Reduced Pressure EB Welding in the Power Generation Industry

Welding and Repair Technology for Power Plants: Sixth International EPRI Conference: June 16-18, 2004, Sandestin Golf and Beach Resort, Sandestin, Florida

Abstract

Electron Beam welding (EBW) is a mature welding process which offers many advantages in terms of weld productivity, avoidance of distortion and minimal metallurgical disturbance. To date, however, the necessity to weld in a high vacuum atmosphere has restricted the application of the process to components and structures that can be entirely contained within a vacuum chamber. This paper describes the development of 'Reduced Pressure' Electron Beam welding where demands upon local sealing and pumping are much less onerous than with high vacuum EBW, thus facilitating the application of the process to large structures and components. A number of practical examples are described of how the process has been used successfully, illustrating the potential for application in the power generation industry.

Introduction

Electron beam welding offers many advantages when considered alongside more conventional multi-pass arc welding processes for the fabrication of heavy section structures and components, particularly in high value materials for strategic applications. The ability to complete a weld in a single pass, without filler, almost irrespective of material thickness leads to huge productivity potential and avoids many of the problems associated with the multi-pass, arc welding processes.

Traditionally, however, to apply EB welding successfully it has been necessary to envelop the components to be joined entirely in a vacuum chamber which has to be pumped to a pressure of the order of 10 ^-3 mbar, Fig.2. This requirement has, to a large extent, inhibited the application of the process in the fabrication of large structures. However, EBW process benefits are potentially greatest for the thick materials often employed in large structures. The prohibitive capital investment necessary for the construction of large vacuum chambers and the attendant cost of pumps has limited the use of the process.

In addition, the high vacuum EB process is intolerant to poor surface condition and it is not always practical or economic to accurately machine joint details on large parts.

Reduced pressure EB welding

The past 10 years, however, have seen the emergence of advanced high power EB welding technology which permits welding at significantly higher working pressure i.e.~1mbar, some one thousand times higher pressure than conventionally used. This has been made possible by the development of an electron beam generator using a differentially pumped transfer column, Fig.3. This generator can produce an electron beam with the workpiece at near to atmospheric pressure; to date optimum welding performance has been achieved at pressures up to 1mbar. In turn this permits the use of a local vacuum arrangement whilst avoiding the need for a sophisticated sealing and pumping arrangement.

With this system it was shown to be possible to establish a welding vacuum ('Reduced Pressure') using a simple mechanical vacuum pump and crude seals. It was quickly realised that the development of this process variant represented a step change as it permitted the use of local sealing and pumping and potentially obviates the need for large chambers to weld big components. This also opens up the opportunity of taking the welding equipment to the structure and applying the welding process 'on-site'.

Furthermore, it was illustrated that in this pressure regime the system was particularly tolerant to many of the variations that previously had previously hindered the adoption of EB welding for large structures viz.

• Component cleanliness

  Welding at high vacuum requires that the both the immediate joint area and the entire assembly are relatively clean otherwise outgassing can occur leading to welding difficulties. When working at 'Reduced Pressure', greatly increased tolerance to component cleanliness is observed and only the immediate joint area needs to be cleaned.

• Gun stand-off distance

  The Reduced Pressure Beam is essentially parallel and has no well-defined focus position. In consequence, the gun to work distance can vary by more than 400mm without detriment to weld quality. In contrast, high vacuum EB welding requires that the gun to work distance is controlled to +/- 2mm to maintain an appropriate beam focus position.

• Pumping time

  Operation at 1 mbar pressure, in contrast to 10 ^-3 mbar significantly reduces pumping time for a given volume. Even for large volumes, pumping times are measured in minutes as opposed to hours.

• Gun discharging

  The combination of the beam transfer column and helium overpressure stage (a small positive flow of helium is maintained at the end of the gun column during welding) eliminates the risk of metal vapour or positive ions entering the electron gun and causing breakdown and interruption of the welding process. This is a serious consideration when welding high value, critical parts using EBW.

• Beam characteristics

  The increased attenuation of the beam caused by the scattering effect of the Reduced Pressure atmosphere results in a 'softer' beam which results in wider weld profiles offering greater tolerance to joint gaps and better weld termination behaviour.

