GB2094694A - Method and apparatus for arc butt welding - Google Patents

Abstract

This invention relates to the joining of two parts by an arc buff welding process in which an arc is struck across a gap between the two faces (10, 12) to be joined and is subjected by solenoids 14, 16 to a magnetic field to cause it to move over the two faces for a period sufficient to heat the faces. The heated faces are then brought together and a forging pressure is applied across them. In the method of the invention, for a final portion of the arc heating period, the value of the arc current is significantly increased above its value during a preceding portion of the arc heating period, and before the final portion of the arc heating period, the value of the arc gap between the faces is increased above its value during the preceding portion of the arc heating period, thereby preventing short-circuit during the final portion of the arc heating period. <IMAGE>

Description

SPECIFICATION Method and apparatus for arc butt welding This invention relates to the joining of two parts by arc butt welding. It is known to generate the heat required for welding in the region of the joint, by the passage of electricity between the parts to be joined.

One such process is resistance butt welding; another is flash butt welding, or flash welding, in which both resistance and electric arc heating are employed.

Arcs are formed across the gap between the components to be welded when their initial points of contact become molten and the molten bridge then ruptures.

Another form of butt welding, known as arc butt welding, employs a radial magnetic field around the two components, for example two tubes whose end faces are to be joined. British Patent No.1089178 relates to such a process. The magnetic field, generated by coils surrounding the tubes, causes an arc struck between the end faces of the tubes to travel rapidly around in a ring between the annular faces. This gives a more even distribution of heat, but the arc still has a limited width, and the maximum thickness of tubing which may suitably be joined by this process is around 4to 5 millimetres.

British Patent No. 1475450 describes an improved arc butt welding process in which a second magnetic field causes the rotating arc to deviate periodically from its narrow circular path around the surfaces to be welded. A central conductor is placed within the workpiece during welding. Preferably, the workpieces are connected to a source of alternating current, and the central conductor is connected to a source of direct current such that its associated magenticfield causes the rotating arcto move periodically outwards and inwards from the axis of the tubes. The perturbed motion of the arc causes it to cover much more of the surface area of the tube ends, and thicker tubes can be used. However, it is not always possible to use this central conductor.

British Patent No. 2035177 describes an alternative to the use of a central conductor in arc butt welding, in which the two components to be welded are moved in a plane parallel to their faces, once an arc has been struck. The magnetic field due to the flow of arc current, together with the relative movement of the ends of the workpieces, constrain the arc roots to move over substantially the whole of each face.

The faces are then brought into registration and forgoing pressure is applied across the faces.

The arc current for arc butt welding is generally alternating circuit when a central conductor is used and either alternating or direct current when the method of British Patent No. 2035177 is used. It has been found that the amplitude, or level, of arc current should be increased from a low initial value to a higherfinal forging value, for the best results.

This two stage programme comprising the longer duration low current stage and short duration final stage is useful since it gives sufficiently high heat input and soak back to allow an efficient final forging operation. Such a current programme is indicated in Figure 10 of British Patent No. 1475450, and is described on page 3, column 2, lines 87 to 92. With all magnetically impelled arc butt welding processes, globules of metal may extend across local regions of the two arcing faces, causing short circuits. These may be sufficient to cause the arcing process to halt, if there is insufficient power to expel the molten globules and rupture the short circuits.

This problem is described in British Patent No.

2035177, page 2, lines 39 to 69. A secondary source of current is provided which takes over when short circuits occur. However, this is only a partial solution to the problem of short-circuiting; although both methods of arc butt welding described above produce good results, defective welds are still made when using such a two-stage current programme due to severe short circuiting which occurs in the final high current period. These short circuit across the weld interface cause local expulsion of molten material and result in poor adhesion across the weld joint in this area on final forging.

The present invention eliminates this short circuiting problem. A method of joining two parts by an arc butt welding process, comprises the steps of striking an arc across a gap between the two faces to be joined and subjecting the arc to a magnetic field to cause it to move over the two faces, maintaining the arc current for a period to heat the faces, and then bringing the heated faces together and applying forging pressure across them, and in which for a final portion of the arc heating period, the value of the arc current is significantly increased above its value during a preceding portion of the heating period, the method further comprising, before the final portion of the arc heating period, the step of increasing the value of the arc gap between the faces above its value during the preceding portion of the heating period.

