One prior art method for partial heating of metal sheets is by means of a gas flame which, however, means bad precision, and low efficiency due to high heat leakage and it is also difficult simultaneously to heat both sides of the metal sheet. Also, it is difficult accurately to control the temperature of the sheet and the heating-process is very fine-consuming. The same problem burdens IR heating. Light arc heating, as in tig welding, does broadly speaking involve the same problems as those mentioned above. Laser heating requires high investments and has low efficiency and it is only possible to heat one side at a time, meaning considerable waste of time. The big problem in con- nection with resistance heating, involving use of an equipment of the point welding type with electrodes of copper or another metal material, is that the metal sheet can practically not at all be moved once it is in contact with the electrodes and feeding current supplied. Further, the electrodes cool the sur- face of the sheet, meaning that the highest temperature will be formed at the center of the sheet. Induction heating is in- accurate and, when heating jaws are used, the temperature rise is very slow to which should be added that such device is very inflexible.

The object of the present invention is to provide a method and a device for partial heating in the context mentioned above, and

to eliminate the disadvantages enumerated here. The features characterizing the invention are set out in the subsequent claims.

Thanks to the invention there have now become available a method and a device which in an excellent manner satisfies its purpose.

In addition thereto the method can be carried out con-veniently and the device can be manufactured at low cost. According to the invention use is made of a resistance heating device provided with at least one electrode of carbon, graphite or a non- metallic material having similar properties and ex-hibiting low friction values when the electrode, or the elec-trodes when they are two in number, mounted one on each side of the sheet, and are displaced while pressed against it. The use of this method and of this device results in a heating pattern having great advantages in comparison with the previously known methods.

When, in the case where two electrodes are used, the current feed is initiated, the electrode points will get very hot, meaning that the sheet is heated from both sides, right out to the outermost portion, i. e. not mainly in the center, and simultaneously a resistance heating is generated due to the current passage. Thanks to the invention a very quick heating occurs and the friction between the metal sheets and the electrodes is very low. Moreover, the extension of the heated zone is precise because it is possible to complete the process very quickly and because it can be directed. There is also a possibility to influence the heating process by variation of the electrode pressure, the feeding speed and the current charac- teristic resulting in different balances in response to the re- sistance heating and to the heating emanating the hot elec- trodes.

The invention is below described more in detail with reference to a preferred embodiment.

The field of application of the invention is partial heating of metal sheets in order to establish zones which are to be plas- tically deformed subsequently to the sheets having, by hardening or cold working, been given a higher yield point, and hence generally a lower ductility. Its structure could be such that it comprises two opposite electrodes, facing each other at a vari- able distance between them determined by the thickness of the metal sheet which is to be used. The electrodes consist of car- bon, graphite or of a non-metallic material with similar pro- perties. They have a low friction value when pressed against the metal sheet from opposite sides thereof. The electrodes and the sheet can be moved relatively each other during the heating pro- cess. They are mounted in electrically conductive electrode holders which enclose them so that only a very short portion of them projects. If copper-coated electrodes are used, the pro- jecting portion may however be longer. The electrode diameter can vary from a few mms and upwards and the electrode pressure does not have to be higher than within the order of magnitude of 1,5-15 N/mm2. Also, the electrode diameter could be different for those above and those below the sheet, to create an unsym- metric heating. The current source could be constituted by a simple welding transformer. When two electrodes are used they may be mounted one on each side of the sheet making it possible for the current through the sheet to flow from the one electrode to the other. When feed current is switched on the electrode tips turn very hot, even glowing, and they heat the sheet from both sides. The current passage yields a symmetric heating of the sheet.

In the case when only one electrode is utilized the heating takes place from the one side of the sheet, where the current flows from that electrode to the sheet, which is connected to ground or in contact with an underlying, grounded metal plate, this results in a more unilateral heating, i. e. a non- symmetrical heating in the thickness direction, than what is the case in the embodiment utilizing two electrodes.