Page 22 - ITAtube Journal 1-2021

Technical Papers
requires normalized is only the weld HAZ, but it’s difficult to heat more of the tube without distortion. However, the traditional way isn’t the only way; several years ago, research
and a field implementation proved that it is possible to normalize just the HAZ on tubes that measure less than 2 in. diamter with controlled distortion.
Doing so efficiently and effectively hinges on three main factors: the distance between the inductors and the tube’s surface, the align- ment of the weld seam and the inductors, and tracking the weld seam to maintain the alignment.
Keeping the Proper Coupling Distance. Using induction heating efficiently is a matter of keeping the induction coils as close to the surface as possible, and the standoff distance in this application ideally is not more than 0.200 or 0.300 in. If the standoff distance is greater than 0.300 in., the efficiency drops exponentially.
The problem with keeping a minimal standoff distance is that a variation or
disturbance on the mill can
decrease the gap, which risks
creating a short circuit where the tube makes contact with the inductor. Considering the amount of electrical power in this application, a short-cir- cuit is a potential disaster for the tubing and the induction coil. Ceramic standoff wheels can help to prevent contact. Still, the wheels are just single points of contact spaced along the induction coil’s length, so
Caption: As annotated on these charts, angular displacement refers to the heat distribution of an annealing line and the resulting heat-affected zone. The center of the chart, at 0 degrees, represents the weld seam center, which is the center of the annealing coil placement. The heat distributions vary, so to be effective, a normalizing system must be designed to capture the widest heat profile of the pipe to be produced on this mill.
maintaining the proper standoff distance is still challenging on smaller diameters.
Another problem is that the induction process generates magnetic lines of flux that create vertical forces called Lorentz forces. Research has shown that magnetic lifting forces can exert nearly 90 lbs. of upward force on predominately 2.0-in.-dia. and smaller tubes. This wreaks havoc on the tube’s dimensional consistency, leading to a distorted tube and adding to the possibility of short circuit between the induction coil and the tube.
Seam Width.
Typically, a single inductor of a specified width is used for a range of tubing sizes. Knowing the width of the weld HAZ is necessary to design induction coils that work as efficiently as possible. They have to cover enough of the tube’s circumference to normalize the entire width of the HAZ
to the ID of the tube. Still, if there is too much overlap between the weld HAZ and the inductor width, electrical energy waste
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