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Development of Welding Parameters of Automatic GTAW Process for Pipes with Variant edge preparation

Hung-Ta Kuo
Kuan-Chywan Tu
Institute of Nuclear Energy Research
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Abstract

Introduction

Experiments

Results and Discussions

  1. Bead-on-Plate Experiment
  2. Pipe Welding Test

  3. In many pipe welding practices, the pressurized backing gas can really offer a positive effect to influence the weld bead appearance in solidification process. In circumferential welding path, the backing gas pressure can give the best effect within the sector between 11 o'clock to 1 o'clock, where can easily simulate the position of flat welding. Also, in this sector the gravity force of molten metal pointed downward making the sagging molten metal be supported by gas cushion with most convenience.

    When welding along the circumferential path, the arc impact force, the molten metal surface tension force, gravitational force, and the outward backing gas pressure combine their own forcing influences on molten metal and made the forcing environment in welding puddle a complicate one. Table 1 shows the individual influences of each force at different circumferencial regions.

    Arc Position Molten Metal Flow Direction Acting Force Pattern
    1 o'clock to 11 o'clock flows toward and stays at the rear of arc
    11 o'clock to 7 o'clock flows toward to arc
    7 o'clock to 5 o'clock tends to be sagging
    5 o'clock to 1 o'clock more tends to flow forward to the rear of arc and accumulates
    Table 1: Force Patterns vs Arc Position

Fig 16: Pierced Weld under DP=300 mm Fig 17: Even Weld Contour under DP=1.7 mm

Conclusions:

  1. In pipe welding, the molten metal beneath the welding arc experienced the resultant force which was the combination of gravity, arc impact force, and the molten metal surface tension force. These forces change the direction of their own whilst welding was in progress.
  2. The method of adjusting the pressure level of internal shielding gas do help supporting the weld beads and have an effect to reduce the formation of welding dross.
  3. The BGPA method could be a good method to make a easier root pass bridging.
  4. The shielding gas can affect the acting pattern of surface tension on molten metal. Therefore, the weld bead contour and the heat flux flow pattern in molten metal were also affected.
  5. The different shielding gas made no obvious change in the weld microstructures.

References

  1. Recommended Practices for Root Pass Welding of Pipe Without Backing, AWS D10.11-87.
  2. Weld Discontinuities- Causes and Cures, Welding Journal, P37~P42,June 1998.
  3. AWS Welding Handbook
  4. Control of Purge Gas Pressure in Orbital Welding of Electro-Polished Stainless Steel 316L Tubes , by Kuang-Hua Hou; Yu-Chich Chen; and J.R. Chou; Chang Gung University, Taiwan.

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