Patent Publication Number: US-2009224530-A1

Title: Welded butt joints on tubes having dissimilar end preparations

Description:
FIELD OF THE INVENTION  
     This invention relates to the field of welding, in particular, the field of welding butt joints in vertically arranged tubing. 
     BACKGROUND OF THE INVENTION  
     Joining tubes for industrial applications generally involves the butt-welding of tubes in bundled arrays. In such arrangements, spacing between the tubes may be less than an inch, thereby making it very difficult, if not impossible, to joint weld individual tubes using manual or conventional machine welding techniques. Such tube configurations are typically used for critical processes, such as conveyance of high pressure steam, and so demand high-quality welded joints for safety and reliability. When these critical systems require maintenance and repair, time is of the essence, and the use of low production methods to achieve the necessary quality are not desirable and may cause substantial adverse economic impact. 
     Conventional tube joint butt welding is performed with standard and symmetrical weld joint designs, and is mostly performed manually, although some machine welding systems are available. These designs and techniques require the addition of weld filler materials in the form of welding rods and wires. Multiple passes are typically required to complete the joint by filling it with weld material. 
     As illustrated in  FIG. 1A , a conventional method for preparing a weld joint  10  involves beveling the mating tube ends  12 ,  14  of tube sections  16 ,  18  to form a joint groove  20 . The beveling provides, a wider joint groove  20  at the outside surfaces  22 ,  24  of the tube sections  16 ,  18  compared to the mating inside surfaces  26 ,  28 . The inner surfaces  26 ,  28  are typically counter-bored to assure that their respective diameters match. This symmetrical design is classically known as an open butt weld preparation. Turning to  FIG. 1B , the first weld bead  30   a,  using a consumable weld rod or wire of the desired metallurgical chemistry, is placed at the joint root  32  of the groove opening  32  and formed circumferentially around the two-tube weld joint  10 . Subsequent weld beads  30   b,    30   c,    30   d  are placed in the same circumferential movement until the weld joint  10  is completely filled, being at least flush with the exterior surfaces  22 ,  24  of the tube sections  16 ,  18 . With each weld pass to deposit another bead of weld filler material in the weld joint  10 , heat is applied to the base material, creating a “heat-affected” zone of altered base material metallurgy and physical characteristics, which generally has a negative effect on system performance. 
     Manual methods of applying weld beads typically require a number of starts and stops in applying each weld bead as the weld applicator repositions each time to continue a circumferential bead. Each start and stop creates another point of high heat, which adds significantly to the heat-affected zone in the base materials, thereby increasing the residual stresses that remain in the weld joint. Some advanced alloy materials are quite susceptible to failure if they are exposed to sufficient localized heating. Minimizing the heat input for weldjoining these materials preserves the performance expected of the advanced alloys. 
     In contrast to the variability of manual joints, the consistency of machine-applied weld joints provides substantial value for critical processes where reliability is necessary. The frequency of starts and stops can be substantially reduced because the welding equipment revolves around the tube and, therefore, does not require the repositioning that a manual applicator must perform. However, the operating constraints of machine welding impose a minimum size on machine weld heads. Conventional machine weld tooling must enable the feeding of weld wire as a consumable filler for the joint. The feeding of weld wire adds complexity and size to the weld head and creates a physical limitation on the size to which the equipment may be reduced. Thus, the minimum practical equipment size is larger than that which may be required for the confined space and limited access encountered in tube bundle configurations. In addition, the choice of weld filler materials and associated weld parameters may require either waiting between weld bead passes on a given joint or moving through a sequence of other tube joints so that the heat in a joint can dissipate and the interpass temperature will not be exceeded. Such requirements may result in the need for multiple set ups on the same joint. 
     Current welding practices require multiple passes using minimal amounts of material because of lack of control of the puddle during weld out of the joint. Particularly in the vertically-extended (2G) position, gravity effects and heat flow affect the weld puddle geometry before it sets. Too large of a puddle will result in puddle sag with undercutting of the upper boundary interfaces and excess material and bulging at the lower interface. 
