September 1, 2009
Most tube and pipe producers weld the seam as it is—without additives or fillers—and risk the problems associated with oxidation. A specially formulated brazing flux, in liquid or paste form, dissolves and removes oxides, prevents re-oxidation, and helps transfer weld heat to the seam.
Technological improvements often are simple concepts that can have profound effects on existing processes. New technologies and process efficiencies allow manufacturers to improve products or increase throughput. For example, developments in high-frequency power supplies allow tube and pipe manufacturers to select the optimal frequency for the application. Consumables also continue to evolve and find new applications. Welding flux is one such consumable. Many tube and pipe manufacturers have found that using a flux reduces defects, thereby improving yield.
Fluxes for tube welding processes are available in two forms: paste and liquid. Both have pros and cons, and the choice depends on several factors. Introducing fluxes is easy in the tube and pipe production process, and the result often is a cost-effective return in quality, efficiency, consistency, and time management.
Before choosing a paste or a liquid, it is necessary to learn about the chemical makeup and vitreous (glassy) nature of tube and pipe welding fluxes; the chemical mechanisms whereby the flux removes refractory oxides from the surfaces of 409 stainless steel, 409 aluminized stainless steel (MO), and 439 stainless steel; and the role of fluorine.
Fluxes are essential when joining metals in ambient air, whether by soldering brazing, or welding. When heated, fluxes perform four tasks:
It is important to distinguish brazing fluxes from welding fluxes. The difference is a matter of temperature: Brazing takes place from 900 degrees F to 2,200 degrees F; welding is from 1,500 degrees F to 3,000 degrees F.
Tube and pipe mill weld boxes develop very high temperatures (2,600 to 2,800 degrees F), so a welding flux seems to be appropriate for this application. However, the heat cycle used in tube and pipe production is too short and too intense to activate welding fluxes. Moreover, the fluoride content of these fluxes is quite low. High fluoride content is critical in tube and pipe welding applications.
In fact, various silver brazing flux formulations are suitable for this operation. These fluxes are active in the temperature range of 1,000 to 1,600 degrees F and have numerous industrial uses. The formulations activate in the intense heat of the weld box and remain active during the very short exposure time. The most common inorganic raw materials used in these fluxes are:
Formulations for tube and pipe welding fluxes are available in paste and liquid forms. The flux is applied to the mating surfaces. Paste is applied directly on both coil edges, while liquids are sprayed onto the joining surfaces immediately prior to entering the weld box.
Ferrous alloys, especially mild steel, are the most common materials for manufacturing tube and pipe. These metals exhibit surface oxides. For mild steel these are iron oxides such as FeO and F2O3. Generally, iron oxides are either reduced at elevated temperatures or are displaced in the welding process. An exception is 409 stainless steel, which contains approximately 12 percent chromium. The chromium oxidizes to chromium oxide (Cr2O3), a tenacious and refractory oxide that tends to remain on the joining surface and interferes with the welding process. This is an even greater problem with 439 stainless steel, which contains approximately 18 percent chromium. The aluminum in alloys such as 409 aluminized stainless steel exacerbates the problem: Heating this material causes both aluminum oxide and chromium oxide to form. Aluminum oxide is also refractory and remains on the joining surface. Using argon gas or another inert shielding gas affords some protection against high-temperature oxidation. However, these gases do not remove the original metal oxide tarnish, and they are only partially protective in the dynamic environment of the weld box.
This is where fluxes perform their fundamental process function. At approximately 1,000 degrees F, the assemblage of inorganic chemicals is transformed into a glassy complex. Glass viscosity is important: It must be high enough to serve as a protective blanket, keeping oxygen diffusion to a minimum, while at the same time low enough to dissolve surface oxides effectively.
The primary means for oxide removal by alkali fluoborate fluxes is dissolution. Oxides are absorbed into the glass, leaving a very clean, deoxidized metal surface, which is properly prepared and protected for the welding stage. Flux activity reaches its peak in the short induction welding cycle.
Boron oxide (B2O3) is the glass former in the silver brazing flux formulation. It provides the protective blanket for the base metals and the filler alloys, lowering the rate of oxygen diffusion to the metal surface so that it is below the rate of oxide removal from the joining surfaces. Potassium oxide (K2O) and sodium oxide (Na2O) are the glass modifiers, lowering the melting point and viscosity of the glass, improving flow. Fluorides reduce the melting point and viscosity further because they disrupt the cross-linkages in the borate glass structure. However, the improved fluxing action results mainly from the greater oxide dissolution capability of the glass because of its fluorine content. Fluorine is especially important because it aggressively attacks such oxides as Al2O3 and Cr2O3, accelerating their solution in the borate glass.
Silver brazing fluxes usually are used in conjunction with silver-based (BAg) alloys. The flux cleans and protects both the base metal and the filler alloy, allowing the filler alloy to flow and wet the base metal. This is not the case with the tube and pipe welding application because it uses no filler alloys or electrodes. Yet the outcome is the same. The flux removes oxides and protects the surface against re-oxidation, yielding a strong, consistent weld seam. The simple addition of flux improves metallurgy, cosmetics, consistency, and stability of the tube welding process.
Tube and pipe welding fluxes come in two forms: paste and liquid (low-solids and high-solids). Paste is applied by brush to both sides of the steel coil and is ready for use after all solvents evaporate. For liquid application, flux is sprayed onto joining surfaces just before the edges enter the weld box. Conventional tube and pipe pastes are effective on 409 stainless steel, 409 aluminized stainless steel, and 439 stainless steel. On the other hand, conventional low-solids liquid fluxes work effectively on 409 stainless steel but aren't compatible with 409 aluminized stainless steel. A new high-solids liquid flux has been developed that solves this problem—it is effective on 409 aluminized stainless steel and 409 stainless steel.
In terms of choosing a flux, some operators prefer the paste because it requires just one step; they do not have to do anything after applying the flux. The downside is that this process does not use the flux effectively; a fair amount flakes off before reaching the weld box. The flaking is associated with material moving through the accumulator, where it travels up to 220 feet per minute. These flakes and particles must be removed from the area continuously to prevent health, safety, and equipment efficiency problems.
Spraying flux onto joining surfaces is a simple process, but it requires constant monitoring. For example, if the spraying is inconsistent, the result is a visual difference between the fluxed and unfluxed weld seams. The liquid results in a cleaner operation because flux is applied close to the weld box and it does not flake.
Learning about what flux does and how it does it isn't enough. One more question remains: How does flux benefit a tube and pipe operation?
Finally, because it reduces tube rejects, it frees engineers and operators to spend more time on manufacturing and process improvements and less time dealing with problems.
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