Open root welds on pipes can be made three to four times faster than GTAW by using the Surface Tension Transfer® process. When integrated with an internal spacer clamp into a new automatic orbital pipe welding system, even faster production is possible, with no lack of fusion.
Pipe welding codes, whether for applications in the field or in the plant, require high-quality root pass welding. To ensure that the joints will not leak, especially for steam or pressurized applications, a weld must penetrate completely through the pipe.
In the past, pipe welding was done by one of three methods, each of which has its advantages and disadvantages. These are the methods that have been used.
Gas tungsten arc welding (GTAW) is popularly known as TIG. Travel speeds are slow, heat input is usually high, and it requires high operator skill level.
Gas metal arc welding (GMAW) - also known as MIG - is a much faster process than GTAW. However, because operator skill level is hight and heat input difficult to control, fusion may not always be 100 percent.
Shielded metal arc welding (SMAW), also known as stick, can be cost effective in terms of equipment but requires high operator skill. Frequent starts and stops are another potential problem.
By contrast, the Surface Tension Transfer (STT) process makes it possible to complete open root welds three or four times faster than GTAW, with low heat input and no lack of fusion. The STT process uses high frequency inverter technology with advanced waveform control to produce a high-quality weld with less spatter and smoke. For pipe welding, the process also makes it easier to perform open gap root pass welding, with better back beads and edge fusion. It is easier to operate than other processes, yet produces consistent, X-ray quality welds. The STT process results in a complete back bead without shrinkage from the 12 to 6 o'clock weld positions. Also, because current control is independent of wire feed speed, the process allows greater flexibility under all conditions.
Controlling Spatter and Smoke
STT is a proprietary Lincoln Electric process that makes use of Wave Form Control Technology™ to control current precisely and rapidly during the entire welding cycle. It is unique in that it is neither constant current (CC) nor constant voltage (CV). Instead, the power source adjusts current automatically to the instantaneous heat requirements of the arc.
Spatter and smoke are reduced with this process, whether the arc shielding gas is 100 percent CO2, blends of argon and CO2 or helium mixtures for use with stainless steel. Reducing spatter minimizes final weld surface preparation and allows the operator more welding time before the gun nozzle must be cleaned of accumulated spatter.
Reduced spatter also translates into significant cost savings because more of the electrode is applied to the weld joint, not as spatter on the pipe and surrounding fixtures. Further cost savings are realized because larger diameter wire can be used.
At the start of the cycle, when the electrode shorts, the current is reduced immediately, eliminating the incipient short. This low-level current is maintained for a short time so that the surface tension forces can begin transferring the drop to the puddle, forming a solid mechanical bridge. A high level of pinch current is then applied to accelerate the transfer of the drop. The necking down or squeezing of the shorted electrode is monitored. When a specific value is reached, the pinch current is reduced quickly to a low value before the fuse separates. When a short breaks, it does so at a low current, which produces very little spatter.
Next, the arc is reestablished and a high current known as peak current is applied. This momentary pulse of current establishes the arc length and causes the arc to broaden and melt a wide surface area, which eliminates cold lapping and promotes good fusion.
Better Pipe Welding Results
The constant voltage GMAW process normally used for pipe welding does not control the current directly. Instead it controls the average voltage. This can cause the weld puddle temperature or fluidity to be too high, and the internal bead may be flat or shrink back into the root. This is known as "suck back." Also, when using conventional short arc GMAW, the operator must concentrate the arc on the lip or leading edge of the puddle to ensure proper penetration and fusion. If the arc is too far back on the puddle, penetration will be incomplete. If the arc is too far ahead, the electrode shoots through the gap and causes whiskers to form inside the pipe.
Because Surface Tension Transfer controls the welding current independently of wire feed speed, the process makes it easy to control the temperature or fluidity of the puddle to ensure proper penetration and fusion. This is what makes it so attractive for open root pipe welding applications. In the 5G position, the operator simply has to stay in the puddle. Experienced pipe welders almost always find the process a welcome improvement, both in ease of welding and comfort. They particularly appreciate the reduction in spatter when welding in the 6 o'clock position.
As the decision process evolves, the vendor and the fabricator will continue working together to determine the appropriate system accessories, including safety devices, the optimal layout for the robotic cell, manpower and training requirements, and service and maintenance requirements (internal vs. outside vendor support).
The STT process is gaining acceptance in pipe welding and similar applications, which require precise control of heat input as well as smoke and spatter reduction. Since the heat is controlled directly, the internal backbead profile is also controlled. Welders find that not only are open root welds easier to make, but their mechanical and metallurgical properties are excellent. Superior welding bead profiles can be attained with improved properties in the heat affected zone. Moreover, open root welds are made without the use of ceramic or copper internal backup. In the case of copper, corrosion is thus eliminated by avoiding the possibility of copper inclusions.
The process is effective for welding mild and high-strength steels, as well as stainless steel and related alloys. On steel, it offers the advantages of low hydrogen and 100 percent CO2 shielding with low spatter. When welding duplex stainless, critical pitting temperature is significantly better with STT than with GTAW, and travel speeds three or four times that of GTAW can be obtained, with much less skill.