Essentials of Shielded Metal-Arc Welding Technology
The objective of this chapter is to provide a review of shielded metal-arc welding technology. By learning the principles treated here, the welder will have a better understanding of how and why to make adjustments to the machine and in his welding technique.
Basic Concept of Electric Arc Welding. In shielded metal-arc welding, an electrical circuit is established between the workpiece and the welding machine. The current in this circuit may be either alternating current (AC) or direct current (DC), although DC welding is preferred for welding pipe.
When using DC current, the polarity may be straight or reversed. Straight polarity means that the electrode is negative and the work is positive. The current in this case flows from the electrode to the work. For reversed polarity the above conditions are reversed; the work is negative, the electrode is positive, and the current flows from the work to the electrode.
Current flow is measured in amperes, and the term “amperage” is sometimes used to refer to current flow. Voltage refers to electrical pressure, which is measured in volts and which can be likened to hydraulic pressure. Voltage is the force that causes the current to flow. It is important to realize that a force, as well as a voltage, cannot exist unless there is a resistance to overcome. Feel the force built up in your arm when you press against a wall and then try to do this by “pressing” into the open space around you.
Open circuit voltage is a term encountered in electric arc welding. It is the voltage existing when there is no contact between the electrode and the work and when there is no arc. No current can flow in this case because the resistance is too great for the voltage or electrical pressure to overcome. However, the electric generator is trying to overcome this resistance by generating the maximum voltage of which it is capable.
When the electrode touches the workpiece, the resistance to the current flow is lowered and the current will flow in the circuit. Because there is less resistance, the electrical pressure required to “push” the current is less; i. e., the voltage is much lower than the
open circuit voltage. In this case so much current will flow that if the electrode is stuck to the workpiece, it will overheat.
However, if the electrode is backed away slightly from the workpiece to form the arc, less current will flow because there is increased resistance to the flow of current caused by the gap. Since there is more resistance to the flow of current, more voltage will be generated to overcome this resistance.
When the arc is long and the gap is larger, more voltage is generated and less current will flow than when the arc is short, say normal, for welding. The increased current flow encountered when a normal arc length exists causes a greater temperature rise in the workpiece and deeper penetration, as compared to a long arc length.
When current flows, it encounters a resistance to this flow in the workpiece which causes its temperature to rise. An even greater resistance is encountered in “jumping” the gap, which creates much heat within the arc. The effect is to cause the metal in the workpiece and at the end of the electrode to melt.
The metal at the end of the electrode forms a droplet or globule which is transferred across the arc to the workpiece. If a molten pool of liquid metal exists on the workpiece, this globule or filler metal will mix with the metal in the molten pool, or base metal, forming an alloy, consisting of filler metal and base metal.
When the arc moves on, the puddle of molten metal is maintained by melting additional base metal ahead of the arc and adding filler metal from the electrode. However, some of the metal behind the arc solidifies to form the bead.
Metal Transfer. Of particular importance is that the pipe welder should understand how the metal is transferred from the electrode to the workpiece.
The droplet or globule is transferred from the electrode to the base metal by the propellant force of the arc and by an attractive force exerted on the globule by the base metal. In passing from the electrode to the puddle, the force of gravity also acts on the globule. When welding in the flat position the transfer of the globule is assisted by gravity. However, in the overhead position it opposes this transfer.
With the normal arc length used in overhead welding, the combined effect of the propellant force and the attractive force is large enough to overcome the pull of gravity and the filler metal will be deposited on the workpiece. When a long arc is maintained in overhead welding, the globule has a longer path to travel and more time is available for the pull of gravity to act, causing the globule to slow down. In this case, the globule will sometimes not reach the base metal and if it does, it wiil be moving at a slower speed. If, as is usually the case when the arc is long, the workpiece is not hot enough, the slow moving globule will not attach itself to the workpiece and will drop to the ground. Therefore, when a long arc is used in overhead welding, no filler metal is transferred to the workpiece. This is important when striking an arc to start a bead in the overhead position.
Electrode Coating. Almost all modern electrodes are coated. The function of the coating is to form a gaseous shield over the molten metal to protect it from the effect of the atmosphere (see Fig. 2-І).
Certain ingredients in the coating enter the molten puddle to deoxidize the metal. The compounds thus formed are lighter than the molten metal and rise to the top of the puddle to form a slag coating when the metal solidifies. This coating, while it is still hot, protects the solidified metal from the atmosphere.
In some electrodes there are additional ingredients, certain o? which are included to stabilize the arc. Others may be added in th< form of powdered metals to provide alloying elements or additions iron for the puddle.
In welding, care must be exercised to manipulate the puddle of liquid metal so as not to trap the slag within the metal as it solidifies. It must be allowed to rise to the surface of the weld bead. Also, for the same reason, the slag coating or crust must be completely removed before a second bead is welded over the first bead. Entrapped slag can have serious harmful effects on the quality of the weld.
Arc Length. Arc length is the gap or distance between the electrode tip and the surface of the puddle. The correct arc length is primarily
dependent upon the type of electrode used and upon the environmental conditions in which the welding is done.
When a heavily coated and highly alloyed electrode is used, a short arc length must be maintained because the higher alloyed filler and base metal are very sensitive to porosity. Holding the short arc protects the puddle from the atmosphere, which is the cause of porosity.
Since the cooling rate is faster with lightly coated electrodes, the arc length should not be choked. By maintaining a longer arc (approximately /32 to % inch) the voltage will increase slightly, causing the puddle to spread out or enlarge. This, in turn, will allow the
filler metal to flow across the entire puddle, as shown in Fie. 2-2A.
Fig. 2-2. The effect of the arc length. A, Correct arc length; B. Short arc length.
If the arc is too short (approximately Уїв inch), the size of the puddle is reduced considerably and the filler metal has a limited area in which to be deposited. This causes the filler metal to rise, which can result in incomplete fusion along the edge of the bead, as shown in Fig. 2-2B.
The arc length also depends upon the environmental conditions, whether in a closed shop or in the open atmosphere, as in the case of an oil line or a refinery. Generally, a shorter arc is required when welding outdoors; and for heavily coated and highly alloyed electrodes a short arc is used in all situations.