     SUMMARY OF THE INVENTION  
     In a first embodiment, the invention provides a combination of work pieces including a first work piece having a stepped end preparation with a first step and a second step, and a second work piece having a beveled end preparation. The first work piece has a first surface and a second surface which is located opposite the first surface. The first step is located at the end of the first work piece and adjacent to the first surface, and has an edge that is set back laterally from the first surface. The second step is set away longitudinally from the end of the first work piece and adjacent to the second surface. Preferably, the first step is adjacent to the second step. The second end piece has a third surface and a fourth surface which is located opposite the third surface. The angle of the bevel opens outward toward the fourth surface. Further, the ends of the respective work pieces are adapted to cooperate in forming a weld joint. 
     In a second embodiment, the invention provides a weld joint having a joint groove with a lower face formed by a stepped end preparation and an upper face formed by a beveled end preparation. The weld joint comprises a first work piece and a second work piece. The first work piece has a first surface and a second surface which is located opposite the first surface. The first step is located at the end of the first work piece and adjacent to the first surface. The second step is set away longitudinally from the end of the first work piece and adjacent to the second surface. The second end piece has a third surface and a fourth surface which is located opposite the third surface. The angle of the bevel opens outward toward the fourth surface. The end of the first work piece is juxtaposed the end of the second work piece and oriented such that the first step is above the second step. The end of the second work piece is positioned above the first step so as to form a joint groove within which at least a portion of the first step is exposed when the second surface is aligned with the fourth surface. Preferably, the first and second work pieces are in a vertically-extended (2G) arrangement, and substantially all of the first step is exposed within the joint groove. 
     In a third embodiment, the invention provides a method of welding a first work piece to a second work piece. The steps of the method include: (1) providing a first work piece having a first surface, a second surface which is located opposite the first surface, a first step which is located at an end of the first work piece and is adjacent to the first surface, and a second step that is set away longitudinally from the end of the first work piece and is located adjacent to the second surface; (2) providing a second work piece having a third surface, a fourth surface which is located opposite the third surface, and an end which is beveled at an angle relative to a plane that is perpendicular to the fourth surface, such that the angle opens toward the fourth surface; (3) forming a weld joint by positioning the first and second work pieces such that the first step is above the second step and the end of the second work piece is above the first step such as to form a joint groove within which at least a portion of the first step is exposed, and such that the second surface is aligned with the fourth surface; and (4) performing at least one welding process to weld the first work piece to the second work piece at the weld joint. In the welding process, it is preferred that the welding wire be fed to the welding puddle at the surface of the beveled end near the fourth surface and that the welding tip be moved along an inner edge of the first step of the first work piece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is made to the following detailed description of various exemplary embodiments considered in conjunction with the accompanying drawings, in which: 
         FIG. 1A  is a schematic, cross-sectional view of a weld joint formed by upper and lower tubes in a 2G position having conventional end preparations for joint welding; 
         FIG. 1B  is a schematic, cross-sectional view of the weld joint of  FIG. 1A  filled by multiple beads of weld metal; 
         FIG. 2  is a schematic, cross-sectional view of the weld joint formed by upper and lower tubes in a 2G position having end preparations according to the present invention; 
         FIG. 3  is the schematic, cross-sectional view of the weld joint of  FIG. 2  showing reference dimensions; 
         FIG. 4  is a schematic representation of a front view and partial cross-section of the weld joint of  FIG. 2 , showing a preferred feed entry point for filler wire and touch-off point for an electrode, with the relative positions of the weld wire, electrode and weld pool during the formation of a joint weld; 
         FIG. 5  is a schematic view looking along the length of the upper and lower tubes of  FIG. 2  showing the angles of entry of the weld wire and tungsten electrode during formation of a joint weld; and 
         FIG. 6  is a schematic, cross-sectional view of the upper and lower tubes of  FIG. 2  joined by a weld. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     In the specification, the following terms are used to refer to features of the structures relevant to the invention. A “weld joint” is a structure formed by the prepared ends of two work pieces that are to be welded together. A “joint root” is a portion of a weld joint where the work pieces are closest to each other. A “joint groove” is an open space within the weld joint extending outwardly from the joint root. A “groove face” is a surface at an end of a work piece that is exposed within the joint groove. 