Apparatus for carrying out the invention comprises means for connecting two parts to be joined to opposite poles of a power supply, means for establishing a magnetic field such that an arc formed between the faces of the parts to be joined will travel over the faces, driving means for moving the parts towards and away from one another to form an arc across a gap between faces of the said parts to be joined to heat the faces and, after the arc heating period, to apply a forging pressure across the faces, current control means acting to increase significantly the arc current for a final portion of the arc heating period, and gap control means acting to increase the separation of the two parts before the final portion of the arc heating period.

By increasing the gap between the parts in this manner, for example from 2mm to about 3mm, the local short-circuiting is eliminated, and trials have indicated a good reproducible weld quality using this method.

In one method embodying the invention for welding tubes, the arc gap is increased twice during the arc heating cycle. Typically, in such a cycle the gap has a first value for a little less than one half of the arc heating time, a second value for approximately the same portion of the arc heating time, and a third (largest) value for a small proportion of the total arc heating time at the end of the arc heating cycle, the arc current being increased during this latter portion of the arc heating time.

In order that the invention may be better understood, an example of the preferred method and apparatus will now be described, with reference to the accompanying drawings, in which: Figure la illustrates the general arrangement of two tubular workpieces to be joined in accordance with the method of British Patent No. 2035177; Figure 1b illustrates the ends of the workpieces of Figure 1 a, with their magnetizing solenoids, during the arc-heating operation; Figure 2 shows an example of a two-stage arc current programme; Figure 3 shows the variation of the arc gap with time in accordance with one form of the invention; Figure 4 shows typical variations with time in arc current, forging force and arc gap, in apparatus according to the preferred embodiment of the invention; and Figure 5shows an example of a three-stage arc gap programme.

In Figures 1 a and 1 b, two tubes A and B whose faces 10 and 12 are to be welded, are held in clamps 2 and 4 respectively. A power source 6 supplies direct current through the clamps 2 and 4 to the end faces 10 and 12. The tubes are given a relative orbital motion in a plane parallel to the end faces and are illustrated at a moment of maximum relative offset in the side view shown in Figure 1 b. Magnetizing solenoids 14 and 16, fed from a separate power supply 15, extend around the tube ends and induce magnetic fields in opposite directions, producing a radial magnetic field at the end faces. When an arc (18, 18b) is struck between tubes A and B, this radial magnetic field interacts with the axial magnetic field of the arc to move around the ends of the workpieces at a very fast rate, which for steel is typically 100 to 300 meters/second.

The arc current supplied by source 6 is varied with time by a current control unit 20 (Figure la) according to the schematic diagram of Figure 2. The section AB is the first stage of the two-stage current programme. BC indicates the sharp step-up in current to a second, high-current period CD. The two components are then forged under pressure for a brief interval during which the current may rise still further to a point E. The components are then welded and the current is switched off (F). Typically, the first stage lasts for 14 seconds at a current of 350 amps, and the second stage lasts for 1 second at a current of 1200 amps, for a tube of 50mm diameter with a wall thickness of 6mm.

Severe short circuiting which would normally occur in the final high current period CD, is eliminated according to the invention by controlling the separation between the two faces to be welded.

Figure 3 shows the variation of this separation, i.e.

the variation of the arc gap by a gap control unit 22 (Figure 1 a), with time. At G the arc is struck; the workpieces are then separated to a first value, typically 2mm for the majority HI of the first stage AB of the current programme. The gap is then increased suddenly, IJ, to a larger gap which is maintained at least for the second stage of the current programme CD. The rapid increase in arc gap preferably takes place before the sharp step up in current, and within the last 40% of the total arc time. The second value of the arc gap JK is typically 3mm, but in all cases should be at least 25% greater than the first arc gap HI. When the workpieces are forged the arc is quenched and the separation of the workpieces falls to zero (KL). The timing of the operation of the current control 20 and the gap control 22 is synchronized by a sequence control 24.

