African Fusion talks to Renttech’s Johan Bester about self-shielded flux-cored wires, which can be an ideal substitute for shielded metal arc/stick electrode welding for on-site construction projects.
Self-shielded flux-cored welding or self-shielded gas metal arc welding (FCAW-S) was invented in the mid-1950s by Lincoln Electric as a way of mechanising and increasing the welding speed of the manual stick electrode welding process (SMAW). This also made it possible to weld outdoors without the necessity for gas as a shielding medium, but still using the same equipment as used for GMAW (MIG/MAG) welding.
These wires and the associated process were already perceived as potential substitutes for SMAW. “In addition, FCAW-S was seen as a way of taking the benefits of MIG/MAG outdoors,” Bester tells African Fusion.
When argon gas became more readily available in the 1970s, the argon+CO2 mixed shielding gases emerged for GMAW, offering spectacular weldability in comparison to using 100% CO2, which was prone to excessive spatter. But this did not help to make the GMAW process suitable for use outdoors in windy conditions.
While GMAW-S should have become the ideal outdoor solution, more than 60 years later SMAW still predominates for outdoor construction in South Africa.
“Today, while the self-shielded process has gained popularity in certain niches, little is generally known in industry about how far the self-shielded flux-cored wires have come in the past few decades, or the variety that is now available. We show people the process on a daily basis, most of whom have never seen it before – and they are amazed at the ease of use and weld quality that this process offers,” Bester says.
Bester believes that stick welding can be a challenging process. “To my mind, TIG welding requires the most skill from an operator, but stick welding is not far behind. Also, stick welding, due to the limited length of the electrode, involves a lot of stops and starts that can create potential defects if a welder applies incorrect technique. From a single electrode, you can deposit between 100 mm and 200 mm of a weld bead. This results in up to ten stop starts per metre when using the SMAW process,” he says.
“The FCAW-S process requires less skill and 600 mm to 1.0 m weld lengths can comfortably be completed with welders only needing to stop for repositioning. I have seen skilled welders who can weld for up to 3.0 m in this manner. The risk of weld flaws due to stops and starts thereby reduces, and quality welds are easily achievable, ” Bester argues.
An additional benefit is gained when considering electrode efficiency. SMAW has an electrode efficiency of between 55 and 65%. This means that at least 35% of the weight of every electrode ends up in the stub or as spatter, slag or smoke. FCAW-S has an 85% electrode efficiency, so you get 20 to 30% more weld metal from the wire consumable as compared to electrodes,” Bester estimates.
From a productivity point of view, non-welding activities such as grinding each stop-start, changing electrodes, and removing slag reduces the operational efficiency of the welder for SMAW to between 15 and 35%, depending on how much handling is needed. This means that the welder can spend up to 85% of his day on activities other than actually welding. The operational efficiency for semi-automated processes, such as FCAW-S, can be significantly better, between 35 and 50%, similar to those achievable with gas-shielded processes such as GMAW; but with the added benefit of being able to achieve this increase in productivity in an outdoor environment.
With respect to materials handling, Bester notes that the self-shielded process offers a significant advantage due to its positional capability. This allows the welder to weld the part as it lies or where it stands, without having to manipulate the part or move it to an enclosed workshop to suit the welding process.
As material handling can constitute up to 45% of welding costs, this single factor has a tremendous impact on the cost of production or repair. “With FCAW-S, the flux offers a ‘dam-wall’ effect, where the fast-freezing flux holds the molten metal in place while it solidifies. The resulting slag also helps to shape the weld bead and protect the weld from atmospheric contamination. Another benefit of the fast-freezing slag is that higher currents can be used for out-of-position welding, which produces “great fusion and penetration” at significantly higher deposition rates compared to SMAW.
“With the gasless process, an out-of-position joint can be deposited at 1.9 kg/h and at as high as 2.4 kg/h by a more skilled welder. With a 3.2 mm stick electrode, the current might have to be reduced to 90 A to complete an out-of- position weld, which would reduce the deposition rate to below 1,2 kg/h,” he explains.
Responding to myth that self-shielded wires are associated with poor toughness, Bester says that this is simply not the case: “One has to select the correct wire for the application, as there are various products available, catering for different grades of materials. A common mistake is still made in this regard, where a wire is selected which is not suited for a specific task, resulting in poor toughness and weld failure. With SMAW one can run into the same predicament when selecting an E6013 electrode when your base material or application dictates the use of an E7018-1 low hydrogen electrode,” he says.
“One of the pitfalls of FCAW-S wire selection, however, is that some classifications have weld thickness limitations. Users, therefore, must make 100% certain that they choose the correct classification based on both the grade of material and the weld thickness. These limitations are, however, clearly identified by responsible manufacturers such as Lincoln Electric on the product information sheets and marketing materials.
In debunking the poor toughness myth, Bester cites the new Innershield NR233 wire (AWS E71T-8) from Lincoln Electric, which meets the AWS D1.8 seismic code for construction used in countries on earthquake fault lines – such as California, Japan, New Zealand and Malaysia. The -29 °C Charpy toughness results for this wire are in the 34 to 54 J range. Other benefits include: unlimited weld thickness and ease of welding. “This wire is also an easy switch to make for SMAW welders, due to the similar welding techniques that are required,” Bester adds.
“Self-shielded wire is a great alternative to SMAW with benefits including: better deposition rates, positional capabilities, operational efficiencies, outdoor weldability, welder appeal, and lastly, no shielding gas is required. This is a massive benefit in remote areas, which do not have gas readily available. For piping and tanks there are also various stainless steel TIG rods available as gasless derivatives.”
Describing the equipment requirements for self-shielded welding, he advises on the types of power sources: “The process needs a CV (constant voltage) power source with a built-in feeder or a loose wire feeder. However, unlike GMAW, which runs on dc-positive polarity, almost all gasless wires run on dc-negative polarity.” The polarity on the machine, therefore, has to be able to be changed. Renttech SA offers various solutions via its Uniarc and Lincoln Electric brands, including 220 V single phase, 380 V and 525 V three phase as well engine drives in both petrol or diesel configurations – and for TIG welding using gasless welding rods, CC (constant current) power sources are also available.
He adds that specially designed ‘suitcase’ wire feeders are also a very good idea for welding up, over or inside structures. These feeders are robustly built and are fully enclosed to protect the wire from dust and damage to the spool caused by moving the feeder around in confined spaces.
“Purpose-made gasless torches are also now available with Euro gun-ends. US equipment manufacturers used to offer torches that were designed to fit US wire feeders,” he explains. “However, customers who had European welding power sources had to use a traditional GMAW torch with the shroud removed. These torches got damaged very quickly and were usually destroyed during the course of the project. Today, purpose-built self-shielded torches, such as the TBi MOG 50 are designed specifically for gasless wires and these are now available with the Euro gun-end as a standard – and these torches accommodate self-shielded wire sizes from 2.0 mm down to 1.2 mm in diameter,” Bester informs African Fusion.
“FCAW-S is the ‘tool in the box’ that has great potential, particularly for on-site structural steel construction, mining, ship building, pipeline and tank projects. On a water pipe project in Zambia, for example, our customer has had phenomenal success using the shielded STT process for root runs followed by Innershield NR211 capping runs. The end result was a project finished well before schedule and an extremely satisfied customer,” he says.
“Self-shielded wires are easy to use and it is easy for welders to adapt to this welding process. Although it is not the answer to all outdoor or SMAW applications, it’s well worth investigating for your next project. Our team at Renttech SA will gladly assist and demonstrate the advantages of this great process,” Bester concludes.