       FIG. 2  shows a weld joint  32  formed according to an embodiment of the present invention. Groove faces  34 ,  36  are formed on first work piece  38  and second work piece  40 , respectively, so as to form groove opening  42  when work pieces  38 ,  40  are arranged such that groove faces  34 ,  36  approach each other. The groove opening  42  opens such that it is wider at outer surfaces  44 ,  46  of the respective work pieces  38 ,  40  than at inner surfaces  48 ,  50 , where the joint root  52  is formed. It is to be understood that the terms “outer” and “inner”, as used herein, are relative terms used to denote opposite sides of a portion of a work piece (e.g., a plate or the wall of a tube). The groove face  34  of the first work piece  38  is formed such that it has a notch  54  formed therein and extending along the periphery of the work piece  38  (see  FIG. 4 ). Returning to  FIG. 2 , the notch  54  is inset from the first surface  44 , forming a first step  56  that is adjacent to the inner surface  48 , and a second step  58  that is adjacent to the outer surface  44 . The first step  56  thus is higher than the second step  58  when the work piece  38  is oriented such that the groove face  34  is uppermost. Preferably, the second step  58  is flat, and is parallel to a horizontal plane (not shown), when the work piece  38  is in the aforementioned orientation. The groove face  36  of the second work piece  40  is beveled. 
     An example of the use of the weld joint  32  in welding the ends of a pair of tubes is presented herebelow. In one embodiment of the invention, the first and second work pieces  38 ,  40  are tube sections  38 ,  40 . It will be obvious to one skilled in the welding art that the weld joint  32  can be adapted for use in joining work pieces other than tube sections, such as plates or work pieces of other geometries that are to be joined end to end in a vertically extended (2G) arrangement. 
     Referring to  FIG. 3 , in a working embodiment of the invention, the groove faces  34 ,  36  were prepared in matched 2.5-inch diameter tube sections  38 ,  40  of carbon steel having a wall thickness (t) of 0.188 inch, counterbored to a depth (a) of 0.010 inch along a length (b) of 0.250 inch of the interior surface  48 ,  50  of each respective tube section  38 ,  40 . The groove face  34  of tube section  38  was machined to produce a first step  56  having a thickness (c) of 0.075 inch, with a distance (d) of 0.090 inch between the first step  56  and the second step  58 . The groove face  36  of tube section  40  was beveled at an angle (E)) of 25 degrees. The recited dimensions of the groove faces  34 ,  36  may be varied somewhat, especially for work pieces having wall thicknesses (t) other than that of this example, but it is preferred that the thickness (c) of the first step  56  be about 0.075 inch and the bevel of the upper groove face  36  be about 25 degrees, within a tolerance of about plus-or-minus 2 degrees. 
     The weld joint  32  was welded in a single pass by a machine welding system (Liburdi Dimetrics, Davidson, N.C.) using a tungsten inert gas (TIG) welding process, with the tube sections  38 ,  40  vertically extended in a 2G arrangement. A carbon steel (70S-7) filler wire, having a diameter of 0.035 inch, was selected as the welding wire.  FIG. 4  shows the feed point for the welding wire  60 , which is on the upper groove face  36  near the outer surface  46  of the tube section  40 . The tungsten touch-off point  62 , near the top of the first step  56 , can also be seen in  FIG. 4 . The geometry of the tungsten tip was provided with a nominal 0.020-inch land as an aid in maintaining voltage feedback and control. 
