Patent Publication Number: US-11389889-B2

Title: Arc welding method and arc welding arrangement with first and second electrodes

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of pending U.S. patent application Ser. No. 14/654,994, filed Jun. 23, 2015, which is a 371 application of International Application No. PCT/EP2013/077765, filed Dec. 20, 2013, which claims priority to Swedish Patent Application No. 1200792-8, filed Dec. 28, 2012, the entireties of each of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method for electric arc welding. In particular the present invention relates to a method for electric arc welding according to the preamble of claim  1 , where welding takes place by a welding head having at least two contact devices, each being connected to a respective electrode and a respective power source to enable arc ignition between the electrodes and a work piece. The present invention also relates to an arrangement for carrying out said method. 
     BACKGROUND OF THE INVENTION 
     Electric arc welding with continuously fed electrodes is performed with various techniques. These techniques comprise submerged arc welding, where welding is performed under a protective layer of flux, and gas metal arc welding where welding is performed under a protective gas shield. 
     In order to increase the productivity it has been suggested to use more than one electrode in the welding process. Known such welding processes are tandem and twin welding methods, which are used to increase the production rate when welding. 
     Tandem welding is a welding technology where two independent electrodes are arranged to perform welding in a common weld pool. 
     Each of the two electrodes are connected to a separate power source such that the welding conditions comprising current and voltage can be controlled independently for the electrodes. The electrodes can be mounted in various settings, such as for instance with the electrodes positioned shifted out in a transverse direction with respect to a welding direction or being positioned at a distance from each other in the welding direction. In the event the electrodes are shifted out in the transverse direction, they may be positioned side by side. This is used for surface welding or specific joints where a wide joint is needed. Side by side welding leads to lower penetration and more width. 
     Alternatively the electrodes are positioned at a distance from each other in the welding direction. In this event, the first electrode in the direction of welding is normally referred to as a leading electrode, while the second electrode is normally referred to as a trailing electrode. Normally the leading electrode and the trailing electrodes serve different roles in the welding process. It is for instance known to control the leading electrode such that a desired degree of penetration is obtained, while the trailing electrode controls weld bead appearance, contour and fill. 
     Tandem welding enables increased deposition rates and therefore improves economy of welding. Tandem welding also enables improved weld quality due to the possibility of assigning the leading and trailing electrode different tasks. In order to allow the different electrodes to be assigned different tasks the electrodes should preferably be sufficiently close to enable both electrodes to operate in a common single weld pool. In the event the electrodes are separated too far from each other, the weld pool generated by the leading electrode may solidify before the trailing electrode has reached the puddle. In this event, the two electrodes more or less perform the task of two consecutive welding passes. 
     Twin welding is similar to tandem welding, with the difference that the electrodes are connected to the same power source. 
     Another way to improve the deposition rate is to add one or more consumable electrodes that melt without formation of arcs. These electrodes are sometimes referred to as cold wires, whereas electrodes arranged to generate arcs are referred to as hot wires. A cold wire is continuously fed into the arcs of one or more hot wires, where the cold wire is melted by excess energy generated by said hot wire(s). A current may be transferred through a portion of a cold wire for heating thereof. 
     The introduction of cold wire material into the weld pool may lead to improved control of the composition of the weld alloy, which may lead to improved welds. Feeding of cold wire material into the weld pool may lead to an increase in productivity of up to 100% with optimized welding parameters. In other words, a cold wire allows for higher deposition rates without increasing the heat input. 
     A problem associated with cold wires is that they sometimes increase the welding process instability when the cold wire is not melted at an even pace. The may also strike the parent material through the melt pool. This can cause weld defects and inclusions in the weld metal of unmelted cold wire material. Also, the amount of cold wire that can be added to the weld pool is limited by the amount of excess energy generated by the hot wires. 
     It is a first object of the invention is to provide an efficient electric arc welding method, which makes possible an increased production rate. 
     A second object of the invention is to provide an electric arc welding arrangement, which makes possible an increased production rate and more efficient welding. 
     SUMMARY OF THE INVENTION 
     The first object of the invention is achieved with an electric arc welding method to be used with a welding arrangement. The welding arrangement comprises a first power source, a first wire feeder for feeding a first electrode and a second wire feeder for feeding a second electrode. The welding arrangement further comprises an electric arc welding head comprising a first contact device housing a first duct for guiding the first electrode and providing electrical contact between said first power source and said first electrode and a second contact device housing a second duct for guiding the second electrode and providing electrical contact between said a power source and said second electrode. The first and second contact devices are electrically insulated from each other. The first electrode is adapted to act on a work piece for generating a weld pool via a first electric arc present between the first electrode and the work piece within a first arc region and said second electrode is adapted to act on said work piece for generating said weld pool via a second electric arc present between the second electrode and the work piece within a second arc region. The thus generated heat will assist in melting the electrode material and the thus generated magnetic fields will assist in metal transfer from the electrodes to the weld pool. 
     The first electrode is operated at welding parameters which maintain said first arc ignited and transfer molten electrode material from said first electrode to said weld pool. That is, the welding parameters of the first electrode are such that the first power source alone can maintain an arc ignited and transfer molten electrode material from the first electrode to the weld pool. The second electrode is operated at welding parameters which ensure that excess energy from at least said first electrode is required to maintain said second arc ignited and transfer molten electrode material from said second electrode to said weld pool. 
     The method comprises the step of feeding said second electrode into or in the vicinity of the first arc region to allow said second electrode to consume excess energy from the first electrode to ignite and maintain said second arc and transfer molten electrode material from said second electrode to said weld pool. The welding parameters of the second electrode are such that the power from the power source and the consumed excess energy from the first electrode together are sufficient to ignite and maintain an arc between the second electrode and the work piece, melt a portion of the second electrode and transfer molten electrode material from said second electrode to the weld pool. To put it another way, the second electrode is operated with welding parameters which ensure that the power received directly from the second power source is insufficient to ignite an arc between the second electrode and the work piece. Of course, small amounts of molten metal may be inadvertently and uncontrollably transferred from said second electrode to said weld pool. The transfer of excess energy from the first electrode to the second electrode makes it possible to increase the deposition rate with a minimal energy input from the second power source. This allows for an increased production rate, since a welding process is normally limited by a desire to keep the energy input at an acceptable level (too much energy input will destroy the work piece to be welded). 
