Abstract:
A method for forming a smooth interface between a weld cap and an adjacent base metal utilizing ultrasonic impact treatment. The method improves the geometric profile of a weld while imparting a compressive residual stress layer on the weld metal and base metal thereby alleviating the tensile residual stresses imparted to the metals during welding. The contouring process does not remove material, as in grinding, but plastically deforms the surface being treated producing a densified surface, in turn providing a smooth weld cap and base metal surface finish without the loss of base or weld metal thickness.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to a method for reshaping a weld cap and an adjacent base metal surface, and more particularly, to a method for forming a smooth transition between the weld cap and the adjacent base metal utilizing ultrasonic impact treatment (UIT). 
       BACKGROUND OF THE INVENTION 
       [0002]    Welds are considered to be the weakest point in structures fabricated using welding technologies. This is a result of a number of things coming into play. For example, welds produce poor geometric profiles with respect to fatigue life performance and introduce tensile residual stresses at the same location which are deleterious to metal fatigue life. Furthermore, introducing heat into a base metal during welding can alter the microstructure and properties of a portion of the base metal where the heat is experienced. The area experiencing these alterations is referred to as the heat-affected zone. Heat-affect zone base metal has a weaker relative strength than the base metal and the weld metal. 
       SUMMARY OF INVENTION 
       [0003]    The present invention is directed to a method for forming a smooth interface between a weld metal and an adjacent base metal utilizing UIT thereby improving the geometric profile of a weld seam while imparting a compressive residual stress layer on the weld metal and base metal and alleviating the tensile residual stresses imparted to the metals during welding. The weld metal to base metal interface refers a line defined by the intersection of a lateral edge of the weld metal with the base metal. UIT refers to a process of introducing pulses of ultrasonic wave energy into a metal workpiece through periodic ultrasonic mechanical impulse impacts. UIT is described in detail in U.S. Pat. Nos. 7,431,779; 7,344,609; 7,301,123; 7,276,824; 6,932,876; 6,843,957; 6,289,736, and 6,171,415, which are incorporated herein by reference in their entireties. Generally, UIT serves to plastically deform a metal surface and thus imparting a compressive residual stress layer on the workpiece. Thus, UIT does not remove material, as in grinding, but plastically deforms the surface being treated producing a densified surface, in turn providing a smooth weld cap and base metal surface finish. Thus, the method when used in a post weld environment represents a surface contouring method and an alternate method to surface grinding of weldments. 
         [0004]    According to one aspect of the invention, there is provided a method for modifying a weld seam including a weld metal, a base metal and a first weld metal to base metal interface, wherein the weld metal forms a weld cap produced by one or more cap weld passes. The method includes imparting a desired contour to the weld seam by introducing pulses of ultrasonic wave energy into the weld seam through periodic ultrasonic mechanical impulse impacts using a tool including one or more ultrasonically movable impacting elements. The mechanical impulse impacts are made by multiple passes of the tool along the weld seam. Initially, the weld cap is textured by performing a first roughing pass along the weld seam in a first orbital pattern that is centered on a longitudinal axis of the weld seam. Thereafter, the weld cap, base metal and first weld metal to base metal interface are textured by performing a second roughing pass along a length of the weld seam in a second orbital pattern that is centered on the first weld metal to base metal interface. Following the second roughing pass, the weld cap, base metal and a second weld metal to base metal interface that is opposite the first weld metal to base metal interface are textured by performing a third roughing pass along a length of the weld seam in a third orbital pattern that is centered on the second weld metal to base metal interface. Upon completion of the first, second and third roughing passes, the weld seam exhibits a textured or dimpled pattern. 
         [0005]    Following texturing of the weld seam, which blends together the weld metal laid down by multiple cap weld pass, the textured surface of the weld seam is smoothed by performing a fourth set of contouring passes along a length of the weld seam. Fourth set of contouring passes includes three separate passes, each of which exhibits an orbital pattern having a progressively increasing diameter. Specifically, the fourth set of contouring passes includes a first orbital patterned pass centered on a longitudinal axis of the weld seam, a second orbital patterned pass centered on the first weld metal to base metal interface and a third orbital patterned pass centered over the second weld metal to base metal interface. Lastly, the surface roughness of the weld cap is further smoothed and contoured by performing a fifth set of contouring passes. Fifth set of contouring passes includes three separate passes, each of which exhibits an orbital pattern. Specifically, The fifth set of contouring passes and the fifth orbital pattern include a first orbital patterned pass centered on a longitudinal axis of the weld seam and extending across the first weld metal to base metal interface and the second weld metal to base metal interface, a second orbital patterned pass centered on the first weld metal to base metal interface that extends over the longitudinal axis of the weld seam, and a third orbital patterned pass centered over the second weld metal to base metal interface and that extends over the longitudinal axis of the weld seam. When the passes are completed, the weld seam exhibits the desired profile, which includes a flat, smooth weld metal to base metal interface. 
