Abstract:
The present application relates to an apparatus for and a method of forge welding elongate articles ( 1, 2 ), such as tubes, together. In addition to electrode ( 12, 13 ) assemblies ( 9 ) for heating the article ( 1, 2 ) ends with high frequency resistive heating, the apparatus includes a coil ( 3 ) for induction heating the articles ( 1, 2 ) before or after welding, as well as means for cooling the welding seam.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to welding articles in general and in particular the welding of solid and hollow sections such as oil field tubes and water pipe lines. 
       TECHNICAL BACKGROUND 
       [0002]    The present invention relates to a method for forge or enhanced diffusion welding of two or more metal parts, wherein at least one joint is established between opposed bounding surfaces on the parts to be joined. One such method denoted Shielded Active Gas Forge Welding (SAG-FW) known from, and to a large extent defined by, U.S. Pat. Nos. 4,669,650 and 4,736,084 includes the following features:
       1. The welding process consists of four main stages wherein the metal parts are:
           a. heated electromagnetically to high local temperatures,   b. brought rapidly into close contact,   c. forged together until a metallic bound is established, and   d. cooled by convection, radiation and conduction   
           2. The metal parts have been carefully shaped so that there will be an advantageous triaxial state of stress as well as a high optimal closing contact pressure in the volume close to the weld during forging.   3. The parts are heated, preferably by direct high frequency resistive heating, so that the surface temperature is optimal for the material to be welded and so that the temperature gradient enhances a desirable mode of plastic deformation.   4. A reducing gas is passed between the surfaces of the parts to be welded so that oxides that are detrimental to the quality of the weld are removed before welding/fusion.       
 
         [0011]    The advantages of the method of forge or enhanced diffusion welding described in the above-mentioned patents are the high speed at which welding may be performed. The entire welding cycle may last less than a minute for easily weldable steels. Furthermore, there is no need for expensive machining or other type of trimming of the part shapes after welding since the outer surfaces of the joined parts may be almost completely flush close to the weld. There is also a potential for a high degree of process control and documentation since the temperature is much more closely controlled than for conventional welding methods. 
         [0012]    However, in order to establish a weld of uniform quality and shape it is important to exactly control the viscoplastic deformation of the material. The viscoplastic deformation is to a large extent controlled by the temperature distribution, which may deviate from the desired one in the directions normal to and along the bevel surfaces. Also the material properties may affect viscoplastic deformation and be a cause of variability, which must be detected and compensated for. 
         [0013]    In order to secure the highest possible weld quality it is important to make certain that the temperature of the bevel surfaces is within a certain range. A too high temperature may cause undesirable melting or excessive grain growth while a too low temperature will unavoidably lead to insufficient reduction of surface oxides and poor bounding. Undesirable material phase shifts and brittleness may also be the result of poor temperature control during heating and cooling. 
         [0014]    The heat input and the cooling time after welding are directly related for a given part geometry and material if no special measures are implemented. A large input of heat during the heating stage of the process will produce a heated zone of large extent and cause slow cooling of the material after welding. This may be a problem particularly when welding metals that must be quenched and tempered in order to establish sufficient ductility for a given strength. 
         [0015]    Another problem arises during welding of alloys that require artificially slow cooling after joining. After the weld has been established it is not practical with existing high frequency resistive heating technology to apply a current directly in order to prevent a sharp temperature drop. This would only cause short-circuiting with the current running from one of the electrodes to the other electrode on the same side of the part. 
         [0016]    Hence, with conventional forge welding methods and the standard high frequency resistive heating method, it may be difficult to control the temperature and to establish optimal thermal conditions for plastic deformation, fusion and metallurgical processing at all stages and for any given material and part geometry. 
       SUMMARY OF THE INVENTION 
       [0017]    It is an object of the present invention to provide an apparatus and method for forge welding that improved temperature control of the articles being welded, to allow a better quality weld of certain metal alloys that are responsible to temperature treatments prior to or after the welding step. 
         [0018]    Another object is to provide an apparatus that is easy and quick to operate. 
         [0019]    This is achieved in an apparatus and method for forge welding in which the normal resistive heating method is complemented with one or more inductive heating steps. To facilitate quick shifts between the two heating methods, the inductive heating is performed with a segmented coil which is powered from the power supply powering the electrodes used for resistive heating. 
         [0020]    The scope of the invention is defined in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention will now be described in detail in reference to the appended drawings, in which: 
           [0022]      FIG. 1-4  shows a partly cutaway view of the inventive welding apparatus during a welding sequence, 
           [0023]      FIG. 5  illustrates another embodiment of the invention, 
           [0024]      FIG. 6   a  and  b  is a schematic diagram of a possible electric circuit for powering the inventive welding apparatus. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The induction heating arrangement may be realized in at least two different embodiments:
       1. A two-part coil that shares the transformer supplying the contact assemblies. This is illustrated in  FIGS. 1-4 . Here, the coil part  3   a  is equipped with contact pads  12 ,  13 . During induction heating, the contact assembly  8  is pushed towards the contact pads  12 ,  13 . The other contact assembly  9  is pushed towards corresponding contact pads on the other coil half  3   b . It is a large benefit to avoid a dedicated transformer for feeding the coil, as this component is very bulky and takes up a lot of room. It is also expensive.   2. A four-part coil supplied with current from the transformer(s) supplying the contact assemblies, in which coil segments are fastened to the tips of the contacts. In this case, contact gaps  14 ,  15  are arranged in the coil. The contact gaps are closed before current is applied. This embodiment is illustrated in  FIG. 5 .       