     The second electrode is a semi-hot wire. A semi-hot wire is different from a hot wire in that it is not on its own capable of generating an arc and transferring melted electrode material to the work piece, because the electrode is controlled with welding parameters which limit the amount of energy transferred to the semi-hot wire from the corresponding power source. A semi-hot wire is different from a (preheated) cold wire in that the semi-hot wire is arranged to receive and transfer welding current to the work piece whereas a preheated cold wire is connected to lines adapted to transfer an electrode heating current away from the cold wire. The transfer of welding current to the work piece is accomplished by placing the semi-hot wire in the vicinity of a hot wire, so that the semi-hot wire can absorb excess energy from said hot wire to generate an arc and initiate metal transfer to the work piece. 
     Metal transfer may take place in a short circuit process, in a globular process, in a spray arc process and in submerged arc welding. 
     The second electrode is advantageously fed into the first arc region, to ensure that it consumes excess energy from the first electrode. 
     Optionally, the electric arc welding head comprises a third contact device housing a third duct for guiding a third electrode and providing electrical contact between a power source and said third electrode. The third electrode is adapted to act on said work piece for generation of said weld pool via a third electric arc present between the third electrode and the work piece within a third arc region. The first and third electrodes are fed forward so that said first and third arc regions create an overlapping region. The third electrode may be operated at welding parameters which maintain a third arc ignited and transfer molten metal from said third electrode to said weld pool. The method may comprise the step of feeding the second electrode in the vicinity of or into the overlapping region of the first and third arc regions to allow the second electrode to consume excess energy from the first and third electrodes to ignite and maintain said second arc ignited and transfer molten electrode material from the second electrode to the weld pool. 
     In one embodiment, the first and third electrodes generate arc regions of identical size and cone shape. The electrodes are fed in such a way that the distance between their end portion center axes is less than a cone diameter of an arc cone generated by one of said electrodes and measured at the surface of the weld pool. 
     The second electrode is advantageously fed into the overlapping region of the first and third arc regions, to ensure that the second electrode will consume excess energy from said first and third electrodes. 
     This solution involving a third hot wire allows the second electrode to consume excess energy from said first and third electrodes. A suitable electrode arrangement for this purpose is to position the second electrode in between the first and third electrodes. 
     The first and second electrodes may be powered by the same power source. Alternatively, the first and second electrodes may be powered by different power sources. 
     The first, second and third electrodes may be powered by the same power source. Alternatively, the first and second electrodes may be powered by one power source and the third electrodes by a different power source. It is also possible to power the first and third electrodes with the same power source and to power the second electrode with a different power source. Finally, it is possible to let the first, second and third electrodes be powered by different power sources. 
     The third electrode may also be operated with welding parameters that ensure that excess energy from at least one additional electrode is required to maintain said third arc ignited and transfer molten electrode material from said third electrode to said weld pool. In this embodiment the method comprises the step of feeding said third electrode in the vicinity of, or preferably into, said first arc region to ignite and maintain said third arc and transfer molten electrode material from said third electrode to said weld pool. In this embodiment the third duct is isolated from at least the first duct. That is, a welding arrangement may comprise any number of hot and semi-hot wires. A semi-hot wire can consume excess energy from more than one hot wire and a hot wire may provide excess energy to more than one semi-hot wire. 
     At least two and preferably all three of the first, second and third ducts are fed in parallel to ensure that the end portions of said electrodes are arranged in parallel. 
     The first and third electrodes may be fed in a first plane and the second electrode in a second plane, which is orthogonal to the first plane. This enables a symmetric position of the second electrode in relation to the first and third electrodes. A symmetric positioning of the second electrode with respect to the first and third electrodes allows for more stable arc plasma conditions at the location of the second electrode. Thus a more stable deposition rate of the second electrode can be achieved. Alternatively, the first, second and third electrodes are all fed in a first plane. 
     Advantageously, the end portion of a semi-hot electrode is fed in between the end portions of the two or more hot wires. This allows the semi-hot electrode to consume excess energy from more than one hot wire. 
     Advantageously, the end portion of a hot wire is fed in between two semi-hot electrodes. The semi-hot wires are preferably located on opposite sides of the hot wire. This allows the hot wire to transfer excess energy to more than one semi-hot wire. The result is a more energy efficient solution. The hot wire may have a relatively large diameter in comparison to the second and third electrodes. 
     Optionally, the wire feed speed of the second wire feeder is controlled in dependence on one or more welding parameters of at least one hot wire. This is advantageous in that the welding parameters of the hot wire determines the magnitude of the excess energy available to assist in the melting of the second electrode and thus at which wire feed speed the second electrode can be fed towards the work piece. The second electrode is advantageously fed at a wire feed speed of between 0.2 and 0.9, more preferably 0.6 and 0.8, of the power feed speed wmax, wherein wmax is the wire feed speed at which the excess energy consumed by the second electrode corresponds to the energy needed (in addition to the power generated by the second power source) for igniting an arc at the second electrode. 
     It is advantageous to keep the ratio between the input power to wire feed volume much higher for the electrode at which transfer of molten material would take place even without excess energy from a neighboring electrode than the ratio for the electrode at which transfer of molten material would not occur. In this way, it is ensured that the heat input can be kept at a low level and that high production rates can be achieved. 
     The invention contemplates the use of welding parameters for the second electrode which comprise current, voltage and wire feed speed, wherein the current, voltage and wire feed speed for the second electrode are selectable independently of each other. 
