TandemNarrowGrooveSubmergedArcWelding_Nov11

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NOVEMBER 201132

As the section thickness of vessels forthe nuclear and petroleum industry in-crease, the desire for reduced weld time

and material consumption has driven thejoints on these vessels to become narrowerand deeper. Narrow joints significantly re-duce the number of passes, time, and ma-terials needed to complete each weld.Whenever the needed material and spec-ification allow, the submerged arc weld-ing (SAW) process ideally, the tandemprocess using two torches is preferred

due to its ability to provide higher depo-sition rates and to produce high-quality,reliable weldments.

To achieve the desired weld properties,these joints are typically produced by de-positing one or more root passes and thenmaking two passes per layer until the jointis filled Fig. 1. The two passes per layermethod minimizes undercut, trapped ormechanically locked slag, and concave

weld profiles. This method requires theability to swing the weld wire nozzles fromside to side as each pass is made. Theprocess may also require changing themode of the power supplies from DC to

AC as the weld transitions from the rootpasses to the fill passes to achieve thegreater deposition associated with ACSAW. In some cases, the use of AC-only

welding may be necessary due to arc blowin a deep narrow joint.

Consider looking down 12 in. insidethe 1.5-in.-wide weld joint of a thick-sec-tion reactor vessel, 16 ft in diameter, 40 ftlong, weighing more than 500 tons, thatssitting on turning rolls, and is preheatedto greater than 250C, while trying to con-trol the vertical and horizontal position oftwo SAW torches to maintain exactly 0.110in. off the moving side wall for more than24 h. Dont forget that you also need tosmoothly switch to the other sidewallevery 360 deg. These are only a few of theimportant requirements of this process.

Tandem narrow groove SAW is amongthe most challenging welding processes to

perform and automate. Consistent per-formance is a balance between wire posi-tion from the sidewall and the depositionrate. For a given deposition rate, if the

wire is too close to the side, undercuttingand underfilling will likely occur; if toofar, the bead becomes more concave andincomplete fusion and flux entrapmentcan occur. Successful automation of thisprocess requires critically accurate con-trol of the wire position with respect tothe joint sidewall as well as the power de-livered to each weld wire. However, thelatest technologies are now being appliedto make this demanding process more au-tomated, manageable, and successful.

There are four areas in which signifi-cant advances have been applied to thisapplication: 1) Rigid and precise servo-controlled weld heads; 2) scanning spotlaser tracking systems; 3) accurate digi-tally controlled power supplies; and 4) to-tally integrated digital control and processmonitoring systems.

Narrow Groove Weld Heads

Successful narrow groove weldingmust begin with the head Fig. 2. A nar-row groove head needs to be relativelylong and thin to reach into these joints,and it must also be designed to be rigidand to maintain dimensional stability athigh temperatures for long periods oftime. Designs and materials are optimizedfor the long, thin profiles while providing

Advanced Technologiesfor Tandem SAW Narrow

Groove ApplicationsAutomation technology takes on the challenges of the tandem narrow groove

submerged arc welding process

BY DON SCHWEMMER, BOB BEATTIE, AND PATRICK WAHLEN

DON SCHWEMMER ([email protected]) is with AMET, Inc., Rexberg, Idaho. BOB BEATTIE is with Meta Vision Systems, Eynsham,Oxfordshire, UK. PATRICK WAHLEN is with The Lincoln Electric Co., Cleveland, Ohio.

The use of narrow, deep joints reduces thenumber of passes as well as the time andmaterials it takes to build pressure vesselssuch as the one shown above.


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as massive a frame as possible for rigid-ity, current transfer, and heat dissipation.The latest heads are a composite of ma-terials to provide structural integrity, ther-mal stability, and electrical conductivity,as well as isolation and contaminationprotection from potential contact with thesidewall of the part in these tight joints.

