S. Keitel, J. Neubert, and M. Ströfer
For years, well tested and proven arc welding processes have been applied for the welding of large pipes of oil and gas pipelines, with a scope extending from manual arc welding with stick electrodes up to the application of so-called orbital welding units using the metal active gas (MAG) process. If permitted by the length of the pipeline and the profile of the ground, a number of these orbital units are used at the same time, with every single station having been designed for welding of one or two passes and then being displaced to the next pipe joint to produce the same weld seam there. Such production aggregates often rely on several welding heads per unit thus representing a high state-of-the art1, both in relation to equipment and welding. This, however, is connected with a high expenditure on personnel and plant engineering.
FIGURE 1. Formation of the weld in hybrid welding.
A further performance increase in this area poses some problems since the arc processes applied have attained their physical limits concerning deposition efficiency and welding speed, such that no essential increases can be achieved by optimizing the arc welding technology.
The development of welding processes for increased performance must be carried out under the following aspects:
- Reduction of the number of passes at constant and improved seam quality, respectively
- Reduction of the number of welding stations and thus the expenditure on equipment and personnel
The application of laser beam gas metal arc (GMA) hybrid welding is a promising technology for the future.
In laser-GMA-hybrid welding, both processes are combined such that the laser beam and the arc act in a common melting pool. The result is more than simply adding both of the energy sources and the filler metal; it is rather that the resulting synergetic effects combine and enhance the advantages of the single processes. Thus, a joint profile is generated that is similarly deep as that one obtained by laser welding but has a considerably better gap bridging capability. Hence, in the field of thin sheets, very high welding speeds can be obtained that partly are many times the amounts of the state-of-the-art of GMA-welding. With larger plate thicknesses, the advantages are not within the area of welding speed but there is the possibility of reducing the number of layers by single pass layers, often without additional joint preparation. A typical formation of the seam for a sheet plate thickness of 8 mm using GMA, laser hybrid welding is shown in FIGURE 1.
FIGURE 2. Welding head with hybrid equipment for root pass welding and arc torch for filler pass welding.
The fact that today the application of laser beam sources under construction site conditions is possible is based on the rapid developments in this field. Thus, fiber lasers that perform within the two-digit kW range also are distinguished by a sturdy and compact structure. In addition to a very high operating efficiency and an excellent beam quality, the preconditions for a mobile application are achieved; these could not be met using conventional current, state-of-the-art laser beam sources (CO2 or Nd:YAG lasers) . In the last five years, fiber lasers have been used as a mobile application in shipbuilding and in the production of pipes2.
FIGURE 3. Entire test built-up on the pipe.
Laser GMA hybrid girth welding
The objective of the investigation of technology and equipment described in the following was the transfer of the state of knowledge of laser GMA hybrid welding to the production of pipe joints incorporating all necessary aspects such as tolerances, environmental influences, equipment mobility, and welding out-of-position.
FIGURE 4. Seam preparation and macro sections for a) 4.6 kW at a root face of 6 mm and for b) 6.5 kW laser output at a root face of 8 mm.
The focus was to use the typical laser deep welding effect to produce a high quality free root pass at root faces of 6-10 mm. To this end, the different arrangements of the laser beam and the arc possible for hybrid welding of butt joints were compared to the different types of joint preparation.
FIGURE 5. Influence of the distribution of hardness by the trailing arc3.
The approach for the production of pipe joints was welding of two vertical-down seams, being a common practice in pipeline construction and considerably reducing the types of freedom in the arrangement of laser beam and arc required for the technological optima of the seam formation.
For the generation of a closed seam profile, the weld head was extended by a further arc torch, thus enabling welding of the first pass using hybrid welding and the cover pass using GMA welding during one welding run. The objective was to produce a closed seam profile up to a plate thickness of 12 mm in one rotation. Further, this trailing process was a good opportunity to have a positive influence on the mechanical-technological properties of the weld seam.
FIGURE 2 shows the completed welding head with the equipment for hybrid welding and the integrated second arc torch for welding the cover pass during one vertical-down weld movement. Through the integration of the components described, the entire test set-up shown in FIGURE 3 was realized.