     Welding was performed with pulsed electrical current having a primary (peak) pulse of 170 Amps and a background current of 40 Amps. The low pulse current occurred every 0.6 seconds at a normal sinusoidal transition from peak (50% pulse width). The travel speed of the tungsten tip (T) was 2.7 inches per minute along the circumference of the weld joint  32 , following a delay of 8 seconds after energizing the tungsten tip. Following a similar, but shorter delay, the filler wire  60  was fed to the weld puddle  64  in a pulse fashion to coordinate with the current pulse. The filler wire was fed to the weld puddle  64  at a rate of 65 inches per minute at the peak current pulse and at a rate of 35 inches per minute during the background current pulse. Referring to  FIG. 5 , the lead angle of the wire feed (Θ W ) and the lead angle of the tungsten tip (Θ T ) were 30 degrees and 5 to 10 degrees, respectively. 
     Prior to the welding, an inert gas back purge (i.e., a purge of the interior of the tube sections  38 ,  40 , or the backside of the joint root  52  (see  FIG. 2 )) was established using 100% argon gas at a back pressure of less than 7 psi. During the welding, an arc shielding gas, comprising 50% argon and 50% helium, was used at a flow rate of 30 cubic inches per minute using a number 10 gas cup to maintain the arc environment. 
     The weld was completed in a single pass over a period of about 3 minutes for the typical 2.5-inch diameter tube sections with 0.0188-inch wall thicknesses, where conventional methods would have required 20-30 minutes to complete, plus time required for set up and delay time between passes. The consumption of filler wire was about 60% less than with conventional methods. 
     It should be understood that, for any weld joint, multiple tests may be performed to optimize particular welding parameters for performing a satisfactory weld. With reference to  FIGS. 2 and 4 , a fine balance should be maintained between heating the filler wire  60  and chilling the weld puddle  64  (through addition of the filler wire  60 ) to allow wetting of the upper groove face  36  and rapid fixing of the filler metal to the weld joint  32 . During the welding process, the arc from the tungsten tip consumes the first and second steps  56 ,  58  of the exposed lower groove face  34 . The heat generated thereby rises to maintain enough heat in the molten puddle  64  to melt the filler wire  60 . The weld puddle  64  should not become so molten that gravity pulls the weld puddle  64  away from the upper groove face  36 , causing the weld defect known as undercutting. When an effective combination of weld parameters is applied, the wetting of the upper groove face  36  provides enough surface tension to guide a properly formed weld puddle  64  to follow the contour of the weld joint  32  and keep the weld puddle  64  from falling out of the weld joint  32 . In the weld joint  32 , the consumption of the steps  56  and  58 , along with consumption of the filler wire  60 , allows gravity to assist in filling in the groove opening  42 . The design of the steps  56 ,  58  and the remaining lower internal joint surface area provides a balance of filler material and heat transfer area to assure full fusion and sufficient chilling of the weld puddle  64  such that the weld puddle  64  remains in place without falling out of place or slumping excessively. A schematic, cross-sectional illustration of a completed, single-pass weld of the weld joint  32  is presented in  FIG. 5 , showing the weld bead  66 , exemplary lower and upper boundaries  68 ,  70  of the fused weld and tube sections  38 ,  40 . 
     It should be noted that alternative welding processes may use consumable inserts of welding metal, rather than welding wire. Such inserts are specially engineered to mate with prescribed weld joint preparations. Specially-designed machine welding equipment consume the insert in lieu of adding wire to fill the joint groove. Depending on the dimensions of the weld joint, it is possible to consume the insert to create a completed weld in a single pass. 
     It should be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications thereto without departing from the spirit and scope of the present invention. It will be recognized that the weld joint disclosed herein can be applied to a tube, pipe, plate or work piece of other geometry. It will further be recognized that, for tubes or pipes, the weld joint can be reversed for application of weld filler material from the inside of the work piece. Besides being applicable to work pieces in a 2G position, the weld joint  32  may also be applied in situations where the work pieces are off of a vertical orientation by as much as 15 degrees. Further, the dimensions of the lower and upper groove faces  34 ,  36  may be varied depending on the thickness of the base material and the filler metal selected. The selection of suitable filler metals for the materials to be welded, and for the uses to which the welded materials will be put, is routine in the welding arts. All such variations and modifications, including those discussed above, are intended to be included within the scope of the invention, which is described, in part, in the claims presented below.