A case for metal-cored consumables
July 12, 2005
For welding in the 1G position (in which the tube or pipe rotates), solid wire is traditional filler metal. However, metal-cored wire is making headway as an alternative. Metal-cored wire requires no land at the bevel, is more forgiving of welding dirty metal, produces less spatter, and allows travel speeds up to inches per minute.
When welding rolled pipe, the welder holds or fixes the welding torch at a 90-degree angle to the ground and at the 1 o'clock position in the joint.
This type of pipe welding involves setting a pipe horizontally, in the 1G position, on a rotating table that rolls the pipe in place. The welder then holds or fixes a torch at a 90-degree angle to the ground and at the 1 o'clock position in the joint that connects the pipe to either another pipe or a fitting (see Figure 1).
While solid wire is the most commonly used consumable for this application, metal-cored wire is another viable option.
Although more costly per unit than solid wire, metal-cored wire is becoming increasingly prevalent in the industry, while meeting and sometimes exceeding the same demanding American Welding Society (AWS) standards as solid wire.
Metal-cored wire is a composite tubular wire consisting of a metal sheath and a core of powdered metals. It can be used with the same equipment as solid wire for welding rolled pipe in the 1G position.
Welding rolled pipe with metal-cored wire provides good gap bridging on root passes (see bottom portion of the pipe) and a consistent weld on fill passes (see top portion of the pipe), while eliminating the need for grinding between passes.
The chemical and physical composition of metal-cored wire helps eliminate silicon deposits and spatter, as well as the need to grind each weld before making the next pass. Although precleaning is recommended whenever possible, the deoxidizers in metal-cored wire help provide cleaning action and allow for welding of dirty metal with minimal contamination. These features help create X-ray-quality welds from root to cap without stopping on many 1G pipe applications and speed the process of welding rolled pipe.
Metal-cored wire transfers the welding current through a thin outer sheath. This factor increases current density and lowers the voltage at which a welder 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 wide area. This wide 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 (see Figure 2).
The spray transfer of the metal-cored wire produces little spatter, even at a high rate of metal deposition, because the small molten droplets create low turbulence in the weld pool.
Using metal-cored wire for welding rolled pipe often allows welders to standardize wire size. For instance, depending on the application, a 0.045- or 1/16-inch-diameter metal-cored wire could be used throughout an entire process. Not having to switch over helps reduce downtime while increasing deposition rates and reducing cycle times.
What are the effects of metal-cored wire on throughput and overall weld cost? While metal-cored wire has a high initial purchase price, using a 1/16-in.-dia. metal-cored wire, for example, to weld on 6-in. Schedule 40 pipe provides travel speeds from 25 to 45 inches per minute (IPM). This travel speed, and the absence of grinding after passes, allows for welding a significant amount of material in a given time frame.
While the high travel speed of metal-cored wire benefits productivity, it may require alterations to existing welding equipment. Specifically, the equipment that rotates the pipe needs to keep up with the travel speed. Small-diameter pipe is a special consideration, as a 25-IPM travel speed on a 4-in.-dia. pipe requires significantly faster rotational speeds than the same travel speed on a 10-in.-dia. pipe.
Whether the work is being done for an oil refinery or a big-city high-rise office building, pipe welding can be a costly, labor-intensive application. Eliminating cleanup and increasing travel speeds ultimately help to lower overall weld costs. Using metal-cored wire consumables can be the first step in that direction.