Philadelphia Pa., assignor to Selas Corporation of America, Philadelphia, Pa., a co'poration of Pennsylvania Application July 1, 1955, Serial No. 519,549

6 'ClaimS. (Cl. 263-52) The present invention relates to furnaces, and more particularly to furnaces that are adapted' to heat steel billets to forging temperature and a method of heating billets.

It is generally customary in the heating. of large billets for forging to heat them gradually as they are being moved through a furnace from the entrance to the exit thereof. When the surface of the billet has reached the desired temperature, it is held in a furnace zone of substantially constnt temperature as the billet soaks, while its interior heats up to the proper temperature. This soaking period requires a rather long time during which scal'e is forming on the surface of the billet, and the grain of the metal is increasing in' size. Before the billet can be forged, the scale must be removed.

In the present invention, the billets are heated rapidly from the time they are placed in the furnace until the billet interior reaches substantially its transformation temperature. At that point, the exterior of the billet is kept at the temperature that it has attained, while the interior is heated by conduction until it has passed the transformation point. Thereafter, the billet is again heated rapidly until it has reached the temperature required-for forging or other hot working operation. During the final portion of the heating, 'the interior heats more rapidly relative to the speed of heating of the exterior than was the case at the first portion of the heating, so that both the interior and exterior 'of the billet are at substantially the same temperature by the time the final temperature is reached. j f

The furnace of the present invention comprises an elongated, tunnel like structure through which the billets are moved. Adjacent to the entrance, there .is provided a heating zone, and adjacent to the exit of the turnace, there is provided another heating zone. Between these two, there is provided a zone whose temperature is maintained by hot products of combustion drawn .toward the center from the end zones.

It is an object of the invention to provide a furnace for heating billets, particularly of steel, to a high temperature such as the temperature required for forging. It is a further object of the invention to provide a method of heating billets. v y

A more specific object of the invention is to provide a tunnel type furnace in which the ends are fired and the center is heated by products of combustion from the ends.

A further object of the invention is to provide a method and apparatus for rapidly heating steel billets to forging temperature in such a manner that the center and.

surface of the billets reach the desired temperatures at substantially the same time.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and tforming a part of this specification. For a better Understanding of the invention, however, tits advantages (and specific objects attained with its use, reference should be had to the acconpanying drawingsand descriptive matter in which I have illustrated and' described a preferred embodiment of the invention.

In the drawings:

Figure 1 is a diagrammatic plan View in section showing the fumace of the present. invention; V

Figure 2 is a view taken on line 2-2 of Figure 1, and

Figure 3 is a set of curves showing the manner in which the heating takes place. i

Referring to Figure l, there is shown an elongated furnace 5 of the car bottom type which is constructed in accordance with ordinary furnace practice of refractory 6 that is backed up by sheet metal 7 and reenforced by conventional bnck stays 8. The furnace is divided into sections or Zones which include an entrance zone 9, a central or soaking zone 11 and an exit zone 12. As is shown herein, the zones are separated from each other by means of partitions that extend partly across the furnace chamber. Zones 9 and 11 are separated by a partition 13, while zones 11 and 12 are separated by a partition 14.

The entrance and the exit zones, 9 and 12 respectively, of the furnace are heated by burners that are located therein, while the central section 11 does not have any burners in it. Preferably, and as shown diagrammatically herein, the burners 15 are of the type disclosed in Furczyk Patent 2,561,793. As shown herein, a plurality of burners are located in each side of the furnace in zones 9 and 12, so that they point forward the work. These burners are operated in the manner set forth in the above patent to heat the cups thereof and the turnace walls to incandescence, so that radiant heat is projected against the work passing in front of them. Some heat is also transferred to the work by convection from the products of combustion. Air and gas are supplied to these burners through suitable pipes that are connected in a conventional manner, so that the firing of the burners can be controlled and the furnace temperature varied if necessary. v

The products of combustion from the burners are drawn toward the center of the furnace where they are discharged through ducts or chimneys 16 located at substantially the center of the central section or zone 11. A-fan may be used to withdraw the gases if desired. It is noted that the temperature of the hot products of combuston fiowing in opposite directions to the central section is sufiicient to maintain the temperatures of the billets as they are moving through this section of the furnace.