         [0006]    According to another aspect of the invention, there is provided a method for contouring a weld seam including a weld metal, a base metal and a first weld metal to base metal interface, wherein the weld metal forms a weld cap produced by one or more cap weld passes. The method includes making a plurality of roughing passes along a length of the weld seam whereby the weld seam is imparted with a texture, and making a plurality of contouring passes along the length of the weld seam whereby the texture is removed and the weld seam is provided a substantially smooth surface. The plurality of roughing passes and the plurality of contouring passes are performed by impacting the weld seam with one or more ultrasonically movable impacting elements longitudinally along the weld seam in both directions and in orbital patterns. 
         [0007]    The plurality of roughing passes include a first pass along the weld cap which is centered along a longitudinal axis of the weld seam, a second pass along the first weld metal to base metal interface which is centered along the first weld metal to base metal interface and a third pass along a second weld metal to base metal interface opposite the first weld metal to base metal interface, the third pass being centered along the first weld metal to base metal interface. The plurality of contouring passes include a first pass along the weld cap which is centered along a longitudinal axis of the weld seam, a second pass along the first weld metal to base metal interface which is centered along the first weld metal to base metal interface and a third pass along a second weld metal to base metal interface opposite the first weld metal to base metal interface, the third pass being centered along the first weld metal to base metal interface. The plurality of roughing passes are performed using impacting elements having a first diameter and the plurality of contouring passes are performed using impacting elements having a second diameter wherein the second diameter is usually but not restricted to being larger than the first diameter. Preferably, the first diameter is about 2.0-5.0 mm and the second diameter is about 5.5 to 7.5 mm. 
         [0008]    According to yet another aspect of the invention, there is provided a workpiece including a weld seam having smooth weld metal to base metal interface constructed and arranged by introducing pulses of ultrasonic wave energy into the weld seam through periodic ultrasonic mechanical impulse impacts. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  depicts positioning of UIT tools relative to a weld seam in accordance a preferred embodiment of the invention. 
           [0010]      FIG. 2  depicts a first roughing pass along the weld seam of  FIG. 1   
           [0011]      FIG. 3  depicts a second roughing pass and a third roughing pass along the weld seam of  FIG. 1 . 
           [0012]      FIG. 4  depicts a set of fourth contouring passes along the weld seam of  FIG. 1 . 
           [0013]      FIG. 5  depicts a set of fifth contouring passed along the weld seam of  FIG. 1 . 
           [0014]      FIG. 6  depicts a desired weld seam profile created utilizing a method in accordance with a preferred embodiment of the present invention. 
           [0015]      FIG. 7  depicts an alternative weld seam profile created utilizing a method in accordance with a preferred embodiment of the present invention. 
           [0016]      FIG. 8  depicts yet another alternative weld seam profile created utilizing a method in accordance with a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The UIT contouring process provides a weld seam having a smooth weld cap and a smooth weld metal to base metal interface clear of surface irregularities. The process further provides an improved stress profile at the weld metal to base metal interface and it is believed a surface finish suitable for post weld non-destructive inspection methods including dye penetration and ultrasonic processes. This is accomplished by imparting a compressive stress layer to the weld seam having a depth of up to 2.5 mm while plastically deforming the weld cap and base metal surface into the desired weld seam profile. The UIT contouring process can be used on base metal and/or welded structures, including fillet and butt welds, when post weld contour grinding has been specified by a design engineer as a means of improving surface finish and/or applying compressive residual stress to the surface of a welded structure. The UIT weld contouring process is a post weld treatment process that should only be applied when the cap weld is complete, the weld has been inspected and weld is accepted by a certified welding inspector or quality control engineer. 
