Electron beam welding in the construction and engineering industry
Electron beam technologies have unique characteristics that make them suited to a variety of applications in the construction and general engineering sectors. EB welding has the capability to make high integrity welds in thick section materials with low distortion; this makes it attractive for the manufacture of a range of specialist products. Other EB processing technologies have their own desirable characteristics.
The range of materials that may be EB welded and processed in this industry is vast; from the specialist high strength alloy steels, through copper alloys, bronzes, aluminium alloys, nickel alloys, stainless steels, to maraging steels and titanium alloys.
Submarine hull penetrations and valves
| Submarine hull valve |
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High strength steels are used to make submarine hull components. Wherever the hull is penetrated by a tube or other component, a complex assembly is required to withstand the service conditions. Typically these parts are made via large, complex castings. These parts are expensive, and have very long lead times. In addition, castings are difficult to make without the presence of a variety of defects that may compromise the integrity of the structure if they are not repaired via arc welding. By contrast, wrought material of intrinsically high integrity may be used in EB welded fabrications that may be used to replace complex castings. These components, as well as having other advantages, are made on a short lead-time, from stock plate material and a small inventory of parts.
Cutting edges via EBW
| EBW - Bi-metallic saw blade |
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Cutting tools of many kinds place conflicting requirements on the materials that are used to make them. On the one hand, a cutting edge must be hard and sharp; but the support for it needs to be tough, and strength is not so important. Exotic forging techniques have been practised for hundreds of years in the manufacture of sword blades that possess the right qualities. A more prosaic, but equally tough problem is the manufacture of saw blades. High-speed steels make for teeth with good cutting edges, but have neither the properties nor the desirably low cost to make a perfect material for the bulk of the blade.
A widely implemented solution is to make bi-metal sawblades via EBW. A narrow strip of high speed steel is EB welded to a wider strip of low carbon steel. This is typically done in batches, each batch being a reel many hundreds of metres in length. Although the welding operation is performed in vacuum, strip material may be fed on reels in and out of a small 'local vacuum' chamber if required. The EB welded composite material is then processed to make a wide variety of blades, in stages that include annealing, tooth grinding and hardening operations. Most bandsaw and hacksaw blades are made this way; in some cases the hard steel strip is continuous along the edge of the blade, in others the tooth root cuts through the weld, leaving each tooth isolated. In all cases the bimetal blade outperforms the alternatives in both cost and performance. A bi-metal hacksaw blade will not shatter like a traditional 'all hard' blade, yet cuts better and lasts longer.
Power transmission products via EBW
| Tractor crankshaft |
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The ability to join complex machined components via EBW to make high integrity fabrications with low distortion has been shown to provide great benefits to the end user. One design study has shown that it is possible to fabricate large diesel engine crankshafts via EBW; allowing the replacement of complex forgings with simpler parts. This would bring cost benefits, as well as allow repairs and refurbishments to be carried out. Once heat treated, tests of all kinds on dressed EB welded sections have given parent metal specification levels of strength and fatigue performance.
| Clutch reverser |
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In another application, a compact EB welded gear assembly replaces a complex bulky assembly. Several EB welds are required, each requiring high integrity welds and low distortion. Some require a very small access area, at a great distance from the weld itself. This immediately rules out many other processes, and in fact EB welding is the only process that combines the required characteristics for this application.
Surface processing for improved corrosion performance
| EB processed material, as processed. This is an SEM image; under normal viewing this surface just looks smooth and shiny |
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Both cast and wrought materials are used in corrosive environments. In these conditions, segregation of solute elements in the parent material may ultimately compromise the corrosion resistance of the alloy. The harmful segregation occurs when the alloy is first cast, as it cools relatively slowly from the melt. In wrought materials it persists despite many intermediate heat treatments and other processing steps. In many cases it has been shown that a significant improvement in corrosion performance may be obtained by utilising an EB surface treatment technique. The surface processing technique typically utilises an electron beam to remelt the surface to a depth of hundreds of microns, or even several millimetres. To accomplish this, an electron beam of controlled shape is manipulated at high speed via electromagnetic beam deflection coils. This movement is combined with workpiece motion, allowing continuous processing of large areas if required. At each point on the surface, the material is melted and resolidifies rapidly, so that partition of solute and segregation are virtually eliminated. The controlled processing sequence restricts overlapping of remelted zones, whilst inhibiting formation of defects during processing.
In addition to the reduced segregation, the processed surface may be rendered smooth at an atomic level. This can help to inhibit the formation of corrosion pits in the processed surface, further improving the overall corrosion performance.
Processing is rapid, with a typical treatment taking 1 second per square inch to carry out; with minimal changes in shape, the EB treatment may be used as a finishing operation.
More detailed information is available to TWI's Industrial members:
Contact: electronbeam@twi.co.uk
Copyright ©2006 TWI Ltd
