Abstract:
A process for control of a butt welding of tubes is disclosed wherein readily measurable parameters of a process of welding are read, voltage and current, and parameters such as impedance or time interval across a portion of a pulse are derived from the voltage and current. The derived values are then used in conjunction with parameters of the welding machine to set the values of the welding machine and the torch or torches connected to it to control the power of penetration of a welding arc for no piercing but with penetration.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     An arc welding process for butt welding of parts, especially designed for orbital butt welding of tubes for piping applications. 
     2. Brief Description of the Prior Art 
     1. Preliminary 
     Welding has been used for many years. As it is well know in the art, when butt welding parts, the items to be welded must be held in position relative to one another. In the case of the parts being tubes welded together, a clamp is used for this purpose, generally placed inside the tubes and positioned at the junction of the two tubes. This clamp is equipped with rows of pistons, each row holding a tube. 
     The ends of the parts to be welded may first be chamfered  50  (depending on the thickness of the parts, the arc welding process used, the procedure—automatic or manual—and the like), to form ring  13 . See FIGS. 1 and 2 for illustrations of the prior art components that are welded together. 
     2. Welding Principle 
     The process used for butt welding parts of the present invention is the MIG/MAG or GMAW metal arc welding with gas shield process. A metal wire  25  is connected at the weld torch  11  to one of the polarities of a power source and is fed to the welding location. An electric arc is created between this wire and the parts to be assembled, which parts are at the polarity different from that of the wire polarity (See FIG. 5 for an illustration of the prior art equipment and process discussed in more detail below). 
     The electric arc causes the wire (sometimes referred to as the “filler metal”) to melt, as well as partial melt of the parts to be assembled. The wire feed rate (called V f ) is made equal to its melt rate so as to provide for a stable arc. The first weld bead, which produces a perfect joint at the inner skin (with admissible imperfections according to implemented standard) between two chamfered parts is know as “the penetration pass.” 
     The current wave delivered by the generator is either continuous or sequential. Arc welding is a periodic phenomenon whose period is a few hundredths of a second. During the welding process, the electrical parameters therefore also vary in a periodic fashion. 
     The current and voltage waveforms of the welding arc change according to the type of transfer of the weld metal from the filler metal (See FIGS.  3  and  4 ). 
     3. Types of Generators Used 
     MIG/MAG arc welding generators capable of generating sequential type arc modes (including the pulse and/or controlled short-circuit modes) illustrate the ability to have controlled waveforms as shown in FIGS. 3 and 4. 
     Parameters adjustable by the operator on such generators as shown in FIGS. 3 and 4 are: 
     Ipeak  1 , Ibase  2 , Upeak  3 , Ubase  4 , peak time  5 , up ramp  6 , down ramp  7  (depending on source manufacturers), V f , frequency (inverse of cycle  51 ) (adjustable or self-regulated), cool time  8  (adjustable or self-regulated). 
     In automatic welding, overall parameters managed by the program logic controller (“PLC”) are: 
     Welding unit speed (equivalent to welding speed), oscillation amplitude and frequency of the welding torch. 
     For Sequential mode: pure pulse mode (FIG.  3 ), the pulse phase corresponds to the Ipeak dwell time  5 . 
     For Sequential mode: controlled short-circuit (FIG.  4 ), Upeak: peak voltage. Voltage value  3  at which the current stops increasing, i.e. voltage at the inception of Ipeak  1 . Umax  9 : maximum achieved voltage. This is a phenomenon due to system inertia. 
     The pulse phase starts when the voltage becomes equal to Upeak and ends at the end of Ipeak. 
     In such generators, an intelligent system (microprocessor+programmable logic device, for example) allows certain parameters (Ipeak, frequency, cool time, and the like) to be adapted in real time according to the weld pool conditions (thermal emissivity of the weld pool, for example). 
     4. For Tube Welding (Penetration Pass), Several Techniques are Available 
     None of the techniques presented below makes use of real-time, self-assessment of the penetrating power of the electric welding arc. 
     a. Welding from the Inside: 
     An inner welding clamp is used. The clamp is usually equipped with two rows of pistons to hold the tubes in position. Further, mobile welding torches are mounted on the clamp for the purpose of completing the penetration from the inside. Parameters can be changed to fit predefined angular positions. The problem with this are that implementing this device is a complex process (positioning in the mating plane is difficult, centering of the welding torches, no direct check possible during welding, poor cost-effectiveness, complex machine with limited diameter range, and the like). 
