The last 50 years have seen big improvements in productivity for most arc welding processes and arc welding still creates the bulk of welding output in engineering construction today. Increasingly, high power laser, hybridlaser-arc, electron beam (EB) and friction welding techniques are beginning to feature in the fabrication of large scale structures in several countries, and usage is expected to grow. For example, laser and laser-arc hybrid welding are now employed in several European ship panel lines whilst friction stud welding is routinely used for many building and bridge applications worldwide.
It will be some years before newly developed processes are used in bridge, pipeline, building and other public infrastructure constructions and they are only likely to displace arc welding in those applications requiring large assemblies with straight welds, where step improvements in productivity are achievable, where tonnage throughput can justify the initial capital outlay for specialised welding equipment and where distortion control is critical.
This paper reviews current trends in arc welding processes used for C-Mn steel construction in civil infrastructure projects and will discuss some of the newer processes which are beginning to replace arc welding for such steels, or have potential to do so in the next 10 years.
1. Arc welding
Significant improvements in productivity are nowadays hard to gain in this mature process. In gas-shielded MIG/MAG processes, the growth of tubular wire over solid wire is still slow despite the deposition and bead shape benefits ofthe former although tubular wire usage is higher in the USA and Japan compared to other parts of the world. The lower price of solid filler wire, particularly in Europe, is still a dominant factor that continues to favour their use. Self-shielded cored wires are still widely used in the USA but less so in Europe because of the fall-off in fabrication of large offshore installations. Care is always needed in selecting self-shielded wires if weld metal toughness isa critical design factor.
Small inverter controlled electronic power sources are standard today, and gas mixtures are chosen on the basis of price and application. In Europe, argon and helium are cheaper compared to prices in, for example, Japan. European industry uses Ar-CO 2 mixtures a great deal for MAG welding, while Japan usually employs 100% CO 2 and has developed more sophisticated power sources to cope with the tendency for greater spatter generation.
Tandem wire MAG welding has become more popular over the last five years and by using a special torch feeding two wires with separate power sources, deposition rates can be more than doubled compared to single wire MIG/MAG, (e.g.15Kg/hour can be achieved at travel speeds of 5m/min). The process ( Fig.1) seems quite suited to long fillet welds where high travel speeds can bring economic benefits. Of course narrow gap techniques offer an additional route for improving productivity in thicker plate but high capital costs remains an obstacle to widespread use.