Abstract:
A process for applying weld overlay material to a tube with a single pass of a weld head. A gas tungsten arc welding process with high frequency wave pulse current is preferably used to apply the overlay material. The overlay material is preferably in the form of weld wire and is pre-heated and mechanically fed into the resultant weld pool on the tube. The wire is preferably resistance-heated to a temperature lower than its melting point prior to contact of the wire with the molten weld pool, and the weld pool is preferably oscillated during the overlay process. Use of the gas tungsten arc welding process provides sufficient weld fusion to allow the overlaid tube to be subsequently bent and formed, and use of the high frequency wave pulse current provides for reduced dilution of the overlay.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims benefit of U.S. Provisional application Ser. No. 60/310,961, filed Aug. 8, 2001, the entirety of the disclosure of which is incorporated herein by reference thereto. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method and equipment for weld overlay of metal tubing. More specifically, the invention relates to a system which employs a welding process to overlay conventional metal tubing in a single pass. 
     Boilers are used in the power generation, paper, steel and waste-to-energy industries, and these industries have an interest in finding means to protect and extend the operating cycles of their boilers. The concerns of boiler operating cycles arise from the fuel used by the boiler, methods of combustion, and/or impact of environmental regulations. Boilers generally use tubing, and such tubing may have structural characteristics and must be corrosion resistant as well, in view of the high temperature and corrosive conditions within boilers. 
     In certain types of boilers, such as coal-fired, black liquor recovery and waste-to-energy boilers, there is a particular need for corrosion and/or erosion resistance. In such instances, a solution is normally found in applying overlay to tubing used in the boilers with an alloy having desired corrosion and erosion protection. 
     When tubing is weld overlaid, the weld penetration of the tubing needs to be carefully controlled to ensure good fusion between a weld overlay bead and tube stock, as well as between each weld bead. The exterior surfaces of the weld overlay need to be smooth, and the thickness of the weld overlay needs to be uniform. Conventional welding processes may produce excessive weld penetration, with dilution of the base metal into the weld metal in excess of 40%. Weld overlays with this high dilution are not acceptable. 
     In conventional application of weld overlay, if the voltage and current of the weld head are controlled to ultimately provide a smooth exterior surface on the weld overlays, then weld penetration of the tube stock may tend to be excessive and uneven, and this can result in excessive dilution. 
     On the other hand, if the voltage and current are controlled to provide optimum weld penetration, then the exterior surface of the weld overlay may tend to be rough and uneven. A rough surface finish provides greater chance for corrosion media to deposit and start localized corrosion sites in the overlays. This can also make tubes more difficult to bend, which can result in an uneven distribution of the stresses during bending. A rough and uneven weld overlay surface can also affect the thickness and resulting dimensions of the tube. 
     Certain weld overlay methods have been patented. For example, U.S. Pat. No. 6,013,890, issued to Hulsizer, discloses a dual weld pass overlay method and apparatus which uses a first weld head to apply a bead of weld overlay material onto a tube, and thereby create a heat-affected zone in the tube. A second weld head is then used to heat the material within the heat-affected zone to “a temperature higher than its tempering temperature but lower than its Ae 1  temperature.” U.S. Pat. No. 6,204,477 B1, issued to Lai, also discloses a weld overlay method. 
     However, a need still exists for a weld overlay process which can provide uniform weld penetration on tube stock, uniform fusion, a smooth and even exterior surface, and also, which can control the thickness of the overlay and the dimensions of the pipe. 
     SUMMARY OF THE INVENTION 
     It is, therefore, the principal object of this invention to provide a method and apparatus for performing a single pass weld overlay. 
     Another object of the present invention is to provide a weld overlay system. 
     Another object of the present invention is to provide a weld overlay system using a gas tungsten arc welding head for applying a weld overlay bead to a tube. 
     Still another object of the present invention is to provide a weld overlay system using a gas tungsten arc welding head with high frequency wave pulse current to apply a weld overlay bead onto a tube. 
     Yet another object of the present invention is to provide a weld overlay system wherein heat is applied to weld material prior to being applied to a tube. 
     A still further object of the present invention is to provide a weld overlay system using a gas tungsten arc welding process which provides reduced dilution of the base metal into the weld metal of the tube. 
     Another object of the present invention is to provide a weld overlay system for minimizing weld penetration into the tubing, providing proper uniform fusion, and for providing a smooth and uniform exterior surface of the weld overlay. 
