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MODERN STEEL CONSTRUCTION may 2009

welding

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WhILE U.S. DEMaND fOR STRUCTURaL STEEL has

increased signicantly over the last ve years, construction

spending prospects have declined rapidly or the beginning o2009 as the credit reeze recession spreads rapidly through most

economies around the world. Now that the economic stimulus

plan has passed, questions remain as to how soon we might begin

to see the inrastructure projects moving, and whether they will

be sucient in sparking a turnaround or the economy.

The inrastructure section o the package includes unding

or building and repairing highway bridges, expanding transit

systems, upgrading airports and rail systems, and building and

repairing ederal buildings. Lawmakers indicate the some o the

projects could be up and running within three to six months,

spurring steel abricators to keep a vigilant eye out or technol-

ogy and processes that give them a competitive advantage in tak-

ing advantage o this stimulus. This includes the pursuit o weld-

ing productivity improvements and leveraging opportunities that

combine equipment, consumables, advanced welding processes,

and automation.

Improved Predictbility

Construction material prices have varied signicantly since

late 2003 creating signicant challenges or project cost estima-

tors. The downturn in construction demand may result in a sta-

bilization o construction material prices in 2009. Yet only 30%

to 35% o the abricated and erected cost o structural steel is theactual cost o the material. Thereore, it remains very important

to be able to accurately predict the cost o shop activities. The

use o robotic automation can increase predictability o the vari-

able shop costs. Robots provide the means to secure precision

and repeatability via tightly controlled procedures in combina-

tion with heavy-duty positioning equipment. They also provide

improved weld joint accessibility and numerous other benets to

the structural steel industry, including improved quality, produc-

tivity, and manuacturing fexibility. While over-welding is com-

mon in manual processes, a robot can be programmed and quali-

ed to produce cost-eective, repeatable procedures that match

the correct weld size to the load that is to be applied, reducing

waste.

Designers have typically provided or a single-sided bevel

rather than a double-bevel to avoid labor-wasting positioning

time, which can be a signicant hindrance or large abrication.

However, this actually doubles the amount o weld metal or the

same eective weld throat. Robotic automation allows or larger

abrications to be automatically positioned or easier accessibility

and reduced welding time, resulting in the improved ability to

For structural steel welding, automation oers a

viable path towards efciency.

By Geo Lipnevicius

Sop

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may 2009 MODERN STEEL CONSTRUCTION

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reduce and control shrinkage stresses and reduce overall

cycle times.

Robot Intelligence Trends

Vision is becoming an increasingly important compo-

nent o many automation opportunities in the structural

steel industry. The integration o vision to robotics has

been made easier and more cost-eective in recent years.

Robots can use a vision sensor to see the location

and orientation o parts, examine and veriy part t-up,nd eatures pre-weld, measure the joint position, detect

what is going on ahead o the arc, provide or real-time

seam tracking, and signal changes to user-dened pro-

cess parameters using adaptive parameter control. Laser

vision systems are also commonly used or multi-pass

welding sequence management and can also be used or

error proong.

Error proong in automation relates to the ability o

a system to either prevent an error in a process or detect

it beore urther operations can be perormed. It can be

perormed on every weld in a process or to monitor criti-

cal welds o a process.

automted Monitoring

Robots are increasingly integrating digital technology

to network welding equipment and bring data rom the

actory foor to the business arena. Production monitor-

ing enables any networked power source to be set up so

that weld data can be monitored, les can be stored and

shared, production tasks can be monitored, weld limits

and tolerances can be set, consumable inventory can be

tracked, serial number traceability can be perormed, and

diagnostic troubleshooting can be perormed remotely.

Excellent fusionHistorically, gas metal arc welding (GMAW) has been

associated with incomplete usion deects in the struc-

tural steel industry. Pulsed spray metal transer (GMAW-

P), however, takes advantage o the high energy o axial

spray metal transer and alternates this high energy (peak)

current with a lower energy (background) current. Many

aspects o the GMAW-P waveorm can be controlled, and

the benet o the pulsed energy is that it produces excel-

lent weld usion characteristics and considerably reduces

the heat input. The dynamics o the pulse also permit the

use o GMAW-P or out-o-position welding. Out-o-

position welding, coupled with lower heat input, assistsin achieving lower dilution rates, excellent weld metal

mechanical properties, and improved Charpy Impact test

values.

In semi-automatic pulse applications, an operator can

manually adjust the torch height to adjust or variation.

In automated applications, to control the length o the

arc despite changes in the contact tip-to-work distance

(CTWD), welding equipment with adaptive control is

required. Adaptive control adds energy to the arc as the

CTWD decreases, and it takes energy out o the wave-

orm as the CTWD is increased. The adaptive loop pro-

vides stability to the arc length, and increases the overall

usability o the pulse waveorm in automated structural

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MODERN STEEL CONSTRUCTION may 2009

applications, especially when xturing is

prohibitive.

Where hydrogen-induced weld crack-

ing is an issue, the lower hydrogen weld

deposit o GMAW-P (