Process offers customizability, flexibility for engineered structural sections
June 1, 2010
Architects and engineers are designing structures with new and innovative shapes. To meet these ever-changing requirements, manufacturers may turn to solid-state, high-frequency electric resistance welding (HF ERW) to produce engineered structural sections at high speeds with unlimited beam profiles, better structural performance, and with lighter weights.
Architects and engineers are designing structures with new and innovative shapes due in part to new steel chemistries and improved flat rolling technologies. To meet these ever-changing requirements, manufacturers may turn to solid-state, high-frequency electric resistance welding (HF ERW) to produce engineered structural sections at high speeds with unlimited beam profiles and higher structural performance at lighter weights.
By controlling welding power and frequency, manufacturers of engineered structural sections (ESS) are welding different materials successfully, including carbon steel, stainless steel, advanced high-strength steel, weathering steel, titanium, and aluminum.
Until recently structural sections have been limited to the standard catalog of symmetrical shapes and sizes that are practical to produce by rolling hot ingot metal to the required shape. Designers, architects, engineers, and manufacturers, however, are looking for new ways to lower costs, improve product performance, and reduce lead-times.
Weld Quality. Solid-state, high-frequency welding for ESS in general requires minimal heat input, produces a narrow heat-affected zone (HAZ), and results in improved weld properties. The solid-state weld process uses less heat input because high-frequency (150-400 kHz) electromagnetic energy is used in conjunction with high pressure to join two materials. The material is joined under heat and pressure instead of melted together with a filler material. Controlling key welding process parameters, such as frequency, enables you to control the time and temperature, resulting in a narrow HAZ and improved weld properties in the weld zone.
The strength of the weld zone in solid-state welded structures is near that of the parent material, whereas the strength of fusion weld zones is approximately 70 percent that of the parent material for typical steels.
Flexibility. HF ERW has the flexibility to weld low- and high-strength materials, dissimilar metals, and various sizes and shapes.
Efficiency. With this process, producing ESS inline enables you to yield greater operational efficiencies when compared to traditional hot rolling lines.
Production Rates. The process can accommodate speeds from 15 to 30 meters per minute.
No Restraightening. Through the control of key parameters such as time and temperature, you can produce sections that are straight as produced off the line.
Welded beam has a degree of customization. In addition to asymmetric and custom geometries, since welded beam is processed continuously, it can be delivered cut to the required length, reducing the scrap, costs, and frequency of beam splices. This ultimately helps make fit-up easier while decreasing weld metal and labor time. The smaller tonnage requirements of high-frequency beam welding lines makes them more suitable for small- and medium-size beam markets. The flexibility of custom geometries helps producers meet various requirements of many different beam sizes in varying quantities.
In addition to custom sizing, the process helps reduce material costs because of the thinner cross-sectional area. These geometry changes have little impact on strength but dramatic impact on weight reduction.
ESS with HF ERW can help to eliminate the waste and inefficiencies inherent to fusion welding and the piece production processes of extrusion mills. To compete on a global basis, manufacturers require more process control and repeatability as well as less direct labor contact to maintain a viable business.
Welded ESS are formed from slit coils. For H- and I-beam sections, three slit coils are delivered to the coil ramp from the coil storage area for loading onto the uncoilers. The coil is delivered to the center of the uncoiler on a coil car which is operated by hydraulic motors on rails. The coil is loaded onto the uncoiler by means of a mandrel expanded by hydraulic power. A coil peeler flattens the leading edge of the coil so that it can be inserted into a pinch and leveler easily.
The shear and end welder cuts the leading and trailing edges of each coil at the same time and welds them together for continuous operation. The shearing is done by two shears, and welding is done by semiautomatic gas metal arc welding (GMAW) or gas tungsten arc welding (GTAW).
The three accumulators allow continuous flow of strip to the weld fixture, enabling the line to run without interruption. The three strips are then fed into the welding fixture, where the edges are upset to have a wider welding area. At the point where welding occurs, the proper squeeze pressure is provided by a set of squeeze rolls.
Two high-frequency welding machines transfer energy to the weld via copper contact tips. High-frequency current flows along the strip edge and subsequently heats those strip edges before they pass between the weld squeeze rolls. The top and bottom flanges are then forge-welded together, and any impurities within the molten metal are forced out during the solid-state weld process. This results in a clean weld zone at both the top and bottom flanges.
One common argument against the use of welded beam is that the irregularities that can occur within the squeezeout can result in localized crevice corrosion. Beam lines with bead conditioning systems solve this problem by removing the squeezeout as the beam is produced. Galvanization provides additional protection against corrosion both inside and outside of the welding area.
The bead conditioning system removes the excess weld flash, resulting in higher corrosion resistance, improved fatigue life, and a more aesthetic weld. Removal of the weld flash is critical for dynamically loaded and fracture-critical members.
The HF ERW sections are straightened in the beam straightening section, enabling the top and bottom flanges to be aligned. To further optimize the efficiency of the production process, the beam is cut to length inline.
Process technology has enabled ESS to be constructed of high-carbon steels, advanced high-strength steels, and all manner of materials for extreme applications. Recent advancements in welding technology have played a role in improving control of the HAZ around a solid-state weld. These HAZ control technologies broaden the possible materials and applications for HF ERW.
Popular uses for ESS are recreational applications, transportation, and both residential and commercial construction. Developing countries utilize ESS extensively in construction projects. The ability to forge-weld galvanized, stainless, and aluminum has led to their use in extreme environments, leading to much longer life spans.
Mick Nallen and Kris Livermore, Engineered Structural Sections, Thermatool Corp, East Haven, Conn., 2009.
Thermatool Corp. HF Welded Beams, Corrosion Resistance Assessment, East Haven, Conn., 2009.
A high-frequency welding line produces H-beams from three coiled steel strips with the following specifications:
Standard: ASTM A-6, A-36, JIS G3353 Material: Hot-Rolled Steel, Cold-Rolled Steel, Stainless, Dual-Phase Steels, HSLA
Web Thickness: 3~9 mm; Flange Thickness: 3.2~12 mm; Beam Height: 100~500 mm; Flange Width: 100~300 mm; Beam Length: 6 M~15 M
Electric resistance welding was modified and improved in the 1950s by Thermatool's Wallace Rudd. Rudd developed high-frequency welding and, in doing so, solved some of the inherent process limitations of DC welding and low-frequency welding, such as excess heat and low weld quality and speed. Today's high-frequency welders can produce a range of products, from titanium to corrosion-resistant steels used in oil and gas pipelines.
Recent innovations in variable-frequency welding have made HF ERW techniques available across a broad scope of materials and geometries. Now the same HF ERW fundamentals that have made the tube and pipe industry more efficient can be utilized for manufacturing ESS. Lower capital costs, improved efficiencies, and flexible production capabilities are especially important as today's global market is burdened with rising material costs and greater pressure from competitive international suppliers.