TWI Knowledge Summary

Reduced pressure electron beam welding

by Keith Nightingale

Process description

Like conventional electron beam (EB) welding, reduced pressure EB welding is performed using a beam of electrons from an electron gun having a power up to 100kW. The beam is focused onto the joint-line of the two parts to be welded and either the electron gun or the parts are moved so that the beam traverses along the joint-line, penetrating the full depth (up to 200mm) in a single pass.

The difference between reduced pressure (RP) and conventional EB welding is in the design of the electron gun which allows the pressure in the welding enclosure to be several orders of magnitude higher: 10 ^-1 - 10 mbar for RP compared with 10 ^-5 - 10 ^-2 mbar for conventional EB welding.

Current status

Reduced pressure EB welding is in the process of being adopted for two major applications -

Nuclear waste encapsulation

SKB in Sweden required a process to seal the lids of 5m long x 1m diameter copper canisters, after filling them with high-level nuclear waste. The difficulty of vacuum pumping this assembly, due to the long and tortuous evacuation paths, meant that conventional EB welding was not feasible.

The nozzle arrangement, between the electron gun and weld chamber, ensures that no copper particles or vapour reach the electron gun to cause discharging. The weld is non-penetrating but needs to be about 60mm deep with no root porosity, even in the slope-down region. A beam power of around 70kW and a welding speed of 200mm/min gave a wide (8mm) round-bottomed weld profile ( Fig.1), ideal for avoiding root defects.

Fig.1. Reduced pressure EB weld in copper showing round-bottomed weld profile

J-lay pipe girth weld

Saipem SpA. in Italy required a fast welding process to join pipes on a lay-barge using the J-lay system in deep (>2000m) water. It is anticipated that by using two electron guns for the girth weld of a 41mm thick, 710mm diameter pipe (see Fig.2), the pump-down time for the inflatable-sealed vacuum chamber could be around 30 seconds and the weld time less than 5 minutes.

Fig.2. Transverse weld section from 25mm wall thickness (left) and 41mm wall thickness (right) carbon manganese steel pipe

For both of the above applications, TWI designed and manufactured a 'special' reduced pressure EB system (consisting of an electron gun and transfer column) and integrated it with an intelligent, inverter-driven high voltage power supply and a programmable logic control (PLC) system. The prototype pipe welding equipment is shown in Fig.3.

Fig.3. Prototype reduced pressure EB pipe welding equipment

Current issues

TWI operates three, reduced pressure EB systems at its research laboratories. These are used for basic research and investigating potential applications for TWI Research Member companies.

The effect of the higher pressure on fusion zone composition, and how this affects weld properties, is an important issue and is under investigation at TWI.

Benefits

The benefits and characteristics of reduced pressure EB welding include -

weld profiles and properties largely the same as conventional EB welding
welds thick-section components in a single pass (up to 200mm in steels)
uses a locally sealed chamber to weld components too large to fit in a vacuum chamber
welds components which cannot be pumped to low pressure, e.g. complicated structures or dirty/damp components
greatly reduced effect of weld-pool emissions on electron gun stability - virtual elimination of gun discharges
reduced capital and running costs of pumping system
reduced pump-down times: up to 3 times faster than conventional EB systems

Precautions

Apart from the usual precautions required for conventional EB welding (x- rays, high voltage, vacuum entrapment), the weld metal vapour accumulates as dust in the welding enclosure, rather than being deposited as an adherent layer as in conventional EB welding. The airborne dust should be extracted during the enclosure venting cycle. Dust which has settled on surfaces can be readily removed by brushing but, again, adequate dust extraction and personal breathing protection equipment must be employed.

Further information

FAQ: What are the benefits of reduced pressure EB welding?
FAQ: What is EBW at Reduced Pressure?

Also, you can contact TWI's Electron Beam Group direct

Use the Weldasearch literature database to supplement what you find in JoinIT.