     The invention further contemplates use of welding parameters for the first electrode comprising current, voltage and wire feed speed. The current and the voltage may be set parameters and the wire feed speed a result parameter selected to enable a stable arc. Alternatively, the wire feed speed may be a set parameter and the current automatically adjusted to maintain an arc voltage level. The welding current may also remain essentially constant whereas the arc voltage is dependent on the rate at which the hot wire is fed towards the work piece. 
     The second object of the invention is achieved with an arc welding arrangement for carrying out the above described method. The arc welding arrangement comprises a first power source, a first wire feeder for feeding a first electrode and a second wire feeder for feeding a second electrode. The arc welding arrangement further comprises an electric arc welding head comprising a first contact device housing a first duct for guiding the first electrode and providing electrical contact between said first power source and said first electrode, a second contact device housing a second duct for guiding the second electrode and providing electrical contact between a second power source and said second electrode. The first and second contact devices are electrically insulated from each other, said first electrode being adapted to act on a work piece for generating a weld pool via a first electric arc present between the first electrode and the work piece within a first arc region and said second electrode being adapted to act on said work piece for generating said weld pool via a second electric arc present between the second electrode and the work piece within a second arc region. 
     The first electrode is arranged to be operated at welding parameters which maintain said first arc ignited and transfers molten electrode material from said first electrode to said weld pool. That is, the first electrode does not require excess energy from other electrodes to generate said arc. 
     The second electrode is arranged to be operated at welding parameters which ensure that excess energy from at least one additional electrode is required to maintain said second arc ignited and transfer molten electrode material from said second electrode to said work piece. 
     The first and second ducts are arranged such that said second electrode is fed in the vicinity of or into the vicinity of said first arc region to allow said second electrode to consume excess energy from said first electrode to ignite and maintain said second arc ignited and transfer molten electrode material from said second electrode to said weld pool. 
     Advantageously, the first and second ducts are arranged such that said second electrode can be fed into said first arc region. That is, the distance between the center axis of the first duct and the center axis of the second duct should not exceed half the diameter of an arc cone generated by the first electrode and measured at the surface of the weld pool. 
     In an alternative embodiment, it may be sufficient to arrange the first and second ducts such that the first and second arc regions at least partially overlap. 
     The distance between the center axis of the end portion of the first electrode and the center axis of the end portion of the second electrode required to allow said second electrode to consume excess energy from said first electrode depends on a plurality of factors, for example wire dimension, wire material, stick-out, current, voltage, process mode (CA, CW, CC), frequency and welding speed. 
     The first arc generates a substantial amount of energy while the second electrode absorbs excess energy from the first electrode and thus increases the deposition rate with a minimal energy input. This allows for an increased production rate, since a welding process is normally limited by a desire to keep the energy input at an acceptable level, and a more energy efficient welding procedure. 
     The arc welding head may comprise a third contact device housing a third duct for guiding a third electrode and providing electrical contact between a power source and said third electrode. The third electrode is adapted to act on the work piece for generating said weld pool via a third electric arc present between said third electrode and said work piece within a third arc region, wherein said first and third ducts are arranged to allow said first and third arc regions to have an overlapping region. The third electrode is adapted to be operated at welding parameters which maintain a third arc ignited and transfer molten metal from said third electrode to said weld pool. The second duct is arranged to allow said second electrode to be fed in the vicinity of or into said overlapping region of the first and third arc regions to consume excess energy from the first and third electrodes to ignite and transfer molten electrode material from the second electrode to said weld pool. 
     Advantageously, the second first, second and third ducts are arranged such that the second electrode is fed into the overlapping region of the first and third arc regions. This would ensure that the second electrode can consume excess energy from more than one electrode and may thus provide a more stable welding procedure, as the second arc can be maintained also when one of the first and third arcs is extinguished. 
     Alternatively, the arc welding head may comprise a third contact device housing a third duct for guiding the third electrode and providing electrical contact between a power and said third electrode. The third electrode is adapted to act on said work piece for generating said weld pool via a third electric arc present between the third electrode and the work piece within a third arc region. The third electrode is adapted to be operated at welding parameters, which ensure that the third electrode must consume excess energy from at least one additional electrode to maintain said third arc ignited and transfer molten electrode material to said weld pool. The first and third ducts are arranged such that said third electrode is allowed to be fed into or in the vicinity of the first arc region to allow said third electrode to consume excess energy from said first power electrode to ignite and maintain said third arc and transfer molten electrode material from said third electrode to said weld pool. 
     Advantageously, the first and third ducts are arranged such that the third electrode can be fed into the first arc area. It is in any case arranged such that the third electrode can be brought close enough to the first electrode to absorb excess energy from said first electrode. 
     Advantageously, the first, second and third ducts are arranged on opposite sides of the first duct, to ensure that the end portions of the second and third electrodes during welding are located on opposite sides of the first duct. This is particularly the case in embodiments wherein both the second and third electrodes are adapted to consume excess energy from the first electrode. 
     The second duct may also be located in between said first and third ducts. This is advantageous in that the second electrode may consume energy from said first and third electrodes. 
     In some embodiments, at least two of said first, second and third ducts are arranged in parallel. 
     In some embodiments, the first and third ducts are arranged in a first plane and the second duct in a second plane, which is orthogonal to the first plane. This enables a symmetric position of the second electrode in relation to the first and third electrodes. A symmetric positioning of the second electrode with respect to the first and third electrodes allows for more stable arc plasma conditions at the location of the second electrode. Thus a more stable deposition rate of the second electrode can be achieved. Alternatively, the first, second and third ducts are all arranged in a first plane. 
     Advantageously, the welding arrangement comprises a controller adapted to control one or more of the wire feeding means and the power sources. 
