Just Roll With It: Maximize the Efficiency of Welding Rolled Pipe in the 1G Position
Welding rolled pipe in the 1G position is standard practice in industries that require endless lengths of pipe for pumping large amounts of liquid (petrochemical plants, high-rise buildings, etc.). Using solid wire in this application is an equally common practice. Take a closer look at solid versus metal cored wire, however, and the differences in labor and weld quality may cause you to reevaluate your consumable use.
This type of pipe welding involves setting a pipe horizontally (in the 1G position) on a rotating table that rolls the pipe in place. A welding torch is then held or fixed at a 90-degree angle to the ground and at the 1:00 position in the joint that connects the pipe to either another pipe or a fitting (elbows, t-joints, etc.). This process is normally automated by fixing the torch in place, but it can be done semi-automatically by holding the torch. And though pipe diameter and wall thickness often vary according to a specific application, the examination of consumables in this article is universal to this welding technique.
Solid Wire Provides Upfront Savings, a Comfortable Process for Welders
Using solid wire for welding rolled pipe is an industry standard because of its low unit cost and high availability. Even so, solid wire often creates labor-intensive actions that can prolong the welding cycle and increase overall welding costs. When welding rolled pipe with solid wire, you need to bevel the pipe/fitting edges to a given angle (30-37.5 is standard) and then grind a “land” at the beveled edge of each pipe. Creating this type of edge helps support the heat intensity of the .035-in. diameter solid wire generally used for the root pass and also gives the weld bead a place to set. Once you shape the land edge, you need to perform light grinding and cleaning of mill scale and/or scarf (leftover from flame cutting) to remove impurities. If you mechanically cut the pipe, you need to de-burr the edge to prevent inclusions in the weld and ensure proper fit-up.
After pre-cleaning and preparation, you can butt together each section of pipe and check for misalignment, which is almost always present in pipe welding due to variance in roundness and cut. Normally you would use .035-in. diameter solid wire for the root pass with a root opening. The root pass and at least one hot pass is traditionally done with short circuit transfer. Using this process deposits a cold, manageable puddle, which makes it easy to bridge the root gap.
Cycle times using this process in this application are slower than with metal cored wire for a number of reasons. First, in order to fill the root gap properly with solid wire, you need to run your power source at lower voltages, typically 18-20 volts. The welding current must also be kept low, approximately 130 amps. Often you need to oscillate the torch from side to side when performing the hot pass with solid wire. Doing so helps ensure proper sidewall fusion and penetration into the root pass, but it can also lead to operator fatigue. As a result of these factors, using solid wire with short circuit provides slow travel speeds (amount of surface passing the welding torch)—approximately 5 to 13 inches per minute (IPM).
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| When welding rolled pipe, you can fix the welding torch at a 90-degree angle to the ground and at the 1 o’clock position to the joint. |
Once you create the root pass and a hot pass, you can often upsize to a .045-in. diameter solid wire and make the remaining fill passes at higher weld currents. The higher welding currents used on the .045 diameter will result in increased deposition rates, which will allow for increased travel speeds. Still, regardless of how fast you lay the weld bead, one factor still slows down cycle times with solid wire: the need for grinding and cleanup between each pass.
After passes with solid wire, you often need to grind the weld bead to remove silicon deposits or spatter that may have formed in or near the toe line. If not removed properly, these contaminants could cause weld flaws which would require costly weld repair, especially in critical pipe applications where welds must pass x-ray inspection. Once you clean each weld bead, you can proceed with the next fill pass, but using solid wire requires that you continue the welding/cleaning/ cycle on each pass until you complete the final cap pass.
In terms of investment, the need for grinding contributes to the 85 percent of labor that comprises your total welding cost. Still solid wire, despite the additional labor it incurs, remains popular for its low upfront unit costs. But how does this low unit cost approach impact your total process cost and cycle times – and can metal cored wires help you improve the overall pace of production?
No Longer an Alternative: Making a Case for Metal Cored Wire
Although more costly per unit, metal cored wires are becoming increasingly prevalent in the industry, while meeting and exceeding the same demanding AWS standards of solid wires.