     b. Welding from the Outside: 
     (1) For high wire feed rates, e.g. in the case of mechanized or automatic welding where the welding head movements are controlled by a carriage-type electromechanical assembly, the weld pool must be maintained using backing strips (copper, ceramics, and the like) to prevent the weld pool from collapsing. The backing strips are slaved to the piston rows and applied flat against the back of the joint to be welded when the pistons extend. During welding, parameters can be changed to fit predefined angular positions. The problem is that regardless of the type of medium, the backing strips gradually deteriorate as welding passes accumulate with current MIG-MAG type arc welding processes. 
     (2) In manual welding, or in automatic welding at low wire feed rates, the outside can be welded without backing strips. The welders are generally assisted by an operator who is requested to adjust the mean welding current according to the weld pool behavior. The problem is that low output occurs, with a welder-dependent process. 
     In the prior art, whether welding from the outside or from the inside, the welding parameters can be regulated according to the weld pool conditions (temperature, luminosity . . . etc.) or the bevel shape (width, gap, high-low . . . etc.) to control the penetration. To do so, some sensors (thermographic vision, laser, camera, pictures/vision analysis and the like) are available and can be used to measure the thermal evolution of the weld pool, the bevel shape evolution, and the like in real time while welding. 
     But these devices which need to be handled by the welding system are more cumbersome, fragile, expensive, and less time responsive to adapt the parameters. 
     5. Example of Configuration of Automatic Outside Welding 
     As shown in FIGS. 5 and 13, a motor-driven welding unit  10  is mounted on fixed tube  12  by means well known in the art. It is connected to a welding generator  14  and a wire reel  15 . It is equipped with one or several welding torches  11  and moves along a ring  13  integral with the fixed tube  12 , the welding unit  10  welds, for example, a half-circumference of ring  13 . 
     6. General Problems Experienced When Completing the Penetration Pass 
     a. A Requirement Exists to Control Weld Pool Fluidity in all Positions (Orbital Welding). 
     Fast cooling is controlled by varying the welding arc heat input. With no support, or with a support made of a low thermal conduction material, cooling of the weld pool during the penetration pass is slower than on a metal support (such a copper). The weld pool must therefore otherwise cool off fast enough to limit the effect of gravity on the fluid pool, thus preventing it from collapsing and avoiding gaps, overpenetration and concavity. 
     b. The Electric Arc Penetrating Power Varies as a Function of Angular Position. 
     Even under perfect pipe fit-up conditions (no gaps or high-low areas), the penetrating power of an electric arc is not constant. According to the local angular tilt of the interface of the parts to be welded, the position of the weld pool relative to the arc varies. Thus, for example, for tube welding, at the 12 and 6 o&#39;clock positions, the weld pool is located vertically juxtaposed to the arc, as it is pushed away by the arc pressure: in this case, the arc penetrating power into the metal is high. 
     Conversely, at the 3 and 9 o&#39;clock positions, the weld pool tends to run under the arc, owing to gravity, as the pressure exerted by the arc is not always adequate, according to the pool fluidity, to push the weld pool back. The arc penetrating power is therefore reduced as compared to the 12 and 6 o&#39;clock positions, under comparable fit-up conditions. 
     c. Penetration is Harder to Achieve on Fit-up Defects (See FIGS. 6. 7.  8  and  9  for Illustrations of Defects). 
     In the presence of fit-up defects (gap and/or high-low), the welding arc penetrating power naturally increases, and the weld pool cooling rate is slower (as side thermal pumping by the parent metal is less effective), and gaps may be created (owing to the lack of metal or collapse of the weld pool). When using backing strips, the backing strips may be severely damaged as the electric arc comes into contact with them. 
     d. Penetration is Harder when the Machining Dimensions of the Ends to be Assembled Vary. 
