Short Circuit Pipe Welding

The invention relates to the art of welding with an electric arc and more particularly to an improved method and apparatus for controlling a short circuiting type welding system during the welding together of two steel plates such as a two pipe sections.

The present invention relates to an improvement in spatter controlled systems and heat control systems of the general type described in the U.S. Pat. Nos. 5,148,001; 5,001,326; 4,972,064; 4,897,523; 4,866,247; and 4,717,807. The prior issued patents are incorporated by reference herein and background information and for their discussion of concepts in the spatter control area to which the present invention is specifically directed.

Also incorporated by reference is U.S. Pat. No. 5,676,857. This prior issue patent is incorporated by reference herein as background information and for its discussion of welding sections of pipe together.

This invention relates to the field of arc welding using a consumable electrode and more particularly to an improved apparatus and method of short circuiting arc welding two steel plates, such as two pipe sections, together by use of a cored electrode.

In the art of welding the ends of large diameter pipe, it is conventional to machine the ends of each pipe to provide an external bevel and a narrow flat land; and to bring the machined ends into axle alignment with the lands in close but usually spaced relationship to form a weld groove which includes a gap between the two ends of the pipe. Once the pipes are in position, one or more welding heads are moved around the pipe so as to effect a 360.degree. weld. The weld is usually made in several steps. First, a root pass is made where at least the inner edges or lands of the pipes are fused and the gap between the lands filled with weld metal. Thereafter, several filler passes are made wherein the space formed by the bevel is filled so that the weld metal is at least flush with the outer surface of the pipe.

Just before and during the root pass, the ends of the pipe must be in accurate alignment with one another so as to form a quality weld between the two pipe ends. The root pass is a very critical part of the welding operation. Once the root pass is completed, the alignment of the pipes is assured and the welding of the next joint down the line can be commenced. The filler passes on the previous joint can be made at a more leisurely pace. Thus, during the root pass, a 100% sound weld bead must be laid. Soundness of the weld bead means the complete fusion of both the lands clear through to the inner surface of the pipes and the complete filling of the gap between the lands with the weld metal. Depositing of the weld metal in the gap is difficult because the weld must be made by moving the weld heads around the pipe such that the welding position varies from down-hand welding, vertical up or down welding, to overhead weld as the root pass is formed around the pipe. During the root pass, the pipe sections must be maintained in alignment throughout the root pass so as to form a quality weld. Typically, the pipes are clamped together to maintain the pipe alignment during welding. In addition to weld position complications and pipe alignment concerns, the weld metal formed during the root pass should fill the gap between the pipe sections, but the weld metal should not be allowed to pass through the gap and accumulate on the interior surface of the pipe. The weld bead should form a relatively smooth surface with respect to the interior of the pipe which has very little, if any, protrusion into the interior of the pipe. Excessive protrusion of the weld bead in the pipe can: 1) create problems with apparatuses running inside the pipes to detect the soundness of the pipe system, and 2) cause unwanted fluid mixing and turbulence as the fluids are transported through the pipe system.

In order to overcome the problem of the weld bead protruding into the interior of the pipe, it is common practice to make a root pass from the interior of the pipe. Such a welding method insures that the land of the weld bead during the root pass is controlled so as to prevent unwanted protrusion into the interior of the pipeline. However, such a welding method requires specially designed and costly equipment. In addition, such a welding method is very time-consuming and cost-preventative in various types of applications. Furthermore, such a welding method can only be used for large diameter pipes. Smaller diameter pipes cannot accommodate the welding apparatus inside the pipe. Another method of welding which prevents protrusion of the weld bead into the interior of the pipe is the use of backplates or back-up shoes positioned on the interior of the pipe and over the gap between the pipe sections. The backplate prevents the weld bead from protruding into the interior of the pipe sections during the root pass of the weld bead. However, the use of backplates is also very time-consuming and is limited to relatively large diameter pipes. In addition, the backplate, in many instances, becomes welded to the interior of the pipe section during the laying of the root pass. Consequently, the backplate must later be removed which can be both costly and can adversely damage the weld bead during the removal of the welding plate.

A welding apparatus which overcomes many of the problems associated with past pipe welding methods is disclosed in U.S. Pat. No. 5,676,857. This patent discloses an improvement in the welding of the end of two pipe sections by the use of a welding apparatus having two welding bugs which continuously move on a track around the periphery of the pipe. The welding bugs include a special short circuiting power source to apply a root bead between the two ends of a pipe. The patent discloses that by using this system of welding with the proper bug speed and welding wire speed, only a slight burn through each edge of the bevel occurs and a small flat weld is formed on the interior of the pipe, thus eliminating the need to run an initial root pass from the interior of the pipe or to use other types of equipment inside the pipe such as a backplate. Although the method of welding disclosed in U.S. Pat. No. 5,676,857 overcomes several of the problems associated with the welding of pipe sections together, problems associated with the alloy composition of the weld metal of the root bead and the shielding of the weld metal of the root bead from the adverse affects of the atmosphere remain a problem.