Metal cored wire is a composite tubular wire consisting of a metal sheath and a core of powdered metals. Whether using solid or metal cored wire when welding rolled pipe in the 1G position, the equipment is identical. Metal cored wire, however, provides certain advantages and improvements to weld quality that increase productivity and reduce cost.
The chemical and physical composition of metal cored wires help eliminate silicon deposits and spatter, while also eliminating the need to grind each weld before making the next pass. Metal cored wire’s added deoxidizers also provide better cleaning action and allow for welding dirty metal with minimal contamination, although pre-cleaning is always recommended when possible. These features help create x-ray quality welds from root to cap without stopping on many 1G pipe applications and make the entire process of welding rolled pipe much faster.
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| Welding rolled pipe with metal cored wire provides good gap bridging on root passes (see bottom portion of pipe) and consistent weld on fill passes (see top portion of the pipe), while eliminating the need for grinding between passes. |
Metal cored wire transfers the welding current through a thin outer sheath compared to the current being transferring through the entire cross section of a solid wire. This factor increases current density and lowers the voltage at which you can use spray transfer, allowing for welding on the root pass without blowing through the joint. Spray transfer with metal cored wire transmits the metal in a funnel-like manner over a wider area. This wider arc cone provides a penetration profile that eliminates the need to grind the land on the pipe during preparation and also eliminates the need for a root gap. The spray transfer of the metal core wire produces very little spatter. The small molten droplets create less turbulence in the weld pool than the short circuit transfer used with solid wire. This reduces the amount of spatter created even at a higher rate of metal deposition– two factors that increase overall productivity.
Using metal cored wire for welding rolled pipe often allows you to standardize on one wire size, which in turn reduces purchasing hassles and inventory concerns. Whereas with solid wire, you may need to change from a .035-in. to a .045-in. diameter wire as you move from root to cap pass, you could use a .045- or 1/16-inch diameter metal cored wire throughout your entire process. Using one diameter metal cored wire reduces downtime, while also increasing deposition rates and reducing cycle times.
Implementing Metal Cored Wire in the Field – Challenges and Results
Despite the slightly higher purchase price for metal cored wire, you can achieve an impressive improvement in throughput and reduce your overall weld cost. For example, using a 1/16-in. diameter metal cored wire to weld on 6-in. schedule 40 pipe provides travel speeds starting at 25 IPM and as high as 40-45 IPM. These figures far exceed those of solid wire whose travel speeds generally fall in the low teens. Such an increase in travel speeds provides the opportunity to weld at least twice as much material in the same amount of time without requiring grinding after passes.
The increased travel speed of metal cored wire is a major benefit to productivity, but in some cases it may require you to alter your existing equipment. Specifically, your equipment needs to rotate the pipe fast enough to accommodate metal cored wire’s increased travel speeds, especially on smaller diameter pipe. For instance, the travel speed of 25 IPM on a 4-in. diameter pipe requires significantly faster rotational speeds than 25 IPM travel speeds on a 10-in. diameter pipe.
Compared to using solid wire, metal cored wire’s travel speeds can reap real world benefits. Take into consideration the following comparison: A company welds 8-in. schedule 40 pipe using .035-in. diameter solid wire for the root pass followed by .045-in. diameter solid wire for each following pass. With these wires, each weld cycle takes 5.5 minutes to complete, including grinding and cleaning after each pass to remove silicon deposits and/or spatter. By changing to 1/16-in. diameter metal cored wire, such a company can reduce its weld cycle time to 1.75 minutes. Eliminating the need to grind the weld bead after each pass reduces much of the cycle time. In this example, a company could perform the same amount of work in one-third of the time and also automate the process by fixing the torch in place in order to increase operator comfort. Furthermore, time can be saved by eliminating pre-weld joint preparation (such as preparing a land on the pipe ends or gap joint).
Conclusion
Whether you work in an oil refinery or weld the piping through a big city high-rise office building, welding can be a costly, labor-intensive application. Any amount of labor you can remove by ridding the process of cleanup will ultimately lower your overall weld costs. The best part is that you may find improving productivity and saving on costs is as simple as changing your welding consumable from solid to metal cored wire.