The Tube & Pipe Journal - September 2009

LOOKING TO IMPROVE
WELD SEAMS?
Try fluxes for tube, pipe production
By Yehuda Baskin, Ph.D.

TECHNOLOGICAL improvements often are simple concepts that can have profound effects on existing processes. New technologies and process efficien-cies 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.

FLUX TYPES

Fluxes are essential when joining metals in ambient air, whether by soldering, brazing, or welding. When heated, fluxes perform four 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.

Photo courtesy of Abbey International Ltd., Perrysburg, Ohio.

4. Remove surface oxides, allowing surface or filler metals to flow and wet the joining surfaces.

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.