Regulated Metal Deposition (RMD™) MIG Welding Process Improves Stainless Steel Pipe Fabrication
To meet demand, stainless pipe fabrication shops already work hard. Now they have an opportunity to work smarter by substituting a modified short circuit GMAW processes, called Regulated Metal Deposition (RMD™), for the traditional GMAW and GTAW processes for the root pass.
Pipe fabricators are particularly skeptical about short circuit GMAW, and with good reason. With traditional GMAW, the short circuits occur at erratic intervals and with varying intensity. As a result, the weld puddle experiences a great deal of agitation. To prevent cold lap, or lack of fusion, the operator must work to control and manipulate the weld puddle. High-speed video demonstrates how the short circuit “explosion” causes the weld puddle to splash up and freeze on the sidewall of the pipe, which is how cold lap occurs. It also leads to spatter and extensive clean-up time.
Because it takes a great deal of skill to produce code-quality root welds with traditional short circuit GMAW, many fabricators shun the process, and many end users do not include it in their list of approved procedures. Fortunately, technology advances are changing the game.
With RMD™ technology, the welding system anticipates and controls the short circuit, then reduces available welding current to create a consistent metal transfer. Precisely controlled metal transfer provides uniform droplet deposition, making it easier for the welder to control the puddle. High-speed video proves that stable short circuits create only small ripples in the weld puddle, which in turn allow consistent tie-in to the sidewall. With a stable and more controllable weld puddle, apprentice operators can quickly and easily learn to create uniform, high-quality welds.
As with any welding process, success with the RMD™ process requires establishing and maintaining good preparation and welding techniques. The following guidelines lead to proven success and increased productivity for using RMD to weld stainless steel pipe.
Start with pipe joint sections that have the standard 37.5-degree bevels, for a total included angle of 75 degrees. The lands can range from a knife edge to 3/32 in. Use a minimum 1/8-in. root opening to ensure proper root reinforcement on the weld’s backside. An easy way to space the gap is with a filler metal rod that matches the desired gap size.
Tack the pipe with the RMD process, making tacks (in this order) at the 12-, 6-, 3- and 9-o’clock positions. Remove the filler metal spacer after making the first tack, then check the gap with a tool designed for that purpose. Tacks on smaller diameter pipe can be 1/4- to 1/2-in. long. Tacks on larger pipe may be 1 in. or longer. Note that tack welds will shrink during cooling, causing the gap to close up. In areas with less than a 1/8-in. gap, grind the joint using a 3/32-in. cutting wheel to open the root. Finish preparing the pass by grinding each tack weld to a feather edge to ensure that the root pass consumes the tack weld
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Benefits of the RMD process include:
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Start the arc in the center of a tack around the 1:30- to 2-o’clock position. Hold the gun perpendicular to the pipe with a 5- to 10-degree drag angle. Use a 3/8- to 5/8-in. electrode stick-out. In some cases, this may require a recessed contact tip to help maintain correct stick-out.
Establish the weld puddle and position the electrode in the center as the pipe rolls away from the operator (essentially, the operator is dragging the weld puddle). Watch the puddle closely to ensure that it ties into the sidewalls. Normally, do not use a weave technique. However, if the gap is greater than 3/16 in., the operator may need to weave the electrode slightly across the gap and up the sidewall to bridge it.
When the electrode is properly positioned in the weld puddle, the RMD process creates a muted buzzing sound much softer than the “crackling bacon” sound of traditional short circuit GMAW.
Although the RMD process appears colder than typical GMAW, the weld puddle fuses into the sidewall and penetrates the joint due to the calm metal transfer and stable arc. The face of a good root weld appears flat (neither concave or convex) and, as noted, it is thicker than a traditional GMAW root
With traditional GMAW, operators position the arc on the leading edge of the puddle. Do not do this with RMD, as the arc will stutter and create spatter and greater penetration on the inside of the pipe (note that an optimum root has about a 1/16-in. reinforcement). If travel speeds become too slow and the electrode becomes positioned too far back in the puddle, the arc becomes unstable (listen for a sound more like traditional GMAW. Also, the weld face will be convex. If this happens, grind out the high spots to prevent areas of lack of fusion on the next pass).
If the joint is misaligned, continue to concentrate the arc in the center of the joint. Do not favor the high side of the joint; the new technology will automatically compensate. Let the arc do the work!
Begin welding in the 12-o’clock position. As with the 1G position, start the arc in the center of a tack weld using a 5- to 10-degree drag angle and a 3/8- to 5/8-in. stick-out.
At the start of the weld, keep the arc in the center of the puddle, but move the electrode back and forth across the gap using a half moon motion (with the face of the moon pointing down).
At about the 1-o’clock position, gravity starts to push the puddle down the joint. Once gravity takes over, stop weaving and concentrate on directing the electrode into the center of the weld puddle. At about the 5-o’clock position, use a slight side-to-side motion until reaching 6 o’clock, ending the bead on the feathered tack weld. The side-to-side motion flattens the weld bead and minimizes grinding.
If the weld does not end on a tack weld (e.g., the operator breaks the arc for whatever reason), this may lead to a pinhole at the end of the weld. Grind out the end of the weld before resuming. After completing the root pass, also grind out starts, stops and high points before making the first fill pass (remember that the root pass with the modified process can eliminate the hot pass).
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| The RMD™ process creates a root bead that matches the quality of a GTAW root bead. Unlike GTAW, which requires a high learning curve, the new GMAW process enables an apprentice operator to make production-quality welds after two days of training. |
The techniques for welding carbon steel pipe are the same as those described here for welding stainless. However, to qualify procedures for welding 300 series stainless steel pipe without backing gas, fabricators must do the following:
Note that using the RMD™ process without a backing gas does produce a small amount of oxide scale on the backside of the weld, which usually flakes off as the weld cools. While within the standards for oil and petrochemical applications, it does not meet the “high purity” standard found in the pharmaceutical, semi-conductor or food industries.
Pipe fabricators have long memories. Chances are, most have tried, and many have rejected, the other GMAW procedures for root pass welding. However, GMAW technology advances in recent years now provide dramatically better results. It sounds cliché, but you actually have to experience the new modified short circuit transfer to believe how easily an operator can learn it and how easily it creates a quality root bead. Hopefully, the 100- to 400-percent productivity improvements delivered by RMD welding will be enough of an incentive for stainless steel pipe fabricators to re-examine the GMAW process.
