FACT BOX
Benefits:
- High deposition rate
- High thermal efficiency
- Deep and safe penetration
- No spatter from the weld pool, which is covered by the flux layer
- No need for fume extraction
Limitations:
- No observation of arc and process during welding possible
Submerged Arc Welding (SAW) is a welding process in which the arc is contained beneath a covering layer of flux – hence ‘submerged arc’. The heat required to form the melt pool is generated by the arc formed by an electric current passing between a metallic wire, which is part of the electrode, and the work piece.
A flux hopper (see the illustration for single wire SAW) feeds the process with a layer of granulated mineral material known as flux, which covers the tip of the welding wire, the work piece and the arc. There is no visible arc, no sparks, spatter or fumes.
Submerged arc welding (SAW) is the most important welding process in today’s production of tubular wind turbine towers. Based on technical discussions with manufacturers, LORC Knowledge estimates that more than 90 percent of the total welding for offshore towers is performed by submerged arc welding.
In submerged arc welding a layer of flux covers the tip of the welding wire, the arc and the work piece, shielding the weld pool from contact with the atmosphere
Shielding and deposition rates
The substantial amount of heat produced by the passage of the electric current through the welding zone melts the top of the wire and the adjacent edges of the work-pieces, creating a pool of molten metal - the so-called weld pool. Shielding of the weld pool is mandatory because of the metals’ high affinity to oxygen. If unshielded, thick layers of oxides would form on top of the melt causing unacceptably poor welding qualities.
In submerged arc welding, the weld pool is shielded from contact with the atmosphere by the flux, which cools and hardens into a brittle, glass-like material as the welding zone moves along the joint. When cool, it usually detaches itself completely from the weld.
SAW is (principally) carried out with fully automatic equipment. Since the operator cannot observe the weld pool, great reliance is placed on parameter setting and positioning of the electrode. Yet an operator is needed for real time monitoring of the weld process i.e. to hear if the process sounds “normal” and to correct for joint misalignments. The process is considered as very efficient due to high welding speed and deposition rates, which describe the amount of welding material deposited per unit of time. Likewise SAW involves a high-energy gain, because of the isolating effect of the flux, which entails that a substantial amount of the supplied energy is transferred directly to the welding.
In wind turbine tower production, deposition rates of up to 50 kg/hour are common with submerged arc welding. However, the deposition rates vary based on wire type, range and the number of wires.
Electrodes
The electrode used for submerged arc welding is commonly a solid wire but recently cored wires have been introduced to increase deposition rates.
Electrode dimensions
The smaller the wire diameter, the higher the current density at the same applied current (amperes), because of a higher current density. This yields more deeply penetrating and narrower weld beads the smaller the wire diameter and/or higher currents, and consequently higher deposition rates, the larger the wire diameter.
Multiple electrodes
To increase productivity a multi-electrode arrangement can be utilized, which allows high deposition rates and welding speeds. In wind turbine tower production, six electrodes are considered to be optimum due to the curvatures of the welding areas. Thus, an arrangement with more than six electrodes would expand the weld pool uncontrollably, resulting in an increased risk of molten metal overflowing the banks of the weld pool. The multi-wired SAW variants are known as twin, tandem, tandem twin and multi-wire.
Single wire SAW (LORC 2011)
SAW variants
The simplest SAW variant involves one single wire as illustrated above. A solid core wire used for single-wire SAW typically has a diameter in the range of 2.0 - 4.0 millimetres, and deposition rates are up to 12 kg/hour.
For twin-wire welding, two wires are connected to the same power source. It offers up to 30 times higher deposition rates than single wire SAW, up to 15 kg/hour with a solid wire, and can be used at higher currents and speeds. The wire diameter range is typically 1.2 – 3.0 mm.
In tandem SAW, two wires are connected to their own power source, in contrast to the twin wire variant. The two wires are normally of diameters within the range of 3.0 – 5.0 mm (commonly referred to as large), and deposition rates are up to 25 kg/hour.
The tandem-twin process involves two twin wire heads placed in sequence, which according to the above implies that each head is operating two wires. The wires are typically 2.5 – 3.0 mm and deposition rates of up to 38 kg/hour can be archived. The tandem twin is typically used for circumferential welding of the wind tower sections.
In wind tower fabrication, up to six wires can be used together. In a six-wire arrangement, two of the wires are sometimes cold feed, i.e. not supplied by a power source. Thus, the heat from the weld pool provides the deposition of the two cold feed wires. Deposition rates of up to 50 kg/hour can be archived by this arrangement.