Desarrollo psicomotor y Sistema Nervioso Central (SNC)

Arc welding also known as shielded arc welding is accomplished by producing an electric arc between the work to be welded and the tip of the electrode.

The arc column is generated between an anode, which is the positive pole of a DC power supply, and the cathode, the negative pole. Metal ions pass from the positive to the negative pole because they are positively charged and thus attracted to the negative pole.

Fig. 5.35 Types of welding processes.

The arc is one of the most efficient means for producing heat that is available to modern technology. Approximately, 50% of the electric energy put into the arc system comes out in the form of heat energy. Approximately two-third of the energy released in the arc column system is always at the anode (the positive pole). This is true in all DC systems. Another type of arc power source used in shielded-arc welding is alternating current (AC). When an AC power supply is used, the heat in the arc column generally is equalized between the anode and the cathode areas, so that the area of medium heat is then in the plasma area (Fig. 5.36).

Fig. 5.36 Heat liberation in arc welding.

The arc column and power sources. The welding circuit consists of a power source, the electrode

direct current and alternating current. Each of these two power supplies has distinct advantages. In DC welding, the electron flow is in one direction and in AC welding, the electron flow is in both directions. In DC welding, the direction can be changed by simply reversing the cables at the terminals located on the generator. The different settings on the terminals indicate that the electron flow will be either from the electrode to the work, which is the positive ground, or from the work to the electrode, which is the negative ground.

Two-thirds of the heat is developed near the positive pole while the remaining one-third is developed near the negative pole. As a result, an electrode that is connected to the positive pole will burn away approximately 50% faster than one that is connected to the negative pole. Knowing this information helps a welder to obtain the desired penetration of the base metal. If the positive ground is used, the penetration will be greater because of the amount of heat energy supplied to the work by the electrode force. At the same time, the electrode will burn away slowly. If the poles are reversed and there is a negative ground, two-third of the heat will remain in the tip of the electrode. For this reason, the

penetration of the heat zone in the base metal will be shallow when compared to the penetration depth of the positive ground arc column. Alternating current yields a penetration depth that is approximately half that achieved by the DC positive ground. Since electron flow switches ground every time the AC cycle changes, the penetration of the heart zone in the base metal is approximately between the two DC types.

In straight polarity, the electrode is negative and the work is positive. The electron flow goes from the electrode into the work. When the electrode is positive and the work is negative, the electron flow is from the work to the anode, a characteristic called reverse polarity (Fig. 5.37). When reverse polarity is used, the work remains cooler than when straight polarity is used.

Fig. 5.37 DC arc welding.

In both the AC and DC power sources, the arc serves the same purpose: producing heat to melt metal. If two pieces of metal that are to be joined are placed so that they touch or almost touch one another and the arc from the electrode is directed at this junction, the heat generated by the arc causes a small section of the edges of both pieces of metal to melt. These molten portion along with the molten portions of the electrode flow together. As the arc column is moved, the molten puddle solidifies, joining the two pieces of metal with a combination of electrode metal and base metal.

The coatings on the electrodes burn as the electrode wire is melted by the intense heat of the arc. As the electrode wire melts, the electrode covering, or the flux, provides a gaseous shield around the arc, preventing contamination. The force of the arc column striking the workpiece digs a crater in the base metal. This crater fills with molten metal. As the flux melts, part of it mixes with the impurities in the molten pool causing them to float to the top of the weld (Fig. 5.38). This slag protects the bead from the atmosphere and causes the bead to cool more uniformly. The slag also helps to design the contour of the weld bead by acting as an insulator. By insulating the heat-affected zone, located in the parent metal or the base metal and completely surrounding the weld bead, the slag allows an even heat loss from this heat affected zone, thus helping to control the crystal or grain size of the metal.

The arc column reaches temperatures from 5000 to 7000°F. These temperatures have a harsh effect on the parent metal. The molten pool, which must maintain a temperature of approximately 2800°F,

radiates heat outward and changes the crystals surrounding the weld bead. Many times after welding, a part must be heat treated to change the size of the grains in the weld bead and the surrounding area. The heating of the base metal by the arc stream and the resultant molten weld puddle or crater

generally extend deep into the base metal. The extent of the heat-affected zone can be observed by studying the crystalline structure of the base metal in this zone. It generally is represented by a large grain. The grains in the unaffected areas of the metal are smaller. Because of the protection of the flux, the weld bead itself has medium-sized grains that extend to large grains at deeper penetration. It is not necessary to heat treat mild steel; but with many metals, the heart from welding will result in locked-in stresses that must be relieved either through peening or further heat treatment of the entire piece of metal.

Power supplies. Power is needed to supply the current that supports the arc column for fusion

welding. There are three types of welding power supplies: DC motor generators, AC transformers, and AC transformers with DC rectifiers.

Voltage characteristics. There are three major current–voltage characteristics commonly used in

today’s arc welding machines to help control the fluctuating currents caused by the arc column.

They are: (1) the drooping-arc voltage (DAV), (2) the constant-arc voltage (CAV), and (3) the rising- arc voltage (RAV).

The machine that is designed with the DAV characteristics provides the highest potential voltage when the welding current circuit is open and no current is flowing. As the arc column is started, the voltage drops to a minimum and the amperage rises rapidly. With DAV, when the length of the arc column is increased, the voltage rises and the amperage decreases (Fig. 5.39(a)). The DAV is the type of voltage–amperage relationship preferred for standard shielded-arc welding that is manually done.

Fig. 5.39 Voltage–Current characteristics in arc welding.

The CAV and a modification of it called the RAV are characteristics preferred for semiautomatic or automatic welding processes because they maintain a pre-set voltage regardless of the amount of current being drawn from the machine (Fig. 5.39(b)). These voltage amperage characteristics are sensitive to the short-circuiting effect of the shielded-arc mechanism of metal transfer. With these types, the spray are method rather than the short-circuit arc method of metal transfer is used. The spray arc, much like a spray gun spraying paint, works on a principle different from that of the short- circuit pinch-off effect of welding with a stick electrode. An advantage of the RAV over the CAV characteristic is that as the amperage requirement is increased, the voltage is increased automatically, thus helping to maintain a constant-arc gap even if short-circuiting occurs. This RAV is adaptable to the fully automatic processes.