     In one embodiment, the controller is connected to a wire feeder arranged to feed a semi-hot electrode. The controller may be adapted to control the wire feed speed of the semi-hot electrode in dependence on welding parameters of at least one hot wire adapted to provide excess energy to said semi-hot electrode. That is, the wire feed speed of the semi-hot electrode is not dependent on the current or voltage applied to the semi-hot electrode. For example, the second wire feeder may be controlled in dependence on welding parameters of the first and/or third power electrodes. 
     The controller may also be adapted to control the wire feed speed of the wire feeder arranged to feed the semi-hot electrode in dependence on the wire feed speed of at least one wire feeder arranged to feed an electrode arranged to provide excess energy to said semi-hot electrode. For example, the wire feed speed of the second wire feeder may be controlled in dependence on the wire feed speed(s) of the first and/or third wire feeder. 
     An electrode that must consume excess energy from at least one adjacent electrode to generate an arc may be adapted to have a wire feed speed higher than the wire feed speed of the electrode adapted to provide said excess energy. 
     The welding parameters for a semi-hot wire may comprise current, voltage and wire feed speed. The current, voltage and wire feed speed for the semi-hot electrode can be selectable independently of each other. 
     The welding parameters for a hot wire may comprise current, voltage and wire feed speed. The current and the voltage may be set parameters and the wire feed speed a result parameter selected to enable a stable arc. Alternatively, the wire feed speed may be a set parameter and the current automatically adjusted to maintain an arc voltage level. The welding current may also remain essentially constant whereas the arc voltage is dependent on the rate at which the hot wire is fed towards the work piece. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will now be described in further detail with reference to the appended drawings, wherein: 
         FIG. 1  shows a front view of a welding head according to the invention; 
         FIG. 2  shows a side view of an embodiment of the welding head in  FIG. 1 ; 
         FIG. 3  shows a side view of another embodiment of the welding head in  FIG. 1 ; 
         FIG. 4 a, b    show cross-sections taken at A-A and B-B in  FIG. 1  of the embodiment also shown in  FIG. 2 ; 
         FIG. 4 c, d    show cross-sections taken at A-A and B-B in  FIG. 1  of the embodiment also shown in  FIG. 3 ; 
         FIG. 5  shows a cross-section taken at D-D in  FIG. 2 ; 
         FIG. 6  shows a cross-section taken at F-F in  FIG. 3 ; 
         FIG. 7  shows a welding head arranged for welding a work piece; 
         FIG. 8  shows a view of an embodiment of an arc-welding welding arrangement for submerged arc welding; 
         FIG. 9  shows the arc-welding welding arrangement of  FIG. 8  turned counterclockwise by 90°; 
         FIG. 10  shows a perspective view or the arc-welding arrangement of  FIG. 8 ; and 
         FIG. 11  shows an arc welding system. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     A first embodiment of an arc electric welding head according to the invention is shown in  FIGS. 1 and 4   a, b.    
       FIG. 1  shows a front view of an electric arc welding head  1 . A front view is a view toward the welding head in a direction opposite to a welding direction. The welding direction is the direction in which the welding head or one or more electrodes are moved during welding. The arc welding head  1  in  FIG. 1  comprises a first contact device  3  housing a first duct  5  for feeding a first electrode  2  and providing electrical contact between a first power source (not shown) and the first electrode  2 . 
       FIGS. 4 a, b    show cross sections taken at A-A and B- 8  in  FIG. 1 , respectively. As evident from  FIGS. 4 a, b   , a second contact device  11  is positioned behind the first contact device  3  in the arc welding head  1 . The second contact device  11  is housing a second duct  13  for feeding a second electrode  7  and providing electrical contact between a second power source (not shown) and said second electrode  7 . The first and second contact devices  3 ,  11  are electrically insulated from each other by means of an insulating body  15 . 
     The ducts  5 ,  13  for the first and second electrodes  2 ,  7  have the same diameter. The first and second electrodes  2 ,  7  also have the same diameter. In alternative embodiments, the ducts  5 ,  13  and the electrodes  2 ,  7  may have different diameters; for example, the diameter of the first electrode  2  may be larger than the diameter of the second electrode  7 . 
     The first and second contact devices  3 ,  11  are retained in a recess  39  in a retainer  41 . As shown in  FIG. 4 b    the retainer  41  comprises a first and a second retainer element  43   a ,  43   b . The first retainer element  43   a  comprises a first part  43   a   1  having a jaw portion for securing a first contact plate  3   a  (part of the first contact device  3 ) and an attachment portion for securing the first part  43   a   1  to a carrier  53  ( FIG. 1 ). The first retainer element  43   a  also comprises a second part  43   a   2  having a jaw portion for securing a second contact plate  3   b  (part of the first contact device  3 ) and an attachment portion for securing the second part  43   a   2  to said carrier  53 . 
     The first retainer element  43   a  has first and second wall portions  45   a ,  45   b  facing the first and second contact plates  3   a, b , respectively. The first wall portion  45   a  is arranged in abutment with horizontal wall portion  47   a  of the first contact plate  3   a  and the second wall portion  45   b  is arranged in abutment with horizontal wall portion  47   b  of the second contact plate  3   b.    
     Likewise, the second retainer element  43   b  comprises a first part  43   b   1  having a jaw portion for securing a first contact plate  11   c  (part of the second contact device  11 ) and an attachment portion for securing the first part  43   b   1  to said carrier  53 . The second retainer element  43   b  also comprises a second part  43   b   2  having a jaw portion for securing a second contact plate  11   d  (part of the second contact device) and an attachment portion for securing the second part  43   b   2  to said carrier  53 . 
     The second retainer element  43   b  has a first wall portion and a second wall portion  45   c ,  45   d  facing the first and second contact plates  11   c, d , respectively. The first wall portion  45   c  is arranged in abutment with horizontal wall portion  47   c  of the first contact plate  11   c  and the second wall portion  45   d  is arranged in abutment with horizontal wall portion  47   d  of the second contact plate  11   d.    
