Welding
Welding is a process in which metal components are fused together by the application of pressure or heat.
Welding is a jointing technique that almost is unique to metal. In essence, it means that two pieces of the same material can be fused together to form an almost continuous piece of material. With soft metals like gold and silver, this can be done simply by squeezing together two pieces of clean metal. Since ancient times, soft metals like these have been hammered together. Even iron can be welded together by hammering and heat, as is demonstrated by the medieval sword that had to be a fusion of low carbon iron for flexibility and high carbon iron for sharpness. Structural joints are normally rather too large to be hammered together so a technique of fusion welding developed through the 19th century. It was based on a similar principle to soldering whereby molten metal is introduced between the two faces to be joined. The liquid metal connects into the crystalline structure of the parent metal so that when it cools down and solidifies, there is for practical purposes, just one continuous piece of metal. Ideally, this makes a joint that is in no way weaker than the original metal. While this may be the ideal joint, it required complex technology and skilled operatives to achieve full strength.
This illustration shows four types of butt weld with which metal sections can be connected end to end.
Different welds require different degrees of preparation and the amount of weld is critical to the cost. A further point that is blindingly obvious to anyone doing the job but not so obvious to anyone just looking at the final product is that the two pieces of metal have to be held rigidly together while the weld metal is being laid. This may mean while the joint is turned upside down to work on the far side.
A. Square butt weld which requires no preparation but is only suitable for thin plates.
B. Single V-butt for which the preparation can be by flame cutting or chipping.
C. Double V-butt which must be welded from both sides.
D. Single U-butt which uses less weld metal than the V-butt but the preparation must be machined.
This illustration shows six types of weld that join metal at right angles.
A. Single fillet weld is cheap but may be subject to lack of fit.
B. Corner butt weld with which allowance must be made for angular distortion.
C. Partial penetration corner butt weld with inner fillet which requires good fit-up but reduces angular distortion.
D. T-butt weld which ensures complete continuity.
E. Partial penetration T-butt which is a useful intermediate between a double weld and a full penetration T- butt weld.
F. Double fillet weld which is cheaper than D but may be subject to lack of fit.
The heat for welding was provided either by an oxy-acetylene flame (gas welding) or by an electrical arc (electric welding).
The first problem of producing the molten metal was solved at the end of the last century by heating iron/steel welding rods with the very hot flame produced by the burning of acetylene gas in a stream of oxygen. Later the heat came from passing a high electric current through the rod and "arcing" it across to the parent metal.
A number of technical problems had to be solved in the development of a reliable welding technique.
A number of technical difficulties had to be overcome before welding became a reliable means of jointing structural steel. These included:
- The heat generated has problematic side effects. It causes local expansion and contraction in the parent metal and it encourages chemical reaction between the metal and the surrounding atmosphere.
- For the molten metal to intimately engage with the parent metal there must be no barrier of dirt or grease at the interface.
- There must be no relative movement between the two faces while the weld is cooling and solidifying.
- The operatives must be extremely consistent in their work to ensure that the weld is laid down in an even way.
- Once the weld has solidified it is difficult to check the efficacy of the fusion between the weld metal and the parent metal. It is not visible to the human eye so machines that use either X-rays or ultra-sonic waves were developed for checking welds.
Welding has been in common use for structural steelwork since the mid twentieth century.
Until the 1939-45 war welding was a specialised process only carried out where the necessary skills and equipment could be concentrated economically. After the War in every fabricating shop welding became the normally preferred means of connecting.
Initially, welding was not carried out on site. Structural components of transportable size were prefabricated by welding but site connections were made by bolting.
Simple shop logic dictated that the largest possible assembly was welded together in the fabricating works where skilled people worked in a controlled environment. Then the steel components were transported to the site and connected up using bolts that could be seen, counted and easily checked. Until the 1980s, site welding was an expensive and hazardous operation, generally only used to get out of an unforeseen difficulty. In recent years, portable equipment has made site welding a much more common technique. The contrast between a welded joint and other methods of jointing can be seen in the series of examples all found in Central Station,
Modern techniques enable thick plates of steel to be cut to shape and then welded together at the edges, offering a variety of simple shapes but often resulting in bland forms.
Traditional cast iron allowed a much richer vocabulary of forms to be used to connect disparate structural members.
In many railway stations the contrasting methods of the nineteenth and twentieth centuries can be seen in one view. At the same station a large rolled section joined by a visually elaborate bolt can be contrasted with sections built up from relatively small angles and flats and connected by the much less visually intrusive rivets.
The introduction of welding has led to the widespread use of tubular elements for structural steelwork.
A major consequence of the widespread use of welding was the development of tubes for structural purposes. Both rivets and bolts required flat plates that could be overlapped, and these were provided by the open sections. Circular and square hollow tube sections could only be connected by welding, since while it was possible to drill a hole in the side of the tube, it was not possible to get inside to tighten a nut or squash a rivet. It was possible to swage the ends of tubular sections which could then be joined by either welding or bolting but this produced a particular visual effect where the joint was obviously expressed. Welding cut tubes together produced a joint of much simpler appearance. The interface of the circular tube forms an interesting geometry much beloved of experts in engineering drawing, but happily firms such as Stewarts and Lloyds developed machines that could automatically cut the correct profile for any meeting of tubes. This has turned out to be one of the most interesting new techniques available to designers in the second half of the 20th century. It has opened up a freedom of three-dimensional geometry, particularly in latticed roof trusses, that has only been fully exploited in recent years.
The most recent development in jointing of steelwork has been the re-introduction of the casting technique.
The production of steels that are suitable for both casting and welding is a relatively recent development that has greatly simplified the problem of making the joints between sub-elements in complex three-dimensional frameworks. The joints in the trusses in the roof structure of the Waterloo Terminal in
Cast steelwork joint elements greatly simplified the appearance of this structure.
