Regulated Metal Deposition

China Product


Development

Miller Electric Mfg. Co. developed RMD technology in 2004 to address issues related to welding thin metal subject to burn-through, welding heat-sensitive alloys such as stainless steel, overcoming less-than-ideal fit-up (gaps) and welding root passes on pipe.

Compared to conventional short circuit GMAW, the RMD modified short circuit transfer process promotes smoother metal transfer, prevents excess puddle agitation and lowers heat input. When specified correctly, RMD Process can accomplish the following: disinfectant cleaner

In pipe welding applications, the RMD Process provides the following benefits: ice blasting

Reduce training time. The SMAW and GTAW processes, require a higher degree of hand-eye coordination to master. compared to RMD. sand blaster

Provide welding travel speeds of 6 to 12 inches per minute in fixed position pipe welding applications

Potential to eliminate the hot pass.

Potential to eliminate the need for backing gas for some of the 300 series stainless steel grades and chrome-moly steels.

It also provides these additional benefits on plate and pipe applications.

Minimize clean-up time, as the process creates no slag and little or no spatter.

Create the ability to bridge wide gaps and compensate for less-than-optimal fit-up.

Enable using the same wire and shielding gas for subsequent passes with other GMAW processes.

Equipment

Software

The RMD Process is software-driven. Miller factory-installs the software on its welding systems designed specifically for pipe welding; pipe-specific software is designated RMD-Pro. On welding systems designed for semi-automatic multi-MIG welding and automated/robotic welding multi-MIG welding, the optional RMD software can be factory installed at the time of product purchase, or users can upgrade at a later date.

Hardware

RMD is designed to operate with inverter-based welding systems, currently designed as the PipePro 450 RFC Welding System, PipeWorx Welding System, Axcess series and Auto-Axcess series. These systems include an inverter-based welding power source, wire feed control, and MIG gun/torch.

Electrode and shielding gas

The RMD Process uses standard solid wire electrodes for GMAW welding (AWS ERXXS-X) from 0.035 in. to 0.045 in. or 0.9 mm to 1.2 mm respectively. Acceptable shielding gases include most of those commonly used for conventional GMAW processes, e.g. 100% CO2, 90% argon/10% CO2 (C10), and 75% argon/25% CO2 (C25) for carbon steels and 98% argon/2% O2, 98% argon/2% CO2, and Tri H for stainless steels. Other gas variations can also be used and others are possible.

As long as the electrode and shielding gas support the process, some systems with RMD software can switch on-the-fly between RMD and any other process within the system, such as standard short circuit GMAW, spray transfer GMAW, conventional pulsed spray transfer GMAW Accu-Pulse pulsed transfer GMAW or Pro-Pulse pulsed transfer GMAW.

Operation

The RMD Process provides the benefits of advanced welding processes without a complex operator interface and extensive set-up. To begin welding with the RMD Process, operators use the system controls to select (from a pre-programmed menu) the material type, wire diameter and gas combination. The system then selects the optimum welding parameters from its library of application programs, available in the product. After that, operator sets the desired wire feed speed (WFS) to match the application and begins welding. The system automatically maintains all other variables as operators increase WFS (to weld thicker sections and/or increase travel speeds) or decrease WFS (to weld thinner sections or decrease travel speeds).

Welding with the RMD Process requires similar techniques to short circuit GMAW, but with slight modifications specific to the process (learn more).

Metal transfer process

Understanding short circuit GMAW

Standard short circuit transfer GMAW is preferred for thinner metals, as it is designed to operate at lower heat levels, most commonly 60 to 200 amperes and 16 to 22 volts. However, it has limitations. To understand RMD benefits, it is necessary to understand standard short circuit GMAW.

In standard short circuit transfer, the welding wire (electrode) initially contacts the work piece, creating a short reducing the voltage to a low value. At that point, the welding power source reacts to the short, attempting to maintain the preset voltage value by rapidly adding current to the wire to clear the short. As the wire heats from resistance to the high current flow, it forms an electromagnetic field at the end. The electrical force from this field then causes the molten bridge to constrict and subsequently clears the short. An arc is then reestablished, forming a ball at the end of the wire, for the next short-circuit. This process happens repetitively at a rate of about 90 to 150 times per second.

Observation of short circuit events using slow motion video emphasizes that the standard short circuit process can be viewed as a chaotic, unpredictable event that creates spatter and affects the next short circuit. Further, the uncontrolled rush of current adds more heat than necessary. Excess heat increases the likelihood of warping, may alter the metallurgical/mechanical properties of heat-sensitive metals and may produce burn-through in situations where the metal cannot support much heat, such as the edge of a thin plate or beveled pipe. The combination of current spikes and spatter may also lead to lack of confidence in fusion and/or consistent penetration.

Controlled RMD process transfer

The RMD transfer operates similarly to the short circuit transfer, but with some important differences. The RMD software program, working with an inverter-based welding system and closed-loop feedback, closely monitors and controls the electrode current at speeds up to 50 microseconds (50 millionths of a second). By closely monitoring the change in energy and understanding exactly how much is required for a specific wire diameter, speed and gas combination, the welding system can predict future arc conditions and control the metal transfer accordingly. It then applies a precisely controlled waveform at critical points in the short circuit cycle. With RMD technology, the welding system anticipates and controls the short circuit, then reduces available welding current to create 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 allows consistent tie-in to the sidewall. As a result of a stable and more controllable weld puddle, apprentice operators can quickly and easily learn to create uniform, high quality welds.

Pipe welding

The RMD modified short circuit process provides several benefits for pipe welding:

1. Smooth metal transfer compensates for a high-low misalignment between pipe sections. The process easily bridges gaps of up to 3/16 inch.

2. Smooth metal transfer creates more consistent root reinforcement on the inside of the pipe.

3. The shielding gas coming out of the gun remains relatively undisturbed by the controlled transfer. As a result, enough shielding gas gets pushed through the root opening to prevent oxidation on the backside of the weld. Some fabricators have qualified procedures to weld some of the 300 series stainless steels and chrome-moly steels without a backing gas, improving productivity (large diameter pipes take a long time to purge, and the gas is costly).

4. The RMD Process maintains a consistent arc length regardless of electrode stick-out. It compensates for operators that have problems holding a constant stick-out, and it enables a better view of the weld puddle (as welders weld from the 4- to 6-olock position, they tend to increase their wire stick-out. With older technology, a long stick-out skews welding parameters and often compromises quality).

5. The RMD process creates a root pass weld with a 1/8 to 3/16 in throat. In many instances, the amount of root pass metal deposited will be sufficient to support the heat input requirements of the first pulsed GMAW or FCAW fill pass. Fabricators can eliminate the GTAW hot pass.

The same electrode and shielding gas used for the RMD process can also be used for the fill and cap passes using a next-generation pulsed GMAW process, called Pro-Pulse. This process improves performance and operator acceptance compared to traditional pulsed welding, and it improves both travel speeds and deposition rates while lowering overall heat input.

References

"Blasting the Bottleneck Blues." Practical Welding Today. 8 May 2007 <http://www.thefabricator.com/ArcWelding/ArcWelding_Article.cfm?ID=1628>

"Pipe Welding Video Available for Download." TheFabricator.com. 12 April 2008 <http://www.thefabricator.com/TubePipeFabrication/TubePipeFabrication_Products.cfm?ProductsID=993>

"Pipe Welding Technique Videos Now Available at MillerWelds.com." Miller Electric Mfg. Co. 23 Jun 2008 <http://www.carmenelectrode.com/pipe-welding-technique-videos-now-available-at-millerweldscom/>

External links

A case study

RMD Stainless Tech Story

Process overview

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