     The values of the various machined dimensions of the ends to be assembled, essentially those of the part to be fused during the penetration pass (usually referred to as the root face), i.e. the dimensions of the chamfers (FIG. 10) such as root face  21 , lip  22 , radius  20 , must, as far as practicable, need to be accurate so as to ensure the quality in manual or automated welding. In production, however, the machining conditions for chamfers and the critical root face are often less than ideal and defects may occur. Thus, if the root face thickness is smaller and/or if the width of the lip is greater than nominal values, the ability of the parent metal to cool off the weld pool will be lessened and the electric arc penetrating power will subsequently increase, with the additional risk of creating defects such as gaps or collapsing, under substantially constant mean welding arc electrical parameters. 
     Conversely, if the root face is thicker and/or the lip narrower than the nominal values, the ability of the parent metal to cool off the weld pool will increase and the arc penetrating power decrease, but a risk exists that not all the parent metal will melt (i.e. the total thickness of the root face), thus creating a defect known as lack of penetration. 
     It is an object of the present invention to control the weld pool by simple sensory readings and to adapt the welding parameters to changes in the weld pool conditions. 
     It is a further object of the present invention to have such control using reliable sensors to consistently regulate in real time the weld pool conditions through the welding parameters. 
     SUMMARY OF THE INVENTION 
     A process for real-time assessment of the penetrating power of the electric welding arc during a butt weld penetration pass is disclosed. The penetrating power is defined as the ability of the electric arc to fuse the metal to be melted (usually referred to as the parent metal). 
     The purpose of the invention, based on real-time analysis of the welding arc waveforms (current, voltage), is to enable real-time assessment and adjustment of the welding arc penetrating power during the butt weld penetration pass. 
     The penetrating power changes constantly during the welding phase, and until now, welding current sources and waveform programming only took into consideration a mean penetrating power estimated for constant or steady state welding conditions (fit-up, chamfers). However, these conditions may change locally. Thus the piercing or collapse or burning through phenomenon is due to excessive welding arc penetrating power under given fit-up and local weld pool fluidity, and is detectable through real-time analysis of electrical parameters. Likewise, the lack of penetration phenomenon is due to a low welding arc penetrating power under given fit-up and local weld pool fluidity conditions, and is also detectable through real-time analysis of electrical parameters. 
     Both burning through and lack of penetration are unacceptable defects which require repair. Sequential modes provide for different weld pool fluidity and penetrating powers (high velocity of the metal transferred during the Ipeak current) according to programming of the waveform parameters (current, voltage, frequency, and the like). However, for given programmed parameters, stabilizing penetration is difficult, owing to the critical character of the thermal balance (heat input/removal) obtained under good fit-up and chamfer conditions, and to the differences of power levels necessary to achieve correct penetration under poor fit-up conditions or with poorly machined chamfers. 
     The invention will enable the arc mode and all the overall parameters of the installation to be slaved in real time so as to regulate the arc penetrating power and achieve constant penetration without backing or with backing (ceramics, copper, and the like) whether joint preparation is perfect or exhibits imperfections through the use of the voltage and the current of the welding machine. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For further understanding of the nature and objects of the present invention, reference is had to the following drawings in which like parts are given like reference numerals and wherein: 
     FIG. 1 is a side cross sectional view of two tubes forming a single-V groove (broad chamfer); 
     FIG. 2 is a side cross sectional view of two tubes forming a single-U groove (narrow chamfer); 
     FIG. 3 is a graphic display of voltage and current versus time for a welding machine in sequential mode: pure pulse; 
     FIG. 4 is a graphic display of voltage and current versus time for a welding machine in sequential mode: controlled short-circuit; 
     FIG. 5 is an end view of a motor driven welding unit of the prior art as shown mounted on the tube, partially in phantom line; 
     FIG. 6 is a partial, side cross-sectional view in cross section of two tubes as in FIG. 2, showing perfect fit-up; 
     FIG. 7 is a partial, side cross sectional view of two tubes as in FIG. 2 in step fit-up; 
     FIG. 8 is a partial, side cross sectional view of two tubes as in FIG. 2 showing fit-up with gap; 
     FIG. 9 is a partial cross sectional view of two tubes as in FIG. 2 showing fit-up with gap and step; 
     FIG. 10 is a partial cross sectional view of a single tube showing the various aspects of the section; 
     FIG. 11 is a graphic display of voltage and current waveforms versus time for a welding machine in mode-penetrating, non-piercing arc with low penetrating power; 
