Comparison of Welding Processes and Control Technology Options for Clad Welding Machines

Overlay welding of nickel alloys for corrosive and sour service is commercially accomplished with most welding processes ranging from shielded metal arc (stick) to laser. The valve industry has generally standardized on hot wire gas tungsten arc (GTA-HW), pulsed gas metal arc (GMA-P) and submerged arc (SAW) welding. This document will focus on these three but a little history will help explain the reasons for using these processes. In the 1980s most of the 625 overlay welding was done using GMA. At that time, three modes of transfer were used; short circuiting (short arc), spray and pulse. Over time short circuiting transfer was largely abandoned because of its tendency to produce lack of fusion defects. This tendency is due to the extremely low heat input of the process. Spray transfer is the other extreme, a high energy, deep penetrating process. Deep penetration is undesirable for low dilution overlay welding. Pulsed GMA is a medium heat input process so it has become the preferred GMA welding mode. Largely due to the weld defect problems experienced with early GMA welding systems most bore cladding switched to GTA HW. The primary advantage is that GTA HW has an inherently low defect rate, the disadvantage is that <5% dilution is only possible in 2 layer welds. SAW is a high productivity welding process with a low defect rate.