common welding defect

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common welding defect

مُساهمة من طرف مؤتمن عبدالرحيم في الأحد مايو 02, 2010 12:20 am

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Welding Defect




[size=12]Weld Defects-Their Causes and How to Correct Them

With the correct welding conditions, techniques and material quality standards, the mig process will yield avery high quality weld deposit. However, as with any other welding process, weld defects can occur. Mostdefects encountered in welding are due to an improper welding procedure. Once the causes aredetermined, the operator can easily correct the problem.Defects usually encountered include incomplete penetration, incomplete fusion, undercutting, porosity, andlongitudinal cracking. This section deals with the corrective action that should be taken.



1- INCOMPLETE PENETRATION



This type of defect is found in any of three ways:1) When the weld bead does not penetrate the entire thickness of the base plate.2) When two opposing weld beads do not interpenetrate.3) When the weld bead does not penetrate the toe of a fillet weld but only bridges across it.Welding current has the greatest effect on penetration. Incomplete penetration is usually caused by the useof too low a welding current and can be eliminated by simply increasing the amperage. Other causes canbe the use of too slow a travel speed and an incorrect torch angle. Both will allow the molten weld metal toroll in front of the arc, acting as a cushion to prevent penetration. The arc must be kept on the leading edgeof the weld puddle.Figure 10-1 - Examples of Lack of Penetration





2-LACK OF FUSION



Lack of fusion, also called cold lapping or cold shuts, occurs when there is no fusion between the weld metaland the surfaces of the base plate. This defect can be seen in Figure 10-2. The most common cause of lackof fusion is a poor welding technique. Either the weld puddle is too large (travel speed too slow) and/or theweld metal has been permitted to roll in front of the arc. Again, the arc must be kept on the leading edge ofthe puddle. When this is done, the weld puddle will not get too large and cannot cushion the arc.Another cause is the use of a very wide weld joint. If the arc is directed down the center of the joint, themolten weld metal will only flow and cast against the side walls of the base plate without melting them. Theheat of the arc must be used to melt the base plate. This is accomplished by making the joint narrower or bydirecting the arc towards the side wall of the base plate. When multipass welding thick material, a split beadtechnique should be used whenever possible after the root passes. Large weld beads bridging the entiregap must be avoided.Lack of fusion can also occur in the form of a rolled over bead crown. Again, it is generally caused by a verylow travel speed and attempting to make too large a weld in a single pass. However, it is also very oftencaused by too low a welding voltage. As a result, the wetting of the bead will be poor.When welding aluminum, the common cause of this type of defect is the presence of aluminum oxide. Thisoxide is a refractory with a melting point of approximately 35000F (19270C). It is also insoluble in moltenaluminum. If this oxide is present on the surfaces to be welded, fusion with the weld metal will be hampered.

The best safeguard against this is to remove all oxide as soon before welding as possible.Although iron oxide (rust, mill scale) can be welded over in mild steel, an excessive amount can cause lackof fusion.



3-UNDERCUTTING



As shown in Figure 10-3, undercutting is a defect that appears as a groove in the parent metal directly alongthe edges of the weld. It is most common in lap fillet welds, but can also be encountered in fillet and buttjoints. This type of defect is most commonly caused by improper welding parameters; particularly the travelspeed and arc voltage.



When the travel speed is too high, the weld bead will be very peaked because of its extremely fastsolidification. The forces of surface tension have drawn the molten metal along the edges of the weld beadand piled it up along the center. Melted portions of the base plate are affected in the same way. Theundercut groove is where melted base material has been drawn into the weld and not allowed to wet backproperly because of the rapid solidification. Decreasing the arc travel speed will gradually reduce the size ofthe undercut and eventually eliminate it. When only small or intermittent undercuts are present, raising thearc voltage or using a leading torch angle are also corrective actions. In both cases, the weld bead willbecome flatter and wetting will improve.

However, as the arc voltage is raised to excessive levels, undercutting may again appear. This is particularlytrue in spray arc welding. When the arc becomes very long, it also becomes too wide. This results in anincreased amount of base material being melted. However, the heat transfer of a long arc is relatively poor,so actually thearc is supplying no more total heat to the weld zone. The outermost areas are very quicklycooled and again proper wetting is prevented. The arc length should be kept short, not only to avoidundercutting but to increase penetration and weld soundness.Excessive welding currents can also cause undercutting. The arc force, arc heat and penetration are sogreat that the base plate under the arc is actually ”blown” away. Again, the outermost areas of the basematerial are melted but solidify quickly. Puddle turbulence and surface tension prevent the puddle fromwetting properly. It is always advisable to remain within the current ranges specified for each wire size.



