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Technical Papers
increases. In addition, on smaller diameters, excessive overlap causes heat to saturate the tube’s circumference, increasing the steel’s elasticity and rendering it more susceptible to distortion.
Finite element analysis (FEA) can deter- mine the width of the HAZ in the annealing process, but this is too big a tool to use for every project. In addition, developing an FEA is a time-consuming and expensive process. A slightly less exact method, but a much faster one, is to cut and etch an as-welded tube sample to determine the actual width of the HAZ. A heat distribution profile graph of the normalizing process then can be pro- duced that allows the prediction of the seam annealing HAZ.
Seam Tracking.
A third element in induction efficiency is maintaining the inductors’ location relative to the seam. Ideally, the weld seam and the inductors are perfectly aligned, centered at the 12 o’clock position, but the seam does tend to wander left and right as the tube moves through the mill. In most cases, the seam deflects less than +/- 7.5 degrees, so an orbital coil positioner that moves a total of 15 degrees covers the typical deflection.
Big Reels Keep on Turning
Quite a bit of engineering goes into making very long lengths of tubing and doing so efficiently.
First, these tubes don’t have a single wall thickness. The first section down the hole has the lightest wall; subsequent sections have heavier walls to provide more strength to support the increasing amount of weight.
Second, while the process could use full- body annealing, this is too wasteful. In
some common tube applications—say, a few hundred lengths of 20 ft. each—a full-body anneal would make sense because setting
up a custom-made inline normalizing system with a seam tracker and an orbital unit is an engineering-, capital-, and time-intensive undertaking at best. For a small contract, the return on investment isn’t likely to materi- alize. However, for a program that involves making hundreds or thousands of flexible steel lines that run for miles, the full-body
ITAtube Journal 2021 – Special Edition
anneal wastes far too much energy, and a custom annealing line can be justified.
Third, when developing a normalizing system that anneals several diameters and wall thicknesses, the smallest sizes get more heat saturation around the circumference, thus increasing the metal’s plasticity. The Lorentz forces come into play and can cause some distortion.
These forces can be countered by using a larger number of smaller inductors. In a properly designed system using this princi- ple, the total amount of heat the inductors generate is the same, but each generates less magnetic flux, thereby generating less verti- cal force, thereby reducing the distortion.
Using these inductors, known as twin series inductors, allows the use of additional steel standoff wheels, which are placed between the inductors. In other words, whereas a conventional system has ceramic stand-
off wheels, this system has those and two additional modes of protection against short circuits: less vertical lift from the magnetic flux, and additional guide wheels located between the induction stations.
Depending on the level of expertise and the time put into designing such a system, two competing normalizing lines can differ quite a bit in their layouts. For example, a conven- tional system designed more or less along traditional principles might use four sepa- rate induction stations. In contrast, a more sophisticated, updated approach can result in a system that does the same amount of work with just three induction stations, con- serving floor space and energy. This equates to less equipment on a smaller footprint operating more efficiently.
Donald Gibeaut is global bar and tubular products manager for Ajax TOCCO Magnether- mic Corp., 1745 Overland Ave., Warren, OH 44483, 800-547-1527, www.ajaxtocco.com, dgibeaut@ajaxtocco.com.
Donald is also Chairman of the board of the ITA, International Tube Association for North and South America. Donald also serves advi- sory board of directors for the TPA, Tube and Pipe Association.
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