     FIG. 12 shows a graph of voltage and current waveforms versus time for a welding machine in critical or high mode-piercing arc with high penetrating power, and illustrating different cases; 
     FIG. 13 shows a graphic representation of the preferred embodiment of the present invention showing control of the various parameters of a welding machine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     1. Observation of Waveforms in the Case of a Piercing Arc and a Non-piercing Arc 
     A non-piercing arc is defined as an arc whose penetrating power is sufficient to fuse the parts to be joined (i.e. to ensure penetration), but whose pressure, created by the arc power, exerted on the weld pool does not eject the melted metal before solidification. Conversely, a piercing arc is defined as an arc with high penetrating power, whose power is too high and ejects the melted metal with a risk of creating gaps or bum-through (the arc is said to have pierced). 
     In the present invention, the voltage and current waveforms allow the electric arc penetration power to be determined (See FIGS. 11 and 12 for the extreme situation of lack of penetration potential (FIG.  11 ), with ΔZ&lt;ΔZo or T&lt;To and piercing (FIG. 12) with T&gt;Tr or ΔZ&gt;ΔZo). 
     a. Normal Mode—Penetrating, Non-piercing Arc (FIG.  11 ). 
     During the pulse phase  55 , voltage U and current I remain near constant. Thus, the ratio U/I=Z (impedance of the wire+arc system) remains near constant during the pulse phase. Time T 56  to achieve maximum voltage-to-current ratio is very short. 
     b. Critical Mode—Piercing-arc (FIG.  12 ). 
     (1) For the “a” portion of FIG. 12, voltage increases during the pulse phase and current remains constant. The ratio U/I=Z increases between the beginning and the end of the pulse  55 . Time T  57  to achieve maximum voltage-to-current ratio is long, and close to the pulse time (voltage rises slowly, which accounts for the fact that time T  57  becomes close to the pulse time). 
     (2) For the “b” portion of the FIG. 12, voltage increases and current drops between the beginning and the end of the pulse phase  55 . The ratio U/I increases between the beginning and the end of the pulse. The instantaneous current drops and the voltage increases during the pulse phase. Therefore time T  58  is also close to the pulse time. 
     (3) For the “c” portion of FIG. 12, voltage remains near constant and current drops between the beginning and the end of the pulse phase  55 . The ratio U/I increases between the beginning and the end of the pulse. The instantaneous current drops and voltage remains near constant during the pulse phase. Therefore, time T  59  is also close to the pulse time. 
     2. Assessment of Penetrating Power 
     Assessment is conducted using an electronic computer microprocessor  20  which provides general functions of reading instantaneous analog data  30  associated with the welding torch  11  operating on tube  12  with weld wire  25  forming arc  26 , welding torch  11  mounted on welding unit  10  and connected to welding generator  14  and oscillator  16  and supporting wire  25  from reel  15 . The analog data is converted to a digital representation  30  and then used in equations  31  discussed below to derive parameters from the data for calculations associated with time  32  for final calculation  33  of control signals. These signals are then converted from digital data to analog signals by a digital to analog converter  34 . See FIG.  13 . 
     Several methods are available for assessing the arc penetrating power, in particular for detecting the time where the arc begins to pierce. As examples: 
     a. Assessment Mode No.  1  (FIG. 13) 
     Assessment is provided by the calculation of impedance, Z, of the wire  25  and arc  26  as a system during the pulse phase, through the equation:          Z   =     U   /   I       ,                where                                        Z                 is                 the                 impedance                 of                 the                 wire     +     arc                 system                                I                 is                 the                 instantaneous                 current                 in                 the                 arc                              U                 is                 the                 instantaneous                 voltage                 in                 the                 arc                                      
     Using an analog-to-digital converter  30 , signals U and I taken from the electric arc  26  are acquired and processed. Then, using software-programmed equation the software  31  calculates impedance Z=U/I. The impedance is first measured at the beginning of the pulse, Z 1 ; measurement starts at the time where voltage becomes equal to Upeak  3 . When timer  32  expires, the impedance is measured a second time, Z 2 . The computer  70  then calculates the impedance variation, for example as follows: 
     