4-POROSITY



Porosity is gas pores found in the solidified weld bead. As seen in Figure 10-4, these pores may vary in sizeand are generally distributed in a random manner. However, it is possible that porosity can only be found atthe weld center. Pores can occur either under or on the weld surface.The most common causes of porosity are atmosphere contamination, excessively oxidized work piecesurfaces, inadequate deoxidizing alloys in the wire and the presence of foreign matter. Atmosphericcontamination can be caused by:1) Inadequate shielding gas flow.2) Excessive shielding gas flow. This can cause aspiration of air into the gas stream.3) Severely clogged gas nozzle or damaged gas supply system (leaking hoses, fittings, etc.)4) An excessive wind in the welding area. This can blow away the gas shield.



The atmospheric gases that are primarily responsible for porosity in steel are nitrogen and excessiveoxygen. However, considerable oxygen can be tolerated without porosity in the absence of nitrogen. Oxygenin the atmosphere can cause severe problems with aluminum because of its rapid oxide formation. The gassupply should be inspected at regular intervals to insure freedom from leakage. In addition, excessivemoisture in the atmosphere can cause porosity in steel and particularly aluminum. Care should be exercisedin humid climates. For example, a continuous coolant flow in water cooled torches can cause condensationduring periods of high humidity and consequent contamination of the shielding gas.Excessive oxidation of the work pieces is an obvious source of oxygen as well as entrapped moisture.Again, this is particularly true for aluminum where a hydrated oxide may exist. Anodized coatings onaluminum must be removed prior to welding because they contain water as well as being an insulator.Porosity can be caused by inadequate wire deoxidation when welding semi-killed or rimmed steels. Theoxygen in the steel can cause CO porosity if the proper deoxidizing elements are not present.Foreign matter can be a source of porosity. An example is excessive lubricant on the welding wire. Thesehydrocarbons are sources of hydrogen which is particularly harmful for aluminum.Other causes of porosity may be extremely fast weld solidification rates and erratic arc characteristics.When solidification rates are extremely rapid, any gas that would normally escape is trapped. Extremelyhigh travel speeds and low welding current levels should be avoided.Erratic arc characteristics can be caused by poor welding conditions (voltage too low or high, poor metaltransfer) and fluctuation in the wire feed speed. All these occurrences cause severe weld puddle turbulence.This turbulence will tend to break up the shielding gas envelope and cause the molten weld metal to becontaminated by the atmosphere.



2-LONGITUDINAL CRACKING



Longitudinal or centerline cracking, of the weld bead is not often encountered in mig welding. However, thatwhich does occur can be one of two types: hot cracks and cold cracks. Typical hot cracks are shown inFigure 10-5. Hot cracks are those that occur while the weld bead is between the liquidus (melting) andsolidus (solidifying) temperatures. In this temperature range the weld bead is ”mushy”. Hot cracks usuallyresult from the use of an incorrect wire electrode (particularly in aluminum and stainless steel alloys). Thechemistry of the base plate can also promote this defect (an example would be any high carbon stainlesssteel casting). Any combination of the joint design, welding conditions and welding techniques that results ina weld bead with an excessively concave surface can promote cracking.One form of this defect which may often be encountered, particularly with any 5000 series aluminum, iscalled a crater crack. These are small cracks which appear at the end of the weld where the arc has beenbroken. Although small, these cracks are troublesome since they can propagate into the weld bead. A cratercrack is shown in Figure 10-6. The major reason for this defect is the incorrect technique for ending theweld. To properly end a weld, the crater should be filled. This is done by reversing the arc travel directionbefore breaking the arc. This technique is depicted in Figure 10-7. In addition, if the welding control isdesigned to supply gas for a short time after the arc is broken, the crater should be shielded until it iscompletely solidified.

Figure 10-5 - Example of Longitudinal

Cracking

Figure 10-6 - Example of Crater Cracking


Those cracks that occur after the weld bead has completely solidified are called cold cracks. These defectsoccur only when the weld is too small to withstand the service stresses involved.For your convenience and quick reference, Table 10-1 lists all possible defects, their cause and correctiveaction.
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مُساهمة من طرف مؤتمن عبدالرحيم في الأحد مايو 02, 2010 12:36 am

و ده الرابط و فيه الملف بصورة أفضل مع الشرح و الصور

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مُساهمة من طرف ابراهيم في الإثنين مايو 03, 2010 10:21 am

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مُساهمة من طرف مؤتمن عبدالرحيم في الأحد مايو 16, 2010 2:20 pm

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