Application Stories

Structural Steel Welding: Automation Provides New Levels of Productivity
Geoff Lipnevicius, Engineering Manager, Automation Division

Since 2004, U.S. demand for structural steel used in the construction of buildings, power plants, petrochemical facilities, bridges and other projects has grown by 25% – providing increased market opportunities for the structural steel industry.

While facing other challenges, such as a skilled labor shortage and increased material and energy costs, fabricators are on the lookout for technology and processes that give them a competitive advantage in taking advantage of this growth. This includes the pursuit of welding productivity improvements and leveraging opportunities that combine equipment, consumables, advanced welding processes and automation.

Flexible automation (robots) offers numerous benefits to the structural steel industry – improved quality, productivity and manufacturing flexibility – and as a result, is becoming increasingly popular.

Automation Provides Improved Predictability of Manufacturing Costs

For the structural steel industry, robotic automation can increase predictability of actual costs. Robots provide the means to secure precision and repeatability and tightly control procedures. In combination with heavy-duty positioning equipment, they also often provide for improved part accessibility.

Though over-welding is common in manual processes, a robot can be programmed and qualified to deliver cost-effective, repeatable procedures that match the correct size weld to the load.

Where manual process positioning time is a hindrance for large fabrication, designers typically have provided for a single-sided bevel rather than a double-bevel to avoid labor-wasting positioning time. This actually doubles the amount of weld metal for the same effective throat. Robotic automation allows larger fabrications to be automatically positioned for easier accessibility and reduces welding time, resulting in the improved ability to reduce and control shrinkage stress.

Robotic Pulsed Spray Process for Excellent Weld Fusion Characteristics & Low Hydrogen Weld Deposits

Historically, gas metal arc welding (GMAW) has been associated with incomplete fusion defects in the structural steel industry. Pulsed spray metal transfer (GMAW-P) takes advantage of the high energy of axial spray metal transfer and alternates this high energy (peak) current with a lower energy (background) current. Many aspects of the GMAW-P waveform can be controlled, and the benefit of the pulsed energy is that it produces excellent weld fusion characteristics and considerably reduces the heat input. The dynamics of the pulse also permit the use of GMAW-P for out-of-position welding. Out-of-position welding, coupled with lower heat input, assists in achieving lower dilution rates, excellent weld metal mechanical properties and improved Charpy Impact test values.

Where hydrogen-induced weld cracking is an issue, the lower hydrogen weld deposit of GMAW-P (<5 mL H(2) / 100 grams) is also an excellent choice.

GMAW-P typically provides higher efficiency metal transfer (98%) for solid or metal-cored electrodes. Comparably, the lower heat input of the GMAW-P process can result in lower weld fume generation, helping to meet EPA and OSHA standards.

Synchronized Tandem MIG Process for Increased Productivity

The dual-wire synchronized tandem MIG process continues to gain popularity as a means to increase production in automated arc welding applications. The process follows early industry trends of reducing welding costs by developing dual-wire processes for greater productivity. Early developments in multiple wire welding focused on the submerged arc process. The availability of high-powered inverter power sources has enabled dual-wire welding using the GMAW and GMAW-P processes.

Since the introduction of tandem MIG in the early 1990s, the estimated installed base of dual-wire systems has grown to more than 1,500 units worldwide. The majority of systems have replaced single-wire processes that had been pushed to the extreme high end of the useable operating range in an attempt to improve productivity and lower cost by depositing as much metal in the shortest time frame possible. Synchronized tandem MIG extends the welding productivity range beyond what is possible with conventional single-wire processes.

The synchronized tandem MIG process employs two electrically isolated wire electrodes positioned in line, one behind the other, in the direction of welding. The first electrode is referred to as the lead electrode, and the second electrode is referred to as the trail electrode. The spacing between the two wires is usually less than 3/4” so that both welding arcs are delivering to a common weld puddle. The function of the lead wire is to generate the majority of the base plate penetration, while the trail wire performs the function of controlling the weld puddle for bead contour, edge wetting and adding to the overall weld metal deposit rate.

The synchronized tandem MIG process can, on average, represent a 30-80% increase in deposition potential when compared to conventional single-wire processes

Synchronized tandem GMAW enjoys expanded use in girder fabrication in the offshore industry segment for several cost-effective reasons, including higher deposition rates, faster travel speeds, lower heat input and reduced distortion. The lower hydrogen deposit makes it a primary choice for use on high-strength low alloy or thermo-mechanical controlled processing (TMCP) type steels. And its use on complete penetration type welds and joining the web to the flange eliminates the need for back-gouging operations.

The system components specified depend upon the level of automation. Automated side beam delivery, tractors, sidecars and welding bugs are all involved. In some cases, the use of robotic welding automation, which both tracks and welds web to flange connections for girder fabrication, is quite viable.

AC/DC Submerged Arc Productivity Advantages Applied to Robotics/Automation

Submerged arc welding (SAW) that combines the advantages of AC and DC SAW welding was not possible until a few years ago. This technology now is increasingly applied to structural steel automation applications.

The latest technology provides for control over the ratio of positive to negative amplitude, as well as the amount of time spent at each polarity. The limiting factor for SAW AC welding is that it takes too long to cross from electrode positive (EP) to electrode negative (EN). This lag can cause arc instability, penetration and deposition problems in certain structural applications. AC/DC SAW solves this problem by controlling amplitude and frequency, allowing the automated process to take full advantage of the reduction in arc blow experienced with AC, while maintaining the penetration advantages of DC positive and the advantageous deposition rate of DC negative. Using these controls, the shape of the output waveform is changed, and in turn the welding characteristics are controlled. With AC/DC submerged arc welding, you get the best of both worlds: the speed, deposition rate and penetration that DC SAW offers and the resistance to arc blow that AC SAW offers.

