US2676232A - Arrangement for thoroughly heating of large billets - Google Patents

Description

April 20, 1954 L. DREYFUS 2 ARRANGEMENT FOR THOROUGHLY HEATING OF LARGE BILLETS Filed June 24, 1952 IN VEN TOR.

L up W/G DREYFUS omey.

Patented Apr. 20, 1954 UNITED STATES ATENT OFFICE ARRANGEMENT F-OR THOROUGHLY HEATING OF LARGE BILLETS Application June 24, 1952, Serial No. 295,240

Claims priority, application Sweden J une 25, 1951 3 Claims.

The invention relates to a method for the electrical heating of metal bodies and to an apparatus for carrying out this method. More particularly, it relates to a method and apparatus for heating large billets of steel or the like.

It requires comparatively long periods of time for the thorough heating of large cylindrical steel billets, that is, those having an average diameter greater than cm., or of steel plates having a thickness greater than 10- cm. In heating such pieces with electric fields of the frequencies produced by rotating electrical machines (that is, in the range of about 50 to 3000 cycles), the heat is generated in a limited zone near the surface of the work. The coeflicient of heat conductivity Watt cm.C.

is so small that the heat in this surface zone is distributed rather slowly to the central part of the body. Heating at such frequencies can be carriedout economically only if the furnace is effectively insulated, if the heating period is is kept as low as possible by a proper control of the power and proper choice of the frequency, and if the internal power consumption of the control device is kept low.

The primary object of the present invention is to provide an improved method and apparatus for the electrical heating of large pieces of metal.

Another object of the invention is to provide a process and apparatus for achieving the criteria set out above, so as to obtain economic heating of such pieces.

Further objects and advantages of the invention will appear more fully from the following description, especially when taken in conjunction with the accompanying drawing which forms a part thereof.

The drawing shows diagrammatically the circuit of a heating apparatus which can be used for carrying out the invention.

According to the invention, billets having a major dimension greater than 10- cm. (which exprcssion is used herein to designate either cylindrical billets having a diameter greater than 10 cm. or plates having a thickness greater than 10 cm.) are heated in two heating periods.

respectively. During the-first heating period to, the surface temperature of the billet is heated to the highest permissible temperature. The duration of the period to is preferably between f1: and 3T1, where T). is the time constant at These periods are designated as to and ts-to 2 which the temperature difierence between the surface of the billet and the interior thereof is equalized when the supply of power is discontinned.

At the end of the period to, the supply of power (which equals is abruptly reduced. The difference of temperature. between the surface layer and the center of the billet is equalized during the period ts-tu. during which period the temperature of the surface is maintained substantially at the value r which it had at the end of period to by proper control of the power. 7

Another feature of the invention is the provision of apparatus for carrying out the method described above. This includes a plurality of furnace units, a source of power, a voltage divider delivering different voltages, a separate voltage controlling device in series with the voltage divider for delivering an adjustable voltage to' each furnace unit, and an individual change-over device for each furnace unit for connecting the units selectively to different voltages. V

In the following description, 6 has been used to indicate the diameter of a cylindrical billet and A the thickness of a plate, measured in centimeters. D has been used as generic to both 5 and A, where the same formula is applicable to both. Where there are different formulas,

they are designated by B for cylindrical billets and S for plates or sheets, respectively; as (1B) and (1S) First heating period to (see) It is useless to begin heating with the largest power 110 (that is,

Watt

where c designates the specific heat per unit of volume Watt. sec. cm. C.

But under these conditions the surface tem perature will increase still more rapidly, and an excess temperature will exist corresponding to The time constant at which the excess temperature -s is generated during the heating period and equalized during the cooling period is represented by the formulas:

It will be seen that the excess temperature, as the time t increases, approaches A high speed of heating thus results in a high excess temperature S&,,, of the surface of the billet, especially if the time constant T0 is not kept .as low as possible.

