Welding Faults and Their Prevention
Undercutting. When the base metal along the edge of the weld is reduced from its original thickness, as shown in Fig. 13-3, the weld is said to be undercut. There are several reasons why undercutting occurs.
Excessive current can cause the edge of the joint to melt and the molten metal will wash into the weld, leaving a drain-like impression at the edges of the weld. This can occur in varying degrees; even with a normal arc length, undercutting can occur if the electrode is not manipulated in such a manner as to supply an adequate amount of filler metal to the molten edge of the weld. Because the edge of the weld cools more rapidly than the center of the weld, the arc should pause at the edges, when weaving, in order to deposit filler metal and to supply additional heat to the area.
Pausing at the edge also has the effect of churning the metal in this region, thereby obtaining a better mixture of the base metal and
F ig. і3-3. Undercut.
the filler metal. If the composition of the weld metal varies greatly from the edge to the center, it will solidify at a different temperature, and for this reason, the correct composition of filler metal should always be used. Using incorrect filler metal can also contribute to undercutting.
If the electrode angle is too small, the arc force will tend to wash away the molten metal at the edges of the joint to cause undercutting. To avoid this cause of undercutting, it is important to maintain the correct electrode angle while welding. Other causes of undercutting are dampened electrodes, using an excessive arc length, and a welding speed that is too slow.
In summary, undercutting is caused by any one or a combination of the following factors;
1. Excessive welding current
2. Incorrect electrode manipulation
3. Using an incorrect welding rod which provides filler metal of the wrong composition
4. Electrode angle too small
5. Arc length too long
6. Welding speed too slow
7. Using dampened electrode.
Slag Inclusions. Slag inclusions (nonmetallic particles of slag embedded in the weld) can have a serious, adverse effect on the quality of a weld. Usually the slag is from the electrode coating, although, in some instances, particles of slag (slag inclusions) appear in the base metal and they can be retained in the weld. Figure 13-4 illustrates a weld in which a large amount of slag inclusions appear.
These slag inclusions frequently appear at the edges of a weld if the correct welding procedure is not used. The molten slag, being lighter than the weld metal, rises to the surface of the liquid pool, rapidly forming a blanket that covers the metal. Furthermore, the slag solidifies at a lower temperature than the metal. Thus, when an edge is not completely filled with metal and has a drooping contour, the slag blanket slips around this contour and settles in the corner. When the metal solidifies, more slag will settle in the edge to form a tightly adhering inclusion.
During weaving, the arc should pause at the edge of the joint to provide sufficient heat in this area so that it will not cool too rapidly and will deposit additional filler metal to join the surfaces of the solid and liquid metal without an undercut appearing. By pausing, any previously trapped slag in this area will also be remelted and will have time to rise to the surface of the weld metal.
Fig. 13-4. Inclusions in weld metal.
The arc force churns the metal and forces some of the slag into the body of molten metal. Under certain conditions, some of this slag can become entrapped below the surface of the metal by other metal that is solidifying from the walls of the joint and up from the bottom of the weld as well. This can result from erratic electrode manipulation. Most frequently, however, it is caused by maintaining a pool of molten metal that is too large in relation to the electrode diameter.
If the body of liquid metal is too large it will have a tendency to roll, especially if it is also excessively hot and very fluid. As shown in Fig. 13-5, the layers of liquid metal above the fusion line will move
Fig. 5 3-5. Liquid layers above the fusion line will move out further than liquid
layer at fusion line.
out further than those close to the fusion line. The liquid slag which blankets the molten metal will be rolled over toward the fusion line where it will be trapped as the metal solidifies. To prevent slag inclusions from this cause, the puddle must not be allowed to become too large in relation to the electrode size and an excessively high current setting should not be used.
