Arc weding processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. The intense heat produced by the arc quickly melts a portion of the base metal, resulting in the formation of a weld. This electrical arc is around 3590°C in its center. Filler metal is added in most welding processes to increase the volume and strength of the weld joint. A pool of molten metal, consisting of base and filler metal is formed near the tip of the electrode. As the electrode is moved along the joint, the molten metal solidifies in its wake.
The welding power supply can use either direct current (DC) or alternating current (AC), and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas. In arc welding, the length of the arc is directly related to the voltage, and the amount of heat input is related to the current. The voltage supplied by power companies for industrial purposes-120 volts (V), 230 V, 380 V, or 480 V is too high for use in arc welding. Therefore, the first function of an arc welding power source is to reduce the high input or line voltage to a suitable output voltage range, 20 V to 80 V. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate.
Submerged Arc Welding
Submerged arc welding (SAW) is a high-quality welding method, which involves submerging the welding arc beneath a mound of granular flux particles (consisting of lime, silica, manganese oxide, calcium fluoride) as the arc is initiated. Additional flux is continually added in front of the electrode as weld travel progresses. The flux protects the arc and molten weld metal from ambient atmosphere, thereby preventing the formation of oxides. Filler metal is obtained primarily from an electrode wire that is continuously fed through the blanket of flux into the arc and pool of molten flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. Melted flux becomes slag, which is waste material and must be removed after welding. During the submerged arc welding process, not all flux turns into slag. Depending on the welding process, 50% to 90% of the flux can be reused.
Submerged arc welding is normally operated in the automatic or mechanized mode. Submerged arc welding is ideally suited for any application involving long, continuous welds. The ability to readily weld thick plates, sometimes with simple joint configurations, makes SAW the method of choice for welding components of large and thick structural assemblies. For example, a reactor pressure vessel is a cylindrical vessel with a hemispherical bottom head and a flanged and gasketed upper head. The bottom head is welded to the cylindrical shell, which consist of seceral rings welded together by submerged arc welding with narrow gap. Narrow gap welding offers two major benefits. It is an economical joint configuration with less weld volume to fill compared to other joint configurations and the automation friendly joint welded with moderate parameter limits weld defects and gives a high quality weld.
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