As shown in Figure 2, the bottom of the furnace is provided with a slot 17 into which the floor 18 of a car extends .to form the remainder of the floor of the furnace. This floor of the car is supported on the framework 19 that is provided with wheels 21. The slots extending between the sides of the car bottom and the edges of the furnace wall are closed in order to prevent infiltration of the air at this point. To this end, the sides of the furnace are provided with a pair of sand filled troughs 22 into which extend aprons 23 that projectdownwardly from the edges of the car bottom. The cars travel on tracks 24 that may be laid on the floor beneath the furnace or that may be supported by cross beams 25 as shown. herein. In any event, these tracks extend throughout the length of the furnace and in a loop around the outside of the furnace, so that cars moving from one end of the furnace' may be emptied and reloaded and supplied to the other end. It is noted that the ends of the furnace are provided with doors 26 to close the chamber and thereby prevent loss of heat except When cars are being placed in and removed from the furnace chamber.

Each car is provided with a plurality of pedestals 27 upon which the work, in this case a block of steel 28 to be heated to forgng temperature, is mounted. Enough cars are kept in the furnace, so that they completely fill the length of the furnace chamber and thereby provide a continuous floor for the chamber; Additional cars are provided upon which heated Work is mounted, and unheated work can be placed to be removed' into the furnace chamber.

In the operation of the furnace, a cold billet is introduced into the entrance end as a hot billet is removed from the exit end, so that the various cars at all times form a complete floor. Ordinarily, when heating billets for forging, zones 9 and 12 will be maintained at a temperature of 2450 F., and the billets will be heated to approximately 2300 F. When billets at ambient tempera-` ture are introduced into the furnace, the temperature difference between them and the furnace chamber is so great that the heating rate of the surface will be extremely rapid as shown in the curves of Figure '3 at a. The center of the billet will heat more slowly, however, since it can only be heated by conduction from the surface. This is shown on the curves at b.

The heating of the billets will continue as the cars are gradually moved through zone 9. The rate of heating of the surface, which is heated predominantly by radiatior, will decrease as the temperature difference between the furnace and the billet decreases. The rate oftheating of the center of the billet will increase gradually, but the' temperature difference between the surface and center of the billet will remain high. This is due in large part to the heat absorbed by the mass of the billet as it passes through the transformation temperature range. The transformation range is generally between 1300 F. and 1400 F., although this is subject to variation and raises sornewhat as the speed of heating is increased.

`By the time the billet reaches the end of. the entering zone 9, the surface will have passed the transformation` range, almost instantaneously, and the center will be at about that point. The billet will then enter the soaking or` holding zone, where the only heat that is obtaned comes from the hot products of combustion. During the time the billet is in this zone, its surface temperature will remain substantially constant as shown on the curves at c, but the temperature of the center will continue to rise as shown at d. By the time the billet leaves zone 11, its center will have passed through the transformation range. After this, its rate of temperature increase will be greater than that of the surface, so that the temperature difference between them will decrease.

When the billet reaches the last zone 12, it again comes under the influence of the burners, so that its temperature as a whole begins to rise again. The rate of heating of the surface slows down, however, as its temperature approaches that of the furnace or the differential tem perature between the furnace and billet dirninishes. The rate of heat transfer between the surface and center remains substantially the same, however, so that the temperature difference between the center and surface will gradually decrease.

By the time the billet leaves the end of the furnace, its surface temperature is slightly higher than desired, and its center temperature is slightly lower. As the billet is moved from the furnace to the point of hot working, the surface temperature will drop, but the center temperature will continue to rise slightly. When it is worked upon, the billet will have a uniform temperature throughout.

