US2841692A - Method of and an apparatus for the electrical resistance heating of semi-finished metllic articles - Google Patents

Description

July 1, 1958 G. SEULEN 2,841,692

METHOD OF AND AN APPARATUS FOR THE ELECTRICAL RESISTANCE HEATING OF SEMI-FINISHED METALLIC ARTICLES Filed Dec. 21, 1955 Inventor United States Patent METHOD OF AND AN APPARATUS FOR THE ELECTRICAL RESISTANCE HEATING OF SEMI-FINISHED METALLIC ARTICLES Gerhard Seulen, Remscheid, Germany, assignor to Deutsche Edelstahlwerke Aktiengesellschaft, Krefeld, Germany Application December 21, 1955, Serial No. 554,446 Claims priority, application Germany December 27, 1954 3 Claims. (Cl. 219-156) The invention relates to a method for the electrical resistance heating of semi-finished workpieces made of ferrous or non-ferrous metals or alloys thereof (herein generically referred to as metallic workpieces) more particularly workpieces of steel, for preparing said workpieces for immediately following hot-working.

In order to fulfil the various requirements which are involved in a method of this kind to adjust the heating of the workpieces to the speed of operation of the hotworking apparatus, for example a rolling mill, the workpieces are raised to the requisite temperature for hot deformation in stages by treatment at successive stations. These heating stations are fed by two or more phases of a source of alternating current, each workpiece being heated to a temperature within a predetermined range at one station, then raised to a temperature within a predetermined higher range at the following station and so on from station .to station until the requisite temperature for hot-working is reached. The number of heating stations corresponds to the number of phases or a multiple thereof. Because of the succession of individual stages of heating achieved by the successive conveyance of the workpieces from station to station, the resistance of the workpiece being heated is increased in accordance with the increase in temperature, i. e. the resistance value of the workpiece being heated varies from one heating station to the next. When employing a normal alternating or three-phase mains current source a non-uniform phase loading is imposed on the supply by reason of the differing resistance values of the workpieces being heated. The workpiece with the higher or highest resistance value provides the lower or lowest loading in the phase which serves to heat this workpiece. With the high power involved in such heating installations, which are particularly used for bringing billets, ingots, bars, tubes or similar semi-finished articles to hot-working temperature for treatment in rolling mills, presses, hammer forging or the like, such nonuniform phase loadings have a particularly disturbing influence on the supply mains and zero displacements may be caused thereby.

According to the invention, a substantially uniform loading for all the phases of the supply mains is promoted in that in each phase a voltage differing from that in each other phase is selected so as to be consonant with the resistance resulting from the temperature of the workpiece at the relevant station.

In order to carry the method into effect, according to the invention an alternating current installation is used which has a multi-phase transformer (which term when used herein includes a two-phase transformer), the secondary phases of which supply to the workpieces voltages which differ inter se. For facilitating choice of these differing voltages, a transformer may be employed which is provided with two or more tappings for each phase. By this means, either by preliminary calculation or empirically, a suitable tapping for the phase in question and therefore for the corresponding heating sta- ICC tion can be chosen, adapted to the prevailing conditions. In this way the workpieces can be heated substantially uniformly despite their continually varying resistance values, and the mains supply is substantially uniformly loaded in all phases.

One specific embodiment of the invention is illustrated purely by way of example in the accompanying drawing and will now be specifically described in connection therewith.

The drawing illustrates a diagram of connections showing in principle a preferred arrangement according to the invention. To the 3-phases R, S, T of a 50 cycles/sec. mains network a three-phase transformer is connected having primary windings marked IR, 18 and IT and secondary windings marked 2R, 28, 21" and provided with tappings 4 whereby in each phase different voltages can be obtained. The workpieces 3R, 3S and 3T are connected by way of the terminals 5 with the chosen tappings of the secondary windings of the separate phases in such a manner that the voltage in each is appropriate to the temperature of the corresponding workpiece and therefore to the resistance thereof, so that although each workpiece is at a different temperature and has therefore a different resistance from the others all three phases are substantially equally loaded. The workpiece with the lowest temperature and therefore having the lowest resistance is supplied at the lowest voltage and the workpiece at the highest temperature and having the highest resistance is supplied at the highest voltage, the voltage for the workpiece(s) at intermediate temperature(s) being correspondingly intermediate the lowest and highest voltages. The voltages are chosen so that all three phases are equally loaded.

By way of example only and not in any sense meant to restrict the scope of the invention, the work-pieces may be equal-size cylindrical workpieces (diameter 6 cm., length cm.) of hot-deformable steel, e. g., a carbon steel. The workpieces are mounted to be moved together, e. g., mounted on a common carrier, and periodically displaced from station to station and from terminals to terminals by any suitable mechanical or other means which are not illustrated as they form no part per se of the present invention. To fulfil the stage-by-stage heating, the workpieces are stationary at each successive station for a period of 50 see. before being displaced to the next station, or in the case of the workpiece at the last station, before being removed or displaced to the hot-working apparatus. Workpiece 3R at the first station is supplied at a voltage of 20 v. and heated to a temperature of 400 C. The workpiece 38 will have been already similarly heat-treated at the said station and has been displaced to the second station where it is supplied at a voltage of 25 v. its temperature at this station is raised to 800 C. The workpiece 3T will have been already heated at the two previous stations and is now heated further to a temperature of l200 C., viz., the temperature requisite for hot-working of the workpiece, the voltage being 28 v.

Whatl claim is:

1. Method for the electrical resistance heating of semifinished metallic workpieces and particularly steel workpieces for preparing said workpieces for an immediately follownig hot-working process, which consists in supplying the heating current in at least two phases to workpieces connected in circuit to the different phases at a number of different heating stations corresponding to the number of phases, feeding the workpieces from station to station so that the temperature of each workpiece is raised by the resistance heating stage-by-stage by connection in circuit to first one phase and then another and employing different voltages in the different phases in dependence upon the resistance of the workpieces as affected by the temperatures thereof so that the phases of the supply mains are substantially uniformly loaded.

2. Method for the electrical resistance heating of semifinished metallic workpieces and particularly steel workpieces for preparing said workpieces for an immediately following hot-working process, which consists in supply ing the heating current in at least two phases toworkpieces at different heating stations in number at least corresponding to the number of phases, feeding theworkpieces from station to station so that the temperature of each workpiece is raised by the resistance heating stageby-stage, supplying the said heating current to the Workpieces at the said stations by electrical connections between the workpieces and secondary windings of a transformer for passage of the current through the workpieces, and tapping at least some of the said connections to the said windings to render the voltages at the different stations dependent upon the resistance of the particular workpiece as afiected by the prevailing temperature thereof and thereby ensuring the phases of the supply to be substantially uniformly loaded.

3. Apparatus for the electrical alternating-current rea 4 sistance heating of semi-finished workpieces comprising a multiphase transformer and a plurality of stations with terminals between which separate workpieces may be electrically connected to the secondary windings of the said transformer for passage'therethrough of the heating current, means for moving the workpieces from station to station for heating through a ditierent range of temperatures at each station and means whereby some at least of the said terminals may be ditferently tapped to the said secondary windings and the voltages at the different stations thereby varied depending upon the resistance of the particular workpiece as affected by the prevailing temperature thereof.

Great Britain Feb. 2, 1955

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