 
         [0028]      FIG. 1-4  illustrates a welding sequence showing an embodiment of the inventive welding apparatus in detail. In the illustrated embodiment, a two-part coil is employed, as well as a separate cooling ring. The cooling ring  18  includes a central chamber  19  with a is number of apertures or nozzles  20  in the inner wall. Packings  21 ,  22  are proved at the upper and lower part of the cooling ring, respectively, for restricting the area affected by the cooling fluid ejected through the apertures  20 . 
         [0029]    In  FIG. 1 , a two-part coil  3  has been positioned outside the gap between two tube sections  1 ,  2 . The figure shows that contacts  16 ,  17  in contact assembly  9  are being pushed against the contact pads  12 ,  13 . Thus, the coil circuit is completed through the contact assemblies. Power is supplied through the contacts, and the tube sections are heated inductively. 
         [0030]    In  FIG. 2 , power has been removed and the coil sections have been retracted in a radial direction by positioning devices (not shown). Then, the contacts in the contact assemblies have been pushed onto the tube sections. Power has been applied again; this time creating a localized heating of the tube ends by resistance heating. A stinger  23  on the inside of the tubes supplies a flushing fluid through the gap between the tube ends. The flushing fluid works in particular to prevent oxides from forming and for reducing oxides on the tube ends. 
         [0031]    In  FIG. 3 , the contact assemblies  8 ,  9  have been retracted and the hot tubes forced together. Then, the welding per se is completed. Actuators have positioned the cooling ring  18  outside the welding seam, and a cooling fluid is supplied through the apertures  20  for quench cooling the area around the welding seam. The stinger  23  has been relocated inside the tube to bring apertures  24  in proximity of the welding seam. The apertures  24  are supplying a cooling fluid from a channel inside the stinger, and are adapted to cool the tube from the inside. It is also possible not to move the stinger before cooling and to merely use the apertures for the reducing gas to apply the cooling gas internally. 
         [0032]    In  FIG. 4 , the induction coil  3  has been positioned outside the welding seam for an after welding heat treatment of the tube. Also the stinger  23  has been moved to position an induction coil  25  near the welding seam. Thus, the welding seam and the adjacent area are heated both from the inside and outside of the tube. The thickness of the pipe and the characteristics of the material determines whether it is necessary to use an internal coil in addition to an external. 
         [0033]    The invention enables heat treatment of the metal prior to welding or immediately after welding as an integrated part of the process. The heat treatment may include reduced cooling rate for normalizing the metal, or annealing subsequent to welding and quenching. These steps may be necessary or not dependent on the properties of the metal used in the tubes, as explained earlier. 
         [0034]    For some material qualities pre-heating with induction coils give a temperature distribution (more widely distributed) that is better suited for slow plastic deformation and establishment of microstructure with small grains. The improved temperature distribution includes a more even distribution along the periphery of the tubes. 
         [0035]    In embodiments of the inventive apparatus that includes a stinger positioned on the inside of the tubes, a temperature gradient between the inside and the outside of the tubes may be achieved, which is beneficial for the welding of bi-metal tubes. 
         [0036]    The induction coil may also be used for drying the articles prior to welding. 
         [0037]    The combined use of induction and resistive heating has a synergic effect. Resistive heating enables quick heating and forging with a narrow temperature field, i.e. a temperature field with a steep thermal gradient. Inductive heating achieves a more widely distributed temperature field and is suited for pre-heating and heat treatment. By using an apparatus that quickly (0.5-2 seconds) shifts between inductive and resistive heating, it is possible to weld and heat treat tubes in succession, and join tubes that earlier could not be welded in a short time. Then it is necessary that the apparatus includes cooling means for quenching. Earlier processes were very time-consuming, and were not feasible to adopt in a single welding apparatus. 
         [0038]    In addition to pre-heating, normalizing and annealing steps, the tubes may be induction heated during the forging step when the tubes are brought together. This heating may be performed with coils both inside and outside the tubes. Subsequently, the tubes may be cooled in a way that is optimal for the metal in question. The cooling step may include cooling in air, rapid quenching by applying gas or liquid through an array of nozzles, or reduced rate cooling with simultaneous inductive cooling with reduced effect. In case annealing is required, this is done by induction heating to the proper temperature (e.g. 680-700° C.). If a full cycle with heat treatment is required, the tubes are induction heated before the material is quenched and annealed. 
         [0039]      FIG. 5  shows another embodiment of the invention. Here, the coil is divided into four segments,  71   a - d . Each segment is mounted at the tip of a respective electrode  72   a - d . A small point protrudes on the inside of each coil segment, and during resistive heating, these points are pressed against the tubes  73 ,  74 . For inductive heating, the coil segments are retracted a small distance to break the contact between the points and the pipe walls. Then, shorting devices (not shown) are adapted to short the coil segments at protruding parts  75   a - d . The shorting devices may be solenoids pressing the protruding parts together, or solenoid operated shorting bars adapted to close the gap between the protruding parts  75   a - d.    
         [0040]      FIG. 6   a - b  illustrates a possible circuit for powering the inventive devices, and in particular the embodiment of the invention illustrated in  FIGS. 1-4 . A high frequency generator  81  feeds two field transformers  83   a - b . Each field transformer  83   a - b  includes two primary windings, which are connected in series through capacitors  82   a - d .  FIG. 6   a  shows the circuit during resistive heating, when contacts or electrodes  84   a - d  are pressed against the tube walls. The contacts are connected with secondary windings of the transformers, and leads current through the pipe walls. 
         [0041]    In  FIG. 6   b  the contacts has been retracted from the tube walls and instead connected with two coil segments  87 ,  88  to complete the circuit. The pipe segment  87  is on the visible side of the tubes, while the segment  88  (shown in broken line) is on the other side of the tubes. Several more contacts and coil segments may also be used to sequentially heat the tubes in segments of their circumferences.