     Yet a further object of the present invention is to provide a weld overlay system offering improved control of the thickness of the overlay applied to a tube. 
     The present invention includes a process for applying a weld overlay to a tube with a single pass of a weld head. In a preferred embodiment, a gas tungsten arc welding process with high frequency wave pulse current is used to apply heat to a tube. Overlay material in the form of weld wire is pre-heated and mechanically fed into the resultant weld pool. The wire is resistance heated to a temperature lower than its melting point just before the wire contacts the molten weld pool. 
     The pre-heating of the wire, together with use of the high frequency wave pulse gas tungsten arc welding head, are significant features of the present invention. The control of these parameters produces weld beads with improved uniformity of weld penetration and fusion. Also, the present invention provides for improvements in the control of dilution, composition of the weld overlay, and in the smoothness and evenness of the exterior surface finish. The present invention provides relatively accurate control of thickness of the overlay and, consequently, of the final dimensions of the overlaid tube. 
     Preferably, the tube is positioned on rollers to support the tube&#39;s weight, and the tube is rotated with respect to the weld head while the weld head moves along the longitudinal axis of the tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing, as well as other objects of the present invention, will be further apparent from the following detailed description of the preferred embodiment of the invention, when taken together with the accompanying specification and the drawings, in which: 
     FIG. 1 is a schematic illustration of a weld overlay system constructed in accordance with the present invention, and shows a welding head and the pre-heating of wire used for weld metal overlay for a metal tube; 
     FIG. 2 is a schematic illustration of equipment used for performing weld overlay in accordance with the present invention; and 
     FIG. 3 is a longitudinal cross sectional view of a tube with weld overlay applied in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The accompanying drawings and the description which follows set forth this invention in its preferred embodiment. However, it is contemplated that persons generally familiar with welding systems and techniques will be able to apply the novel characteristics of the structures illustrated and described herein in other contexts by modification of certain details. Accordingly, the drawings and description are not to be taken as restrictive on the scope of this invention, but are to be understood as broad and general teachings. 
     Referring now to the drawings in detail, wherein like reference characters represent like elements or features throughout the various views, the welding system of the present invention is indicated generally in the figures by reference character  10 . 
     Referring now in more detail to the embodiment chosen for the purpose of illustrating the present invention, reference numeral  10   a  in FIG. 1 denotes a metal tube having weld metal overlay  11 . A carbon and low alloy steel material is typical stock for tube  10   a , and alloy 625 is suitable weld overlay material  11 . However, the method and apparatus of the present invention are equally suitable for any other tube stock or other weld overlay material. 
     The weld overlay material in the form of wire  11   a  is applied at a first location  12  on tube  10   a  by heating the tube boa using a high frequency wave pulse gas tungsten arc welding (GTAW) torch, or welding head, generally  13 , which is also known as a tungsten inert gas (TIG) torch. Gas tungsten arc welding power supply  20  provides power to the welding head  13 , and a wire feeder  15  associated with pre-heat holder  14  feeds metal overlay wire  11   a  to location  12 . 
     Alternating current (a.c.) power supply  16  preheats the weld wire overlay  11   a  by resistance heating through cable  17 , and wire  11   a  is fed through a holder  14  into the weld overlay location  12 . 
     Inert gas  19  is directed against weld overlay location  12  to protect the wire  11   a  from oxidation. The inert gas is, in one preferred embodiment, a mixture of Argon and Helium, or a mixture of Argon and Hydrogen, with a flow rate of between 30 and 40 CFH (cubic feet per hour). 
     The pre-heating of weld wire overlay  11   a  is controlled by adjusting the current supplied to pre-heat holder  14  from the power supply  16 . The preheat holder  14  is, in one preferred embodiment, positioned at an angle of 30 to 60 degrees with respect to the welding head  13 . 
     The gas tungsten arc welding head  13 , together with preheat holder  14 , is longitudinally advanced using a welding robot  22  mounted on a track  24 , while the tube stock  10  is simultaneously rotated beneath the torch  13  in the direction indicated by the arrow  18 . As shown in FIG. 1, when viewed from the right end of tube stock  10  the tube (FIG.  2 ), tube  10  rotates in a clockwise direction, and the GTAW torch head  13  is preferably positioned at 20 to 35 degrees from the top of the tube stock. By, and in conjunction with, the addition of high frequency wave pulsing to the GTAW torch head, the tube stock overlay  28  can be produced with dilution of less than 20%. 
