August 6, 2013
When welding process pipe, welders need to determine which wires can provide the most appropriate results for every weld pass—root, fill, and cap—and be certain that they are selecting the highest-quality filler metal.
Like any other welding application, process piping has its own unique requirements and challenges. Welders must adhere to strict codes when welding in these applications, as well as take precautions to avoid potential defects like lack of fusion, porosity, or other weld defects that could compromise the integrity of the completed weld.
Process piping welders also encounter many of the same constraints found in other jobs. Tight deadlines, lack of skilled labor, and the drive to reduce costs are considerations, as is the need to turn out quality welds.
As a means to gain a competitive advantage, some welders have chosen to move away from the shielded metal arc welding (SMAW) or gas tungsten arc welding (GTAW) processes common in this industry and convert to wire processes. Advanced technologies like modified short-circuit GMAW or pulsed gas metal arc welding (GMAW-P), as well as conventional GMAW and flux-cored arc welding (FCAW), can help speed production, ease training, and produce high-quality welds in process pipe applications.
With this conversion, however, comes the question of filler metal selection. Welders need to determine which wires can provide the most appropriate results for every weld pass—root, fill, and cap—and be certain that they are selecting the highest-quality filler metal.
The material used in process piping applications is predominantly carbon steel, ranging in diameter from 4 to 36 inches. The most common diameters welded with the semiautomatic process are 18 in. or smaller. As a productivity-enhancing measure, most operators weld the pipe in the 1GR position whenever possible—a horizontal position in which the pipe rolls.
The three main filler metal wire options for this application are solid wire; gas-shielded, flux-cored wire; and metal-cored wire. Each has unique features, advantages, and limitations. These factors must be weighed carefully to determine the best option for the job. Following are descriptions to help with the decision.
A standard in the welding industry for many years, solid wire is well-known by many process piping welders. As its name implies, solid wire is solid throughout and, thus, carries the current through the cross-sectional area. It offers good deposition efficiencies (a greater amount of filler metal goes into the weld joint) compared to SMAW electrodes, and it also provides low hydrogen levels to help minimize the potential for cracking.
Solid wire can be used to weld root passes on process pipe when coupled with a conventional GMAW or a modified short-circuit GMAW process. The latter process makes it easier to maintain a consistent arc length, while also maintaining a calm and stable weld pool. This combination helps minimize the potential for rejects. It also can increase productivity. Welders can sometimes skip the hot pass when using this wire/process because it deposits a thicker root pass than SMAW or GTAW.
When combined with a GMAW-P process, solid wire also can be used to weld the fill and cap passes on process piping. The combination provides a consistent weld bead appearance and helps reduce spatter to minimize postweld cleaning. It also enables welders to keep the pipe in position if rolling isn’t an option for them, since the combination allows for all-position welding.
Many welders find using solid wire with these technologies to be particularly advantageous, because it allows them to weld from root to cap with one wire and one shielding gas, eliminating the need for wire changeover.
When selecting a solid wire, it is important to look for a product that is consistent in diameter and in its outer coating and lubricant; it also is important that this wire has a smooth surface. This helps to ensure consistent wire feeding and prevent buildup in the liner that can cause downtime to address feeding issues.
Some process pipe welding applications call for an SMAW or GTAW root pass instead of a wire-welding process. Yet there is a filler metal/wire process that can be used to help speed productivity on the fill and cap passes—gas-shielded, flux-cored wires in conjunction with a GMAW spray transfer process.
These wires are tubular and have flux inside the wire that solidifies more quickly than the molten weld material, resulting in a "shelf" that is capable of holding the molten pool when welding overhead or vertical-up, in particular. Gas-shielded, flux-cored wires also are available in a wide array of alloys.
These wires meet industry mechanical properties, including high strengths and strong impact values, as well as good side-wall fusion to help reduce the opportunity for weld defects. They also are relatively easy for operators to use, making them a good choice for less experienced welders. These wires provide excellent deposition rates—possibly higher than a solid wire/GMAW-P combination—for enhanced productivity.
Because gas-shielded, flux-cored wires produce slag, they do require cleaning in between passes and after completing the final weld, along with disposal of the slag—factors that need to be weighed in relation to the productivity increases these filler metals provide.
Other considerations include the possibility of worm tracking and environmental factors, as these wires tend to produce higher levels of visible smoke than solid wires or the metal-cored wires discussed in the next section.
Metal-cored wires are another option for welding the root, fill, and cap passes on process pipe. Like solid wires, these wires can be used with a modified short-circuit GMAW process to weld the root pass and with a GMAW-P for the fill and cap passes, again allowing welders to standardize on one wire and gas for the entire application. Also similar to solid wires, metal-cored wires (when used in combination with these welding processes) provide a stable arc; low spatter; and a calm, easy-to-control weld pool. In some cases, these wires are available in a wider variety of alloys than solid wires.
Metal-cored wires consist of a hollow metal sheath filled with metallic powders, alloys, and arc stabilizers. Because of these wires’ structure, the current goes through the outer sheath to produce a broad arc (compared to solid wires), which in turn provides a broader penetration profile.
These wires excel at welding the fill and cap passes in the 1G rolled position. They provide high deposition rates similar to flux-cored wires, but with lower visible smoke. Metal-cored wires do not produce slag, which eliminates any concern about slag entrapment and reduces interpass and postweld cleaning. Some silicon islands may appear after welding, which may require brushing to clean up.
When a welder makes the decision to convert from a SMAW or GTAW process to a wire process—whether it’s modified short-circuit GMAW, GMAW-P, or a conventional GMAW or FCAW—it’s necessary to adjust and qualify the new procedures to be certain that this shift is feasible for the application. Working with a trusted welding distributor or equipment or filler metal supplier can help ensure access to the equipment and wires that are best-suited for the application.
Some welders may find that using a solid or metal-cored wire for the root to cap passes provides the most productivity, while others may require a combination of other processes. Whatever the decision, it is critical to select high-quality filler metals for the job to prevent potential defects and ensure the weld quality and productivity needed to gain a competitive advantage from a wire process.