The deep, narrow profile of thesejoints, especially beyond 6 in., prohibitsactually tilting of the whole head from side

to side as the joint fills, so the head musthave articulating nozzles that allow the

weld tips to be angled into each sidewall.After each pass, the nozzles must swingto the other side to provide the appropri-ate entry angle. This also has to be coor-dinated with the head cross-seam slide asthe entire head will need to move slightlyto maintain the same sidewall offset withthis entry angle as the joint widens and isfilled.

Although torch angle articulation hasbeen historically performed pneumati-cally or with relatively simple mechanicaldevices, new generations of heads are uti-

lizing precision servomotor- and encoder-based systems to be able to move underprogrammable control. This provides sev-eral significant advantages including theability to vary the entry angle as the weldprogresses, to have different angles foreach torch nozzle (lead or trail torch), andthe ability to control the speed or smooth-ness of the transition from one sidewallto the next after completion of each pass.Servo-encoder-based control of the torchangle as well as the cross-seam and heightposition slides allows for the coordinationof all of these positioning devices to bet-ter ensure the desired position of the weld

wire.The overall narrow groove torch heads

have now, appropriately, become massiverigid structures with heavy-duty precisionslide assemblies, redundant wire straight-eners, integrated video cameras, laser sen-sors, and flux delivery/recovery systems Fig. 3. This is all to ensure the accuracyand repeatability of this process.

Scanning Spot LaserTracking Systems

As important as accurately placing the

wires is, having the ability to preciselyknow where to place them is equally cru-cial. Thus, being able to determine the

joint profile in real time is vital.Mechanical tracking with probes has

been a common and reliable method oftracking joints to accommodate for move-ment of the part, or variations in the partor weld geometries. However, mechani-cal tracking systems must contact the partand can only give position information

where the probe is in contact. Also, un-less there is a probe on both sidewalls,

transitioning from one side to the othercan be difficult and usually requires theprocess to stop and be reset for the oppo-site sidewall. When there are two probesfor the sidewalls and one for the base ofthe joint, the amount of equipment insidethe joint becomes significant and cumber-some.

Laser tracking would be an ideal solu-tion as it is noncontact (located away fromthe joint) and could provide a completeprofile of the joint. However, as joints be-came deeper and narrower, the reliability

of conventional laser tracking systems tosee these joints became more and moreof a problem. Fortunately, an unconven-tional approach to laser tracking providesa viable solution.

There are two main approaches tolaser-based weld joint tracking: one usesa laser stripe and the other a laser spot.The simpler, more common approach tolaser tracking is to project a laser stripeonto the part and then image the com-plete stripe with an area detector (cam-era). However, there are potential imagequality problems with this approach fornarrow groove joints. Since the entire

laser stripe is always on and the entireimage is always illuminated, shiny sur-faces, typical of narrow groove joints, cre-ate secondary reflections, which may re-turn more light to the sensing camera thanthe primary surface of interest. This re-sults in difficulty analyzing the image todetermine the actual weld profile. Due tothe steep and long sidewalls of these

joints, significant secondary reflectionscan be present, thus reducing the reliabil-ity of the system to accurately track andcharacterize the joint.

Fig. 1 Typical narrow groove profile (topleft) and cross section of a weld.


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A laser spot scanner uses a laser spotinstead of a laser stripe. The spot scanner

makes only a single point measurementat any one time and thereby requires ascanning mechanism (Galvo Scanner) tosweep the spot (and linear detector)across the weld joint. This principle is il-lustrated conceptually in Fig. 4.

The reflection of light from one sideof the joint to the other and then reflect-ing back to the imager associated with thelaser stripe method is not a problem withthe scanning spot sensor. Since the detec-tor is only looking on a line perpendicu-lar to the scan, it literally does not see the

reflection. Scanning spot laser sensorsalso perform well in situations where there

is a requirement for a small width of fieldbut a very large depth of field, such as

welding deep, narrow grooves. Since aprofile of the weld joint is obtained, otheradvanced software solutions can be ap-plied to calculate the volume of the jointand adaptively control the process to com-pensate for variances in the groove andsidewalls to achieve more even fill. Finally,because it is measuring at a single point,it is easy to implement an effective auto-matic gain control on a laser spot scannerso that the sensor compensates dynami-

cally and automatically for variations insurface conditions.

