WELD DEFECTS – PRECAUTIONS & PROCEDURES TO IMPROVE SHOP-FLOOR PRODUCTIVITY Weld defects lead to re-work and rejections, thereby becoming one of the leading reasons for diminished productivity. In this issue we provide you with a synopsis of the most common weld defects and some precautionary measures for your shop-floor. With the right product, adherence to correct process norms, your welding costs and productivity will become more efficient. ARC BLOW Appears only while welding with DC power source in which an arc becomes uncontrollable and electrode end burns taper and resemble an eccentrically coated electrode. Precautions / Corrective action Current may be slightly reduced (Recommended current should be used) Weld towards a heavy tack weld or the weld bead already made Use back-step welding technique on long seam joints Place ground connection closer to the arc position Wrap the ground cable around the job to neutralize the effect of magnetic arc blow Switching over to AC arc may be the final and last option for some typical job design CRACKS Weld Cracks are predominantly caused when the :- Weld metal is not ductile enough. (it may be too low in manganese or too high in carbon.) Base metal is high in carbon or sulphur or contains hardening elements like Mo, Cr, V, Ni, etc. Change the base metal or use low hydrogen electrodes and preheat if necessary Base metal is too thick or the assembly is under very heavy restraint Electrodes are very damp. Dry the electrodes as per recommendations In high carbon steels and alloy steels especially when the sections are thick, “under bead cracks” are found at the junction between the weld metal and the base metal. They are invisible and open out to the surface after sometime. They can be avoided by using low hydrogen electrodes freshly dried in an oven, and preheating the joint areas adequately Precautions / Corrective action Correct recommendation of correct welding consumables Use no hydrogen type for good results Preheating the job on the basis of thickness is must. Follow the corrected welding sequence Storage condition must be in damp free atmosphere. Use dry electrodes only To preheat the job on the basis of thickness and use of low hydrogen type dried electrodes only avoid under bead cracks LACK OF FUSION This defect is caused by using too little welding current and use of too small or too large an electrode in relation to the joint size, improper arrangements of weld passes, too low welding current for given size of electrode. Precautions / Corrective action Correct fit-up of joints with suitable size of the electrodes following recommended welding parameters are very important to avoid Lack of Fusion. OVERLAPPING Overlapping is caused by using a higher than recommended welding current or too slow travel speed of welding. It may be also due to incorrect electrode angle and weaving technique. Precautions / Corrective action Strictly following the recommended welding parameters can avoid this defect. POROSITY May be seen over the surface or may be hidden inside the weld. Caused by damp electrodes, rusty or dirty plates, high sulphur in the plates or pipe materials, or sluggishness of the weld puddle. If the weld is sluggish, increase the welding current and weave electrode briskly. Cluster porosity indicates the need of changing your arc striking technique from tapping to scratching type. Linear porosity appearing longitudinally in the middle of weld bead is a sign of lack of penetration and if it is observed at the toe then that is a sign of lack of fusion or overlapping. Damp electrodes particularly low hydrogen electrodes to achieve porosity free welds. For more on R2U packaging, click here adorwelding Precautions / Corrective action Impurities in the base metal non cleanliness of base metal moist electrodes in use. SLAG INCLUSION It may be in the surface or hidden. It is cause by the dirty and uneven fusion faces, inadequate cleaning after each pass, and improper arrangements of passes. Too convex bead, too large size of an electrode applied in a narrow groove will also contribute to slag inclusion. Improper electrode angle, unstable long arc too invite the problem of slag inclusion. Precautions / Corrective action Clean the weld area adequately, cleaning at each pass, correctly arrange passes only. Large size of the electrodes than required for narrow groove. UNDER CUT It is cause by using too high welding current and too fast technique and electrode angle. Precautions / Corrective action Avoid too high current and too fast traveling speed control the weaving during welding.
Posted on: Thu, 12 Sep 2013 04:40:24 +0000
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