US2308995A - Method of electrically heating metallic strip - Google Patents

Description

F. MlESS Jan. 19, 1943.

METHOD OF .ELECTRICALLY HEATING METALLIC STRIP Filed Dec. 20, 1941 FEE Patented Jan. 19, 1943 UNITED STATES PATENT OFFICE METHOD OF ELECTRICALLY HEATING METALLIC STRIP 3 Claims.

This invention relates to a method of electrically heating metallic strip, and more particularly to the heat treatment of strip steel, although it is not limited thereto.

In the manufacture of strip steel, it is at frequent stagesof its manufacture in the form of coils of substantial width and large diameter. In order to heat treat it, the strip, while in coiled form, may be placed in a furnace and subjected to heat. material to be treated in the furnace necessitates a long period of time for the heat to penetrate to the center of the mass and for all parts to attain equality of temperature.

The greater, the rate at which heat flows in, the greater the difference in temperature between the outside surface and the center of the mass. Therefore when the surface reaches the limiting temperature and the temperature of the center begins to approach the temperature of the surface, the difference between the two temperatures becomes less and the rate of rise at the center becomes slower.

Under these conditions it may require from 50 to 75 hours (including soaking period) to bring a charge of coiled steel strip to a reasonably uniform annealing temperature. Even then temperatures are not as nearly uniform as might be desired and different parts of the mass have received different heat treatments due to the great differences in time that they were held at the maximum temperatures.

With the gas-fired furnaces, heat loss through furnace walls goes on for from 50 to 75 hours per charge, and in addition there is an enormous loss of heat in the products of combustion, leaving the furnaceat 1300" to 1400 F. for the 50- to 75- hour run, so that the coiled steel strip being treated receives only about 15% of the heat of the fuel consumed.

Another method of heat treating strip steel is to pass it through an elongated furnace in continuous, as distinguished from coiled, form. This method of heat treating is not only expensive because of the high cost of the elongated furnace, but also because of the necessity of rather elaborate handling apparatus. In addition, the construction of the elongated furnace is such that there are large heat radiating surfaces which result in the loss of heat, and the output of heat-treated strip is slow.

It is among the objects of the present invention to electrically heat treat metallic strip in a rapid and inexpensive manner.

Another object is to provide a method of the The placing of such a large massof' type described in which there is a large mass/surface ratio, thus permitting the treatment of a large quantity of metallic strip in a furnace of minimum dimensions, whereby heat loss is minimized.

Still another object is the provision of a method of electrically heat treating metallic strip while in coiled form when using practicable currents and voltages, as distingished from prior art practices which require impractically large currents and low voltages.

A further'object is the provision of a method of heat treating metallic strip wherein it is heated in such manner that all parts thereof may be'brought to desired temperature simultaneously.

The invention, then, comprises the features hereinafter fully described and as particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail certain illustrative embodiments of the invention, these being indicative of several of the number of ways in which the principles of the invention may be employed.

In said drawing:

Figure 1 is a plan of an upright coil of metallic strip and illustrating diagrammatically instrumentalities which may be used to heat it in accordance with the teachings of the present invention.

Figure 2 is a sectional view on the line 11-11 of Figure 1.

Figure 3 is a fragmentary plan 01' the coil of Figure 1, illustrating the manner in which insulating material is disposed between the convolutions of the coil. I

Figure 4 is a sectional view on the line IV-IV of Figure 3.

Referring more particularly to the drawing, the numeral 2 designates a furnace or insulating cover which is provided with a top portion 3, the bottom thereof being open and adapted to rest upon a base 4.

As shown more clearly in Figure 2, the base 4 is provided adjacent one end with a pair of adiacent but spaced vertical apertures 5 and 0 and adjacent the other end with a single aperan alternative, a continuous Strip of asbestos may be coiled with the metallic strip in such manner as to maintain the convolutions thereof in insulated relationship. Still another method is to coat the strip on one or both sides with a suitable insulating paint or cement of which there are so many commercially obtainable varities that their specific mention herein is deemed unnecessary.