     During operation, the weld head  13  oscillates in a direction generally parallel to the longitudinal axis of the tube stock  10 , and the height of the weld head  13  is controlled through use of an automatic arc voltage control, which provides precise arc length. 
     To control the temperature of the tube stock  10   a  and to control the rate of cooling of the weld overlay metal, water flows through the tube stock  10  during the welding process, as shown in FIG.  2 . 
     As also shown in FIG. 2, welding robot  22  is mounted on track  24 , and moves along the length of the tube stock  10   a  while applying the overlay to tube  10   a  in a single pass. 
     In a preferred embodiment, one end of the tube stock is gripped in a chuck  30 , which is rotationally driven by an electric motor (not shown) or other suitable means. Each end of the tube stock is capped by rotary unions with hoses  32  coupled to the rotary unions, which introduce water into one end of the tube stock and withdraws it from the other. 
     The welding robot  22  arm extends through a suitable mounting bracket to hold welding head  13  and pre-heat holder  14 , preferably at an angle of 20 to 35 degrees from the top of the tube stock. 
     During operation, the welding robot  22  moves along the longitudinal axis of the tube stock. The tube stock simultaneously rotates underneath the welding robot arm, while the weld pool is mechanically oscillated by the welding robot head in the longitudinal axis of the tube stock to provide a smooth surface finish. 
     The speed at which the robot  22  moves along the axis of the tube stock is controlled with respect to the speed at which the tube is rotated, so the weld head  13  is advanced by a distance equal to the width of the weld bead for each rotation of the tube. 
     To support the weight of the tube stock along its length, metal roller supports  34  are provided. These metal rollers  34  are positioned along the length of the tube stock. 
     At the initiation of the weld overlay process, the robot  22  starts at the chuck end of the tube stock and moves on the track  24  along the axis of the tube stock until the desired length of weld overlay is completed. The position and movements of the welding robot  22  are controlled through control pendants or other operator interfaces, and specific welding parameters are preferably controlled through robot software interfaces. 
     FIG. 3 is a longitudinal cross sectional view of a tube with weld overlay. There are three primary zones in an overlaid pipe: (1) the weld; (2) the heat-affected zone; and (3) unaffected base metal. The boundary between the weld and the heat-affected zone is also known as the “fusion boundary”. The nature of the gas tungsten arc high frequency wave pulse welding process allows for extremely close control of the welding parameters. By close control of the welding parameters, the heat input on the tube stock can also be controlled. Controlling the heat input during welding produces weld overlay with a generally accurate control of dilution, and a reduced heat-affected zone and solid uniform fusion line. This, along with a smooth and even exterior finish provided by the weld overlay system of the present invention, makes the overlaid tubes easier to bend and should prolong tube life. 
     Example 
     In one example of application of the present invention, tube stock  10  two and half inches (2.50″ O.D) diameter SA-210 material was weld-overlaid using gas tungsten arc high frequency pulse current with pre-heat of weld wire overlay. The tube stock was rotated at four (4) to seven (7) rotations per minute (RPM). Cooling water with temperature of 80 to 120 degrees F. flowed through the tube at a rate of seven (7) to twelve (12) gallons per minute. Overlay material alloy 625 in a form of weld wire with diameter of 0.045 inches was pre-heated by AC power supply  16  with frequency set at 30 to 80 Hz at current amperage of 75 to 100 AC amps and 6 to 8 AC volts. The weld wire was shielded by a gas mixture of Argon and Helium, or a gas mixture of Argon and Hydrogen, at 35 to 40 cubic feet per hour (CFH). Gas tungsten arc (GTAW) weld torch  13  was positioned at 20 to 35 degrees head angle and applied heat to tube stock  10 . The GTAW weld torch operated at a wave pulse frequency of between 300 and 600 Hz, 200 to 400 amps, and voltage of 8 to 12 volts with a gas mixture of Argon (25%) and Helium (75%), or a gas mixture of Argon (95%) and Hydrogen (5%), and flow rate of 35 to 40 cubic feet per minute (CFM). Weld overlay with deposit thickness of 0.070″ was deposited on the outer surface of the tube stock with a smooth surface finish. 
     While preferred embodiments of the invention have been described using specific terms, such description is for present illustrative purposes only, and it is to be understood that changes and variations to such embodiments, including but not limited to the substitution of equivalent features or parts, and the reversal of various features thereof, may be practiced by those of ordinary skill in the art without departing from the spirit or scope of the present invention and the claims.