     The first contact device  3  comprises the first contact plate  3   a , which may be constituted by an essentially rectangular plate having a longitudinally extending recess  51   a  that forms a portion of the duct  5  for the first electrode  2 . The first contact device  3  also comprises the second contact plate  3   b , which may be constituted by an essentially rectangular plate having a longitudinally extending recess  51   b , which is a portion of the duct  5  for the first electrode  2 . 
     In the same manner, the second contact device  11  comprises the first contact plate  11   c , which may be constituted by an essentially rectangular plate having a longitudinally extending recess  51   c  that forms a portion of the duct  13  for the second electrode  7 . The second contact device  11  also comprises the second contact plate  11   d , which may be constituted by an essentially rectangular plate having a longitudinally extending recess  51   d , which is a portion of the duct  13  for the second electrode  7 . 
     The first and second contact plates  3   a, b  of the first contact device  3  may be biased against each other by spring action. The first and second contact plates  11   c, d  of the second contact device  11  may be biased against each other by spring action. 
     The first retainer element  43   a  and the first contact device  3  with its first and second contact plates  3   a, b  are shown at a front side of the welding head  1  ( FIG. 1 ). The first and second contact plates  3   a, b  are separated by a gap such that an electrode  2  may be interposed in a space there between and secured in a longitudinally extending recess. The first and second parts  43   a   1 ,  43   a   2  of the first retainer element  43   a  are arranged to be movable with respect to each other such that the gap between the first and second contact plates can be made larger or narrower. Springs  55  are arranged to control a force between the contact plates  3   a, b  and the electrode  2  positioned in between the contact plates  3   a, b.    
     The contact plates  3   a, b  are secured to a respective first and second part  43   a   1 ,  43   a   2  by means of screws  57 . The first part is secured to the carrier  53  by screws  59 . The second part is secured to the carrier  53  by screws  60  having resilient means in the form of springs  55  such that the contact plates  3   a, b  can be biased against each other. 
     The second retainer element  43   b  and the second contact device  11  with its first and second contact plates  11   c ,  11   d  are located at a back side of the welding head  1 . The first and second contact plates  11   c ,  11   d  are separated by a gap such that an electrode  7  may be interposed in a space there between and secured in a longitudinally extending recess. The first and second parts  43   b   1 ,  43   b   2  of the second retainer element  43   b  are arranged to be movable with respect to each other such that the gap between the first and second contact plates  11   c ,  11   d  can be made larger or narrower. Springs are arranged to control a force between the contact plates and a wire electrode  7  positioned in between the contact plates  11   c, d.    
     Now referring to  FIG. 4 b   , the first and second retainer elements  43   a ,  43   b  and the first and second contact devices  3 ,  11  are separated by the insulating body  15  such that on one side of the insulating body  15  are located the first part  43   a   1  of the first retaining element  43   a  carrying the first contact plate  3   a  and the second part  43   a   2  of the first retaining element  43   a  carrying the second contact plate  3   b , and on the other side of the insulating body  15  are located the first part  43   b   1  of the second retaining element  43   b  carrying the first contact plate  11   c  and the second part  43   b   2  of the second retaining element  43   b  carrying the second contact plate  11   d.    
     The first and second parts  43   a   1 ,  43   a   2 ,  43   b   1 ,  43   b   2  of the first and second retaining elements  43   a ,  43   b  are attached to the carrier  53  at a nose portion  65  of the carrier  53 . 
       FIG. 2  is a side view of an embodiment of the welding head  1  shown in  FIG. 1 . The welding head  1  in  FIG. 2  is arranged to receive two electrodes  2 ,  7 . As is evident from  FIG. 2 , an insulating body  15  extends through the welding head  1  to separate the welding head  1  into first and second halves. The first half comprises the first retainer element  43   a  and the first and second contact plates  3   a ,  3   b  connected to the first retainer element  43   a . The second half comprises the second retainer element  43   b  and the first and second contact plates  11   c ,  11   d  connected to the second retainer element  43   b.    
     The carrier  53  is likewise separated into two halves  53   a ,  53   b  which are electrically insulated from each other by the insulating body  15 . A single insulating body  15  separating the carrier halves  53   a ,  53   b  as well as the first and second retainer elements  43   a ,  43   b  can be used. Alternatively, a plurality of insulating bodies can be used to form the insulating layer between the two halves  53   a ,  53   b . The two halves  53   a ,  53   b  are connected together by insulated connections  61 . The insulated connections  61  can be formed by a pipe of insulating material. A screw  63  may be extending through an insulating pipe to secure the two halves  53   a ,  53   b  and the insulating body  15  together. 
     As shown in  FIG. 4 a   , the carrier  53  is formed by a first and a second plate  53   a, b  with an insulating body  15  interposed in between. The first and second plates  53   a , bare secured to each other via an insulating connection  61 . The insulating connection  61  may be formed by a pipe of insulating material extending through the first and second plates  53   a, b . A screw  63  extends through the pipe of insulating material. 
     Referring to  FIGS. 1 and 2 , the carrier  53  may have a nose portion  65 . The first and second retainer elements  43   a ,  43   b  are attached to the nose portion  65 . The first and second retainer elements  43   a ,  43   b  are separated by the insulating body  15 . The first and second retainer elements  43   a ,  43   b  are connected via an insulating element  61 . This insulating element  61  may be formed by an insulating pipe through which a screw  63  may extend. Preferably insulating washers of insulating material are used to insulate the side walls of the welding head  1  from the screws. 
