In the case of flat-position weld overlay, several processes enable high productivity levels to be achieved: pulsed MIG welding, solid-flux welding or even variants with foil, and plasma welding with deposition via filler wire or powder. However, several particular aspects plead in favour of TIG welding. In fact, when a significant quantity of metal is to be deposited, TIGer technology enables pipe cladding deposition rates approaching 6 kg/h to be achieved with a single torch and up to 10 to 12 kg/h with a double torch for flat cladding applications. Let’s see what aspects of this technology are to be considered.
Advantages of TIG Weld Overlay in pipe cladding
Weld overlay, or cladding, involves covering the ‘wetted’ surfaces of a workpiece (those exposed to hostile media) with a protective layer, usually of stainless steel or nickel-based alloy.
The use of TIG welding in this clad welding process has certain advantages:
- The accuracy of the process when welding overlay complex surfaces, especially when welding overlay zones with corners, curves or inside or outside edges, avoids the need for manual touch-ups.
- The separate control of arc energy and the quantity of metal deposited is advantageous when cladding tricky geometries.
- Welding energy transferred to the workpiece is controlled, optimising the ratio of welding energy-to-deposition rate according to workpiece typology.
- The welding sequences can be interrupted and resumed at any time, avoiding microcracks and craters at the start and end of beads.
In TIG weld overlay, programming of precise parameters: cladding speed, currents, voltage, wire feed speed and preheating temperature, allows for greater control of the dilution rate (characterised by the mix of molten filler material with parent metal) thereby guaranteeing correct chemical composition of deposits.
The position welding capability of TIG cladding, gives operators greater ﬂexibility and versatility with workpieces, which cannot always be suitably arranged for ﬂat-position welding. In TIG pipe cladding, due to the ﬂexibility of the process, it is possible to work with a very wide energy range with welding currents from 80 to 450 A. Ease of automation makes it possible to work with several torches, or wires to achieve productivity without affecting operability and the quality of the deposit. In TIG cladding, all the conventional functions used in TIG welding remain fully applicable (AVC, oscillation, synchronisation of movements with the welding currents, etc.).
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Weld overlay –Technique and Sequence
The coating of pipes in clad welding requires the deposition of various layers, usually two but for operations such as in the Nuclear industry, more than three may be required.
Configuration of Components
Use of torch conﬁgurations with a wire feed from the front or side avoids exposing support materials to the direct action of the arc. This arrangement has the advantage of reducing parent metal fusion, as the ﬁller metal is positioned as a screen to capture most of the heat energy.
In welding overlay, wire feed from the weld pool side is possible, which simpliﬁes torch design with the wire sheath running parallel to the torch body over the entire length up to the point where it enters the wire guide. The wire angle in the weld pool inﬂuences dilution, it is therefore better to work with wire feed angles of 60 to 70°, where interaction between welding and wire heating currents is neutral. Other factors in limiting dilution are welding and electrode position.
Number of layers
Layers are deposited by making multiple passes, during which control of dilution is vital.
Dilution in the ﬁrst layer between the ﬁller metal and the parent metal does not generally allow the expected properties for the coating to be guaranteed. Two cases are generally to be considered in deﬁning the number of layers required:
- Where the deposition alloy can be in direct contact with the parent metal, then two layers sufﬁce. For example, an unalloyed steel coated with a nickel-based alloy.
- Welding metallurgy recommends the use of an intermediate layer to control the percentage of ferrite and limit the effects of ﬁssuring. Consequently, the second layer (corresponding to the deposition grade) must be used with a third layer, which can then be considered as all weld metal.
Welding overlay operations used in manufacturing and maintenance
Many industrial processes cause damage to piping and fittings by the continuous assault of corrosive and abrasive media, together with elevated process temperature and pressure. To increase the endurance of highly exposed parts, pipe surfaces are protected by clad welding with more resistant materials.
Cladding of piping and manifolds is common in applications in gas and oil, the petrochemical industry and for power plant equipment; in the aeronautic industry, components for aircraft engine are also often reinforced using this process.
TIG weld overlay applications
Weld overlay operations using the TIG process generate good results in the following types of application :
Part repair operations
These concern repairs on worn parts (usually stainless steel), in service or following a manufacturing anomaly. The parts may require a machining or equivalent operation to eliminate the damaged areas, followed by a cladding phase by redepositing materials identical to the composition of the substrate. Welding does not fundamentally change the metallurgical characteristics of the parent materials but does increase the stress level in the repaired part. This type of repair is common for piping used in the nuclear reactor environment. Parts scrapped as a result of machining anomalies can also be restored by cladding.
Preventive weld overlay for new pipe manufacture
The manufacturing cycle of new parts comprising coated bearing surfaces, end pieces or grooves is a conventional process, to protect pipe from corrosion and wear. Pipe cladding is performed in a workshop with dedicated means and greater ﬂexibility for performance (rotating parts, heat treatment, machining adjustment, etc.).
This involves forming a deposit, not to protect the parent metal from an external hazard, but rather to create a metallurgical transition zone with the aim of making a heterogeneous joint. Buttering plays an integral part in the mechanical strength of the weld once the joint has been made.
Who we are
Polysoude specialises in the design, development and manufacture of innovative arc welding solutions. The brand is synonymous with world-leading expertise in 3 core sectors: automatic orbital TIG welding systems, automated TIG and plasma welding solutions and TIGer™ weld overlay.
Situated in Nantes since 1961, the company manufactures a variety of power sources, weld heads and weld overlay equipment, like a wide range of open and closed orbital weld heads.
Nowadays, as a smart factory oriented company, Polysoude products are all designed to be ready for Industry 4.0.