Powder plasma arc welds thick pipe efficiently

An innovative plasma arc welding process for the butt-joint welding of thick-walled pipes and powder finishing in one process and without defects is described by Mario Marconi.

The use of plasma welding technique (PAW process) together with a tailored equipment facing the process new demands proves that structural welds on thick-wall pipes is possible, by joining two or more sections of spun-cast tube materials.

Plasma arc welding technique, in keyhole mode, assures a complete root penetration in the first pass (convex root surface) with high quality from both the production demand and the control of structural characteristics of the joint.

The seam finishing by Powder Plasma Arc Welding (PPAW), performed in single pass after the first in keyhole mode (melt-through), represents the most innovative and attractive aspect of this technique, gaining a perfect near-net shape bead at face of weld, thus leaving the two joint beads (face and root of weld ) in the as-welded state.

The results of this innovative technique for structural welding of thick pipes have been presented at 24th Bi-Mu machine tool exhibition in Milan, Italy, by Plasma Team Company.

They can be summarized in the following notes: 1.

A stable and well penetrated keyhole has been gained thanks to a correct choice of anodic nozzle constriction shapes, as well as of the geometrical sizes of electrode tip and set-back distance; 2.

Keyhole setting-up and maintenance have been achieved by plasma forming gas flowrate, with a reliable and repetitive control of its variations during the upslope and downslope phases; 3.

In the second pass, a near net shape bead has been reached by an accurate choice of powder deposition rate and groove angle.

Different bevel angles have been tested, to optimize the powder deposition rate; 4.

The innovative technique performed using a filler material in powder form and the same plasma arc welding in automatic runs has been tested and proved to be fully operative; 5.

Microstructure analysis have shown sound and defect free beads, all characterized by a small Heat Affected Zone with reduced sizes of solidification grains; 6.

During the Qualification Procedure Tests at Customer site, powders with the same chemical composition as the base material have been used for the first time in Plasma Arc process; 7.

The results of the full load tests at customer site, considering the fusion behavior, bead flowability and shape, have demonstrated the high degree of flexibility and quality of PAW technique in keyhole mode and of Plasma Team Equipment.

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