by Nestor Gula
Two tungsten electrodes produce smoother arc for weld cladding
Combining two TIG electrodes to provide better heat to the weld overlay process, Polysoude’s TIGer system boasts deposition rates of up to 5.8 kg/hr. A variant of TIG hot wire welding, “we combine two tungsten electrodes into the same torch. This new torch, developed by Polysoude, creates a very specific arc,” says Hervé Penisson, sales manager–automated welding solutions for Polysoude SAS. “This new combined arc is very soft, giving quite a low penetration, so the system will not melt too much of the base material because we are adding the weld overlay by wire. Because of the low penetration of the arc, it gives very good results in terms of dilution.”
Generally, this process is called cladding but Penisson points out that it should be properly called weld overlay. “Cladding is any kind of coating. You can have a plastic coating. This could be called cladding as well. So, at Polysoude, we are fully involved only in weld overlay, which means making this extra surface by using welding solutions.”
The process has many uses. “Cladding can be used to add thickness. It can be used to add a layer inside or outside of a pipe for corrosion resistance,” says Michael McGuire, general manager at MAG Tools, Polysoude’s distributor in Canada. “It can also be used in a process called buttering, which is basically a cladding process where you clad the end of a carbon steel pipe with Inconel material. Then you can weld a stainless steel pipe to that carbon pipe. In other words, joining dissimilar metals.”
Many North American companies are using submerged arc or electro slag systems when performing weld overlays due to the speed of these systems, according to McGuire. “With Polysoude’s technology, you can clad, in some cases, faster than submerged arc at a way better quality. This reduces the need for secondary machining processes, and the system also achieves the dilution percentages that people need in industry at a faster level.”
A nickel alloy, Inconel 625, is the most common substance used for weld overlay cladding worldwide. Used for pipes, valves, fittings, and other applications in the oil and gas industry, “it provides really good corrosion resistance for a variety of chemicals or physical abrasives,” according to McGuire.
One of the benefits of Polysoude’s TIG system is that it uses the TIG process to create the weld overlay. Although the TIG process is usually considered slower and more labour intensive, “the TIGer system uses the TIG process because it has a very high quality of overlay,” says Penisson. “It has a very good approach to the base material. We can adjust all the parameters independently thanks to the TIG process–so we can adjust the thickness of the layer that we want to achieve. We can add many layers. We can keep a very flat face at the ends, which avoids depositing too much expensive material. This also saves time and effort by eliminating the secondary process of removing this excess material by machining it. If the surface is not very flat, you have to put more effort in to remove the material. The TIGer system has a deposition that is of very high quality in terms of dilution, compactness and flatness.”
Thickness of the overlay can be manipulated and layers from 1.5 to 3.5 mm can be achieved. The soft dual tungsten arc of the system ensures that the “penetration is limited to about a few tenths of a millimeter depending on the power we put into the arc–the current and the voltage,” says Penisson. “Most of the time it is between 0.2 and 0.5 mm.”
The ability to adjust layer thickness presents significant cost savings because you are not wasting expensive filler material that then must be machined to reach the desired specification. Polysoude’s overlay system creates surfaces which rarely, if ever, need machining to get the optimum surface condition.
The system also uses a hot wire TIG process that increases the performance of the system and creates a better weld surface. “The hot wire allows us to energize the wire so by the time it hits the welding pool it’s already molten,” says McGuire. “We can then increase our deposition rates as well as decrease our heat input and better our dilution percentages.”
The weld overlay cladding systems come in either horizontal or vertical systems with various roller systems and turntable units to suit the specific job. “One other advantage of our system compared to other processes is that we can clad in any position,” says Penisson. “Then, if you have some big workpieces that you cannot rotate or you cannot move easily, Polysoude’s system can have the torch moving around or inside the workpiece in any position, whereas on some of the other processes you need to always be welding in a flat position.”
The system is flexible in that you can rotate the pipe if that suits the customer’s preference, he adds.
The weld overlay cladding process can be monitored using an optional video system which consists of one or more cameras positioned close to the torches. The operator can view the melting of the wire and the deposition. The camera can be an external type with its own cooling mechanism or a micro-camera integrated into the body of the torch. SMT