       
         ΔZ=Z 2 −Z 1   
       
     
     
       
         ΔZ%=100*(ΔZ)/Z 1   
       
     
     A mean of ΔZ% and ΔZ can be obtained over several cycles. 
     A reference ΔZ, i.e. ΔZr and ΔZr%, is set based on experience, corresponds to a penetrating arc (non-piercing) with a fit-up exhibiting gaps and/or offset. 
     The measured ΔZ is compared  33  to ΔZr and ΔZo by computer  70 . 
     An offset impedence ΔZo, set based on experience, corresponds to a penetrating arc (non-piercing) under coasting conditions exhibiting no gap, with a thick root face or reduced lips  21 ,  22 . 
     ΔZ% measured≧ΔZr% indicates that penetrating power is very high, and the arc is piercing. 
     ΔZ o %&lt;ΔZ% measured&lt;ΔZr% indicates that the arc is in a proper operating zone and not piercing. However, penetrating power is all the higher as ΔZ% measured a high. 
     3. Assessment Mode No.  2   
     Assessment is made by measuring the time since detection of Upeak  3  to achievement of Zmax (nulling of derivative dZ/dT). 
     The evolution of the wire+arc system impedance Z is measured by determining at what time the impedance Z is maximum Zmax  9 . 
     Using the analog-to-digital converter  30 , signals U and I taken from the electric arc  26  are acquired and processed. Then, using software-programmed equation  31 , impedance Z=U/I is calculated. 
     Using a timer  32 , time T is measured, i.e. the time between the beginning of the pulse and Zmax as calculated by computer  32  from the readings (which represents the nulling of derivative dZ/dT based on the curves of FIG.  12 ). This time T is, as a maximum, equal to the pulse time. 
     A mean of T can be obtained over several weld metal transfer cycles. 
     An offset time To, set based on experience, corresponds to a penetrating arc (non piercing) under coasting conditions exhibiting no gaps, with a thick root face or reduced lip  21 ,  22 . A reference time Tr is, set based on experience, and corresponds to a penetrating arc (non piercing) under coasting conditions exhibiting gaps and/or offset. 
     Tmeasured is compared by software  33 , using the equations, to Tr and To. 
     Tmeasured≧Treference indicates that penetrating power is very high, and the arc is piercing. 
     To&lt;Tmeasured&lt;Tr indicates that the arc is in a proper operating zone and not piercing. However, the penetrating power is all the higher as T is high. 
     4. Regulation (See FIG. 13) 
     In the preferred embodiment of the present invention, assessment of the electric arc penetrating power permits control to occur through command control micro-processor  70 . The purpose of control is to increase the arc penetrating power when the arc penetrating power is low, and to reduce it when the arc is piercing. 
     a. Control can be achieved by changing at least one of the parameters impacting the arc mode, such as Ipeak, Tbase, Upeak, and the like or parameters impacting the overall power level such as travel speed, and the like. 
     b. Control Principle 
     A welding program is loaded to a PC  36  and stored in the EEPROM  35  for use with the data produced by the command-control micro-processor  70 . This welding control program is determined as follows: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Start values 
                 End values 
                   