       FIGS. 3 and 4   c ,  4   d  show an embodiment of a welding head  1  as shown in  FIG. 1  having three electrodes  2 ,  7 ,  8 , which welding head  1  is arranged  1  to house a central continuously fed electrode  2  and two peripheral continuously fed electrodes  7 ,  8 . The welding head  1  in  FIG. 3  is thus based on a welding head  1  as shown in  FIGS. 1 and 2  with an additional third contact device  4  housing a third duct  6  for feeding a third electrode  8  and providing electrical contact between a power source (not shown) and the third electrode  8 . The first and third contact devices  3 ,  4  are electrically insulated from each other by an insulating body  20 . 
     The ducts  5 ,  6 ,  13  for the first, second and third electrodes  2 ,  7 ,  8  may have the same diameter and house electrodes having the same diameter. Alternatively, the ducts and electrodes may have different diameters. In particular, the diameter of the duct or ducts housing electrodes that are connected operated at welding parameters which maintain an arc ignited and serving to transfer molten material from the electrode to the weld pool may be different than the duct or ducts housing electrodes which are operated at welding parameters, which ensure that the power generated is less than the power required to ignite and maintain an and continuously transfer molten electrode material from the electrode to the weld pool. 
     The third contact device  4  is retained in a recess  39  in a retainer  41  ( FIG. 1 ). 
     As shown in  FIG. 4 c   , a carrier  53  may be formed by a first, a second and a third plate  53   a ,  53   b ,  53   c  with a first insulating body  15  and a second insulating body  20  interposed in between. The plates  53   a ,  53   b ,  53   c  are secured to each other via insulating elements  61 . The insulating elements  61  may be formed by a pipe of insulating material extending through the plates  53   a ,  53   b , and  53   c . A screw  63  extends through the pipe of insulating material. 
     Referring to  FIGS. 1 and 4   d , the carrier  53  has a nose portion  65 . First, second and third retainer elements  43   a ,  43   b ,  43   c  of the retainer  41  are attached to the nose portion  65 . The first, second and third retainer elements  43   a ,  43   b ,  43   c  are separated by the insulating bodies  15  and  20 . The first, second and third retainer elements  43   a ,  43   b  are connected via an insulating element  61 . This insulating element  61  may be formed by an insulating pipe through which a screw  63  may extend. Preferably insulating washers of insulating material are used to insulate the side walls of the welding head from the screws. 
     As shown in  FIG. 4 d   , the first retainer element  43   a  comprises a first part  43   a   1  having a jaw portion for securing a first contact plate  3   a  of the first contact device  3  and an attachment portion for securing the first part  43   a   1  to the carrier  53 . The first retainer element  43   a  furthermore comprises a second part  43   a   2  having a jaw portion for securing a second contact plate  3   b  of the first contact device  3  and an attachment portion for securing the second part  43   a   2  to the carrier  53 . 
     The first retainer element  43   a  has a first wall portion  45   a  facing the first contact plate  3   a  and a second wall portion  45   b  facing the second contact plate  3   b . The first and second wall portions  45   a ,  45   b  are arranged in abutment with horizontal wall portions  47   a ,  47   b , respectively, of the first contact device  3 . 
     Similarly, the second retainer element  43   b  comprises a first part  43   b   1  having a jaw portion for securing a first contact plate  11   c  of the second contact device  11  and an attachment portion for securing the first part  43   b   1  to the carrier  53 . The second retainer element  43   b  furthermore comprises a second part  43   b   2  having a jaw portion for securing a second contact plate  11   d  of the second contact device  11  and an attachment portion for securing the second part  43   b   2  to the carrier  53 . 
     The second retainer element  43   b  has a first wall portion  45   c  facing the first contact plate  11   c  and a second wall portion  45   d  facing the second contact plate  11   d . The first and second wall portions  45   c ,  45   d  are arranged in abutment with horizontal wall portions  47   c ,  47   d , respectively, of the second contact device  11 . 
     Likewise, the third retainer element  43   c  comprises a first part  43   c   1  having a jaw portion for securing a first contact plate  4   e  of the third contact device  4  and an attachment portion for securing the first part  43   c   1  to the carrier  53 . The second retainer element  43   b  furthermore comprises a second part  43   c   2  having a jaw portion for securing a second contact plate  4   d  of the third contact device  4  and an attachment portion for securing the second part  43   c   2  to the carrier  53 . 
     The third retainer element has a first wall portion  45   e  facing the first contact plate  4   e  and a second wall portion  45   d  facing the second contact plate  4   f . The first and second wall portions  45   e ,  45   f  are arranged in abutment with horizontal wall portions  47   e ,  47   f  of the third contact device  4 . 
     The first contact plate  3   a  of the first contact device  3  may be constituted by an essentially rectangular plate having a longitudinally extending recess  51   a , which forms a portion of the duct  5  for the first electrode  2  and the second contact plate  3   b  may be constituted by an essentially rectangular plate having a longitudinally extending recess  51   b , which forms a portion of the duct  5  for the first electrode  2 . 
     In the same manner, the first contact plate  11   c  of the second contact device  11  may be constituted by an essentially rectangular plate having a longitudinally extending recess  51   c , which forms a portion of the duct  13  for the second electrode  7  and the second contact plate  11   d  may be constituted by an essentially rectangular plate having a longitudinally extending recess  51   d , which forms a portion of the duct  13  for the second electrode  7 . 
     The third contact plate  4   e  of the third contact device  4  may be constituted by an essentially rectangular plate having a longitudinally extending recess  51   e , which forms a portion of the duct  6  for the third electrode  8  and the second contact plate  4   f  may be constituted by an essentially rectangular plate having longitudinally extending recess  51   f , which forms a portion of the duct  6  for the third electrode  8 . 
     The first and second contact plates  3   a ,  3   b ,  4   a ,  4   b ,  11   a  and  11   b  of the first, second and third contact devices  3 ,  4 ,  11  may be biased against each other by spring action. 
     The first and second retainer elements  43   a, b  are separated by an insulating body  15  and the first and third retainer elements  43   a, c  are separated by an insulating body  20 . 
     The welding head may comprise any number of retainer elements separated by insulating bodies. 
