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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 Contact

Abstract

Couples of forces, namely, the impact force given by a burning arc where the gravity force which tends to drop the molten metal and the surface tension force acting on the surface of weld puddle, can vitally influence weld bead contour during puddle solidification process. Especially in the root pass welding of a butt joint in a piping system, how to control the bead contour can really affect the service life of pipe line. The interactions of these forces play a very essential role to build a good "bridging" in every specific root pass welding. A poor "bridging" in the first pass can not only downgrade the deposition of subsequent weld passes but also introduce local turbulent flows at the inner surface to induce erosion/corrosion phenomenon in inner weld locals. The quality of a root pass weld often depends on the so-called edge preparation factors. Even some sizing suggestions can be found in some industry Codes or relevant handbooks, however most of them do not meet the more strict requirement in automatic welding process. As a result, the welding defects could be found from time to time in an automatic welding process.
In this research, a method of adjusting the backing gas pressure was used in the automatic GTAW process to weld f 8" Sch 40 SUS 316 and f 6" Sch 10 SUS 316 stainless steel pipes. Besides micro-structure evaluation and micro-hardness analysis, the ultrasonic evaluation was also performed and they demonstrated that the "dross dropping" could be improved by at least 80%with this method.

Introduction

The root pass welding is often considered the most important step in pipe welding. Welding defects such as melt-through, lack of fusion, and dross etc. are very common in root pass welding of pipe system.

Many causes could result weld defects. In the early days, the welding was carried out manually, and the weld quality can be totally controlled by the workmanship of a welder. The welder, when welding, can directly monitor the flow pattern in puddle and make immediate adjustments in arc length, weaving, and handling pattern, etc., to obtain a good weld. Especially in root pass welding, all minute hand movements the welder practiced are to pave a sound continuous supporting surface to let subsequent depositions be easily placed on. In other words, to make a good bridge across the matching joints. Nowadays, more and more jobs had done by welding machines. Machines might have difficulties to practice the techniques commonly used in manual welding. So, the quality requirement will always be the challenge in machine welding.

The backing gas pressure adjustment ( BGPA ) method was tried to improve the surface smoothness of the back side surface of electro-polished ( EP grade tube ) tube welds. By using such a technique, the molten metal which tends to drop or yields holes or dross, can be effectively supported and guided by the pressured gas cushion, and solidify in a flatter pattern rather than a sagging one. The effect of using BGPA method was so obvious in improving the inner surface smoothness of pipe welds, especially those joints having hi-low interface. In this research, the BGPA method was used in welding 8 inch stainless steel pipes.

Experiments

Fig 1: Bead-on-Plate Platform

Fig 2: Box with One Facet Opened

Fig 3: Pipe Welding

Experiments were carried out under the following conditions:
Welding Process : Machine GTAW
Shielding Gas : argon ( 99.95% or better )
Backing Gas : air / argon / nitrogen
Method : bead-on-plate welding, then pipe welding


1.Bead-on-Plate Experiment :
Experiments were carried out to study the dross smoothening effect under the application of backing gas pressure. Air, argon, and nitrogen were used as the backing gases, which were respectively purged into the box to support molten metal and prevent from dross sagging The pressure difference between the box inside and ambient atmosphere can be checked by measuring the water level in a transparent U tube. ( Figures 1&2 )

The purposes of Bead-on-Plate experiment are :

• Evaluate the dross-smoothening effect by introducing pressurized backing gas
• Evaluate the weld appearance change by introducing different backing gases

2.Welding on Pipes
Pipe welding was performed by using ESAB MechTIG 315 welding power source & welding head. Welding head can provide a circumferential welding. Due to the roundness deviation on pipe, there might exist some gaps at pipes matching interface, and minor dimension difference on root face along the circumferential welding path. The experiment facility is shown in Fig 3.

The purposes of this pipe welding practices are :

• Try to adopt the parameters developed in the "Bead-on-Plate" experiments or to make parameter modification, if needed, on pipe welding.
• Study the dross formation patterns in each circumferential sector.

Results and Discussions

1. Bead-on-Plate Experiment
• Backing Gas Pressure vs Dross Contour

  Fig 4: Backing Gas Pressure vs Weld Contour 200A 18cm/min Ar 3t DP=0 weld contour 6.89(w)×0.52(h)

  Fig 5: Backing Gas Pressure vs Weld Contour 200A 18cm/min Ar 3t DP=10 weld contour 6.39(w)×0.33(h)

  A proper backing gas pressure does affect the dross formation. With an adequate pressure applied in internal pipe chamber, the molten metal which tends to form a dross can be supported by gas cushion and solidify in a pattern with flatter surface. Figure 4 to Figure 6 show the results under the backing pressure difference from 0~30H₂O gauge. In the case of Fig 6, the dross is approximately 0.03 mm in height. To compare results in Fig.4 and Fig.5, the weld contours were improved by 90% and 94%, respectively.

Fig 6: Backing Gas Pressure vs Weld Contour 200A 18cm/min Ar 3t DP=30 weld contour 7.65(w)×0.03(h)

Fig 7: Type of Backing Gas vs Weld Contour 200A 18cm/min Air 3t DP=0 weld contour 9.71(W)×3.0(H)

Fig 8: Type of Backing Gas Pressure vs Weld Contour 200A 18cm/min Ar 3t DP=0 weld contour 6.89(W)×0.52(H)

Fig 9: Type of Backing Gas Pressure vs Weld Contour 200A 18cm/min N2 3t DP=0 weld contour 1.21~1.61(W)×7.94~9.15H

• Backing Gas Variation Air, argon, and nitrogen were introduced individually into pipe chamber as the backing gas. Fig. 7 to Fig. 9 revealed the results where argon can offer a better shielding effect due to least oxidation.
• Micro-Hardness Distribution The results are shown in Fig. 10 to Fig. 12.

  Under argon backing, Fig 11 shows a better hardness distribution. Other two figures show a more fluctuating hardness distribution in the weld metal region which might be the result of nitrogen contamination to give rise to the solution strengthening effect.

Fig 10: Micro-hardness Distribution Backing Gas: Air

Fig 11: Micro-hardness Distribution Backing Gas: Ar

Fig 12: Micro-hardness Distribution Backing Gas: N2

• Backing Gas vs Microstructure Backing gases (air, Ar, N₂) offered no obvious contribution to the change of microstructure. In Figures 13 to 15, under 200X magnification, there has no obvious defect in each microstructure pattern, and each microstructure has dendrites in austenitic matrix. The dendritic coring is the characteristic of rapidly frozen austenitic weld metal.

  Fig 13: Micro-structure Analysis Backing Gas: Air

  Fig 14: Micro-structure Analysis Backing Gas: Ar

  Fig 15: Micro-structure Analysis Backing Gas: N2

2. Pipe Welding Test

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.