The test was performed using two laser beam sources of different output. The principal test series for determining the basic parameters for the hybrid arc and for determining the tolerance susceptibility of the hybrid process at continuously changing welding positions across the pipe circumference was performed in Phase 1 using a 4.5 kW fiber laser. Phase 2 served to estimate the potential of the hybrid process at higher laser performance at a simultaneous increase of the root faces for the root pass from 6 mm to 8 mm. To this end, a 10 kW fiber laser system was used.
FIGURE 6. Specialized prototype.
In the following, the results are shown in the form of macrosections both for the first hybrid welded pass and the closed seam profile by the trailing arc at a different variation of seam preparation at a laser output of 4.6 kW (FIGURE 4a) and 6.5 kW (FIGURE 4b).
The examinations were concluded by the determination of the hardness distribution in the root area of the weld seams since this obviously laser-beam-dominated area in the heat affected zones could be susceptible to increased hardening. During these examinations, pure root welding without a cover pass was compared to welding with a closed seam profile through the trailing arc with the results being shown in FIGURE 5.
The objective of the further development of the equipment was to increase the stability of the rotational movement along the pipe and to adapt it to the conditions of the hybrid process. A specialized prototype developed on this basis (FIGURE 6) for the realization of a girth welding movement for laser beam GMA hybrid welding has the following technical data: travelling speeds for positioning were up to 6 m/min and for welding up to 3 m/min. Pipe diameters processed ranged from 500-700 mm with parameter changes depending on position. Seam tracking and a guidance system were used. The integrated laser working head allowed coupling with all fiber-guided solid state lasers of outputs of up to 20 kW.
FIGURE 7. YLS-12000 fiber laser system.
A further focus of the current investigations was on the optimization of the process for pipe wall thicknesses starting from 10 mm at different root faces for the first pass to be welded using a laser, namely the 12 kW fiber laser system, available at the SLV Halle since January 2009 (FIGURE 7).
The focus of the examinations on the one hand was directed to investigate the possibilities of the formation of the seam and the root with the laser output available. On the other hand, the focus was directed on the overlapped areas at the weld start, obligatory when welding two vertical down seams at the circumference of the pipe.
The weld areas were tested metallographically for internal imperfections. Pipe typical tolerances were considered, in order to make statements about the influences of higher laser power and increased welding speeds on the weld formation with regard to different tolerances. FIGURE 8 shows the formation of the seam at the 3 o’clock position for a pipe wall thickness of 10 mm for higher laser outputs.
Conclusion and outlook
To increase efficiency in pipeline construction, examinations of new welding processes for joining of pipes are indispensable with the focus on increasing the welding speed at a reduced number of passes. A possible alternative is the laser GMA hybrid welding process now possible with the development of the fiber laser as a beam source with new fields of application.
FIGURE 8. Macrosection of the seam formation at a 1 mm misalignment of edges.
The objective of the investigations was to prove the principal suitability of hybrid welding for pipeline construction as well as the behavior of this process in out-of-position welding, which is required for its application. Furthermore, both closed seam profiles were produced and the hardening increase of the heat affected areas of the root were reduced for a pipe wall thickness of 10 mm using an arc process trailing the hybrid welding. The results distinctively show the potential of the hybrid process at high laser output and with brilliant beam qualities. In a next step of the examinations the results are to be transferred to larger pipe wall thicknesses. As an alternative to the existing approaches,3 there is the idea of using the hybrid process for the production of a high quality root pass at root faces of 12-15 mm. This approach is the basis of the examinations currently being performed.
1. D. Blackman, V. Dorling, R. Howard, “High-speed tandem GMAW for pipeline welding,” 4th International Pipeline Conference, Calgary, Alberta, Canada, 2002, pp. 517-523.
2. S. Keitel, U. Jasnau, J. Neubert, “Applications of fiber laser based deep penetration welding in shipbuilding, rail car industries and pipe welding,” 4th International Symposium on High-Power Laser and their Applications, 24-26.06.2008, St. Petersburg, Russia.
3. A. Gumenyuk, S. Gook, M. Lammers, M. Rethmeier, “High Power Fibre Laser Welding for Pipeline Applications,” Proceedings of LAMP2009, 5th International Congress on Laser Advanced Materials Processing.
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