TPT March 2025

ARTICLE

Thermatool Corp

One drawback of contact welding is the outer diameter (OD) marking that it leaves on the surface of the tube. In critical applications, the presence of copper deposits on the tube surface raises concerns about corrosion. Therefore, most American Petroleum Institute applications typically require additional processing to eliminate any surface marks. Producing mark-free pipes with HF contact welding is possible; however, it requires careful setup and operation. Lastly, it is important to point out that HF contact welding is better suited for applications with no accumulators (“coil to coil” mills). The HF contact process allows for easy lifting of contact tips when re-threading the mill and allows the tail of the strip to be welded just a few inches from the end. This means that most of each strip can be converted into a tube that is exactly its length, minimising scrap. As a result, the length of the tube or pipe produced is directly related to the length of the strip, making the HF contact process practical and efficient for maximising yield. Applications The HF contact welding method is commonly used for large diameter, thick-walled products. Thermatool welders in contact mode can operate at up to 1000kW and weld materials with dimensions of up to 26" OD and a wall thickness of 1". Although we have extensively examined the efficiency of contact welding – particularly for larger diameter, thick-walled tubes, and pipes – certain applications can only be HF welded using the contact method. For instance, the high frequency contact method is essential for welding engineered structural sections or more commonly referred to as H or I beams. Until recently, structural sections were limited to a standard catalogue of symmetrical shapes and sizes that could be practically produced by rolling hot ingot metal into the required form. However, engineers and manufacturers are now seeking innovative ways to lower costs, enhance product performance and reduce lead times. Engineered HF beams can be welded using the contact method, enabling the successful joining of various materials, including carbon steel, stainless steel, advanced high-strength steel, weathering steel, titanium and aluminium. HF contact welding also enables the welding of adjustable web and flange sizes, as well as various gauges. One special application of HF contact welding is spiral fin welding. This technique is frequently used to join fins to a base tube. The process involves passing an electrical current through the interface where the fin meets the tube, which generates localised heating and melting. Pressure is then applied to combine the materials, resulting in a solid-state weld joint. This method is efficient, reliable and creates a strong bond. In recent years, advancements in HF contact welding include not only new material grades used for contact tips but also new contact designs.

(OD), which often requires multiple coils with varying inner diameters (IDs) when running a wide range of tube sizes. For larger tube diameters, using the contact welding method has another benefit: the use of an impeder is optional. While an impeder can help direct the current along the strip edges it can also complicate the setup. Furthermore, an impeder is not as beneficial when welding large diameter tubes in contact mode due to shorter vee lengths and a larger ID circumference (Figure 3) .

Note: Vee length extends from point of welding to sliding contacts Figure 3: Schematic showing HF contact weld area An impeder introduces another variable to the weld area setup and serves as an additional consumable; if it is not properly cooled, it must be replaced frequently. By eliminating this variable, the mill can avoid unnecessary downtime due to impeder replacement. However, HF contact tips must be replaced as they wear out. The contact tip, which makes direct contact with the tube, is chosen for its current-carrying capacity, wear resistance qualities and compatibility with the material being welded. Traditionally, copper was favoured as the tip material because of its superior current-carrying ability, but it is a soft material that wears quickly. Additionally, the use of copper contact tips can result in copper deposition due to arc burns. To improve wear resistance copper alloys with additions of tungsten and tungsten carbide are now commonly used. Silver, known for its excellent conductivity, is also utilised for contact tips, with silver-tungsten alloys being favoured in critical applications where outer diameter marking is a concern. However, silver is considerably more expensive than copper. Proper selection of contact tips ensures improved weld quality, reduced maintenance and extended tip operational life. The use of advanced materials and coatings in HF welding is expanding, with innovations that enhance the performance and durability of contact tips. Proper cooling of contact tips is essential to extend their lifespan. Another factor that can help optimise contact wear is the application of constant pressure and the use of correct tilt angle.

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MARCH 2025