         [0018]    Referring to  FIG. 1 , the UIT contouring process of the present invention generally includes applying pulse wave energy through ultrasonic impulse impacts to a weld seam  18  by contacting a UIT tool to the center of a weld cap  10  and moving the tool down and across the weld face and on to adjacent base metal  12 . The ultrasonic impacts act first to blend the metal of weld cap  10  and thereafter to smooth and contour weld cap  10  by plastically deforming weld seam  18  surface for a minimum of one inch from both weld toes  15  and  16  to both weld metal to base metal interfaces  19  and  21 . Preferably, the tool is moved across weld seam  18  a minimum of one inch from both weld toes  15 ,  17  to base metal interfaces. 
         [0019]    To impart the requisite pulse wave energy and ultrasonic mechanical impulse impacts to weld seam  18  for obtaining the desired weld seam contouring, an ultrasonic impact operating tool as described in U.S. Pat. No. 6,932,876 can be used. That tool employs a set of ultrasonically movable impacting elements  14 , presented typically as sets of two or four spaced members, for impacting weld seam  18  under control of an ultrasonic transducer head. A periodic pulse energy source, typically operable at ultrasonic frequencies up to 100 kHz, induces oscillations into the transducer head, preferably subject to feedback frequency and phase control processing feedback from the working transducer head to aid in matching resonance characteristics of the head when working on weld seam  18  in the manner more particularly set forth herein and in the parent applications of U.S. Pat. No. 6,932,876. Impacting elements  14  create at the work surface and extending into the sub-surface region of weld seam  18  plasticized metal permitting the surface of weld seam  18  to be imparted with a desired profile. UIT further imparts both ultrasonic relaxation and impulse relaxation within weld seam  18 . These two components of UIT reduce the magnitude of the tensile residual stresses in weld seam  18  at greater depths than the plasticity induced compressive stresses which are a surface phenomenon. 
         [0020]    As described hereafter in more detail, the UIT contouring process is a multipass procedure using a variety of UIT tool power settings and pin geometries to achieve metal blending and a smooth contoured surfaces. For example, UIT tool power setting will vary based upon the desired weld metal to base metal profile, weld conditions and contouring requirements as specified by a structural design engineer. Further, the UIT tool pin geometries will vary depending on whether a tool pass across weld seam  18  is designed to blend metal or contour the metal. Preferably, the UIT tool is equipped with pins having diameters ranging from 3 mm to 9 mm that are arranged in an inline four pin array or single pin configuration. 
         [0021]    A description of an exemplary UIT surface contouring process and tool configurations therefore is described hereafter. It should be noted, that weld cap  18  conditions are can vary thereby requiring adjustments to the general UIT tool contouring setup and contouring process to achieve the optimum weld contour and surface finish. 
         [0022]    Referring to  FIG. 1 , when UIT contouring is being performed, the UIT tool is arranged essentially perpendicular to the surface being contoured. To ensure that a proper pin to surface angle is maintained, the pin angle variation from perpendicular should range within +/−10 degrees. As the UIT tool is moved across weld seam  18 , the motion of the UIT tool is a pushing motion, not sweeping, in straight or orbital paths running the length of weld seam  18 . The orbital paths are linear down the length of weld seam  18  and extend onto base metal  12  using large and small orbital patterns which blend and smooth the weld and base metal surfaces. The treatment speed can vary among operators and is not a critical parameter for the process. The average linear treatment speed ranges between 30 cm per minute and 60 cm per minute. The treatment speed and the number of orbital patterns is determined based on visual inspection of the surface being treated and the smoothness specification of the surface. The critical treatment parameters are weld metal to base metal transition and surface smoothness. 
         [0023]    Referring to  FIG. 2 , the UIT surface contouring process begins with a first contouring pass  16  which is classified as a roughing pass since it imparts a texture to weld cap  10 . The orbital patterns of first contouring pass  16  are uniform traveling the length of weld seam  18  with increasing radial paths or diameters at intervals for blending the weld metal of weld cap  10 . First contouring pass  16  is intended to blend and make uniform the welded surface of weld cap  10  while removing any uneven portions formed between adjacent and overlapping weld cap weld metal passes. First contouring pass  16  is performed using a four, 3 mm diameter, in line pin array. The motion and coverage are shown in  FIG. 2  with the UIT tool traveling up and down weld seam  18  and centered along weld cap  10 . It is important to note that a contouring pass consists of a series of traveling motions up and down weld cap  10  and base metal  12  surface. 
