GB2091616A - Method and apparatus for electric flash butt welding - Google Patents

Abstract

Electric flash butt welding is carried out by using a contact program prior to the actual welding program in order to remove surface asperities from the two surfaces which are to be joined by the weld and to ensure uniform contact resistance and uniform heating of these two surfaces during the actual welding program. The asperities are first of all heated and pressed flat by passing a high current between the two surfaces while pressing the two surfaces together at a high contact pressure, and are subsequently allowed to cool down and reach a uniform temperature prior to starting the actual welding program. The workpieces which are to be welded are preferably clamped close to the weld between well cooled electrodes to facilitate cooling and thermal equalisation at the end of the contact program.

Description

SPECIFICATION Method and apparatus for electric flash butt welding The invention relates to a method of electric flash butt welding (or upset welding), in particular to the electric flash butt welding of workpieces with a large extent of the weld cross-section, for example metal sheets or pipes. I.e. one is principally concerned with workpieces, the crosssectional shape of which differs substantially from that of a circle or a square with the smallest periphery to area ratio, i.e. to workpieces which, for the same cross-sectional area, have large external dimensions.

During electric flash butt welding the sheet metal ends which are to be welded are heated by the passage of current to the welding temperature and are substantially pressed together to effect welding. The precondition for a flash butt weld is uniform heating of both workpiece ends over the whole cross-section of the weld and this is where the real problem lies. Moreover the heating has to take place so quickly that no scale forms at the end faces which then remains in the weld seam during pressing or upsetting. Only very small amounts of scale are pressed out of the seam during upsetting.

For uniform heating it is necessary to obtain uniform contact resistance and uniform current distribution over the whole weld cross-section.

Uniform contact is not however present because of the ever present roughness of the abutting cross-sectional surfaces. One way of obtaining uniform contact of the abutting cross-sectional surfaces resides in heating the asperities of the two surfaces that are to be welded by the passage of current until they reach a docgh-like consistency and then pressing them flat so that uniform contact is achieved between the two cross-sectional surfaces to be welded. The heating and pressing or upsetting of the workpieces, i.e.

the actual welding process, takes place subsequently. However, this method - as determined within the framework of the present invention - also does not lead to a satisfactory result because the initially projecting asperities which are then pressed flat in the dough-like condition already have a temperature which is so high that during subsequent heating over the entire weld cross-section these points reach the welding temperature considerably earlier than the remaining points. Uniform heating is thus not achieved.

The object of the invention is to propose a method with the aid of which the indicated difficulties are avoided and by means of which trouble-free flash butt welding is achieved with simple means.

The solution to this problem lies in the provision, prior to the actual welding program consisting of heating and upsetting, of a special contact program In order to achieve uniform contact over the whole cross-sectional area of the weld.

By temporally separating the contact program preparatory to welding from the subsequent welding program one avoids, on the one hand, excessive pressure forces during flattening of the asperities and, on the other hand, non-uniform temperatures during the actual welding process.

In a further development of the invention the contact program consists of a high contact force and a high current density so that asperities in the contact plane are heated and pressed flat. The contact program is then terminated by a short, current-free, cooling and thermal equalisation interval. In this manner uniform contact is obtained over the whole cross-sectional area of the weld together with a uniform initial temperature and uniform contact resistance for the subsequent welding program.

Finally, in carrying out the method of the invention the workpieces which are to be welded should, in accordance with the invention, be clamped close to the weld between well-cooled electrodes.

The invention makes it possible to keep the duration of the contact program so short that the danger of scale formation is avoided.

For this purpose the two ends, for example of a wheel rim ring, are clamped in two oppositely disposed clamping devices of a flash butt welding machine each of which has a water-cooled electrode and a clamping jaw.

One clamping device is arranged on a carriage which is driven so that the sheet metal edges can be pressed against each other with a preselected force program. The preselectable current is supplied to one electrode and flows via the sheet metal edges and the other electrode back to the power source.

In order to remain largely independent from external influences, and in order to obtain a uniformly reproducible weld temperature, a further embodiment of the method of the invention consists in indicating the end of heating within the actual weld program by movement through a selectable distance with the distance moved with a given contact force being a measure of the welding temperature.

This can likewise be achieved by another embodiment of the method of the invention which is characterised in that the end of heating is indicated on reaching a predetermined quadratic current value (12t) within the actual welding program.

The welding process of the invention has the following elements: 1. Contact time; In this interval uniform contact over the whole width of the sheet metal is achieved by heating and flattening of the asperities.

2. Cooling time; In this interval the non-uniform thermal distribution originating from the contact time is cooled.

3. Heating and upsetting time: In this interval the sheet metal edges are brought uniformly over the whole width to the upsetting temperature and are upset.

Only the cooling time led to success.

Claims (7)

1. A method of electric flash butt welding, in particular electric flash butt welding of workpieces with a large extent of the weld cross-section, for example metal sheets or pipes, characterised in that the actual welding program (heating and upsetting) is preceded by a contact program in order to achieve uniform contact over the whole cross-sectional area of the weld.

2. A method in accordance with claim 1 and characterised in that during the contact program asperities in the contact plane are heated and pressed flat by the use of a high contact pressure and a high current density and in that the contact program is terminated with a short, current-free, cooling and thermal equalisation period.

3. Apparatus for carrying out the method of claims 1 and 2 and characterised in that the workpieces which are to be welded are clamped close to the weld between well cooled electrodes.

4. A method in accordance with claim 1 and characterised in that the end of heating during the actual welding program is indicated by movement through a preselectable distance with the distance for a given contact force being a measure of the weld temperature.

5. A method in accordance with claim 1 and characterised in that the end of heating within the actual welding program is indicated on achieving a given squared current value (12t).

6. A method substantially as described herein.

7. An apparatus substantially as described herein.