Figure 4 shows the variation with time of the arc current, arc gap and forge force in a typical welding cycle employing the present invention. In order to start the arc, there is initially applied a substantial force F1 and the workpiece end faces are brought together (Go) (for the gap trace of Figure 4, the lower the trace, the greater is the arc gap.) During the touch starting of the arc, current rises from zero to a short circuit value (it); the arc gap then increases from zero to about 2mm (G1); the force trace is now at zero value (Fo) The arc current falls to the value 12 which has been set for the first stage of the arc heating programme and in this example the current is held steady at this lower value for about 14 seconds, with the exception of a momentary short circuit.Thereafter, the current is increased to a high value 13 for about 0.6 of a second.

Just before the rapid increase in current, the arc gap increases from approximately 2mm to 3mm (G2). Thereafter, a forging force is applied. so that the force trace rises from zero to the forge value F2.

We have found that for welding tubes with thicknesses greater than about 6mm, it is advantageous in some cases to use three different arc gaps during the arc heating cycle. The increases in gap size will generally be not less than 25% and the final gap size will exist for a small final portion of the cycle. In a typical example, a gap of 2mm is used for 47% of the arc heating time, followed by a gap of 3mm for a further 47%, followed by a gap of 4mm for the final 6% of the arc heating time, the high-level arc current value being established during this final arc heating period. However, the gap values and the proportions of the total cycle time for which these values prevail vary substantially from application to application. A typical trace of arc gap against time for a 3-stage gap process is shown in Figure 5. With the thicker-wall tubes, more heat is necessary and in order to keep weld times reasonably short, the arc current is generally increased. This results in the formation of greater amounts of molten metal. The use of the three different arc gaps substnatially overcomes the short circuiting problems which might otherwise result from the high current.

In the above description, the increase in the arc gap has been in the nature of a step change; however, a progressive increase in arc gap could be used to replace the step changes shown in the drawings.

Although this invention has been described with reference to the welding of tubular components, the method and apparatus are also suitable for welding rods, plates and other components, the invention may be applied not only to arc butt welding processes in which the two components are given a relative orbital movement during arc heating, but also to the process involving the use of a central conductor, and for any other form of arc butt welding in which the arc is subjected to a magnetic field to cause it to move over the faces to be joined.

Claims (9)

1. A method of joining two parts by an arc butt welding process, comprising the steps of striking an arc across a gap between the two faces to be joined and subjecting the arc to a magnetic field to cause it to move over the two faces, maintaining the arc current for a period to heat the faces, and then bringing the heated faces together and applying forging pressure across them, and in which for a final portion of the arc heating period, the value of the arc current is significantly increased above its value during a preceding portion of the heating period, the method further comprising, before the final portion of the arc heating period, the step of increasing the value of the arc gap between the faces above its value during the preceding portion of the heating period.

2. A method in accordance with claim 1, in which the gap between the faces is increased by at least 25% before the final portion of the arc heating period.

3. A method in accordance with claim 1, in which the arc gap prevailing at the beginning of the arc heating period is increased in two subsequent stages prior to the termination of the arc heating period.

4. A method in accordance with claim 3, in which each of the two increases is by at least 25% of the initial gap value, and in which the largest gap exists for a period which is small in relation to the periods for which the preceding gap values prevail.

5. A method in accordance with claim 1, comprising moving at least one of the parts to be joined during the arc heating period in such a manner that the end faces of the parts have a relative motion in a plane parallel to these end faces, the relative movement rendering more even the distribution of heat across the faces.

6. A method in accordance with claim 6, for welding the end faces of two tubes, in which the end faces of the tubes have a relative orbital motion in the said plane during the arc heating period.

7. Apparatus for joining two parts by an arc butt welding process, comprising a circuit for connecting two parts to be joined to opposite poles of a power supply, means for establishing a magnetic field such that an arc formed between the faces of the parts to be joined will travel over the faces, driving means for moving the parts towards and away from one another to form an arc across a gap between faces of the said parts to be joined to heat the faces and, after the arc heating period, to apply a forging pressure across the faces, current control means acting to increase significantly the arc current for a final portion of the arc heating period, and gap control means acting to increase the separation of the two parts before the final portion of the arc heating period.

8. Apparatus in accordance with claim 7, further including driving means for imparting a motion to at least one of the parts to be joined, during the arc heating period, so that there is relative movement of the end faces of the parts in a plane parallel to these end faces, to render more even the distribution of heat during the arc heating period.

9. Apparatus in accordance with claim 8, in which the means for imparting a motion to at least one of the parts to be joined, is such that the relative motion between the end faces of the parts is a relative orbital motion.