Abstract:
The invention relates to a method for forge welding tubulars such that formation of upsets in the welding zone is minimized which involves shaping the tubular ends that are to be welded together into an inwardly sloping configuration such that when the tubular ends are heated during the forge welding process the heated tubular ends deform as a result of thermal expansion into a substantially longitudinally oriented cylindrical shape.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a method for joining pipe ends by forge welding.  
         [0002]     Forge welding involves circumferential heating of the pipe ends that are to be joined and subsequently pressing the pipe ends together to form a metallurgical bond.  
         [0003]     A large variety of heating technologies may be used to make the pipe ends hot enough such that the metallurgical bond can be made. The heating techniques may involve electric, electromagnetic, induction, infrared, sparking and/or friction heating or combinations of these and other heating methods.  
         [0004]     When used in this specification the term forge welding is intended to encompass all techniques which involve circumferential heating of pipe ends and subsequent metallurgical bonding the heated pipe ends, including welding techniques that are generally known as amorphous bonding, diffusion bonding, friction welding, flash butt welding and/or thermodynamic welding.  
         [0005]     It is known from U.S. Pat. Nos. 4,566,625; 4,736,084; 4,669,650 and 5,721,413 issued to Per H. Moe that it may be beneficial to flush the pipe ends just before and during the forge welding operation with a reducing flushing gas, such as hydrogen or carbon monoxide, such that any oxygen skin is removed from the heated pipe ends and a metallurgical bond with a minimal amount of irregularities is obtained. It is also known from U.S. Pat. Nos. 2,719,207 and 4,728,760 to use non explosive mixtures comprising about 95% by volume of a substantially insert gas, such as argon, nitrogen and/or helium, and about 5% by volume of a reducing gas, such as hydrogen and/or carbon monoxide for flash welding and induction butt welding.  
         [0006]     Experiments have shown that forge welding techniques are capable to generate high quality metallurgical bonds between tubular ends, in particular if the pipe ends are flushed with a reducing flush gas mixture during the heating and/or welding operation, but that the red-hot pipe ends are generally deformed such that upsets are formed in the region of the welding zone.  
         [0007]     For many applications it is required to remove upsets after the welding operation, which then involves a grinding or machining operation which is difficult and expensive to accomplish at many sites where pipe ends are welded together, such as on oil rigs, pipe-laying barges and many offshore and onshore sites where underground or above ground pipelines are to be installed.  
         [0008]     The method according to the preamble of claim  1  is known from U.S. Pat. No. 4,669,650. In the known method the tubular ends are shaped in an inwardly sloping configuration. Japanese patent JP 03-243286 also discloses a forge welding method wherein the tubular ends have an inwardly sloping configuration. UK patent GB 793402 discloses a forge welding method wherein the welded pipe ends have complementary concave and convex shapes.  
         [0009]     It is an object of the present invention to provide a method for forge welding of tubulars wherein the generation of upsets in the welding zone is minimized.  
       SUMMARY OF THE INVENTION  
       [0010]     The method according to the inventions comprises shaping the tubular ends that are to be welded together into a sloping configuration such that when the tubular ends are heated during the forge welding process the heated tubular ends deform as a result of thermal expansion into a substantially longitudinally oriented cylindrical shape, wherein the sloping angle of the inner and outer walls of the tubular ends is selected such that the ratio between the average diameter D(t) of the tip of the tubular end and the average diameter D(b) of the base of the tubular end is related to an estimated temperature difference between said tip and base of the tubular end during the forge welding process and a thermal expansion co-efficient of the steel grade or grades of the tubular end.  
         [0011]     In addition the portion of each pipe that is to be forged may be reduced in cross section such that deformation during forging returns it to a dimension substantially the same as its original thickness.  
         [0012]     The precise angles and dimensions of the end preparation depend on the material being joined and its coefficient(s) of expansion, wall thickness, pipe diameter, degree of heat required for welding, the width of the heated zone and the desired forge length. Typical values are provided in Table 1 below for carbon steel tubes approximately 4 mm wall thickness and 70 mm diameter.  
         [0013]     The sloping angle of the inner and/or outer walls of the tubular ends may be selected such that the ratio between the average diameter D(t) of the tip of the tubular end and the average diameter D(b) of the base of the tubular end is related to an estimated temperature difference between said tip and base of the tubular end during the forge welding process and a thermal expansion co-efficient of the steel grade(s) of the tubular end.  
         [0014]     For many forge welding operations said ratio D(t)/D(b) may be selected between 0.8 and 0.99.  
         [0015]     To increase the surface of the forge welded pipe ends and to simultaneously assist alignment of the pipe ends the end face of one of the tubular ends that are to be welded together may a substantially convex shape and the end face of the other tubular end may have a substantially concave shape.  
         [0016]     The forge welded tubulars may comprise a low grade steel base pipe and a higher grade steel cladding on the inner and/or outer surface of the base pipe. In such case it is preferred that the end faces are shaped such that when the tubular ends are pressed together the end faces of the cladding(s) touch each other first the end faces of the base pipe ends subsequently touch each other. It is also preferred that any non-oxidising or reducing flush gas is introduced from the opposite side of the pipe wall to the clad layer.  
         [0017]     It will be understood that the inwardly tapered tubular end may have a large variety of shapes, and that the inward deflection may be determined by iterative calculation and/or experiments in order to asses that the amount of upset of the forge welded tubulars is reduced to a minimum.  
         [0018]     In use the amount of material at the pipe ends deformed by forging is closely controlled to further minimise upset.  
