Function or Failure: Welded Connections in Transmission Structures
The structural integrity of steel transmission pole structures is only as good as their welded connections.
A tubular steel transmission pole generally consists of a series of welded assemblies. The pole shafts and arms are folded plates with seams joining the edges of the steel plates. The resulting tubular steel shafts can then be welded to flanges or other brackets to join them together. Insulators generally are attached to some type of plate welded to the ends of arms or pole shafts. The challenge with the assemblies that make up a typical tubular steel pole often seen by engineers and technicians during transmission line maintenance is that many of the welded connections have virtually no structural redundancy.
During a transmission line inspection, the person in the field will see whether or not the welded connections will either functionally perform as designed or be at risk of catastrophic failure. For example, picture the failure of a typical davit arm shaft-to-bracket weld or a pole shaft-to-base plate weld. These types of welds are commonly referred to by the welding community as fracture-critical welds. For this reason, welding these types of connections is the single most significant quality element in the manufacturing of steel transmission pole assemblies. Welding is one of the least understood elements of steel pole fabrication by most utility engineers and can cause catastrophic outcomes if not caught in the factory or during preventative transmission line inspection.
The American Welding Society (AWS) D1.1 Structural Welding Code‒Steel, is used most commonly by specifiers and fabricators of tapered tubular steel poles for their various welded connections and attachments. Because all welded joints are connections of steel parts, it is essential requirements of AWS D1.1 code, or other specified codes and requirements, be followed diligently during the welding of tubular steel pole assemblies.
Because welding is both an art and a science, it can be challenging to achieve consistently high-quality welds in tubular steel poles. Eight criteria typically define the most common challenges in producing high-quality welds in steel transmission pole structures.
The personnel who are performing the welding must be properly qualified and certified for the welding processes and positions required for the job. Four primary welding positions are used by welders when welding connections on tubular steel poles. In order of increasing difficulty, they are flat (F), horizontal (H), vertical (V), and overhead (OH).
The skill required of the individual welder increases as the difficulty of the position changes. Most steel pole shop welders are tested and qualified or certified in either the flat or horizontal position or both. If not properly qualified or certified to do so, attempting to weld in the vertical or overhead position would be a violation of AWS D1.1 Welding Code.
Also crucial to the quality of a welded connection is the welding process selected by the pole fabricator.
AWS Major Welding Processes:
- Shielded metal arc welding (SMAW)
- Gas metal arc welding (GMAW)
- Submerged arc welding (SAW)
- Flux-cored arc welding (FCAW).
All four of these welding processes are used to varying degrees in the fabrication of tapered tubular steel poles. For shop-welded connections on tapered tubular steel poles, the two more commonly used welding processes are FCAW and, more recently, GMAW. Both have unique advantages and disadvantages that must be considered.
Another important criterion of a quality weld, particularly for the fracture-critical welds that comprise most steel pole connections, is ensuring the weld joint has been fit up properly for welding. Joint tolerances are provided in AWS D1.1 or a weld procedure specification (WPS) for root opening, bevel angles, and weld capsize, among others. All such tolerances need to be followed carefully during joint fit-up and welding.
It is an AWS D1.1 requirement to prepare a highly specific WPS for welding any joint. Additionally, a procedure qualification record (PQR) will help to ensure the welded connection has the mechanical strength, ductility, and, if specified, Charpy toughness characteristics intended and necessary for the welded joint.
As stated earlier, welds are a dilution and combination of the base metals with the addition of filler metal (the welding rod or wire). If a WPS is qualified properly for use based on certain mechanical or chemical properties of the base metals or welding rod or wire, any significant variance of those mechanical properties can result in a change of actual performance of the welded joint. For example, the welding of a connection that has base metals with a significantly higher ultimate tensile strength or higher carbon equivalency than the plates used to weld a sample test for the procedure qualification records (PQR) could lead to significant changes in the ultimate performance of the weld. In turn, this could potentially lead to weld failure.
Joint distortion is another challenge related to welding steel pole connections and is seen time and time again during our in plant steel pole inspection projects. It is important to the welded joint’s integrity to consider and anticipate the distortion caused by the heat input of the welding process. Not doing so in highly restrained and constrained welded connections can create very high residual tensile stresses and potentially lead to weld cracking. The more highly constrained the weldment, the higher the residual tensile stress on the weld as it cools and shrinks.
Monitoring preheat, heat input during welding, interpass temperatures, and post-weld cooldown temperatures is an important consideration when welding the high-strength, low-alloy steels commonly used in steel pole connections. Such criteria are important because all those aspects influence the cooling rate of the weld as it solidifies. Cooling too fast may result in undesirable effects on the deposited weld’s mechanical properties and metallurgical structure, and the heat-affected zone (HAZ).
Other than notch toughness, the mechanical properties of the steel being welded will either increase or decrease with increasing heat input. However, notch toughness has been found to increase slightly and then drop significantly as heat input from welding increases.
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