       FIG. 4 a    relates to the embodiment of the welding head  1  arranged to receive two electrodes  2 ,  7  and shows a cross section taken at A-A in  FIG. 1 . The figure shows a first and a second plate element  53   a ,  53   b , which form part of the carrier  53  and are separated by an insulating body  15 . The first and second plate elements  53   a ,  53   b  each house a duct  5 ,  13  for an electrode  2 ,  7 . The insulating body  15  may be monolithic or be made by two parts where recesses are formed to create a duct  19 . The first plate element  53   a , the second plate element  53   b  and the insulating body  15  are united by an insulating member  61 . 
       FIG. 4 c    relates to the embodiment of the welding head  1  arranged to receive three electrodes  2 ,  7 ,  8  and shows a cross section taken at A-A in  FIG. 1 . The figure shows a first, a second and a third plate element  53   a ,  53   b ,  53   c , which form part of the carrier  53  and are separated by insulating bodies  15  and  20 . The first, second and third plate elements  53   a ,  53   b ,  53   c  each house a duct  5 ,  13 ,  6  for an electrode  2 ,  7 ,  8 . The first plate element  53   a , the second plate element  53   b , the third plate element  53   c  and the insulating bodies  15 ,  20  are united by an insulating member  61 . 
     In  FIG. 5  is shown a cross-section taken at O-D in  FIG. 2 . Here the first plate element  53   a  with a first duct  5  for an electrode  2 , the second plate element  53   b  with a second duct  13  for an electrode  7  and an insulating body  15  are shown. 
     In  FIG. 6  is shown a cross-section taken at F-F in  FIG. 3 . Here the first plate element  53   a  with a first duct  5  for an electrode  2 , the second plate element  53   b  with a second duct  13  for an electrode  7 , the third plate element  53   c  with a third duct  6  for an electrode  8  and two insulating bodies  15 ,  20  separating said plate elements  53   a ,  53   b ,  53   c  are shown. 
       FIG. 7  shows a side view of a welding head  1 . The welding head  1  is arranged for welding a work piece  27 . The welding head  1  propagates in a welding direction as indicated by arrow  25 . The welding head  1  is of similar construction to the welding head  1  shown in  FIG. 3 . However, the first and third electrodes  2 ,  8  in  FIG. 7  are hot wires adapted to produce arcs without the assistance of adjacent electrodes. Each arc is formed between the tip of an electrode  2 ,  8  and the work piece  27 . The second electrode  7  is a semi-hot wire located in between said first and third electrodes  2 ,  8 . The second electrode  7  is unable to produce an arc unless it consumes excess energy from said first and/or third electrodes. 
     In arc welding, an arc is present between the tip of an electrode  2 ,  8  and a work piece  27 . The contact point of the arc at the work piece  27  will be moving in a random manner. However, normally it is assumed that the arc is present within a cone shaped or parabolic arc region  31 ,  33  from the tip of the consumable electrode  2 ,  8  to the work piece  27 . The opening angle of the cone  31 ,  33  may vary from welding case to welding case. However, a normal opening angle a may be around 30°. 
     The second electrode  7  is in  FIG. 7  arranged in between the first and third electrodes  2 ,  8 . This arrangement is preferred, but not necessary. The first and third consumable electrodes  2 ,  8  are preferably mounted at an axial distance being less than a cone diameter of an arc cone  31 ,  33  measured at the surface of a weld pool  28 . The second electrode  7  is preferably introduced into the outer parts of the overlapping portions  35  of the arc cones  31 ,  33  of the first and third electrodes  2 ,  8 , which is beneficial for the weld result. 
     In preferable embodiments the feeding arrangement is arranged to feed said second electrode  7  at an angle of preferably less than 5 degrees, still preferably less than 2 degrees, with respect to a weld  28  surface normal. 
     In a preferred embodiment, the consumable electrodes  2 ,  8  are arranged in parallel and are arranged to be fed toward the weld pool  28  in an essentially orthogonal direction to a surface of the weld pool  28 . 
     Each arc is present within an essentially cone shaped or parabolic arc region  31 ,  33  extending between an electrode  2 ,  8  tip and the work piece  27 . Each cone shaped or parabolic arc region  33  defines a plasma region in which the arc plasma will be present when welding. The cone  31 ,  33  will have an opening angle  2   a , which is dependent on the distance between the work piece  27  and the electrode tip and the voltage between the electrode  2 ,  8  and the work piece  27 . The first and third electrodes  2 ,  8  are positioned in the vicinity of each other such that an overlapping region  35  between the plasma region  31  of the first electrode  2  and the plasma region  33  of the third electrode  8  is created. In order to accomplish this, the first and third ducts  5 ,  6  of the welding head  1  are arranged at a distance which allows for arcs between said first and second electrodes  2 ,  8  and the work piece  27  to be welded to have an overlapping region  35 . 
     The second electrode  7  is fed into the overlapping region  35 . 
     In preferred embodiments, in order to enable creation of an overlapping region  35  suitable for receiving the second electrode  7 , a distance between a center of the second and first ducts  13 ,  5  may be less than 3 times the diameter of the first duct  5 . Further, a distance between a center of the second and third ducts  13 ,  6  may be less than 3 times the diameter of the third duct  6 . 
       FIGS. 8 to 10  depict different views of an example embodiment of an electric arc-welding welding arrangement  100  for submerged arc welding which views are described in combination. 
     Along its longitudinal extension the electric arc-welding welding arrangement  100  comprises an arc welding head  160  at its lower end, which arc welding head  160  during welding is in close proximity to the work piece (not shown) to be welded. The arc welding head  160  is of the type as described in connection to  FIGS. 1-7  and holds the electrodes  2 ,  7 ,  8  of the electrode assembly  170  of the welding arrangement  100 . The electrodes  2 ,  7 ,  8  exit the arc welding head  160  through outlets  162  at the lower end of the arc welding head  160 , which faces the work piece during the welding operation. The wire electrodes  2 ,  7 ,  8  may be fed from respective reservoirs such as coils (not shown) towards the arc welding arrangement  100 . 