Another frequent cause of slag inclusions is carelessness in deslag - ging a previous layer of weld metal. Proper cleaning and deslagging is essential prior to welding any additional beads. If this has not been done, some of the particles of the slag coating may not have enough time to rise to the surface when the weld is restarted, thereby becoming entrapped. At all times, the solidified slag coating of a previously deposited bead must be chipped off and this should be followed by a vigorous application with a wire brush to remove any remaining particles. When restarting the weld to continue a bead, this should only be done for a distance of one or two inches behind the crater and in the crater, in order to retain as much heat as possible in the bead. However, before another bead is deposited over this bead, all of the slag coating must be removed.
Fig. 1 3-6. Porosity below the surface of the weld bead. Part ground to expose
Inclusions can also be caused by heavy oxides such as rust and surface scale. These oxides remain undissolved in the molten metal and do not readily rise to the surface. When the weld metal freezes, the oxide inclusions remain entrapped in the weld.
Porosity. Porosity is caused by oil, grease, or moisture entering the weld metal to form gas bubbles which become trapped in the freezing metal. Porosity can appear in the form of large blow holes or in the form of small pinholes. In Fig. 13-6, the weld bead has been ground down slightly below the surface to show porosity in the form of small blow holes. It is possible that there are additional blow holes or pinholes below those shown.
Moisture is, perhaps, the principal cause of porosity. When moisture enters the molten metal it forms a vapor, and at the temperature of the melt, the water vapor (H2O) quickly disassociates into hydrogen and oxygen gas. A part of these gases rises to the surface and escapes to the atmosphere. However, a part of these gases is dissolved by the molten metal.
The amount of gas that a metal can hold in solution is dependent upon the temperature of the melt. Less gas can be held in solution as the temperature decreases. When the temperature of the molten metal drops, it must precipitate out some of the gas that was in solution. The precipitated gas forms a bubble which grows in size as it rises to the surface of the puddle.
When the temperature of the molten metal drops, the liquid metal also becomes less fluid. Ultimately, as the temperature continues to drop, the metal becomes mushy. When the mushy stage has been reached, gas bubbles rise very slowly and it is easy for them to be trapped in the weld metal upon further cooling and solidification of the melt.
Not all of the porosity is caused by the gases that were precipitated from the molten metal. If the speed of welding is very fast, the molten metal will freeze so rapidly that bubbles of undissolved gas are trapped. Also, excessively long whipping or weaving strokes can cause the molten metal to periodically freeze very rapidly thereby entrapping the bubbles. Moreover, by exposing the surface of the puddle to the atmosphere, additional gases will be dissolved by the molten metal when weaving or whipping excessively.
To prevent porosity, excessive whipping and weaving must be avoided and the speed of welding must not be too fast. All oil and grease must be removed from the weld joint prior to welding. Handling the electrodes with oily, greasy, or damp gloves is an all-too-common cause of porosity, something of which many welders are not aware. Oil, grease, and moisture can be soaked up by the electrode coating and passed on into the weld metal. Electrodes should not be soaked in water and whenever rain or snow can reach the weld joint, welding should be stopped. The weld joint must be free of any moisture; if necessary, the joint should be dried with an
oxyacetylene torch. When they can be used, low-hydrogen electrodes reduce porosity to a significant extent.
Lack of Fusion and Incomplete Penetration. Lack of fusion and incomplete penetration are unacceptable in many industrial piping systems, particularly in systems subjected to high pressures. When these defects are present in root beads they are in the form of small channels and crevices in which corrosive compounds can settle, as shown in Fig. 13-7. These small defects can enlarge into more serious defects, especially when the pipes contain corrosive substances such as acids, liquefied gases, and sulfur compounds.
Primarily, lack of fusion and incomplete penetration occur when the welding procedures described in the previous chapters have not been followed or when the welder is careless in depositing the weld. In any particular case, the reader must refer back to the pertinent chapters when seeking measures to prevent a particular kind of defect. AH that can be said in a general way is that all facets of the welding procedures must be given the most careful attention. This would include the current setting, arc length, electrode angle, electrode manipulation (weave or whipping), the keyhole in the case of root beads, and always the pool of molten metal.
Fig. 13-7. Incomplete penetration at the bottom of a root bead.