The short circuiting power source is designed to be used with a solid wire electrode and, during welding, the weld metal is protected from the atmosphere by use of various types and blends of shielding gas. Although the use of a solid wire and shielding gas produces an excellent weld bead in many environments, the short circuiting welding method has several limitations when used to weld together the pipe sections. Because solid wire electrodes are used, the composition of the weld bead is limited to the available alloy compositions of electrodes for use in short circuiting welding. As a result, the composition of the weld bead may significantly deviate from the composition of the metal of the pipe sections. The composition of the weld metal should closely match the composition of the metal pipe to form a weld bead having optimum weld bead characteristics. Because the composition of the pipe will vary depending on the application of use for the pipe, problems will arise with the obtaining of a solid wire electrode which forms a weld metal that optimally bonds with the pipe sections.

Another limitation of the short circuiting welding process is that a shielding gas must be used to protect the weld bead from the adverse effects of the environment. The welding apparatus must include an arrangement for storing and directing shielding gas to the area of welding. Such an arrangement must include a mounting arrangement for the containers of shielding gas, regulators, flow meters, hoses, and other materials necessary to direct the shielding gas to the welding area during welding. The shielding gas prevents oxygen, nitrogen, hydrogen and other compounds in the atmosphere from reacting with the molten metal and/or being trapped in the molten metal. These elements and compound can cause porosity in the weld bead, cracking of the welding bead, spattering of the weld metal, etc., which can significantly compromise the strength and quality of the weld bead. The use of a shielding gas in a controlled indoor environment is effective in preventing the adverse effects on the weld bead from the environment; however, the use of shielding gases in an outdoor environment are highly susceptible to the effects of wind during the welding process. In order to minimize these effects, special shields must be erected around the perimeter of the electrode to shield the shielding gas from the wind during welding. The use of such shields or other welding configurations to minimize the effects of the atmosphere on the weld bead is both costly and significantly complicates the weld apparatus arrangement.

In view of the problems associated with welding of pipe sections in various types of environments and for producing a high quality weld bead having a composition substantially similar to the composition of the pipes that are being welded together, there is a need for an improved welding method and apparatus which can overcome such problems during the welding of pipe.

The present invention relates to a method and apparatus of short circuit arc welding together two steel plates, preferably on one side of the plates. Preferably, the method and apparatus of short circuiting arc welding is used to weld together pipe sections together and form a root bead between the two pipe sections; however, the invention has broader applications and can be used to weld together a variety of metal objects, in a variety of ways, in a variety of environments.

In accordance with the preferred embodiment of the present invention, there is provided two pipe sections which are positioned together and form a groove between the ends of the two pipe sections, a welding carriage positioned around the groove formed by the two pipe sections, a consumable cored electrode, a power supply with a positive and negative terminal, a welding current circuit which applies a welding current with a given time base profile across the groove between the pipe sections and the consumable cored electrode. The pipe sections are preferably aligned by the use of clamps. The clamps maintain the pipe sections together at least until a root bead has been applied to the groove between the pipe sections. Preferably the pipe sections are aligned such that a small gap exists in the groove that is formed by the two pipe sections. The welding carriage preferably extends at least 180.degree. around the circumference of the groove and preferably 360.degree. around the circumference of the groove. The welding carriage is designed to slide along a track as it moves around the circumference of the groove, which track is secured about the periphery of the pipe. The welding carriage includes a drive motor which in conjunction with rollers or other types of slide bearings allows the welding carriage to slide along the track around the circumference of the groove at a desired speed. The welding carriage includes a mechanism which controllably moves the consumable cored electrode toward the groove during the welding process. The mechanism for controlling the movement of the cored electrode may be integrated with or separate from the mechanism for controllably moving the carriage about the groove during welding. The welding current circuit includes a first circuit for controlling the current flow during the short circuit condition wherein the molten metal at the end of the consumable cored electrode is primarily transferred into the molten metal pool within the groove by surface tension action. The transfer current includes a high current pinch pulse across the shorted melted metal which helps facilitate the transfer of the molten metal from the electrode to the weld pool. The welding current circuit also includes a second circuit to create a melting current. The melting current is a high current pulse which is passed through the arc which preferably has a preselected amount of energy or wattage used to melt a relatively constant volume of metal at the end of the consumable cored electrode when the electrode is spaced from the welding pool.

