September 1, 2009
Shielded metal arc welding (SMAW) is the most common form of arc welding. However, creating a good weld is not always easy, especially for a beginner. Following a few simple tips as well as learning the common weld defect and how to fix them will have you on your way to laying quality SMAW welds.
Shielded metal arc welding (SMAW) is the most common form of arc welding. However, creating a good weld is not always easy, especially for a beginner. Unlike gas metal arc welding (GMAW), in which you basically point and shoot, SMAW requires a higher skill level and a mastery of certain techniques.
By following a few simple tips, even beginners can learn how to spot common weld defects and fix them to create a high-quality weld.
Whenever possible, select AISI-SAE 1015 to 1025 steels with 0.1 percent maximum silicon and sulfur content under 0.035 percent. Using these steels will make the SMAW process easier because they can be welded at fast speeds with minimum cracking tendencies.
If you are welding with low-alloy steels and carbon steels with chemistry compositions higher than this normal range, they will have a greater tendency to crack, particularly when welding on heavy plate and rigid structures. In addition, steels with high sulfur and phosphorus contents are not recommended for production welding. If they must be welded, use small-diameter, low-hydrogen electrodes. A slow travel speed will further keep the puddle molten, allowing gas bubbles time to boil out, creating a better finished weld.
Joint position can greatly affect finished weld quality. When welding on 10- to 18-gauge sheet steel, the fastest travel speeds are obtained with the work positioned at 45 to 75 degrees downhill. Also, don't overweld or make a weld that is larger than necessary for the sake of joint strength—this may lead to burn-through.
When welding mild steel plate that is 3/16 in. thick or more, it is best to position the work flat, as this will make it easier for you to manipulate the electrode. Last, it is best to weld high-carbon and low-alloy steel plate in the level position.
Joint dimensions are chosen with fast welding speed and good weld quality in mind. Proper joint geometry is based on a few simple principles. First, fit-up must be consistent for the entire joint. Since sheet metal and most fillet and lap joints are tightly clamped for their entire length, gaps or bevels must be controlled accurately over the entire joint. Any variations in a joint will force you to slow your welding speed to avoid burn-through and manipulate the electrode to adjust for the fit-up variation.
Second, you need a bevel that will aid in good bead shape and penetration. An insufficient bevel prevents the electrode from getting into the joint. For example, a deep, narrow bead with an insufficient bevel may lack penetration, making it prone to cracking.
Third, a root opening is needed for full penetration. The root opening must be consistent with the diameter of the electrode being used. An excessive root opening wastes weld metal and slows welding speed.
And last, a root face or a backup strip is required for fast welding and good quality. Feather-edge preparations require a slow, costly seal bead. However, double-V butt joints without a land are practical when the seal bead cost is offset by easier edge preparation and the root opening can be limited to approximately 3/32 in.
In general, weld seal beads on flat work with a 3/16-in. E6010 at approximately 150 amps direct current electrode positive (DCEP). Use 1/8 in. at approximately 90 amps DCEP for vertical, overhead, and horizontal butt welds. For low-hydrogen and seal beads, weld with an EXX18 electrode at approximately 170 amps.
Fillets should have equal legs, and the bead surface should be nearly flat. Buildup shouldn't exceed 1/16 in. Extra buildup is costly in material and time, adds little to weld strength, and increases distortion. For example, doubling the size of a fillet requires four times as much weld metal.
To avoid porosity and attain the ideal weld travel speed, it is important to remove excessive scale, rust, moisture, paint, oil, and grease from the surface of joints. If such elements cannot be removed, use E6010 (5P+) or E6011 (35 or 180) electrodes to penetrate the contaminants and deeply into the base metal. Slow the travel speed to allow time for gas bubbles to boil out of the molten weld before it freezes.
Large electrodes are meant for welding at high currents for high deposition rates. Therefore, use the largest electrode that is practical for your application and consistent with good weld quality. Electrode size sometimes may be limited, especially on sheet metal and root passes where burn-through can occur. As a rule, 3/16 in. is the maximum electrode size practical for vertical and overhead welding, while 5/32 in. is the maximum size for low-hydrogen welding. In addition, joint dimensions sometimes limit the electrode diameter that will fit into the joint.
Here are some of the most common stick welding problems and how to correct them.
Spatter. Although it does not affect weld strength, spatter does create poor appearance and increases cleaning costs. There are several ways to control excessive spatter. First, try lowering the current. Make sure it is within the range for the electrode type and size you are welding with and that the polarity is correct. Another way to control spatter is to try a shorter arc length. If the molten metal is running in front of the arc, change the electrode angle. Finally, look for arc blow conditions (commonly referred to as a wandering arc), and be sure the electrode is not wet.
Undercutting. While frequently just an appearance problem, undercutting can impair weld strength when the weld is loaded in tension or subjected to fatigue. To eliminate undercut, reduce the current and travel speed, or simply reduce the puddle size until you have a size you can handle. Then change the electrode angle so the arc force holds the metal in the corners. Use a uniform travel speed and avoid excessive weaving.
Wet Electrodes. If polarity and current are within the electrode manufacturer's recommendations but the arc action is rough and erratic, the electrodes may contain excessive moisture. Try dry electrodes from a fresh container. If the problem recurs frequently, store open containers of electrodes in a heated cabinet.Wandering Arc. With DC welding, stray magnetic fields cause the arc to wander off course. This is a greater problem at high currents and in complex joints. To control a wandering arc, the best option is to change to AC welding. If that doesn't work, try using lower currents and smaller electrodes or reduce the arc length. In addition, you can change the electrical path by shifting the work connection to the other end of the workpiece or by making connections in several locations. You also may do this by welding toward heavy tacks or finished welds, using run-out tabs, adding steel blocks to change the work current path, or tacking small plates across the seam at the weld ends.
Porosity. Most porosity is invisible. However, since severe porosity can weaken the weld, you should know when it tends to occur and how to combat it. Begin by removing scale, rust, paint, moisture, and dirt from the joint. Be sure to keep the puddle molten for a longer time to allow gases to boil out before it freezes. If the steel has a low carbon or manganese content, or a high sulfur (free-machining steel) or phosphorus content, it should be welded with a low-hydrogen electrode. Sometimes the sulfur content of free-machining steels can be high enough to prevent successful welding.
Minimize the mixture of base metal into weld metal by using low current and fast travel speed for less penetration. Or, try using a shorter arc length. A light drag technique is recommended for low-hydrogen electrodes. For surface holes, use the same techniques that are used for porosity. If you are using AWS E6010 or E6011 electrodes, make sure that they are not too dry.
Poor Fusion. Proper fusion means the weld physically bonds strongly to both walls of the joint and forms a solid bead across the joint. Lack of fusion is often visible and must be eliminated for a sound weld. To correct poor fusion, try a higher current and a stringer bead technique. Be sure the edges of the joint are clean, or use an AWS E6010 or E6011 electrode to dig through the dirt. If the gap is excessive, provide better fit-up or use a weave technique to fill the gap.
Shallow Penetration. Penetration refers to the depth the weld enters into the base metal, and usually is not visible. To achieve strong welds, full penetration to the bottom of the joint is key. To overcome shallow penetration, try using higher currents or decreasing your travel speed. Use small electrodes to reach down into deep, narrow grooves. Remember to allow some gap at the bottom of the joint.
Cracking. Cracking is a complex subject because there are many different types of cracks that occur in different locations throughout a weld. All cracks are potentially serious, as they can lead to complete failure of the weld. Most cracking is attributed to high carbon, alloy, or sulfur content in the base metal. To control cracking:
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