CMT is the abbreviation for Cold Metal Transfer. The CMT process was originally intended for use as a method for joining thin gauged materials in the automotive industry. In 2006 CRC-Evans initiated an investigation to understand if this technology was suitable for root pass deposition on pipeline girth welds.
CMT is a modified GMAW process that uses a new method of droplet detachment based on short circuit welding. The moment the power source detects a short circuit, the welding current drops and the filler wire starts to retract. Exactly one droplet is detached into the molten weld puddle. The filler wire then moves forwards again and the cycle is repeated. The filler wire is constantly retracted at very short intervals. The precisely defined retraction of the wire facilitates controlled droplet detachment to give a clean, virtually spatter-free material transfer. For mechanized pipeline girth welding, the CMT process is applied with a conventional bug and band system. CRC-Evans employs the P-450 welding platform with horizontal and vertical tracking capability in conjunction with the Fronius TPS 3200 power supply, a push pull wire feed system, and welding torch also supplied by Fronius. The typical welding procedure incorporates an Argon/CO2 shielding gas with a standard J-Bevel joint design. Travel speeds range from 350 – 406mm/min, with typical heat inputs ranging from 0.47 to 0.75 Kj/mm.
Mechanical testing in both the standard and strain aged conditions reveal properties that meet industry specification standards (DNV OS –F101, API 1104 20th ed. Appendix A).
The potential for CMT use in the pipeline industry is significant.
Steel Cantinary Risers (SCR’s) and fatigue critical flow lines
Mechanized tie ins on HSLA steels (X-80 and greater)
Root pass for single sided closure welds on small and large diameter pressure vessels
Flow line and export production welding
CMT process is an up and coming mechanized root pass welding technology. The technology is just now entering the market and has the potential to increase quality, productivity, and expand capability.