Welding

Welded connections are formed by locally heating the surfaces to be connected allowing them to fuse.

The common characteristic of all welded joints is that the contact faces are united by fusion due to application of localized heat. Electric arc welding of steel uses an electric current to generate a temperature of about 3500°C to melt the two surfaces to be joined. The electrode also melts providing filler metal to the weld. Atmospheric gases must be excluded from the weld zone, since these would otherwise interfere with the weld quality, and this can be achieved in a variety of ways. Welding may be carried out manually or semi-mechanically for long continuous weld deposits.

A variety of welding methods are used for structural connections.

Various methods of welding are commonly used to connect structural steels: manual metal arc (MMA), metal inert gas (MIG), and submerged arc. They differ in the degree of automation and the means of protecting the molten weld from atmospheric contamination.

Manual metal arc is not automated, but is very versatile.

Manual metal arc (MMA) welding employs a hand-held stick electrode which has a core of special steel, surrounded by welding flux. The flux and core are melted by the heat of the electric arc set up between the electrode and the components to be joined. The flux flows around the molten metal as it is placed, and prevents the metal becoming contaminated with harmful gases from the atmosphere. After the molten metal has cooled, the flux forms a glassy covering to the weld metal which must be chipped off. Productivity is also hampered by the need to constantly renew the electrode.

Manual metal arc welding is the most versatile process and is widely used for site welding and for difficult access areas where bulky equipment is unsuitable. However, automatic and semi-automatic processes are often more economical for long weld runs (e.g. plate girders) and for butt welds in thick plate.

Metal-active gas welding is a semi-automatic process, widely used in fabrication but unsuitable for site work.

Metal-active gas welding (MAG) - sometimes known as metal inert gas welding (MIG) - is a semi-automatic, gas shielded process which is now widely used for fabrication. It is similar to the manual metal arc process, but in this case a machine feeds the electrode continuously through the handpiece, and supplies a shielding gas (often CO₂) to protect the arc. Capital and operating costs are higher, but deposition rates are faster, there is no slag to be removed and the incidence of defects is reduced. The equipment used for both MMA and MAG welding is portable allowing it to be transferred easily to different work stations within the workshop. However, for site welding, MMA is almost always used because the MAG gas shield can be dispersed, even by a gentle wind.

Submerged arc welding (SAW) is an automatic process used in the fabrication of elements requiring long, straight weld runs.

In automatic welding the filler material (electrode) is moved automatically by a machine. It is often referred to as the 'submerged arc' process because the arc is shielded by granular flux which is deposited in advance of the welding head, and 'vacuumed' up again after the head has passed.

It is particularly suitable for built up members using plate in the form of box or I girders where long continuous runs of weld are necessary. The weld produced is of very high quality with a smooth and uniform bead making it highly suitable for heavy plate fabrication. Single or multiple weld runs can be laid down quickly, but the equipment is not portable and the process is therefore only used in the fabrication shop, not on site.

Different types of weld in common use are the fillet weld, which does not develop full strength, and the butt weld, which develops higher strengths but requires special preparation.

The most common weld type is the fillet weld, either in a tee or lap configuration. This is normally used where the connection does not need to develop the full strength of the connected components. It is a relatively inexpensive process because plate edges do not need to be prepared (machined), less weld metal is deposited, and inspection is usually less extensive than for butt welds. It is adequate for most applications.

Full penetration butt welds are used in highly stressed connections. The plates to be joined are 'prepared' - the common surfaces are chamfered to allow the weld metal to be placed across the whole plate thickness. To avoid a stress-raising flaw at the root of the weld, the first runs laid are back-gouged before the runs on the 'back' are laid. A major butt weld may consist of many runs, is correspondingly slow to make and is expensive.

Partial penetration butt welds are sometimes used when the weld must be stronger than a fillet weld, but a full penetration butt weld is not required.