To one end of the strip I there is suitably connected, as shown at l2, an electrical tap connecting with a lead l3 which passes through the aperture 5 in the base 4 and is connected to a suitable source of electrical energy generally designated at H. To the other end of the strip in there is suitably connected, as shown at H, an electrical tap to which there is connected a lead i8 which passes through the aperture I in the base 4 and is suitably connected to the source I of electrical energy which was referred to hereinbefore. Electrical energy may be supplied to the strip through the taps l2 and I1, whereby the same may be suitably heated by resistance. In this manner, the electric current flows from end to end of the continuous strip with electrical insulation disposed between the convolutions of the coil, whereby the required current is reduced to a value which is easily handled by leads and connectors of moderate size, and the voltage is raised to values obtainable from commercially acceptable electrical machinery.

It frequently happens that the strip varies in thickness, and because of this or any other reason. it may be deemed desirable for increasing or decreasing the current flowing through any part of the coil. In order to provide for this, a tap 20 is suitably connected to any desired convolution of the coil II), this tap 28 being connected to a lead 2| which is in turn-connected to a means 22 for increasing or decreasing the current in that portion of the coil which is between thetap i2 and the tap 20. This means 22 may take the form of a generator or a shunt.

In the foregoing manner, then,-current from the source 14 is passed through the coil ID of metallic strip in such manner that immediately the whole mass of coil from the exterior to the interior thereof begins to increase in temperature at the same rate, but after a short time, however, the temperature of the exterior of the coil Ill lags behind the temperature of the center of the mass due to radiation of heat from the surface metal, while no such radiation takes place from the metal at the center. To avoid this undesirable feature, the furnace or cover 2 which encloses the coil I0 should have its temperature raised at the same rate as the rising temperature of the coil. Thus the temperatures of the coil and enclosing furnace are balanced, and radiation from the coil is prevented, thereby permitting the coil to heat uniformly throughout by the action of the electrical currents. The heating of the furnace or cover 2 may be accomplished in any of the manners well known in the art, and as such forms no part of the present invention, whereby the illustration of a specific heating instrumentality herein is not deemed necessary.

By placing the tap 20 within a few convolutions of the outside end of the coil III, the uniform heating throughout of the coil to a temperature above the maximum obtainable by the furnace is accomplished. As the electrical current raises the temperature of the coil above the maximum obtainable by the furnace, there is increased the current in the convolutions between the taps l2 and 20 by an amount ju'st suflicient to make up for the heat loss by radiation from the coil to. the somewhat cooler furnace, thus maintaining uniform temperatures throughout the coil in spite of the radiation from the surface of the coil to the furnace.

It will be readily understood by those skilled in the art that the generator or shunt 22 may serve either to put extra electrical current through a cooler section of the coil ill, or the same effect may be obtained by shunting the hotter section of the coil.

While I have shown and described certain specific embodimentsof the present invention, it will be seen that I do not wish to be limited exactly thereto, since various modifications may be made without departing from the scope of the invention as defined in the appended claims.

I claim:

1. The method of electrically heat treating an attenuated metallic article which comprises convoluting the said article, maintaining the respectively adjacent convolutions of the said article out of electrical contact with each other, including at least a portion of the said article in an electrical resistance-heating circuit, applying to said electrical resistance-heating circuit an electric current in sufficient volume to raise the temperature of said article to that required for the desired heat treatment, and imparting to the exterior convolutions of the said article sumcient additional heat to substantially compensate for heat loss from the surface thereof.

2. The method of electrically heat treating an attenuated metallic article which comprises convoluting the said article, maintaining the respectively adjacent convolutions of the said article out of electrical contact with each other, including at least a portion of the said article in an electrical resistance-heating circuit, applying to said electrical resistance-heating circuit an electric current in sufficient volume to raise the temperature of said article to that required for the desired heat treatment, and externally applying to the exterior convolutions of the said article suflicient additional heat to substantially compensate for heat loss from the surface thereof.

3. The method of electrically heat treating an attenuated metallic article which comprises convoluting the said article, maintaining the respectively adjacent convolutions of the said. article out of electrical contact with each other, including a portion of the said article in an electrical resistance-heating circuit, applying to said electrical resistance-heating circuit an electric current in sufficient volume to raise the temperature of said article to that required for the desired heat treatment, including another portion of the said article in another electrical resistance-heating circuit, and applying to said second electrical resistance-heating circuit an electric current of greater magnitude than that applied to said first-named electrical resistanceheating circuit.

' FRED MIESS.

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