To is proportional to T1, but it also depends to a great extent on the frequency 2, the permeability ,1 and the inverse value of the resistivity p (which equals 2 ohm 51 2;

and is a very complicated function of the dimensionless parameter It is clear that, in order to reduce the excess heating of the surface as much as possible, the value of .5 should be kept as low a possible, considering th electrical efiiciency 0f the furnace. This can be accomplished by making 5 as low as possible, but:

In treating pieces having different thickness at the same frequency, the lower limit of the frequency is determined by a consideration of the 4. piece having the smallest dimension Dmin- Assuming for steel that 2 =1.1 ohm the frequency will be:

From this equation, it will be seen that pieces having values of D greater than 25 cm. can be heated with conventional frequencies of 50 to 60 cycles. There is no upper limit for the frequency, but there is ordinarily no particular advantage in going beyond the range:

Upon heatin with a high supplied power 110, the temperature of the surface rises quickly above the Curie point, at which all iron alloys become non-magnetic. Thereafter, it increases slowly to 0 =1,300 6., for instance. This limit should preferably not be exceeded Where forging is .to be carried out at 1200" C. In the vicinity of the highest permissible temperature 0, at the end of period 150, the supplied power is decreased, according to the present invention.

If the total heating period is is considered, from I 10 2 #350 to 2000( the beginning when the billet is cold until it reaches its final temperature 0, the supplied power is applied almost the Whole period to a billet heated to the non-magnetic range, so that it is permissible to base the calculations on the assumptions that ,(L=l and p is constant, even during the period 10.

According to Equation 2, an average temperature is attained when t==t0 where J1 in) l-e T1 l']. P T1 inwhich will have the following values:

to E 0.2 0.1 0.6 0.8 1.0 1.2 1.4

Second heating period, ifs-ta (sec) At the end of period to, the power should be decreased, first by a large amount at once, and

then continuously or step by step at a lower rate. Accordin to the invention, the power should be controlled during the final heating period ts-t0 in S06E53 Way that the surface temperature 0 is kept as exactly as possible at the highest permissible value. average temperature of the billet has reached the forging temperature a the heating is interrupted.

The variation of the temperature is represented by the differential equation:

When t=ts, at which time the From this equation it follows that after interruption of the power supply (that is, when do,,, w

the internal temperature differences -0 are equalized according to the formula:

d(c-c.,.) e0 (13) that is, according to the function:

t-t. z919 =(d19 )e (14:) It also follows that with a constant supply of power (when ie dt is constant) the excess temperature of the surface follows the differential equation:

already has been used in Equation 2.

For the second heating period (ts--t0) Equation 12 must be solved for constant surface temperature (9 being constant,

Equation 12 thus is simplified to The desired forging temperature 0 then is reached after a heating period:

which therefore represents the shortest possible heating time to be attained without exceeding a given value 0 of the surface temperature.

The power control The power supplied to the surface of the work during the first heating period to is expressed according to the Equations 1, 2 and 16 by During the second ifs-t0, the power control must begin according to Equation 19 with a value The power must be reduced abruptly at t=t in the proportion T m T p m 1.T0+T1|:1 e I] (23) The shorter to is made, that is, the higher the heating speed, the more the degree of discontinuity of the high-frequency load increases without shortening the total heating period is in a corresponding degree. For this reason an order of magnitude is recommended for to tszTi up to 2T1 or up to 3T1 (24) and for the power control at t to a power proportion:

p zp =lzgi [0.632 up to 0.865 or up to 0.950]

Whether, after the abrupt decrease at to, the decrease of power to pts at tzts is effected continuously or stepwise in several steps depends on the means used for the control of the voltage of the furnace coil.

All the formulae are approximative, partially since radiation losses from the charge have been neglected. The calculated values, therefore, have to be increased with increases in such losses. It has, however, already been mentioned that only very small heat losses may be counted on because of the comparatively long heating time required for large billets in high-frequency furnaces, and therefore errors involved by neglecting these losses are small.

The arrangement for the power control It follows from the above that the thorough heating of billets having 15 to 30 cm. diameter takes 5 to 30 minutes, which is much more than the time required for the forging or pressing operation itself. In practice, therefore, a great number of (N) billets will always be heated simultaneously. For this purpose there are two alternatives. Either a few (n) furnaces are provided, each accommodating many billets, and provided with several succeeding heating zones acting with different amounts of power on the billets, the billets being conveyed through said zones without interruption; or, a comparatively large number of furnace units are employed each accommodating a few (n) billets, the latter being held stationary within the same furnace during the whole heating period. The last mentioned alternative is to be preferred when it is necessary to avoid oxidation as much as possible, as the furnace units then may be easily closed and filled with a protecting gas. But, with respect to the feeding of the billets, the first alternative is considerably simpler. In this case, it will be noticed, all n furnaces work with the same constant voltage. In the other alternative, however, the control of the supplied power is effected by controlling the voltage of the furnace coil, which control, it is true, is performed according to the same program for all furnaces but with a relative delay of phase in N /n for furnaces which are charged and discharged one after the other. Although the furnace units which are switched in are equal, the amounts of power and the voltages must be different at the same moment in different units. The cells of the furnaces, therefore, cannot be connected in parallel, but their terminal voltages must be adjustable individually, while the total power of the equipment is generated with constant voltage Eg in one or more parallel-connected generators.

