TPi September 2018

Welding technology

Tube welding fluxes make stronger weld seams By Yehuda Baskin, PhD, president of Superior Flux & Mfg Co

A water-based brazing paste is easy to apply by brushing, rolling or high- pressure spray gun, and it is not difficult to coat both sides of the coil before it is mounted onto the mill. The liquid fluxes are water-based and available in two solids contents. They are sprayed on automatically, are clean and do not pollute. When properly selected, installed and aimed, the spray system applies the right amount of flux where it is needed, on both edge surfaces, as the strip feeds into the weld box. Flux functions Fluxes are essential when joining metals. When heated, they perform four vital tasks: 1. Dissolve or react with surface oxides 2. Protect the cleaned surfaces against re-oxidation 3. Transfer heat from the heat source to the joint 4. Remove surface oxides, allowing the clean metal surfaces to join This is where fluxes perform their fundamental process function. At approximately 1,000°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 the metal surfaces 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, deoxidised metal surface that is properly prepared and protected for thewelding stage. 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 from the greater oxide dissolution capability of the glass because of its fluorine content. Fluorine is important because it aggressively attacks such oxides as Al 2 O 3 and Cr 2 O 3 , accelerating their solution in the borate glass.

Chromium is the alloying element that gives 409 and 439 series stainless steels their oxidation resistance. It forms chromium oxide (Cr 2 O 3 ) as a surface coating that essentially prevents further oxidation, much as aluminium oxide (Al 2 O 3 ) does for aluminium metal. Chromium oxide is a tenacious and very stable surface oxide that is not easily removed and does an excellent job of preventing the stainless steels from further oxidation. The downside to providing oxidation resistance is the fact that chromium oxide impedes bonding, especially when the alloy is subjected to the very high temperatures that occur in a tube or pipe weld box in ERW welding. In some cases this can result in weak weld seams. However, many tube and pipe producers that make 400 series tubes weld the seam as is, without additives or fillers, risking the problems associated with the original chrome oxide coatings, and oxidation in the weld box. Those tube producers do have options. One is to completely enclose the weld box and blanket the inside with argon gas. Because it is inert, argon does not remove the original chrome oxide coating, but it does help suppress oxidation in the weld box. Another way to protect the seams from oxidation is to use tube welding fluxes. When heated, these fluxes dissolve and remove the original chrome oxide coating, prevent re-oxidation at the high temperatures in the weld box, and transfer weld heat to the seams. This results in clean metal surfaces, producing stronger tube seams. Special dispensing units are used to spray liquid flux on the strip edges just before they enter the weld box. While generally not necessary, some operators use argon in addition to fluxes, as an added protection. Flux flexibility Fluxes for the seam welding processes come in two forms: paste and liquid. Both have pros and cons, and the best choice depends on several factors.

Spraying flux onto the tube seam

Flux justification Learning about what flux does and how it does it is not enough. One important question remains: How does flux benefit a tube and pipe operation? 1. Better, stronger welds: Removing the centreline oxides results in a better weld. The weld has fewer voids, fewer microcracks, and greater consistency. This is reflected in a sharp reduction of seam splits. 2. Lower tube reject rates: Some flux users have reported reject rate reductions of up to 40 per cent. This reduces scrap, resulting in a large monthly cost reduction. 3. Reduced scarf scrap: Without flux, many operators use higher than necessary welding temperatures to produce a good weld. This can result in a larger-than-normal amount of squeeze-out – material that must be removed by scarfing. 4. Lower electricity consumption: Some flux users have reported a substantial reduction in electricity consumption. 5. Cost savings: The lower tube reject rates and reduced ID scarf scrap result in cost savings. Spraying flux is a fast, simple operation, whereas brushing or applying paste flux onto the surface is an expensive manual operation. Flux application The liquid flux is well suited to an industrial spray system. Its viscosity is compatible with LVLP systems. Modern spray systems can hit a target area smaller than 3mm, and are adjustable to apply just the right amount of flux. Mounting this sort of spray nozzle is a simple affair using an adjustable frame of 1.3cm rod stock (see photo). The best place to mount the spray valves is after the last set of rollers. Because two valves operate at the same time, a single-valve operation channel can control the flow. Superior Flux & Mfg Co – USA

info@superiorflux.com www.superiorflux.com

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TUBE PRODUCTS INTERNATIONAL September 2018

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