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Shielded Metal Arc Welding |
Shielded Metal Arc Welding - Shielded metal arc welding is probably the most widely used process for stainless steels. Its principal advantage is flexibility. The disadvantages, however, are
(i) The slag blanket constitutes a potential source of inclusions.
(ii) Visibility during welding is impaired by slag.
(iii) The process is not adaptable to very thin sections.
(iv)Electrodes are sensitive to moisture pick up.
Electrodes
If the molten pool is not adequately protected from atmospheric oxidation, certain essential alloying elements (such as chromium) may be oxidized and pass into the slag thereby rendering weld metal deficient in corrosion resisting properties.
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For this reason, electrodes, generally, contain a higher percentage of chromium as compared to base metal.
Electrodes should be stored carefully so that they do not pick up moisture. Indian electrodes for welding austenitic stainless steel are, these electrodes can be operated with direct current reverse polarity (electrode + ve). The electrodes operating on AC employ titania-type covering.
Typical electrode specifications may be as follows:
Arc length should be kept as short as possible. Excessive arc lengths and larger diameter electrodes contribute to loss of chromium in the weld deposit.
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The flux should not contain carbonaceous materials (because they will add carbon in weld metal). Moreover, the flux must have adequate fluidity and dissolving power to fuse undesirable oxides from the molten metal.
Surface preparation
All grease, paint or other foreign matter must be removed to prevent carbon pickup and inclusions in the deposited weld metal.Edges/ to be welded need no preparation unless they are more than 3.2 mm thick.
Sheets 4.8 mm thick need only a single bead to be deposited from one side and should be given a Vee angle of 450 to 600 leaving about 1.6 to 2.4 mm (root face) unbevelled at the bottom, and a root gap as wide as the sheet is thick.
Sheets over 4.8 mm thick need two beads or more, and about2.4 mm distance as the root face (or, for double Vees, in the centre). Depending upon plate thickness, square butt, single V (with or without root face), single U, and double V joints with or without copper chill bar (placed at the bottom of the plates to be welded) may be employed for welding purposes.
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For welding thinner sheets tack welds may be employed, the tacks being 50-150 mm apart according to the thickness of the sheet; closer spacing being used with thinner pieces. The tack welds should penetrate right through and be flat on the surface so that it is possible to deposit weld metal upon them without causing irregularity of the weld.
Welding Technique
Fixtures or closely spaced tack welds are employed to control distortion that results owing to higher coefficient of thermal expansion of stainless steels as compared to carbon steels.
The back step and block sequences* are recommended to reduce distortion in sheet, strip and plate assemblies. The lower thermal conductivity of stainless steel and its higher electrical resistively permit greater deposition rates at the same current.
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Since, austenitic stainless steels are not hardenable by heat-treatment, preheating is rarely used. Austenitic stainless steel should be welded in the annealed condition. A stainless steel that has been purposely work hardened to increase its strength will be softened in the heat affected zone.
When welding in flat position, the stringer-bead technique should be used for the first pass. For succeeding passes, stringer beads or a slight weave should be used. Excessive weaving of electrodes of any size should be avoided.
When welding in vertical downward position, the electrode should be tilted so that the force of the arc pushes the molten metal back up the joint. Vertical up welding calls for a triangular weave with 4 mm diameter or smaller electrodes. Overhead welding requires a whipping technique and a slightly circular motion.
When several beads are required to weld the plate, one should prefer a number of light beads rather than to try to fill the groove in one or two passes, because this will associate the danger of shrinkage cracks and decreased corrosion resistance.
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Individual weld beads should be thoroughly cleaned with a blunt tool and then with a stainless steel brush to eliminate possible slag entrapment. Unbacked or double Vee groove welds must be thoroughly cleaned on the backside with the help of grinding, chipping or arc gouging to ensure complete fusion at the root.
Backing rings may be used for better fit up and easier root pass welding of stainless steel piping that cannot be back welded from the inside. Austenitic stainless steels need not be stress relieved after welding.
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