Welding - Steelconstruction. Welding is a core activity in the fabrication factory, undertaken by skilled, qualified operatives working to a welding quality management system under the control of a Responsible Welding Coordinator. It is used to prepare joints for connection in the shop and on site, and for the attachment of other fixtures and fittings. Different welding techniques are used for different activities within the fabrication factory. A separate filler material supplied as a consumable electrode also melts and combines with the parent material to form a molten weld pool. As welding progresses along the joint, the weld pool solidifies fusing the parent and weld metal together. Several passes or runs may be required to fill the joint or to build up the weld to the design size.

Correct prescription of metallurgical requirements and sound practical application is a prerequisite for successful fusion welds. A separate filler material supplied as a consumable electrode also melts and combines with the parent material to form a molten weld pool. The weld pool is susceptible to atmospheric contamination and therefore needs protecting during the critical liquid to solid freezing phase. Protection is achieved either by using a shielding gas, by covering the pool with an inert slag or a combination of both actions. The gas surrounds the arc and effectively excludes the atmosphere. Precise control is needed to maintain the gas supply at the appropriate flow rate as too much can produce turbulence and suck in air and can be as detrimental as too little.

The slag also solidifies and self releases or is easily removed by light chipping. The action of melting the flux also generates a gas shield to assist with protection. Several passes or runs may be required to fill the joint or to build up the weld to the design size. This region of change is known as the heat affected zone (HAZ). Common terminology used in the weld area is illustrated above right. Site welding is also feasible, and guidance on the considerations for site welding is available in GN 7. The positions of these butt welds are allowed for in the design, although material availability constraints or the erection scheme may require agreement of different or additional welds.

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Butt- welded Tee joints may be required where there are substantial loading or fatigue considerations in transverse connections. Full penetration butt welds are designed to transmit the full strength of the section. It is generally possible to weld these joints from one side but, as material thickness increases, welding from both sides is desirable to balance distortion effects, with an in- process back- gouging and/or back- grinding operation to ensure the integrity of the weld root. Single- sided butt welds with backing strips, ceramic or permanent steel, are common for joining large plate areas (such as steel deck plates) and where there are closed box sections, tubes, or stiffeners, which can only be accessed for welding from one side. The design throat thickness determines the depth of penetration required for partial penetration welds.

Note that fatigue considerations may limit the use of partial penetration welds, particularly on bridges. Guidance on weld preparation is available in GN 5. In addition, butt welds tend to have larger volumes of deposited weld metal; this increases weld shrinkage effects and results in higher residual stress levels in the joint. Careful sequencing of welding operations is essential to balance shrinkage and to distribute residual stress, thus minimising distortion.

Dressing flush for aesthetic reasons alone should be avoided because it is difficult to dress the surface to match the adjacent as- rolled surface, and the result is often more visually noticeable than the original weld. Also, grinding is an additional health and safety hazard that is best avoided as far as possible. The dressing of butt welds to a flush finish is usually not required for building steelwork as typically it is not subjected to fatigue. They typically include end plate, stiffener, bearing and bracing connections to rolled sections or plate girders, and the web to flange connections on the plate girders themselves. These are relatively simple to prepare, weld and test in normal configurations, joint fit- up being the principal consideration. BS EN 2. 25. 53. The designer must be careful to ensure that it is clear which dimension is specified and all parties need to be aware of what has been specified.

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The overall time includes setting up equipment, cleaning and checking of the completed weld.). The four main welding processes in regular use in UK steelwork manufacturing are described below. The process numbers are defined in BS EN ISO 4. Variations of these processes have been developed to suit individual manufacturers’ practices and facilities, and other processes also have a place for specific applications but are beyond the scope of this article. A continuous solid wire electrode is passed through a wire feed unit to a . Power is supplied from a rectifier or inverter source along interconnecting cables to the wire feed unit and gun cable; electrical connection to the wire is made in a contact tip at the end of the gun.

The arc is protected by a shielding gas, which is directed to the weld area by a shroud or nozzle surrounding the contact tip. Shielding gases are normally a mixture of argon, carbon dioxide and possibly oxygen or helium. The gas shield is susceptible to being blown away by draughts, which can cause porosity and possible detrimental metallurgical changes in the weld metal.

The process is therefore better suited to factory manufacture, although it is used on site where effective shelters can be provided. It is also more efficient in the flat and horizontal positions; welds in other positions are deposited with lower voltage and amperage parameters and are more prone to fusion defects. The advantage of using these wires is that higher deposition rates can be used, particularly when welding in the vertical position (between two vertical faces) or the overhead position. The presence of thin slag assists in overcoming gravity and enables welds to be deposited in position with relatively high current and voltage, thus reducing the possibility of fusion- type defects. Flux additions also influence the weld chemistry and thus enhance the mechanical properties of the joint.

Alternating current transformers, DC rectifiers or inverters supply electrical power along a cable to an electrode holder or tongs. A flux coated wire electrode (or . The electrode melts at the tip into a molten pool, which fuses with the parent material forming the weld. The flux also melts, forming a protective slag and generating a gas shield to prevent contamination of the weld pool as it solidifies.

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Flux additions and the electrode core are used to influence the chemistry and the mechanical properties of the weld. It is essential to store and handle these electrodes in accordance with the consumable manufacturer’s recommendations in order to preserve their low hydrogen characteristics. This is achieved either by using drying ovens and heated quivers to store and handle the product, or by purchasing electrodes in sealed packages specifically designed to maintain low hydrogen levels. Nevertheless, it remains the main process for site welding and for difficult access areas where bulky equipment is unsuitable. The process feeds a continuous wire via a contact tip, where it makes electrical contact with the power from the rectifier, into the weld area, where it arcs and forms a molten pool.

The weld pool is submerged by flux fed from a hopper. The flux immediately covering the molten weld pool melts, forming a slag and protecting the weld during solidification; surplus flux is collected and re- cycled. As the weld cools, the slag freezes and peels away, leaving high quality, good profile welds.

This also means that personal protection requirements are less.