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Title : Precautions in Welding of Stainless Steels
1.0 Joint preparation is key to quality welding of stainless steel. Joints with narrow gap will create difficulty in welding and slag removals. A wide gap will require more welding electrode consumption thus increasing the cost of welding. A 60 to 70 deg included angle at the butt joint may be suitable for welding up to 15 mm thick plates while U and J groove is recommended for thicker plates. The root gap recommended shall be nearly equal to the electrode wire diameter.
2.0 Surface to be welded must be clean, dry, free from dirt, oxide film, oil, grease and moisture.
3.0 Bake the electrodes at 200-250 deg C for half an hour before use.
4.0 Maintain short arc to minimize loss of Chromium and other alloying elements.
5.0 Maintain current at lower side and use smaller electrodes to minimize heat input residual stresses and weld distortions. Too low current may cause unstable arc, sticking of electrode, and poor bead shape. Hence a compromise has to be made. Too high current may cause undercut, difficulty in removal of slag, poor bead appearance, loss of alloying elements, cracks in weld and heavy distortion.
6.0 Avoid weaving as far as possible while weld depositing. If it is adopted, the maximum weaving shall be within 2.5 times the electrode diameter. Stringer beads are preferred to weaving.
7.0 Lift the electrode slowly so that any crater is filled before arc is broken. This will also avoid cracks.
8.0 Welds shall be cleaned using only the stainless steel brush.
9.0 Stainless steels have tendency of excessive expansions and contraction during welding, particularly for thinner plates or sheets. Use rigid jigs and fixtures to hold the welding job firm. Check the alignment properly. Keep copper chill bar below the joint for faster cooling to avoid distortions. Skip welding or back step welding may be followed. However no preheat or postweld heat treatment is required for austenitic stainless steels.
10.0 Always select electrode with weld metal composition equal or better than the parent plate material. The electrodes with AWS specifications having suffix as 15 are with basic low hydrogen coatings and suitable for DC (+) reverse polarity only, but those with suffix 16 have rutile coating and suitable for both AC or DC(+) reverse polarity.
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