Gas Metal Arc Welding
GMAW (gas metal arc welding), commonly referred to as “MIG” welding, utilizes an automatically fed consumable electrode in the form of wire from a spool for the filler metal. Inert shielding gas is supplied through an annular nozzle at the contact tip of the gun. The process is much better suited for shop fabrication than field fabrication since air movement must be less than 5 mph to maintain the inert gas blanket. (See Figures 600-11 and 600-12.) GMAW is a predominantly a semiautomatic process, but automatic processes are sometimes used for weld overlays.
Depending on the current, voltage (arc length), and shielding gas composition, three modes of metal transfer are commonly used for GMAW:
• Short circuiting transfer (also called short-arc or interrupted-arc transfer). (See Figure 600-13.)
• Globular transfer. (See Figure 600-14.)
• Spray transfer. (Pulsed-current is a variation of spray transfer.) (See Figure 600-15.)
Short circuiting transfer GMAW is a low heat input form of welding which can be used for all positions. However, the process is notorious for producing lack-offusion defects. Therefore this process is not recommended for pressure vessel fabrication, except for the following applications:
• Root passes on circumferential, longitudinal, or nozzle-to-shell weld only if backgouged and backwelded
• Root passes on circumferential piping welds for fabricated nozzles or internal piping
• Root passes on nonpressure containing vessel internals.
Globular and spray transfer GMAW processes are high heat input processes which are acceptable for pressure vessel fabrication. The major drawback of these processes for vessel fabrication is that they are generally limited to the flat position. Therefore, they are typically used only for circumferential and longitudinal welds, rather than for nozzle and attachment welds. They must compete with the faster SAW process (discussed below) for welding applications and are hence not common for pressure vessel fabrication.
A modification of the spray transfer GMAW process, pulsed-current GMAW, utilizes a pulsed current and smaller diameter wire to achieve all-position capabilities. Typical applications are for corrosion-resistant overlays and nozzle welds.
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