![]() ![]() Mixes with little CO 2 usually are used for spray transfer on heavy materials or when low heat input and shallow penetration are desired for thin materials. In the argon/CO 2 mixes, the CO 2 content varies from 5 percent to 25 percent. Most of the commonly used mixes for this material consist of argon/CO 2, argon/O 2, or all three gases together. Many different gas mixtures are used in welding, depending on the process and material (see Figure 5). It is mainly used for the promotion of austenite and to improve corrosion resistance in duplex and superduplex steels. Nitrogen is the least commonly used addition for shielding purposes. Hydrogen also may be used in higher percentages (30 percent to 40 percent) in plasma cutting operations on stainless steel to increase capacity and reduce slag. Hydrogen is not suitable for ferritic or martensitic steels because of cracking issues. As with all of the diatomic molecules, a hotter, wider surface bead results. Hydrogen is primarily used with austenitic stainless steels to promote oxide removal and increase heat input. Hydrogen is a diatomic, active shielding gas component usually used in welding mixes at concentrations of less than 10 percent. ![]() Oxygen also is used in trimixes with CO 2 and argon, where it provides the wetting and spray advantages. Oxygen/argon mixes exhibit a characteristic "nailhead" penetration profile with GMAW carbon steel, which is the most common application. Since the high heat reduces the surface tension of the molten metal, spray transfer is facilitated, as well as wetting at the toe of the weld. Oxygen creates a very wide and fairly shallow penetration profile, with high heat input at the surface of the work. Oxygen has a heat input potential resulting from both its ionization energy and its energy of disassociation (energy liberated by the splitting of the molecule into individual atoms in the arc). Oxygen is a diatomic (two-atom-molecule), active shielding gas component usually used in GMAW mixes at concentrations of less than 10 percent. Produce spray transfer, and it promotes globular transfer, which can cause a great deal of spatter.Ī fabricator has a variety of shielding gases to consider when arc welding. For GMAW applications, pure CO 2 is unable to This tends to give the overall weld a well-balanced width-to-depth penetration profile (see Figure 4). Recombination of the CO/O 2 gives a fairly wide penetration profile at the surface of the weld, while the low ionization potential and thermal conductivity create a hot area at the center of the arc column. This creates the potential for oxidizing of the base metal and dealloying of the weld puddle The CO 2 will disassociate into CO and O 2 at the temperatures encountered in the arc. CO 2 is a compound molecule with fairly complex interactions in the arc column. CO 2 usually is used for GMAW short-circuit transfer and FCAW. Pure helium promotes globular transfer and is rarely used for GMAW, with the exception of pure copper.Ĭarbon Dioxide. Somewhat higher flow rates are recommended because the gas tends to rise in air. The high ionization potential can create difficulty in arc starting unless high-frequency or capacitive arc starting is used for GTAW. Helium provides a wide profile (see Figure 3), good wetting on the edges of the bead, and higher heat input than pure argon. In contrast to argon, helium has a high conductivity and ionization potential, which gives the opposite effects. ![]() Helium also is a monatomic, inert gas, most commonly used for GTAW on nonferrous materials. In the GMAW process, pure argon promotes spray transfer. ![]() There is a slight tendency for undercutting or bead rollover caused by the lack of heat at the outer edges of the puddle in both GTAW and GMAW. This forms a narrow arc column, which in turn creates the traditional pure argon penetration profile: deep and relatively narrow (see Figure 2). This gas has a low thermal conductivity and ionization potential, properties that result in a low transfer of heat to the outer areas of the arc. ![]()
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