Meta Vision Systems has optimizedsuch a scanner specifically for this appli-cation. The joint profile information fromits DLS 300 scanning system can be com-bined with the weld head and torch angleposition to accurately determine and dis-play the position of the wire tip with re-spect to the sidewall and bottom of the

joint. Figure 4 also shows a scan of the

weld profile from a weld in progress. The+ marker on the display indicates the po-sition where the wire intersects the bot-tom of the joint. This scanner also pro-

vides important information for control-ling the head cross-seam and height slidesto maintain the constant sidewall position.The scanning spot laser sensor is, there-fore, a powerful tool and a key componentin the successful automation of theprocess.

Advanced Digital PowerSupplies

All the efforts to accurately character-ize the joint in real time and preciselyplace the wires are for naught if consis-tent and reliable power cannot be deliv-ered to the wire contact tips. This processcan demand more than 700 A per supplyfor more than 24 hours. It requires bothaccurate DC and AC output and the abil-ity to switch between modes. The most ad-

vanced power sources have enhanced dig-ital control circuitry and software thatcompensates for variances in power input,temperature, and demand to ensure a con-sistent, stable output.

In addition to enhanced performancecharacteristics, the ability for direct digi-tal control has also markedly expandedthe automation possibilities of difficult

welding processes. This direct control al-lows these supplies to become an integralpart of the complete system and all oftheir capabilities to be accessed and con-trolled from a single controller. With theability to enable/disable the supplies,change modes between DC and AC, varycurrent, voltage, and AC phasing, allbased on part or torch position, auto-mated continuous root-to-cap multipass

welds are achievable.

The Lincoln Electric Co. has devel-oped its Power Wave digitally controlledpower supplies specifically for these de-manding applications Fig. 5. The PowerWave AC/DC 1000 SD is a high power,high-speed inverter design that deliversWaveform Control Technology for sub-merged arc welding. The power suppliesprovide constant current or constant volt-age operation and allow variable fre-quency and amplitude for AC output. Thesoftware-driven AC, DC-positive, or DC-negative output allows the user to control

Fig. 2 Integrated narrow groove weld head assembly.


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the deposition and penetration. The re-sult is a more consistent, smoother arc ata wider range of parameters, increased

weld speeds, consistently higher-qualitywelds, and improved efficiencies in a sin-gle- or multi-arc environment.

The sophistication of the software-based inverter design allows a significantability to be integrated into an adaptivelycontrolled dynamic system requiredfor the challenging narrow groove

environment. Previous-generation SAWpower sources had a limited range of func-tionality, which greatly limited the capa-bility of the welding process to execute ahigh-quality, high-productivity weld.Newer, more advanced power sources andseamless integration into the overall nar-row groove system architecture have dra-matically enhanced the operating enve-lope and flexibility of the welding process.

It is important to note that the effi-ciency of these inverter-based power sup-plies yields significant savings in operat-ing costs over traditional transformer/rec-tifier designs. In a typical industrial oper-

ating environment fabricating pressurevessels, each machine can potentially saveup to $15,000 per year in operating costsdue to its unique design and use of en-ergy-efficient, high-speed inverters.

Integrated Control andProcess MonitoringSystems

A complex automated system, withservo-controlled weld heads, turning rolls,manipulator boom, two wire feeders, twoarticulating weld torches, scanning spot

laser systems, two advanced power sup-plies, flux delivery/recovery each withits independent control devices canoverwhelm even the most experienced en-gineers and operators. Setting up andmonitoring all of the controls for these de-

vices is a procedural challenge and has po-tential for many costly errors. Process re-peatability, minimized operator setup,ease of programming, and reliable process

veri fication are all associated with thequality and design of the control system.

