Process Pipe Welding: Filler Metal Options to Improve Productivity and Quality
When welding process pipe, contractors 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. Welding operators 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.
In addition, contractors in the process piping industry also encounter many of the same constraints found in other jobs. Tight deadlines, lack of skilled labor and the drive toward cost reductions are all considerations, as is the need to turn out x-ray quality welds.
As a means to gain a competitive advantage, some contractors have chosen to move away from the shielded metal arc welding (SMAW) or gas tungsten arc welding (GTAW) welding processes common in this industry and convert to wire processes. Advanced technologies like modified short circuit GMAW or Pulsed Gas Metal Arc Welding (GMAW), as well as conventional GMAW or Flux-Cored Arc Welding (FCAW) can serve to speed production, ease training and produce high quality in process pipe applications.
With this conversion, however, comes the question of filler metal selection. Contractors 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 and filler metal options
The material found in process piping applications is predominantly carbon steel, ranging in diameter from 4 to 36 inches. The most prevalent diameters welded with the semi-automatic process are typically 18 inches or smaller. As a productivity-enhancing measure, most contractors weld the pipe in the 1GR position whenever possible — a horizontal weld position in which the pipe rolls.
For these applications, there are three main filler metal options from which fabricators can choose: solid wire, gas-shielded flux-cored wire and metal-cored wire. As with all filler metals, there are unique features, advantages and limitations to each, and companies need to weigh each of these factors carefully to determine the best option for the job. Following are descriptions to help with the decision.
Solid wire: a familiar industry choice
Solid wire has been a standard in the welding industry for many years and is a filler metal well known by many contractors in the process piping industry. 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 going into weld joint) compared to SMAW electrodes and 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 allows contractors to maintain a consistent arc length, while also gaining good arc control and arc stability and establishing a calm weld pool. Contractors can often skip the hot pass when using this wire/process because this process deposits a thicker root pass than the SMAW or GTAW processes, which adds further to their productivity.
Solid wire can also be used to weld the fill and cap passes on process piping when used with a Pulsed GMAW process. The combination provides a good weld bead appearance and helps reduce spatter to minimize downtime for post-weld cleaning. It also allows contractors to keep the pipe in position if rolling isn’t an option for them, since the combination allows for all-position welding.
Many contractors find using solid wire with the above technologies to be particularly advantageous, since it allows them to weld from root to cap with one wire and one shielding gas. They don’t have to accrue downtime for wire changeover.
When selecting a solid wire, it is critical for contractors to look for a product that is consistent in diameter and in its outer coating and lubricant; it is also important that this wire has a smooth surface. These features help ensure consistent wire feeding and help prevent build-up in the liner that can cause downtime to address feeding issues.
Gas-shielded flux-cored wire: an option for completing the job
Some process pipe welding applications do not allow the use of wire welding for the root pass, as described previously, but instead call for a GTAW or conventional GMAW root pass. In this situation, contractors still have a filler metal/wire process solution to help speed productivity on the fill and cap passes. Gas-shielded flux-cored wires can be used in conjunction with a GMAW spray transfer process. These wires feature a flux coating 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 vertically up, in particular. Gas-shielded flux-cored wires are also available in a wide array of alloys.
Gas-shielded flux-cored wires offer excellent mechanical properties, including high strengths and strong impact values, as well as good sidewall fusion to help reduce the opportunity for weld defects. They are also relatively easy for operators to use, making them a good choice to address training with newer welders. These wires also provide excellent deposition rates — even higher than a solid wire/Pulsed GMAW combination) — to help contractors further increase their productivity.
Since 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 contractors will need to weigh out in relation to the productivity increases these filler metals provide. They will also need to consider the possibility of issues like worm tracking and consider environmental factors, as these wires tend to produce higher levels of visible smoke than solid or the metal-cored wires discussed in next section.
Metal-cored wire: an alternative filler metal
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 Pulsed GMAW process for the fill and cap passes, again allowing contractors 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 contractors with a stable arc, low spatter and a calm, easy-to-control weld pool. The wires, however, 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 broader arc (compared to solid wires), which in turn provides a broader penetration profile and allows the wires to bridge gaps very effectively on the root passes.
These wires excel at welding the fill and cap passes in the 1G position. They provide similarly high deposition rates as flux-cored wires, but with lower visible smoke. Metal-cored wires also produce no slag, which eliminates any concern for slag entrapment and reduces inter-pass and post-weld cleaning.
Gaining a competitive advantage
When a contractor makes the decision to convert from a SMAW or GTAW process to wire process — whether it’s a modified short circuit GMAW, Pulsed GMAW or a conventional GMAW or FCAW process — it’s necessary to adjust and qualify the new procedures. Contractors will need to be certain that this shift is feasible for the application. If so, they can work with a trusted welding distributor, or equipment or filler metal supplier to ensure they have access to the equipment and wires that are best suited for the application.
Some companies may find that using a solid or metal-cored wire for the root to cap passes is the means to provide them with the most productivity, while others may require a combination of other processes. Whichever 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.