     The electrode assembly  170  comprises by way of example three fusible continuously-fed wire electrodes  2 ,  7 ,  8  arranged in the arc welding head  160 . The continuously fed wire electrodes  2 ,  7 ,  8  are electrically isolated from each other. 
     A wire feeder unit  150  is arranged above the arc welding head  160  to feed an electrode towards said arc welding head. Typically, the feeder unit  150  comprises grooved wheels, which move the wire electrode or the cold wire towards the arc welding head  160 . The feeder unit  150  may be arranged for feeding one or both of the first and third electrodes  2 ,  8  or the second electrode  7  or all electrodes  2 ,  7 ,  8 . In the event the feeder unit  150  feeds more than a single electrode  2 ,  7 ,  8 , the feeder unit  150  will typically comprise electrically insulating portions for feeding through each respective electrode  2 ,  7 ,  8  such that at least the electrodes  7  unable to produce arcs on their own are electrically insulated from the electrodes  2 ,  8  being able to produce arcs on their own. The electrically insulating portions can consist of feeder wheels with an extra insulated groove for the electrically insulated electrodes  7 . 
     The feeder wheels are driven by a driving unit  152 , e.g. an electric motor. Aside from the wire feeder unit  150  a flux hopper  114  is arranged which feeds granular flux to the arc welding head  160  via a nozzle (not shown) for submerged arc welding. Besides the driving unit  152  the wire feeder unit  150  comprises a gear with a drive shaft. On the drive shaft of the gear a feeding wheel is arranged which can be pressurized by another wheel. The feeding wheel drives the electrode  2 ,  7 ,  8  forward in the direction of the arc welding head  160 . 
     A wire straightening unit  140  is arranged above the wire feeder unit  150 . Two rollers depicted in a foremost position of the wire straightening unit  140  are used to exert a pressure on three fixed wheels arranged vertically one over the other in the rear part of the straightening device. The pressure the rollers are exerting on the wheels is adjustable via knobs at the outside of the wire straightening unit  140 . The pressure of the rollers on the three wheels is straightening the wire(s). The wire straightening unit  140  may comprise electrically insulating portions adapted to separate electrodes  2 ,  7 ,  8  from each other. 
     Instead of having a single wire feeding unit  150  for all electrodes, separate wire feeder units can be used for each of the electrodes. In  FIGS. 8-10  a separate feeder unit  130  is provided for feeding the second electrode  7  towards the arc welding head  160 . On the wire feeder unit  130  a driving unit  132 , e.g. an electric motor, is arranged which drives feeder wheels of the wire feeder unit  130 . Besides the driving unit  132 , the wire feeder unit  130  comprises a gear with a drive shaft. On the drive shaft of the gear a feeding wheel is arranged which can be pressurized by another wheel. The feeding wheel drives the second electrode  7  forward in the direction of the arc welding head  160 . 
     Instead of having a single wire straightening unit  140 , separate wire straightening units may be arranged for straightening the individual electrodes. Here a single wire straightening unit  120  for the second electrode  7  is shown. In this embodiment, along the longitudinal extension of the welding head  100 , an electrically insulating conduit  180  is provided for guiding the second electrode  7  from a wire reservoir such as a wire bobbin to the contact nozzle. The electrically insulating duct  180  consists of the electrically insulating portion of the wire straightening unit  140 , the electrically insulating portion of the wire feeder unit  150 , and an electrically insulated portion of the arc welding head  160  as well as electrically insulated wire conduits between and the units  130 ,  140 ,  150 ,  160  and above the wire straightening unit  120  for the second electrode  7 . 
       FIG. 11  shows an arc welding system  200  comprising an arc welding arrangement  202  as shown in  FIGS. 8-10 . The arc welding arrangement  202  comprises an arc welding head  1  according to the invention. The arc welding arrangement  202  further comprises three separate power sources  204 ,  205 ,  206 , which are controlled by a controller  208 . The power sources may be of the inverter type, for example such as described in U.S. Pat. No. 6,291,798. The arc welding system comprises a first wire feeder  210  for feeding a first consumable electrode  2 , a second wire feeder  212  for feeding a second consumable electrode  7  and a third wire feeder  214  for feeding a third consumable electrode  8 . The first power source  204  is connected to a first contact device for providing electrical contact between the first power source  204  and the first electrode  2 . The second power source  205  is connected to a second contact device for providing electrical contact between the second power source  205  and the second electrode  7 . The third power source  206  is connected to a third contact device for providing electrical contact between the third power source  206  and the third electrode  8 . The first power source  204  is operated to maintain a first arc ignited and transfer molten metal from said first electrode to the weld pool. The third power source  206  is operated to maintain a third arc ignited and transfer molten metal from said third electrode  8  to the weld pool. The second power source  205  is operated to ensure that the second electrode  7  on its own cannot generate a second arc. 
     The semi-hot second electrode  7  is moved into the overlapping arc region of the two adjacent first and third hot wires  2 ,  8  to consume excess energy from said hot wires  2 ,  8  in such amounts that an arc is generated at said second electrode  7  and melted electrode material is transferred from the second electrode  7  to the work piece. 
     In an alternative embodiment, an electrode operated at welding parameters which maintain an arc ignited between said electrode and a work piece may be located in between two electrodes operated at welding parameters such that said electrodes cannot on their own generate arcs between the said electrodes and the work piece. 
     The scope of protection is not limited to the above described embodiments. The above described embodiments can be amended and combined in many different ways without parting from the scope of the invention. For example, the welding head  1  in  FIG. 1  can comprise more than three electrodes, the first and third electrodes in  FIG. 11  can be connected to the same power source.