         [0024]    When weld cap  10  surface is uniform, all uneven surfaces along weld cap  10  are removed and the weld metal of weld cap  10  is blended, first contouring pass  16  is complete. Thereafter, a second contouring pass  20  and a third contouring pass  22  are made, which like first contouring pass  16  are roughing passes. Referring to  FIG. 3 , second contouring pass  20  starts the blending process of weld cap  10  to base metal  12  along weld metal to base metal interface  19  of weld seam  18 . Third contouring pass  22  is made along opposing weld metal to base metal interface  21  of weld seam  18 . Second contouring pass  20  and third contouring pass  22  are therefore essentially centered upon weld metal to base metal interfaces  19  and  21 , respectively, and are blending passes to the extent they blend the weld metal and base metal along weld metal to base metal interfaces  19  and  20 . Second contouring pass  20  and third contouring pass  22  use, though they are not required to use, the same pin configuration used in first contouring pass  16  and therefore both impart a textured or dimpled appearance along and adjacent to weld metal to base metal interfaces  19  and  21 . The UIT tool motion and pattern are the same motion and pattern carried out during first contouring pass  16 . That is, the UIT tool is moved linearly in an orbital motion but with the tool positioned at the weld toe interface to base metal  12 . Thus, the motion and position of the UIT tool work in an orbital motion from weld cap  10  to base metal  12 . Second and third contouring passes  20  and  22  are carried out until both base metal  12  and weld cap  10  have been effectively impacted and blended. 
         [0025]    During second and third contouring passes  20  and  22 , the weld cap  18  surface, base metal  12  and weld metal to base metal interfaces  19  and  21  interfaces should be periodically visually inspected to ensure complete treatment and blending has been achieved. If visible lines are present during the inspection insufficient blending has occurred. If this condition is present, additional blending via continued second and third contouring passes  20  and  22  are required in those areas using the four pin, 3 mm array. When performing the visual inspection after second contouring pass  20  and third contouring pass  22 , the surface of weld cap  10  and base metal  12  should be rough or textured having a dimpled surface appearance without visible linearly extending lines. If a linear condition is present additional treatment/blending is required. 
         [0026]    When second contouring pass  20  and third contouring pass  22  are complete, a fourth set of contouring passes  24  are performed. Fourth set of contouring passes  24  are contouring passes meaning they are intended to smooth and impart a desired profile to weld seam  18 . Referring to  FIG. 4 , fourth contouring passes  24  consist of several passes with the UIT tool positioned on weld cap  10  and base metal  12  to achieve the specified surface finish and contoured condition. Those passes include a pass A centered along weld cap  10 , a pass B centered along weld metal to base metal interface  19  and a pass C centered along weld metal to base metal interface  21 . 
         [0027]    Fourth set of contouring passes  24  use a pin configuration based on surface finish requirements which are different from the pin configuration used in the first, second and third contouring passes. In the field, the desired pin geometry and array for fourth set of contouring passes  24  are specified in the work instruction for the specific job based on surface finish requirements. Typically, fewer pins with larger diameters are utilized during the fourth set of contouring passes that are in the first, second and third contouring passes. In this example, the pin array includes a two pin inline array using ¼″ diameter pins with a face radius of six inches. The UIT tool with the 2 pin inline array is worked similarly to the previous passes, moving in a linear fashion up and down weld seam  18  in an orbital pattern. As weld cap  10  and base metal  12  contouring is achieved the orbital patterns and diameter of the radial path increase in size in order to create a uniform and contoured surface. 
         [0028]      FIG. 4  depicts the initial treatment pattern for blending the roughing passes, namely, first contouring pass  16 , second contouring pass  20  and third contouring pass  22 . The orbital patterns used in fourth set of contouring passes  24  start with small orbital patterns that increase in diameter as full blending is achieved. When weld cap  10  and base metal  12  surfaces have been blended and the textured surface is no longer visible, fourth set of contouring passes is complete, and a fifth set of contouring passes  26  is applied. Fifth set of contouring passes  26  are the final contouring passes. Fifth set of contouring passes  26  may require a new pin configuration relative to the pin configuration of the fifth set of contouring passes  26  depending on the surface finish requirements specified in the job specification. 
