17 Modified GMAW-S

Traditional GMAW short circuiting transfer (GMAW-S) produces excessive spatter and is prone to fusion problems because the short circuits occur at erratic intervals and with varying intensity. This is especially true when GMAW-S is used with 100% CO2 shielding gas.

However, modified GMAW-S controls welding current independent of wire feed speed to produce stable short circuits, uniform droplet deposition, and a weld pool with small ripples. The result is a calm weld pool with uniform root penetration, consistent tie-in to groove faces, and significantly reduced spatter, even when 100% CO2 is used for shielding.

Modified GMAW-S was made possible by developments in inverter-based welding power source technology. It was developed primarily for depositing root passes in open root pipe and plate applications.

Modified GMAW-S is known by different trade names: the Lincoln Electric Company calls it Surface Tension Transfer (STT) while the Miller Electric Company calls it Regulated Metal Deposition (RMD).

Some advantages of modified GMAW-S include the following:

• High tolerance for misalignment between pipe sections
• Consistent root reinforcement inside the pipe
• No need for backing gas to prevent sugaring on stainless steel pipe
• Reduced skill level required on the part of the welder
• X-ray quality welds at travel speeds three times faster than GTAW
• Suitability for a variety of metals, including carbon steels, stainless steels, and nickel alloys
• No need for hot passes due to higher deposition rates than GTAW and traditional GMAW-S.