Inspection/Robot Intelligence Trends

Vision is becoming an increasingly important component of many automation opportunities in the structural steel industry, and the integration of vision to robotics has been made easier and more cost effective in recent years.

Robots can use a vision sensor to “see” the location and orientation of parts, examine and verify part fit-up, find features pre-weld, measure the joint position, detect what is going on ahead of the arc, provide for real-time seam tracking and signal changes to user-defined process parameters using adaptive parameter control. Laser vision systems also are commonly used for multi-pass welding sequence management (some offshore platforms require as many as 70 passes) and can also be used for error proofing.

Error proofing in automation relates to the ability of a system to either prevent an error in a process or detect it before further operations can be performed. Error proofing can be performed on every weld in a process or to monitor critical welds of a process.

Advanced Technology Equipment Provides for Production Monitoring

Robots are increasingly integrating digital technology to network welding equipment and bring data from the factory floor to the business arena. Production monitoring enables any networked power source to be set up so that weld data can be monitored, files can be stored and shared, production tasks can be monitored, weld limits and tolerances can be set, consumable inventory can be tracked, welding machine faults can be logged and e-mailed, and diagnostic troubleshooting can be performed remotely.

Conclusion

There are many new opportunities to capitalize on technology to identify cost saving approaches to project design and construction. If you are welding manually, consider automation to improve your process.

Robotic Welding Helps ConXtech Automate Structural Steel Construction

In the fall of 2000, Robert J. Simmons, a 30-year veteran of the structural steel industry, developed a concept for constructing mid-rise residential structures using a steel moment space frame system. From this concept, Simmons founded ConXtech, a company that completes all of its fabricating work in-house and then simply assembles the column and beam components on the construction site by bolting them in place.

Unlike typical structural steel construction, which usually takes seven to eight months using traditional methods, the steel moment space frame system allows ConXtech to slash structural steel erection time to less than two weeks.

Robotic welding systems used in ConXtech’s Hayward, Calif., shop are a critical factor to the company’s success. Compared to ConXtech’s earlier semiautomatic welding operations, the robotic system offers faster travel speeds, high deposition rates and superior quality finished welds.

When welded semi-automatically, it took 40 minutes to weld one collar piece to a beam. Because there are two ends to each beam, this equates to one hour and twenty minutes of welding per beam. With the robotic system, the cell is able to weld collar pieces to both ends in only five minutes and thirty seconds.

Beams, composed of A992 structural steel, are joined to the A572 Grade 50 collar pieces. The plate requires the use of full-penetration welds on the top and bottom flanges and fillet welds on the beam’s web and the back side of flanges. The 24 inches of full penetration welds on each beam are made in four passes, while the 64 inches of fillet welds are completed in a single pass. Past welding projects include welding to the AISC seismic provision as well as applications subject to FEMA 353 guidelines.

Structural Contractor Awarded Grand Canyon Skywalk Work Based on AC/DC Submerged Arc Technology

Suspended 4,000 feet above the Colorado River, a new horseshoe-shaped walkway extends 65 feet from the cliff edge of the Grand Canyon’s western rim. The glass floor and sidewalls ensure heart palpitations to anyone vaguely troubled by heights.

Before the project began in 2004, Mark Steel Corp. of Salt Lake City bid on the fabrication work knowing the competition would be fierce. However, engineers for the steel structure and heavy plate shop needed to speed productivity in their existing submerged arc setup to meet the project’s tight timetable. Mark Steel had been using a DC submerged arc setup for jobs of that scope with typical results. Discovering the Power Wave® AC/DC 1000 from The Lincoln Electric Company, Utah’s largest fabricator learned that a tandem arc setup, one in AC and another in DC, could boost their productivity welding more than a million pounds of steel for the Skywalk.

The main horseshoe itself was formed from two box girders of A572 grade 50 carbon steel. The fabrication is performed in accordance with the structural welding Code of AWS D1.1. The girder sections are 2 inches thick, 6 feet long and 2.5 feet wide. They were shipped in 40-foot sections and assembled on site. While welding the box girders, productivity gains were captured mostly by the tandem submerged arcs.

Deposition rates increased from about 28 pounds per hour with the lone DC set up to about 55 pounds per hour, using 3/16th wire on two arcs. This proved particularly helpful for some of the longer welds, which ran 38 to 40 consecutive feet. Ultrasonic testing revealed the project’s weld reject rate at less than two percent.

The shops fabricators had typically beveled material of this size 30 degrees on each edge to form a combined 60-degree bevel at the joint. Now with greater penetration abilities, the bevels have been reduced to 22.5 degrees at each edge to form a 45-degree total wedge. This narrower gap allowed reduced prep time and grinding with less weld metal required per inch of weld. Overall, Mark Steel saw a productivity gain of 25-30% and a corresponding reduction in consumable cost. The company also realized a 10-15% reduction in electrical costs using the inverter-based equipment.

The Grand Canyon Skywalk is now the highest man-made structure in the world, built with more than one million pounds of steel. It was designed to withstand an 8.0 magnitude earthquake 50 miles away. It is equipped with three oscillating steel plates, each 3,200 pounds, inside the hollow bridge beams that act as shock absorbers. They move up and down to neutralize vibrations from foot traffic and wind gusts. Set atop the box girders, the walkway itself is constructed of 3”-thick, heat-strengthened glass.

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