               
               
                 (0% of weld complete) 
                 (100% of weld complete) 
                 Action on 
               
               
                   
               
             
             
               
                 Ipeak d   
                 Ipeak f   
                   
               
               
                 Ibase d   
                 Ibase f   
               
               
                 Upeak d   
                 Upeak f   
               
               
                 Ubase d   
                 Ubase f   
                 generator 14 
               
               
                 Rise time d   
                 Rise time f   
               
               
                 Peak time d   
                 Peak time f   
               
               
                 Down ramp d   
                 Down ramp f   
               
               
                 Wire feed rate d   
                 Wire feed rate f   
                 reel 15 
               
               
                 Oscillation d   
                 Oscillation f   
                 oscillator 16 
               
               
                 Welding unit speed d   
                 Welding unit speed f   
                 welding unit 10 
               
               
                   
               
             
          
         
       
     
     End values correspond to an arc mode with little penetration allowing welding to be completed on fit-up defects or in case of variations of machined end dimensions (thin root face, wide lip  21 ,  22 ) without causing the arc to pierce while ensuring penetration. 
     Start values correspond to an arc mode with high penetration allowing penetration to be ensured when fit-up is perfect, or in case of variation of machined end dimensions (thick root face, narrow lip  21 ,  22 ). 
     The evolution between start values and end values may be linear, for example, using a 0 to 10 V analog datum for input  30  comparisons representation. 
     c. Control Based on Assessment Mode No.  1   
     In the control system of the preferred embodiment, the analog signals from the digital-to-analog converter  34  are controlled by the comparison of ΔZ% measured with ΔZ reference by the software equations  33 . 
     If ΔZ% measured≦0 then the analog signals are equal to 0V and the parameters are set to controls  10 ,  14 - 16  based on the start values stored in the EEPROM  35 . 
     If 0≦ΔZ% measured≦ΔZ% reference, the analog signals vary, for example in linear fashion, between values of 0 and 10V. For this range, the parameters vary for example in linear or proportional fashion between the start values and end values stored in the EEPROM  35 . 
     If ΔZ% measured≧ΔZ% reference, then the analog signals may be equal to 10V and the parameters are set based on the end values stored in the EEPROM  35 . 
     d. Control Based on Assessment Mode No.  2   
     In this control system, the analog signals from the digital-to-analog converter  34  are controlled by the comparison of Tmeasured with Treference and T offset, Tmeasured being provided by the software equations  33 . 
     If Tmeasured≦To, i.e. one is at the beginning of the pulse, then the analog signals are equal to 0V and the parameters are set based on the start stored in the EEPROM  35 . 
     If To≦Tmeasured≦Treference, the analog signal varies for example in linear fashion between 0 and 10V and the parameters vary, for example in linear or proportional fashion, between the start values and the end values stored in the EEPROM  35 . 
     If Tmeasured≧Treference then the analogue signals may be equal to 10V and the parameters are set based on the end values stored in the EEPROM  35 . 
     While the disclosure above is based on instantaneous readings of voltage and current, it is within the scope of this invention to control modification of certain parameters after 1 cycle or a train of cycles. Further control is not necessarily linear or proportional and may take into consideration, e.g. heavier weighing on the seniority of the last parameter change instructions established over the last cycle trains. 
     There are many variations within the invention herein taught and the disclosure is meant to be illustrative and not limiting.