X-Ray Quality Welds, Fast
For some time, the preferred method of open root pipe welding has been a cellulosic stick electrode (6010) for the root pass or a low hydrogen rod (7018) on the fill/cap passes. However, stick welding pipe has notoriously low arc-on-time and electrode efficiency (65%) and requires a highly trained/experienced welder. In response to these variables, engineers developed modified short circuit.
Because of problems with stick welding, old style short circuit CV MIG has been experimented with and produced mixed results. The hard driving short circuit arc produces a violent amp/volt dance, and often causes root blow through, “wiskers,” “suck back” from excessive heat and poor weld toe wet in (wagon tracks).
In response, Lincoln Electric developed software based low heat input SURFACE TENSION TRANSFER (STT). Miller then followed by developing REGULATED METAL DEPOSITION (RMD). STT and RMD consist of a peak current, background current, and a tail-out time. A tail-out time controls the speed of transition between the two currents. The result significantly reduces short circuit violence, arc heat and problems associated with too much heat (as outlined above). In short, STT and RMD are precisely controlled modified short circuit pulse. Power sources with the subject technology pair with a dual feeder to make fill/cap passes. They use super high deposition processes like solid/metal-cored wire in an axial spray/pulse spray transfer or gas shielded flux-cored wire. STT and RMD have also been implemented successfully on sheet metal applications where blow through and distortion are problematic.
Author: Bob Page