The heating of the billets is on a 'time-temperature basis. For a constant furnace temperature, the temperature of the billets at any point in their path of travel can be controlled by the speed at which they are moved' through the chamber. After this has been determined for billets of a given size and composition, it can always be duplicated. In order to have the temperatures of various parts of the billet at the proper value when it is removed from the furnace, it is essential that a holding period be provided during the heating. This is to give the center a chance to move through the transformation range without a similar increase in surface temperature. The consequent reduction in temperature differential between different portions of the billet during this stage permits more rapid heating, and evener heating, during the final stages thereof. An advantage of this method is that the soaking, if it can be called that, takes place at a relatively low temperature. The billet is maintained at its high temperature where scaling takes place for the shorest possible length of time.

As an example of the heating of billets according to I the present invention, the following is given. A furnace having heated entrance and exit zones each ninety feet long and an unfired central or holding zone forty feet long is used to heat steel billets 17.5" diameter by 21" long and weighing 1800 pounds to forging temperature. The fired entrance and exit zones are maintained at a temperature of 2450 F. The temperature of the unfired central zone will fall somewhat below this value, but will be sufficiently high to supply some heat to the billets as they are moving through the zone. Billets placed on cars and-moved through the furnace at a rate of between two and one-half and three feet per minute will be heated to an average temperature of 2300 F. by

the time they reach the furnace exit. The surface of the billet will be approximately 1950 F., while it is in zone 11, While the temperature of the center will be raised in this zone from about 1l00 F. to l500 F. Thus, enough heat will be supplied to the billet from the products of combustion to carry all portions of the billet through the transformation zone without any appreciable change in the surface temperature. In other words, heat is being supplied to the surface of the billet at substantially the same rate it is being conducted to the center.

It would be possible to bring the temperature of the surface and the center of the billet to the same value, if it is desired, by increasing the furnace length. This, however, is not necessary and, in fact, is not particularly desirable. As soon as the billet is removed from the furnace, its surface will begin to cool. If the billet is at a uniform temperature as it leaves the furnace, the surface cooling will produce an unevenly heated billet by the time it is moved to the point of hot working. On

the other hand, if the surface is at a higher temperature i heating of the center by conduction, after the billet has been removed from the furnace, Will produce a billet of substantially uniform temperature by the time it is moved to the place for hot working. Furthermore, the lower surface temperature and the shorter time it has been at this temperature, produces less scale on the surface of the billet.

It will be obvious that by varying the furnace temperature or the speed of movement of the billet through the furnace that the final billet temperature can be varied.

From the above description, it Will be seen that there is provided a novel furnace by and with which a novel method of heating billets can be cared out.

While in accordance with the provsions of the statutes, I have illustrated and described the best form of ernbodiment of my invention now known to me it will be apparcnt to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit and scope of the invention as set forth in the appended claims, and that in some cases t certain features of my invention maybe used to advane tage without a corresponding use of other features.

heat until the center of the billet reaches transformation temperature, maintaining the surface temperature substantially constant until the center has passed through the transformation range, continuing continuously to raise the temperature of the billet surface to above forging temperature, and discontinuing the heating prior to the time the center comes to forging temperature.

2. The method of heating a steel billet which comprises raising the temperature of the center of the billet continuously from ambient to the desired high temperature above the transformation temperature by conduction from the surface thereof, heating the surface of the billet at a rate faster than heat is conducted into the interior thereof until the center has substantially reached the transformation temperature, maintaining the surface of the billet at a substantially constant temperature as the center is being heated by conduction from the surface until the center has passed through the transformation temperature, and continuing to heat the surface of the billet to the desired temperature at a rate decreasing with respect to the rate of heating of the center thereof.