         [0029]    Referring to  FIG. 5 , fifth set of contouring passes  26  consist of several passes with the UIT tool positioned on weld cap  10  and base metal  12  to achieve the specified surface finish and contoured condition. Those passes include a pass D centered along weld cap  10 , a pass E centered along weld metal to base metal interface  19  and a pass F centered along weld metal to base metal interface  19 . Fifth set of contouring passes  26  do not include a true linear pattern but rather rely upon a motion focused on blending and contouring weld metal  10  surfaces. During fifth set of contouring passes  26 , the orbital patterns progressively increase in diameter with large sweeping patterns focused at contouring so that weld metal  10  and base metal  12  surfaces appear as one surface with all transitions being eliminated. The large sweeping orbital motion is intended to provide surface condition treatment such that all rough or textured surfaces are minimized and weld metal  10  and base metal  12  form a smooth, featureless transition along weld metal to base metal interfaces  19  and  21 . 
         [0030]    Following fifth set of contouring passes  26 , weld metal  10  and base metal  12  surfaces along weld metal to base metal interfaces  19  and  21  are smooth in appearance and to the touch. Thus, the transition between the weld metal and base metal  12  surfaces along interfaces  19  and  20  is smooth so that the transition does not present two uneven surfaces where one surface extends vertically below or above the other surface. 
         [0031]    To determine whether fifth set of contouring passes  26  is complete, weld seam  18  is inspected by wiping the complete weld metal  10  and base metal  12  surface along weld seam  18  with a clean dry 100% nylon cloth, or equivalent. If weld metal  10  and base metal  12  surfaces have any irregular conditions, the nylon cloth is picked by the surface as the wiping occurs. If the nylon cloth is not picked by the surface, each of these areas is then inspected with an 8× magnification glass. If the 8× magnification inspection reveals a sharp or jagged surface condition, further smoothing, contouring passes are required. If no irregular surface conditions are detected, the UIT surface contouring process is complete. 
         [0032]    Upon completion of the UIT surface contouring process, weld seam  18  displays a smooth weld cap  10  surface and smooth weld metal to base metal interfaces  19  and  20 . Referring to  FIG. 6 , to provide smooth weld metal to base metal interfaces  19  and  20 , the weld metal of weld cap weld  10  is across and over base metal  12  to form a thin layer  31  of weld metal over base metal  12  and across heat affected zones  29 . Weld metal layer  31  is as thin 1/64 inch and tapers against base metal  12  to a provide a 180 degree joint there between thereby providing weld metal to base metal interfaces  19  and  20  a smooth or flat profile. 
         [0033]    Referring to  FIGS. 7 and 8 , there are depicted alternative weld seam profiles that can be accomplished using the present UIT surface contouring process. These weld seam profiles are presented to show that the desired smooth and flat weld metal to base metal interfaces forming a 180 degree or flat juncture between the weld metal and base metal  12  do not have to be coincident with cap weld or the base metal. For example,  FIG. 7  illustrates a weld seam  32  including bi-lateral grooves  31  formed in the weld metal of weld cap  10  that extend along a length of weld seam  30  adjacent to heat affected zones  29  and weld metal to base metal interfaces  19  and  20 . Grooves  31  can be created using a UIT tool by moving the tool up and down weld seam  30  in a straight line adjacent to heat affected zones  29  and weld metal to base metal interfaces  19  and  20 . As shown, the UIT treated surfaces of weld metal to base metal interfaces  19  and  20  are smooth and flat and each form a 180 degree joint there between. Fusion lines  35  between the weld metal and base metal  12  intersect weld metal to base metal interfaces  19  and  20  as essentially 90 degree angles. However, the weld metal to base metal interfaces  19  and  20  are not coincident with base metal  12  by virtue of groove. Similarly,  FIG. 8  illustrates a weld seam  32  including bi-lateral grooves  33  formed in base metal  12  that extend along a length of weld seam  32  adjacent to heat affected zones  29  and weld metal to base metal interfaces  19  and  20 . In this embodiment, UIT treated surfaces of weld metal to base metal interfaces  19  and  20  are also smooth and flat and form a 180 degree joint there between. Also, fusion lines  35  between the weld metal and base metal  12  intersect weld metal to base metal interfaces  19  and  20  as essentially 90 degree angles. However, the weld metal to base metal interfaces  19  and  20  are not coincident with grooves  33  by virtue of groove. 
         [0034]    As will be apparent to one skilled in the art, various modifications can be made within the scope of the aforesaid description. Such modifications being within the ability of one skilled in the art form a part of the present invention and are embraced by the claims below.