In accordance with another aspect of the present invention, the cored electrode includes a flux system within the cored electrode to provide a shielding gas during the welding process. Various types of fluxes may be used to produce the shielding gas during welding.

In accordance with another aspect of the present invention, the consumable cored electrode includes alloy metals in the core so as to obtain a weld bead composition which is substantially similar to the composition of the pipes which are being welded together. A weld bead having a composition which closely matches the composition of the pipe sections forms a strong, durable, high quality weld bead.

In accordance with another aspect of the present invention, the second circuit of the welding current circuit provides a high energy boost during the initial portion of the arcing condition. The high current boost preferably has a preselected I(t) area or energy for melting a relatively constant volume of metal on the end of the consumable wire when the wire is spaced from the welding pool. The energy created during the plasma boost is preferably sufficient to create a spherical metal ball having a diameter of no more than twice the diameter of the welding wire. Preferably after the initial high current plasma boost current, the high current is maintained for a preselected period of time and then subsequently decayed over a period of time until the desired amount of energy or wattage is applied to the electrode to melt the desired volume of the electrode.

In accordance with still another aspect of the present invention, the welding current circuit limits the amount of energy directed to the electrode so as to prevent the unnecessary melting of the ends of the pipe sections during the application of the weld bead and/or to maintain too hot of a weld bead during welding to thereby prevent molten metal from passing through the gap between the ends of the pipe sections and into the interior of the pipe sections.

In accordance with another aspect of the present invention, the welding current circuit includes a circuit to produce a background current. The background current is a low level current which is maintained just above the level necessary to sustain an arc after the termination of a short circuit condition. The background current is preferably maintained throughout the welding cycle to insure that the arc is not inadvertently extinguished during welding.

The primary objective of the present invention is the provision of a short circuiting arc welding system and method which forms a high quality weld bead between two metal plates.

Another objective of the present invention is the provision of a short circuiting arc welding system and method which accurately tracks a desired current profile during the welding of two metal plates together.

Yet another object of the present invention is the provision of a short circuiting arc welding system and method which forms a high quality root bead along the groove of the ends of two pipe sections.

Still another objective of the present invention is the provision of a short circuiting arc welding system and method for applying a controlled amount of energy to the electrode to form a weld bead in the groove between two pipe sections without causing molten metal to pass through the groove and into the interior of the pipes.

A further objective of the present invention is the provision of a system and method which forms a weld bead without the need of external shielding gases.

A further objective of the present invention is the provision of a system and method as defined above which produces a weld bead having a composition which is substantially similar to the composition of the pipe being welded.

Yet another objective of the present invention is the use of a cored electrode in a short circuiting weld apparatus to form a high quality weld bead.

Other objectives and advantages will become apparent from the following description taken together with the accompanied drawings.

FIG. 1 is a wire diagram illustrating the broad aspect of the welding control circuit of the present invention;

FIG. 2 is a partial view of the groove between two pipe section ends and the electrode position in the groove to form a root bead;

FIG. 3 is a cross sectional view taken along line 3--3 of FIG. 2 of the electrode nozzle and a cored electrode therein;

FIG. 4 is a current graph illustrating a representative curve profile of a single welding cycle employed in a system or method used in the present invention wherein the electrode is the positive contact and the workpiece is a negative contact; and,

FIG. 5 is an inverted curve profile of a single welding cycle similar to the one shown in FIG. 4 wherein the electrode is the negative contact and the workpiece is the positive contact.

Referring now to the drawings wherein the showings are for the purpose of illustrating the preferred embodiment of the invention only and not for the purpose of limiting same, FIG. 1 illustrates a short circuiting arc welding system connected to the output of a DC power supply. The preferred type of short circuiting welding is SURFACE TENSION TRANSFER or STT type of welding. The welding circuit and control arrangement for such type of welding is disclosed in U.S. Pat. Nos. 5,148,001; 5,001,326; 4,972,064; 4,897,523; 4,866,247; and 4,717,807, which patents are incorporated herein. Therefore, only a general discussion of the welding circuit will be discussed below.

The power supply is a D.C. power supply preferably made up of a motor 10, such as a gas motor, which powers a generator 12 to produce an AC current 13. The AC current 13 is then rectified by rectifier 14 to form a DC current 20. A phase controller 16 controls the rectifier 14 to produce a substantially uniform DC current 20. DC current 20 is then directed into a pulse with modulator 30. The shapes of the pulse widths are controlled by shaping circuit 32 to thereby create a desired pulse with the DC current 20 through output terminals 34, 36. As can be appreciated, the power supply need not be a rectified output but can be any other appropriate DC source.