3. A method of heating steel billets which comprises subjecting the surface of the billet to radiant and convection heat to raise the temperature of the surface, and through conduction the temperature of the center thereof, continuing to apply heat to the surface of the billet to raise its temperature until the center has substantially reached the transformation temperature, n'taintaining the temperature of the surface of the billet substantially constant until the center is heated through the transformation range by conduction fronr the surface, again applying radiant and convection heat to the surface of the billet to heat it to the desired final temperature with the rate of heating of the center by conduction increasing relative to the rate of heating of the surface until the surface and center have reached their respective desired temperatures, and removing the billet frorn the heat before the temperature of the center is equal to the temperature of the surface.

4. The method of heating a steel billet which comprises heating the bllet rapidly by exposing its surface to radiant and convected heat, conducting heat from the surface of the billet to the center thereof, maintaining the surface of the billet at a substantially constant temperature from the time its center has reached substantially the lower point of the transformation range until the center has reached substantially the upper point of the transformation range, and then continuing to raise the temperature of the billet by radiant and convection heat at a decreasing rate until the billet has reached the desired temperature.

5. The method of heating a steel billet which comprises heating the surface thereof by radiant and convection heat at a given temperature faster than it can be conducted to the interior thereof, continuing such heating until the surface has passed the transformation temperature substantially instantaneously and the interior has substantially reached the transformation temperature, maintaning the temperature of the billet surface substantially constant while the interior temperature of the billet passes through the transformation temperature range as a result of conduction from the surface, and subjecting the billet exterior to said given temperature until the billet reaches a desired temperature, the rate of temperature increase of the surface of the billet decreasing relative to the rate of increase in temperature of the center of the billet by conduction as the surface temperature of the billet approaches said given temperature.

6. The combination of structure forming an elorgated furnace having a furnace chamber of substantially uniform size extending from end to end thereof, means to divide said chamber into a plurality of zones lengthwise thereof including an entrance zone, an exit zone and an intermediate zone, a plurality of burners located along the side walls of said entrance zone, a plurality of burners located along the side walls of said exit zone, there being no burners in said intermediate zone, means forming an exhaust opening in said intermediate zone through which products of combustion from said entrance Zone and exit zone are withdrawn, and means to move work pieces to be heated through said furnace chamber.

References Cited in the file of this patent UNITED STATES PATENTS 1,476,142 Bradshaw Dec. 4, 1923 1,799,957 Cope et al Apr. 7, 1931 2,133,673 Spencer et al. Oct. 18, 1938 2,157,221 Spencer et al. May 9, 1939 2,220,585 Spencer .m Nov. 5, 1940 2,504,707 Lloyd Apr. 18, 1950 2,676,008 Munker Apr. 20, 1954

Claims (1)

1. 2. THE METHOD OF HEATING A STEEL BILLET WHICH COMPRISES RAISING THE TEMPERATURE OF THE CENTER OF THE BILLET CONTINUOUSLY FROM AMBIENT TO THE DESIRED HIGH TEMPERATURE ABOVE THE TRANSFORMATION TEMPERATURE BY CONDUCTION FROM THE SURFACE THEREOF, HEATING THE SURFACE OF THE BILLET AT A RATE FASTER THAN HEAT IS CONDUCTED INTO THE INTERIOR THEREOF UNTIL THE CENTER HAS SUBSTANTIALLY REACHED THE TRANSFORMATION TEMPERATURE, MAINTAINING THE SURFACE OF THE BILLET AT A SUBSTANTIALLY CONSTANT TEMPERATURE AS THE CENTER IS BEING HEATED BY CONDUCTION FROM THE SURFACE UNTIL THE CENTER HAS PASSED THROUGH THE TRANSFORMATION TEMPERATURE, AND CONTINUING TO HEAT THE SURFACE OF THE BILLET TO THE DESIRED TEMPERATURE AT A RATE DECREASING WITH RESPECT TO THE RATE OF HEATING OF THE CENTER THEREOF.

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