Traditional control approaches for au-tomated systems are to link the controllerstogether so that the start or change of any

given component is activated, or linked,together with the start or change of an-other component. For example, theprocess may start with the operator push-ing the start button on the turning rolls,

which would immediately activate theflux-delivery control, and then activate thecontrols for the power supplies, beginningthe weld. A change in the part diameteror material would most likely require allsettings and/or dials of several or all con-trollers to be changed. Also, since the

Fig. 3 Articulating nozzle tips (top) of the AMET narrow groove head showing independ-ent (bottom left) and coordinated (bottom right) wire tip position control.

Fig. 4 A Schematic of the scanningspot laser method; B a profile scan show-ing the intersection of the wire with the base

of the joint represented by a +.

A B


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communication is typically one way, thesystem would not necessarily be awarethat any given function did not occur or

was incorrectly set.Consider how individuals in a work-

group have their own computer proces-sors to focus on their individual tasks. Ifthe computers are networked, then the in-dividuals working on the same project canshare all of the relevant information andrespond, if necessary, to all other users.

The most advanced approaches to com-plex automated control are based aroundthis same technology as dedicated proces-sors are assigned to control and monitoreach process task, and share the informa-tion over a network so that other, appro-priate components of the system can re-spond or react. These multiprocessor, net-

worked-based systems have many advan-tages, especially for complex systems thatrequire many tasks to be programmed,controlled, and monitored. One of themost significant is that all of the parame-ters for the process can be programmed,stored, transferred, and displayed through

a single main controller on the network.This comprehensive control of all processparameters through a single controllerprovides enhanced accuracy and signifi-cantly reduces setup and operational er-rors as all process parameters are pro-grammed and stored for each weld or parttype. This method is shown in Fig. 6.

The AMET XM control system usessuch a network to control and monitoreach task through digital signal processorpowered modules that are dedicated toeach task. The ability to share informa-tion and have other components of thesystem respond allows for greater au-

tomation than otherwise achieved.The system can do the following: Obtain the laser sensor information

and combine it with the servo position in-formation of the head position, torchangle, and preset contact tip-to-work dis-tance to control the head position, main-taining the required sidewall offset andtorch height.

Vary any of the process parameters

NOVEMBER 201136

Fig. 5 Power supplies integrated into the weld system.

Fig. 6 A Multiprocessor networked-based control method for a narrow groove application and how each device can be selected and pro-grammed from a single controller; B a typical monitor view.

A

B


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according to the position of the torch orpart.

Constantly monitor each process andprovide process data as well as alerts forout-of-tolerance conditions to the opera-tor at the main controller.

Provide ethernet connectivity so thatthe system performance can be monitoredfrom a central station as well as transfer-ring programs and data.

Provide pre- and postweld automa-

tion programming to allow for pro-grammed timing of flux delivery start,prepositioning of the weld head to aid insetup, and other desired functions ormoves before or after the weld.

Easily integrate with other sensorssuch as temperature measurement be-cause another module for the sensor canbe added to the network and the informa-tion shared.

The overall effectiveness of these con-

trol capabilities also depends on how ef-fectively information is obtained from andprovided to system operators and engi-neers. Systems that can be programmedby selecting desired parameters and en-tering desired values without any specificprogramming language or code are moreintuitive and valuable. Controllers for in-tegrated systems can be specifically de-signed to include not only displays, but ap-propriately placed and integrated joy-sticks, knobs, and buttons with tactile

feedback allowing the operator to focuson the process instead of several con-trollers. Figure 7 shows the operator dis-play and controller for a narrow groovesystem.

Summary

Although the welding community hashistorically struggled with new technolo-gies, the advances in electronics, process-ing capability, laser systems, high currentcontrol, and network communicationshave recently provided reliable solutions

for several of industrys most difficultprocesses. Tandem submerged arc narrowgroove welding is now one of them (Fig.8) as significant improvements in processefficiency and quality can now be achieved

with the appropriate automation.

Acknowledgments

The authors would like to acknowledgethe support of David Mate and theLincoln Electric Automation Center ofExcellence.

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Fig. 7 Integrated weld controller and process monitor displays.

Fig. 8 Integrated narrow groove welding

system.

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TandemNarrowGrooveSubmergedArcWelding_Nov11