SAW welding, Submerged Arc Welding, is an efficient welding process especially suitable for welding thick plates. The process is not sensitive to welding defects, and it is also a safe process for the operator.
In this article, PEMA Welding Engineer Jari Tervolin defines the SAW welding process, discusses its benefits, and explains why it is such a suitable process in the wind energy industry.
SAW welding process: User-friendliness, quality, and efficiency
SAW welding equipment consists of a power source, welding torch, flux feeding, recover and recycle system, wire feeder, wire spool or drum, seam tracking (electromechanical or laser), WeldControl 100 or 500 user interface, movement with for example PEMA Column & Boom or platform, the workpiece handling with e.g., rollerbeds or positioners. Also, induction preheating is used in many cases.
In the SAW welding process, the welding flux protects the arc and molten puddle from the air. A part of the flux melts solidifies and forms a slag on the weld bead. The welding flux also improves arc stability and electrical conductivity. Additionally, the alloying elements of the flux improve the welding metallurgy.
After welding, the rest of the welding flux is recovered and recycled, and the slag is removed. In circumferential welding, the slag is removed by gravity.
Thanks to the protection of the flux, high current, and deep and wide penetration, the weld quality is excellent, and SAW process is not sensitive to welding defects such as lack of fusion, lack of penetration, porosities, or spatters.
“SAW process is really the best fit for thick plates, simple and long welds such as circumferential and longitudinal welds. The process has high efficiency, quality, and operational reliability”, explains Jari Tervolin, PEMA Welding Engineer.
Besides the good weld quality and high reliability and functionality, the SAW welding process is also an ergonomic and safe process for the operator: the welding flux does not only protect the arc and molten puddle, but it also protects the operator – there is no need to wear a mask. The SAW welding process does not expose the operator’s eyes or skin to radiation, and there are no harmful welding fumes.
SAW welding in the wind energy industry
Above mentioned straight and thick plates can be often found in, for example, wind energy towers. Even though SAW welding is not the only suitable process in the wind energy industry, in most cases, it is the best solution. SAW welding process is especially suitable for welding longitudinal and circumferential welds due to its deep and wide penetration and high deposition rate. Furthermore, SAW welding can be carried out with multiple wire electrodes.
“SAW welding process is a very suitable process in the wind energy industry. The thickness of the plates is typically over 10-12 millimeters for onshore and 50-160mm for offshore wind towers. SAW welding process enables the best efficiency and welding quality in circumferential and longitudinal welds of wind energy towers,” explains Tervolin.
Additionally, the SAW welding process can be integrated with welding robots, which enables the welding of diverse workpieces. The process performance and deposition rate can also be enhanced with different variations, discussed in the next chapter.
Enhancing the SAW welding process
The tandem arc SAW welding, with a long stick-out and specially developed tiltable welding head, enables an increased deposition rate and narrower groove by filling welds with large tubular production.
For example, in the manufacturing of offshore wind turbines, when material thicknesses are typically in the range between 50 and 160 mm, the whole process needs to be optimized. Groove geometry should be optimized to minimize the amount of filler material required while maintaining the reliability of the welding process and weld quality. In this way, fewer weld beams are required, thus reducing lead times, deformations, and welding stresses.
When the wire stick-out is longer, Joule heating (I2 x R) pre-heats the wire more, causing it to melt faster, and the deposition rate increases without increasing the heat input. Pemamek has also developed a tiltable welding head for semi-narrow gap grooves, which enables better tolerance for wire positioning and the shape of the welded pass. It enables the grooves to be narrower, and multiple-pass welding can be finished with a smaller number of passes. The increased deposition rate and increased size of the weld pass can be handled better with PEMA tiltable semi-narrow gap welding head.
Milling machines enable various benefits
Milling is crucial for all sectors that have heavy welding in their production: it increases production efficiency, saves welding time, and significantly improves welding quality.
Pemamek’s milling machines are designed for circumferential and longitudinal welds – with milling, a semi-narrow groove decreases groove volume, so a smaller amount of weld passes is required, and consumables can be saved. Also, deformations and tensions are lower. Automatic depth controlling enables the groove to remain constant. After the inside weld has been welded in wind tower manufacturing, the outside is milled a little bit over the inside weld and the root is opened so back gouging can be avoided.
“Semi-narrow groove significantly decreases groove volume and total welding time, and back gouging is not needed anymore. A defect rate can be also decreased using the milled semi-narrow groove“, says Tervolin.