The DC current from the pulse width modulator 30 is directed across a welding area which includes a consumable cored electrode 50 and workpiece 60.

Referring to the welding of the workpiece 60, electrode 50 alternates between a short circuit condition when the electrode 50 engages workpiece 60 and an arcing condition where the electrode 50 is spaced from the workpiece 60. During the arcing condition, an electric arc is created between the workpiece 60 and the electrode 50 for purposes of melting and maintaining molten the end of the electrode as it is fed toward workpiece for a subsequent short circuit condition. This type of welding cycle is schematically illustrated in FIGS. 4 and 5. As shown in FIGS. 4 and 5, the welding cycle alternates between a short circuit condition and a plasma condition.

During the plasma condition, it is necessary that an arc is created and maintained at all times for the purpose of smooth and effective welding. The welding cycle which is repeated several times per second must be accurately controlled for the purpose of reducing spatter at various times during the welding cycle. Pulse width modulator 30 operates at a high frequency. In the preferred embodiment, the operating frequency of the pulse width modulator controller 30 is 20 KHz with a width of the successive current pulse being determined by the voltage on line 33 from shape controller 32. As the feedback control system demands more current in the welding cycle, a higher voltage appears on line 33 causing a wider pulse during the next pulse from the pulse width modulator 30. Thus, the demanded current for the welding cycle is changing 220,000 times each second. Since the highest rate of the welding cycle is generally in the neighborhood of 100 to 400 cycles per second, many update pulses are provided during each welding cycle. In accordance with known practice, the welding current circuit includes a premonition circuit having an output line 40 for controlling switch 42.

The welding circuit directs current to work area in accordance with the operation of the pulse width modulator controller 30 until the dr/dt (where arc is the electrode resistance), di/dt or dv/dt indicates an upcoming fuse during the pinch cycle. When this detection occurs, the logic on line 40 changes polarity to open switch 42. This places resistor or snubber 39 in series with main choke 38. Since the main choke has a small inductive resistance, very little energy is stored in the welding current circuit. Consequently, the current flow caused by the welding circuit between the electrode and the workpiece is immediately dropped to a level determined by resistor 39.

In accordance with the present invention, there is added to the welding current circuit a generally parallel background current circuit. The background current circuit continues to provide five to seven amps of current across the workpiece irrespective of the operating condition of the welding circuit. As a result, the background current insures that at least five to seven amps of current pass between the electrode and the workpiece at all times during the welding cycle, thereby preventing the extinguishment of the arc between the electrode and the workpiece during any phase of the welding cycle.

Referring to FIGS. 2 and 3, pipe edges 70 illustrate each having a beveled surface 72 which forms a groove between the pipe edges. Pipe edges are spaced apart such that a gap 74 exists between the pipe edges. In accordance with known practice, the pipe edges are positioned and secured together, preferably by clamps until at least the root bead is applied to the groove between the pipe edges, thereby filling the gap 74. A pipe ground 78 engages the pipe to complete the arc circuit between electrode 50 and the pipe. Electrode 50 is directed into the groove between the two pipe ends by electrode nozzle 80. During the welding cycle, the electrode is fed through electrode nozzle 80 so as to transfer the molten metal at the end of the electrode into the groove between the pipe ends to form a root bead.

Referring to FIG. 3, electrode 50 is a consumable cored electrode which includes an outer metal sheath 52 and an electrode core 54. Preferably the metal electrode sheath 52 is made up of carbon steel, stainless steel or some other type of metal or metal alloy. Preferably the composition of the metal sheath is selected to be similar to the base metal component of the pipe sections. The electrode core 54 preferably includes fluxing agents 56 and/or alloying metals. Fluxing agents 56 may include compounds to create a slag over the weld bead to protect the weld bead until it solidifies, to retain the weld bead in position until it solidifies and/or to shield the weld metal during the formation of the weld bead. The fluxing agents may also include components which produce a shielding gas to protect the root bead from the adverse effects of the environment. Preferably the fluxing agents include fluoride and/or carbonate to generate a shielding gas during welding so as to eliminate the need for external shielding gases during welding. The use of such an electrode greatly simplifies the welding apparatus. The need for a source and arrangement for an external shielding gas is eliminated. In addition, the need for a special shield to protect the weld bead from the environment since the shield gas forms at the weld bead, thus protects the weld bead in all types of the environment. The slag which forms on the weld bead further shields the weld bead from the environment, thus resulting in the formation of quality weld beads. Heretofore, a flux cored electrode was not used in this type of short circuiting welding. Surprisingly, the flux cored electrode worked well in the short circuiting welder and formed a shielding gas and slag which adequately protected the weld bead from the deleterious effects of the atmosphere. The alloying agents are also preferably included in the electrode core 54. The alloying agents are preferably selected such that the alloying agents in combination with the composition of the metal electrode sheath 52 form a weld bead having a composition substantially similar to the metal composition of the metal pipes. The flux cored electrode provides versatility to the welding method and apparatus in that the electrode both provides adequate protection to the weld bead during welding without need of external shielding gases and forms a weld bead with the same or closely the same composition of the pipe metal, thereby forming a high quality weld bead in all types of environments and with a wide variety of pipe metal compositions.