Application to nuclear waste burial

Use of the Reduced Pressure EB welding process is particularly attractive when considering both the fabrication and sealing of high level nuclear waste containers for a variety of reasons and is under consideration for use both bythe Swedish atomic energy authority ^[1] and the Yucca Mountain project in the USA. ^[2] In the former case the material selected for the canister corrosion barrier is OFHC copper of 50mm thickness and in the latter Alloy 22 has been selected as the preferred material for the waste package corrosion barrier. Thedifference in material selection is due to geological differences in the proposed repositories. Reduced Pressure EB welding was identified as an attractive method for the canister closure for some of the following reasons:

• The process is ideally suited to remote operation and is a true non-contact process.
• The system is fully automatic with on-line seam tracking and no operator dependence.
• The components and elements of the system are radiation hard and not degraded by extended exposure to high radiation flux levels.
• The operation lends itself to rigorous quality control via a combination of in-process monitoring and post weld inspection.
• The weld completion rate is high and thereby minimises the need for several expensive hot cell welding stations to achieve the desired production rate.
• The weld quality is consistently high and reproducible minimising the need for repair.

Residual Stresses

The work on Alloy 22 has also illustrated some potential benefits in improving the distribution and magnitude of surface residual stresses which will not be discussed in detail here but may offer some advantages over conventionalmulti-pass welding in other power generation related applications.

Maintenance and reliability

To be a viable technique for the sealing and fabrication of high level waste containers RPEB welding equipment has to be demonstrably:

1. Stable - that is, capable of maintaining constant welding performance over long periods of continuous operation without interruption.
2. Reproducible - able to replicate welding conditions from day to day, even after long periods of continuous operation or after maintenance periods.
3. Reliable - suitable for high duty cycle operation without frequent maintenance or unpredictable breakdown.

These characteristics have also been designed into the system to accommodate the severe requirements dictated by the application to offshore pipe-lay. In this case the system had to be capable of not only withstanding the rigours of an offshore lay barge environment but also being operated on a 24 hour cycle in which only 30 minutes was allocated for maintenance. This has been achieved by careful design of elements of the electron gun and extended testing.

Joint preparation and weld geometry

RPEB Welding is performed in a single pass without a filler metal addition and thus simple square edge joints are used. This enables fabrication design to be simplified with significant savings in materials and machining, plus radically reduced and controllable distortion. The absence of welding consumables and very low heat input of RPEB welding cost and post-welding correction substantially.

Non destructive testing

Extensive experience with Saipem (Italy) has shown that weld defects are rare with RPEB because of the process reliability and accuracy of weld placement due to the use of real-time seam tracking. RPEB uniquely uses the welding beam itself to track the joint with great accuracy and compensate for machining inaccuracies and thermal distortion during welding. Nevertheless, should any flaws occur, X-ray inspection combined with phased array ultrasonics have proved successful in detection and sizing of planar weld flaws of less than 2mm height.

Economic considerations

Application of RPEB welding involves a significant investment in high technology equipment but at the same time results in uniquely productive, ultra-low cost welding which means that the 'cost of ownership' is very low when repetitive high volume welding envisaged.

Obvious savings are the absence of welding consumables and weld completion achieved in a single pass compared with numerous slow weld passes necessary with other arc welding processes. Less obvious but equally significant is the reduced machining effort required for preparation, base metal savings, reduced welding flaws and post welding correction. A key feature of the process is that it is fully automated and not dependent for success upon skilled manual dexterity or intervention.

Each application of automated RPEB merits individual appraisal and assessment before quantification of the overall savings but early results for Alloy 22, as planned for Yucca Mountain canisters show welding times could be reduced by an order of magnitude ^[2] .

Concluding remarks

The EB welding process offers many advantages when considering heavy section fabrication but is restricted by the need to operate under high vacuum conditions. The ability to operate EB equipment at Reduced Pressure (1mbar) greatly increases process reliability and allows the use of local sealing and pumping opening up opportunities for application of the process to large structures, or on-site.

References

1. K.R. Nightingale, A. Sanderson, C.S. Punshon, L.O. Werme, 'Advances in EB Technology for the Fabrication and Sealing of Large Scale Copper Canisters for High Level Nuclear Waste Burial', Proceedings of 6th International Conference on Welding and Melting by Electron and Laser Beams, Toulon, France (June 1998) vol.1 pp 323-330. [conference paper]
2. F. Wong et al, 'Reduced Pressure Electron Beam Welding Evaluation Activities on a Ni-Cr-Mo Alloy for Nuclear Waste Packages' Proceedings of ANS/ENS 2003 Global conference, New Orleans, November 2003. [conference paper]