Supply Methods & Applications
Supply Methods
MMA electrodes are generally produced in diameters ranging from 1.6mm to 6mm but are available in some types up to 8mm or 10mm, and gravity electrodes may come in even larger diameters. They are manufactured in lengths ranging typically from 300mm to 450mm, but again, gravity electrodes may be supplied in lengths of 700mm or even over 1000mm. Some stainless steel electrodes may be only 250mm long.
Applications
The MMA process is suitable for welding a wide range of materials from carbon steels, alloy steels, stainless and duplex steels, nickel alloys, copper alloys, cast irons, and, to a much lesser extent, aluminium alloys.
It is an extremely versatile and simple process, and is particularly suited to production welding in workshops for on-site fabrication, repair and maintenance. While the productivity is less than with MIG, FCAW or submerged arc, it is still the most widespread general-purpose method of welding. It can be found being used for welding anything from gates and low-technology components to structural steelwork; from general engineering right up to submarines and critical components for offshore applications.
The alloy systems most frequently welded by MMA are steels, from low-carbon to low-alloy, austenitic and duplex stainless steels.
As a general rule of thumb, rutile coated electrodes are suitable for welding mild and low-carbon steels, with low hardenability, up to about 25mm maximum thickness. If welding thicker material, or higher carbon levels, or low-alloy steels with high hardenability, these are more suited to using basic coated rods, to avoid possible hydrogen-induced cracking problems.
Gravity electrodes were developed primarily for the shipbuilding industry, where higher productivity levels could be achieved over normal consumables.
Cellulosic coated carbon and low-alloy steel electrodes are all-positional in all sizes, and in larger diameters (4mm upwards), they find most of their use for welding cross-country pipelines. There they are used vertical-down, in what is known as ‘stovepipe welding’. They are also used in workshops, where the smaller sizes (2.5 and 3.2mm) are used for putting open root runs in pipework.
Stainless steel electrodes come in a variety of coating types, including acid-rutile, rutile and basic, and the suitability of any one type depends largely on the application, the properties required and the cosmetic appearance desired. Acid-rutile types give the best appearance, while the basics give the best properties.
Nickel and copper alloys are still welded with MMA, although other processes are being adopted, particularly MIG for copper alloys. The use of MMA for aluminium has been phased out in favour of MIG and TIG.
The main use for gouging electrodes was for back-gouging weld roots and in repairs. They are gradually being replaced by alternative processes, such as plasma gouging.
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