The problem of carrying out a power control according to the proportion pu:pts-1:0.l by means of cheap control devices for each furnace unit may be solved according to the invention in a manner which is shown on the drawings by way of example.

The voltage Eg produced within the generator 1 or power station is divided by means of a voltage divider 2, arranged in the primary station or in the forge, according to the proportion :rzl-ar, where Pro To each furnace unit the constant voltages s, rr, Eg (1-4:) are applied.

Furthermore, a comparatively small adjustable voltage :E y is inserted in series with the voltage E as, where a /r1 2 w 3 is a voltage control arrangement. If the voltage E y is to be controlled continuously, this voltage may be generated in a booster-generator, an induction regulator or a slide transformer or the like. If only stepwise control is desired, a regulating transformer with a tap-changing switch may be used. In any case the one fgenerator connector tap 4 of the furnace coil I is connected to the free end 5 of the winding producing the voltage Egg while the other generator connector tap 6 is connected through a change-over switch or two switches 1 either to the free end 8 of the voltage divider 2 or to its intermediate pole 9.

By these arrangements the following control program is performed: The furnace unit is started with a voltage Eg (1:11), where the ratio y and its sign is adjusted automatically for constant supplied power pm during the first heating period to, as shown by the position of the switch I in the upper furnace unit. At the end of this period (i=to) the switch I is changed over and connects the coil to the voltage Eg (ac-l-y) as shown by the position of the switch in the lower furnace unit. Thus the power decreases instantaneously according to the proportion (x-i-y) (1+y) =gor :po (28) Thereafter the adjustable auxiliary voltage is altered automatically from E y to E y (as indicated by the changed direction of the arrow in the voltage control means 3) corresponding to a decrease of power in the proportion:

(ry) (as-H1) :121 2221:, after which the furnace coil, when t ts, is disconnected automatically.

The essential feature of this solution of the control is that the expensive control device which generates the adjustable voltage need be dimensioned only for low internal power consumption.

I i designates a capacitance which is automatically controlled in the usual way in resonance with the reactance of the furnace coil.

I claim:

1. Method of thoroughly heating electrically to forging temperature billets having a thickness greater than '10 cm. by currents and frequencies generated in rotating machines, which comprises the billet during a first period and thus increasing the surface temperature of the billet to substantially the highest permissible value, and during a second heating period controlling the power supplied to the billet so as to maintain substantially the same surface temperature, the duration of the said first period being between 1 and 3 times the time constant at which the temperature differences between the surface of the billet and the interior thereof is equalized when the supply of power is discontinued, the power at the end of the first period being abruptly reduced in the proportion where Tois the quotient of the diiference between the surface temperature 5 and the average temperature a of the billet divided by the rate of increase of the temperature dt at which such difference occurs, and during the said second heating period the power being decreased in the proportion 2. Method according to claim 1, in which the current frequency is within the range of about 350 to 2000 times 10 min furnace unit adapted to connect in series with voltage E an a variable voltage :tEgy, and a separate change-over device for each furnace unit for selectively connecting to the furnace unit a voltage Eg (111/), in which 1 is variable, during a first heating period, and a voltage variable from E8 (.r-i-y) to Eg (acy) during a second heating period.

References Cited in the file of this patent UNITED STATES PATENTS Number Number 10 Name Date Cooper et a1 Nov. 28, 1944 Strickland May 14, 1946 Jordan Oct. 28, 1947 Frederick Oct. 26, 1948 Wadhams et a1 June 13, 1950 Grapp Aug. 8, 1950 Storm Sept. 12, 1950 Storm Mar. 4, 1952

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