         [0019]     It is observed that U.S. Pat. No. 4.669.650 discloses a forge welding process wherein the outer walls of the tubular ends are machined away to a greater depth that the inner walls of the tubular ends. The known configuration is, however, not configured such that the heated tubular ends are substantially cylindrical during the forge welding operation. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]     Preferred embodiments of the method according to the present invention will be described with reference to the accompanying drawings, in which:  
         [0021]      FIG. 1  shows a pair of concave and convex pipe ends that have been interconnected by the forge welding method according to the invention;  
         [0022]      FIG. 2  shows how the concave and convex pipe ends are inwardly oriented at an inward sloping angle X which is selected such that the heated concave and convex pipe ends have a substantially longitudinal orientation;  
         [0023]      FIG. 3  is a longitudinal sectional view of a pair of cladded tubular ends just before they are joined by the forge welding method according to the invention;  
         [0024]      FIG. 4   a  is a longitudinal sectional view of a pair of internally cladded tubular ends wherein one tubular end is concave and the other tubular end is convex;  
         [0025]      FIG. 4   b  is a longitudinal sectional view of a pair of externally cladded tubular ends wherein one end is convex and the other end is concave; and  
         [0026]      FIG. 5  is a longitudinal sectional view of an end of a cladded tubular wherein the thickness of the clad layer is increased at the end of the tubular. 
     
    
       [0027]     In  FIG. 1 ,a pair of heated axially aligned pipe ends is shown and in  FIG. 2  the unheated ends are shown, wherein X is the inward sloping angle,  1 =pipe having original wall thickness,  2 =minimum forge length required to complete the weld,  3 , 5 , 6 =typical radii of convex pipe end  9 ,  4 =first contact shoulder,  7 =pipe end having reduced wall thickness,  8 =pipe center line and  10 =concave pipe end. The sloping angles x of the unheated convex and concave pipe ends  9 , 10  illustrated in  FIG. 1  are selected such that the heated pipe ends accurately intermesh and are accurately axially aligned as illustrated such that a seamless forge weld is created when the pipe ends  9  and  10  are pressed together and only minimal upsets are formed at the inner and outer surfaces of the pipes  1  in the region of the forge welded joint.  
         [0028]     An outline of the dimensions of the pipe connection shown in  FIG. 1  and  2  is described in Table 1:  
                             TABLE 1                           Typical values for forge welding weld       preparation - 4 mm WT, 70 mm OD pipe            Identifier (see                 FIG. 1 )   Description   Typical Value               X   Inward sloping   1 to 5°           angle       1   Original wall     4 mm           thickness       3, 5, 6   Preparation radii    0.6 mm       2   Minimum forge   0.05 mm           length       7   Reduced wall     3 mm           thickness       8   Pipe centre line       4   First contact           shoulder                  
 
         [0029]     Referring now to  FIG. 3  there is shown an upper tubular  11  and a lower tubular  12  which each comprise a low grade steel base pipe have an inner cladding of high chromium steel  13 .  
         [0030]     The tubular ends  14  and  15  are wedge shaped such that the tips of the wedge shaped ends are formed by the claddings  13 . This ensures that when the tubular ends are pressed against each other the claddings  13  touch each other before the ends of the base pipes touch each other. Throughout the forge welding operation a flushing gas is flushed around the tubular ends  14  and  15  and to ensure continuation of the flushing between the tubular ends  14  and  15  after the claddings  13  touch each other, flushing gas is injected onto the uncladded outer surfaces of the tubulars  11  and  12 .  
         [0031]      FIG. 4   a  depicts an embodiment where the lower end surface  26  of the upper tubular  20  has a generally concave shape and the upper end surface  23  of the lower tubular  21  has a generally convex shape.  
         [0032]     The inner surface of the tubulars  20  and  21  is cladded with a stainless steel lining  24  and  25  and the concave and convex end surfaces  23 ,  26  are shaped such that the linings  24  and  25  touch each other first and that the base pipes  20 ,  21  touch each other thereafter and that the unheated end surfaces  23 ,  26  are inwardly oriented at a sloping angle X. In this case a reducing non-explosive flushing gas is injected from the exterior of the tubulars and the tubular ends still form a wedge such that the touching zone gradually increases from the outer surface towards the inner surface of the forge welded tubulars. In this way a good bond between the linings  24 , 25  is ensured and inclusion of oxides between the forge welded tubulars  20  and  21  is minimized.  
         [0033]      FIG. 4   b  illustrates a forge welded joint of a pair of low grade steel base pipes  33 ,  34  of which the outer surface is cladded with a stainless steel lining  30 ,  31  and of which the end surfaces  35 ,  36  have an intermeshing convex and concave shape such that the unheated end surfaces  35 ,  36  are inwardly oriented at a sloping angle X and that the stainless steel linings  30 ,  31  touch each other before the base pipes  33 ,  34  touch each other when the end surfaces  35 ,  36  are heated and pressed together during the forge welding operation. In this case a reducing gas is injected from the interior side  44  of the tubulars during the forge welding operation.  
         [0034]      FIG. 5  depicts a tapered wedge shaped end  40  of a pipe  41  which has been clad with a clad layer  43  and where further material  42  compatible with the clad layer  43  has been deposited around the end of the pipe  2  to allow further machining without exposing the base pipe  41 . The inward sloping angle X of the tapered end  40  is selected such that the heated pipe ends are substantially axially aligned and the taper angle is selected such that the clad layers  43  of adjacent pipe ends touch each other first before the base pipes touch each other during a forge welding operation.