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Laser welds cut tube-forming scrap.

Laser welds cut tube-forming scrap

The high chromium content that gives stainless steels their desirable heat-and corrosion-resistance properties also creates weld-seam problems for stainless tubing. High scrap rates have been a fact of life when cold forming stainless tubing that has been seam welded by typical TIG or high-frequency induction welding processes.

Why tube welds fail

These methods create chromium-oxide and iron-oxide layers of porous, non-metallic adhesion at the weld interface that are not a problem for many applications. However, under severe forming pressures (or service pressures), particularly cold forming of bends or flanges, these welds can fail.

A key problem with TIG welding is that it is a relatively slow process. Even with a shielding gas, the surface area of molten metal and the time exposed to oxygen are significant. Oxide films form not only at the weld interface, but also on the inner and outer weld beads and on the adjacent heat-affected zone. In TIG welding, that heat-affected zone is large, and microstructural transformations in this region can lessen the material's formability and corrosion resistance.

With HF induction welding, the induction coils must be located ahead of the tube mill's squeeze-roll assembly. This means the molten edges of the metal are exposed to oxygen, generally without the benefit of a shielding gas. Also, coolant used to cool the induction coils generally finds its way into the weld zone.

As the seam is forced together by the squeeze-roll assembly, oxides and coolant inclusions are trapped in the weld. With TIG, high squeeze-roll pressure is necessary to force out the oxide and coolant inclusions, but some remain and form porosities at the weld interface. This extra pressure also creates severe inner and outer weld beads that must be mechanically removed. This is relatively simple for the outer weld bead, but troublesome and time-consuming for the inner bead.

Although HF induction welding yields the highest stainless welding speeds at the lowest cost per unit length, cold-forming scrap rates can run as high as 20 percent. Other negative factors are coolant cost and coolant's adverse affect on squeeze-roller life and maintenance.

Why laser welds don't

How does laser welding differ from these processes? Because of the laser beam's high energy density and small spot size (0.003" to 0.040"), the volume of molten metal is less than with TIG welding, and a smaller surface area is exposed to oxidation. Also, because laser welding of stainless tubing is two to three times faster than TIG, the time molten metal is exposed to oxidation is greatly reduced. In addition, the smaller heat-affected zone minimizes microstructural changes.

Compared to HF induction welding, the key difference is that laser tube welding occurs at the squeeze-roll assembly. This assures an intimate fit-up accompanied by a flow of shielding gas, unlike the lack of shielding, exposure to oxygen and coolant, and gapped edges with HF induction welding.

As a result, laser-welded tube seams are virtually free of oxidation and associated porosity. Their cold-forming failure rate is approximately 1 percent, compared to 5 percent for TIG and the previously mentioned 20 percent for HF induction. Laser welding also eliminates the need to scarf off weld beads because squeeze-roll pressure is less than that needed for HF induction to squeeze out inclusions.

Since filler material is not generally used, the laser weld nugget is parent material and yields a strong, low-cost weld. However, this lack of filler material and the laser's small spot size mean an intimate fit-up is required: 0.001" tolerances for gap, mismatch, and seam location. Although some laser tube-welding systems can maintain these tolerances without seam tracking, optical and mechanical tracking methods are widely used to correct for both horizontal and vertical variances.
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:May 1, 1990
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