Constructing a steel pole transmission structure typically requires a series of welded assemblies. These include folding the pole shafts and plates, and joining the edges of the plates with seams. The resulting tubular shafts are then welded to flanges or other brackets to join them together, and insulators are attached to some type of plate welded to the arms or pole shafts.
What makes all of these welding connections challenging is they can either perform as they were they were designed or they can be at risk for catastrophic failure. Welding is considered the most significant quality element in steel pole manufacturing, but it can be hard to understand.
We’re here to help make some of it clearer.
What Is Welding?
According to the American Welding Society, welding is defined as a joining process that produces a coalescence of materials by heating them to the welding temperature, with or without the application of pressure or by the application of pressure alone, and with or without the use of filler metal.
When it comes to steel poles, most welded connections use processes that add filler metal, which results in deposited weld metal consisting of a dilution of the base metals and the external filler metal. Welding processes must be properly engineered to produce consistent welds that meet the expected quality and mechanical properties. Additionally, it’s important to understand the strength of the deposited weld metal, as well as its effects on the adjacent base metal, also referred to as the Heat Affected Zone (HAZ).
There are a several challenges you’ll encounter with welding on steel pole transmissions. Here are the top eight to be aware of:
Challenge #1: Proper Certification
First and foremost, the engineers welding your connections must be properly certified for the welding process and positions the joint requires. The skill required for the individual welders will vary based on the positions used, whether flat, horizontal, vertical or overhead.
Challenge #2: Appropriate Process
You’ll need to ensure that the welding process used by the pole supplier is appropriate for the type of connection/joint being welded. All four of the major welding processes—Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), Submerged Arc Welding (SAW), and Flux Cored Arc Welding (FCAW)—are used to varying degrees in the fabrication of tapered tubular steel poles. The two most common processes are FCAW and GMAW.
Challenge #3: Joint Assembly
The joint must be assembled as designed and all tolerances for fit-up must be properly met to prepare the connection/joint for welding. There are joint tolerances provided on a typical welding procedure specification (WPS) for root opening, bevel angles, weld cap size, etc. that need to be carefully followed.
Challenge #4: Material Specifications
The materials being welded must be the ones specified and consistent in mechanical properties with the materials used for the WPS and the tested procedure qualification record (PQR). When you develop a very specific WPS for welding a particular joint and the subsequent testing of the WPS with a PQR, you can ensure the weld performs as it was intended to perform.
Challenge #5: Heat Distortion
You want to assemble welded plate connections in such a way that distortion caused by heat doesn’t create quality issues. With this, you need to consider whether the joint is a highly constrained weldment or a non-constrained weldment. The more highly constrained it is, the higher the residual stress as the weld cools and shrinks.
Challenge #6: Inspectability
Be sure that the quality of your welding will pass the necessary inspections. These inspections will depend on the type of welds and welded joints. Complete Joint Penetration (CJP) welds are expected to have 100% fusion of the plates in the joint and are tested by either ultrasonic or radiographic methods. Partial Joint Penetration (PJP) welds, on the other hand, are only expected to have a partial depth of fusion and are visually inspected for workmanship-related deficiencies.
Challenge #7: Heat Monitoring
It’s important to closely monitor pre-heat, heat input during welding, inter-pass temperatures, and post-weld cool down temperatures. Heat input from welding influences the cooling rate, which may result in undesirable effects on the mechanical properties and the metallurgical structure of the deposited weld and HAZ.
Challenge #8: Hot-dipped Galvanizing
The effects of hot-dipped galvanizing need to be monitored both during welding and after galvanizing. Shaft side, HAZ “toe cracks” can develop within these types of welded connections due to various residual stresses; be sure to watch for these so you can maintain high weldment quality.
Preserve Your Welding Connections
As demonstrated above, it is challenging to ensure high quality welds when fabricating connections in steel transmission poles. But, most of these challenges can be minimized by paying attention to industry standards, as well as the skill of your welders and quality of your materials.