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Popular processes for welding stainless steels

Gas tungsten arc welding (GTAW or TIG)

This is the most widely used process due to its versatility and high quality as well as the aesthetic appearance of the finished weld. The ability to weld at low current, and hence low heat input, plus the ability to add filler wire when required, make it ideal for thin materials and root runs in one sided welding of thicker plate and pipe. The process is easily mechanised and the ability to weld with or without the addition of filler wire, (autogenous welding), make it the process for orbital welding of pipe. Pure argon is the most popular shielding gas, but argon rich mixtures with the addition of hydrogen, helium or nitrogen are also employed for specific purposes. Inert backing gas protection of the weld under-bead is employed with single-sided welding to prevent oxidation and the loss of corrosion resistance.

Plasma arc welding (PAW)

A derivative of the TIG process involving a constructed nozzle system to produce a narrow concentrated transferred plasma arc with deep penetration characteristics. Mainly used in a mechanised system where high speed, high productivity autogenous welding of square-edged butt joints up to 8mm thick is required. With thicker square edged butt joints, a combination of PAW/TIG and filler wire becomes necessary to ensure a full profile weld surface. Thicknesses greater than 10mm employ a partial v-preparation PAW root weld followed by multi-pass joint filling. Argon backing gas protection is necessary to maintain the corrosion resistance of the under-bead.

Shielded metal arc welding (SMAW or MMA)

Manual in operation and the oldest of the arc processes, MMA electrodes are in common use due to their flexibility in accommodating the wide range of materials to be welded. Electrode coating types are produced to give performance characteristics, which make them suitable for differing welding applications. The most widely used, acid rutile coated electrodes, produce a spray arc type metal transfer, self-releasing slag and a finely rippled aesthetic weld profile. Minimal post weld dressing will be required. They are primarily used in the down hand position when producing fillet and butt welds. Electrodes with this coating type can be used in position but are limited in application and size, i.e. 3.2mm maximum.

Basic coated electrodes produce weld metal of higher integrity, with slag micro-inclusions and gas pores, and are extremely useful for fixed pipe weldments. Slag removal and weld profiles are not as attractive as with the acid rutile types. Special coated electrodes are produced for specific applications; e.g. vertical-down and high recovery downhand welding. Electrodes are manufactured in sizes ranging from 2.5 to 5.0mm in diameter, (308L, 347 and 316L types are also available in 1.6 and 2mm diameters).

Gas metal arc welding (GMAW or MIG/MAG)

This semi-automatic welding process, which can be used manually or automated, involves a continuous consumable solid wire electrode and an argon rich shielding gas. It is employed for its high productivity features when welding thin material using ‘short-circuit’ metal transfer mode, or ‘spray arc’ transfer with thicker material. Power sources, which produce a pulsed current supply, have been developed to provide improved weld metal quality when positional welding, and cleaner weld appearance. Gas mixtures, with the addition of oxygen, helium, carbon dioxide etc have been developed to improve arc stability and weld bead ‘wetting’ characteristics.

Flux cored arc welding (FCAW or FCW)

A version of the MIG/MAG process where the solid wire consumable is replaced with a flux, (FCW), or metal powder, (MCW), filled tubular wire and can be used with equipment of the same type. Two variants of wire are produced, one to provide all positional capabilities and one for higher deposition down-hand welding applications. Higher rates of weld deposition and weld metal overlaying are possible than with the MMA or MIG/MAG process. Significant reduction in post weld cleaning and dressing is possible.

Submerged arc welding (SAW)

A fully mechanised wire and flux powder shielded arc process capable of high deposition rate, fast travel speed and weld quality. Applications include continuous down-hand fillet and butt welds in thicker section plate, pipe and vessels and also stainless steel cladding of carbon steel components, particularly where long seams or extended runs are involved. An electroslag process, employing a strip electrode, is also available for overlaying, having some characteristics which are superior to SAW.

Electric resistance welding (ERW)

Resistance spot and seam welding is generally confined to mass production welding of thinner material, where the overlap joint type of weld configuration, and the resultant crevice will not detract from any corrosion resistance expected during service.

Laser welding

The energy concentration reached in the focused spot of a laser beam is very intense and is capable of producing deep penetration welds in thick section stainless steel, with minimal component distortion. The process employs high capital cost equipment and its use is reserved for mass production manufacturing.

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