Referring now to FIGS. 4 and 5, the operation of the preferred embodiment of the present invention is schematically illustrated. FIG. 4 shows a desired current profile to produce low spatter and to prevent the weld bead from passing through the gap 74 and into the interior of the pipe system. This current profile is divided into a pinch portion, a plasma boost portion, a plasma portion and a background portion wherein the arc is to be maintained. In the current profile illustrated in FIG. 4, pinch portion 110 includes a break point 112 and a premonition circuit operating point 114. The plasma boost portion 120 of the current profile includes a decaying portion 122 referred to as the plasma portion. The plasma boost portion, which is critical to the operation of the spatter control system, is the constant current portion prior to the decay portion; however, the decaying portion 122 can be referred to as the end of the plasma boost portion or the start of the plasma portion. Following the decaying portion 122, the current control circuit shifts to the background current level 100 which maintains the plasma or arc.

The current control circuit, in accordance with the present invention, maintains a preselected background current level, thereby preventing the current level through the arc from ever falling below the preselected current low current level and allowing the arc to extinguish.

The current control circuit is designed to produce all the melting of the electrode during the plasma boost and plasma portion of the welding cycle. Further melting of electrode 50 does not take place when the background current level 100 occurs since the IR necessary for melting the electrode is not obtainable through an arc maintained only by the background current. Thus, the background current only serves to maintain the arc and the ball of molten metal in the molten state. The amount of molten metal at the end of electrode 50 which is formed by the plasma boost and plasma is selected to melt a preselected volume of molten metal at the end of the electrode, and the plasma portion of the current is reduced to the background current once the preselected volume is obtained. The duration of the plasma boost and plasma portion is also selected to prevent unnecessary melting of the metal around gap 74 of pipe ends 70. Such over-melting of the metal can result in the weld metal seeping into the interim of the pipe sections. During the formation of the molten metal ball at the end of the electrode during the plasma portion of the current, the jet forces of the high current repel the melted metal from the welding pool until the preselected amount of molten metal has been melted at the end of the electrode. Once the current is reduced, the molten metal is allowed to form into a ball and the molten metal pool in the groove is allowed to stabilize, thereby allowing for a smooth contact between the substantially spherical ball and the quelled weld metal pool. The desired amount of molten metal at the end of the electrode is controlled by directing a preselected amount of energy or wattage into the electrode during the plasma portion of the welding cycle. All during the time the molten metal ball is being formed at the end of the electrode, the core components are releasing shielding gases to shield the molten ball and the weld metal in gap 74 from the atmosphere. The shield gases continue until the molten ball is transferred into the molten metal in gap 74.

Once the molten metal ball is formed during the plasma boost and the plasma portion of the welding cycle, the molten ball is forced into the molten pool by feeding the electrode into the pool, thereby forming a short circuit condition. When the melted metal ball engages the molten metal pool, it is transferred into the pool by surface tension. This action causes an ultimate necking down of the molten metal extending between the pool and the wire in the electrode, and then a rupture and separation of the ball from the wire occurs. Since there is only a low background current during the separation, little if any spatter occurs. Preferably, the current control circuit monitors the necking of the molten metal ball such that when the neck rapidly reduces in diameter by electric pits, the current flow during the pinch curve 110 increases more gradually until a detection of an impending fuse is obtained. Once the detection of an impending fuse occurs, the current is reduced to the background current until the molten metal at the end of the electrode transfers into the weld pool.

The invention has been described with reference to a preferred embodiment and alternates thereof. It is believed that many modifications and alterations to the embodiments disclosed will readily suggest themselves to one skilled in the art upon reading and understanding the detailed description of the invention. It is intended to include all such modifications and alterations in so far as they come within the scope of the present invention.