Increase Productivity in Pipe Fabrication With Advanced Wire Processes
Modified short-circuit and pulsed MIG welding processes increase pipe welding productivity, improve quality and benefit training.
Pipe fabrication processes
Pipe fabrication shops have a few processes to choose from when performing root passes, but which option offers the most benefits for productivity, ease of use and fast training?
TIG welding delivers the potential for high-quality welds, but it requires a highly skilled operator and has slower travel speeds.
Stick welding has faster travel speeds, but it requires the welder to remove slag and grind between passes.
Traditional short-circuit MIG is another option, but many fabricators avoid it for the root pass due to the skill required to produce code-quality welds.
A modified short-circuit MIG process, such as Regulated Metal Deposition (RMD®) from Miller, can provide double or triple the travel speed of other process options, in addition to possibly eliminating the hot pass. RMD is also very tolerant of differences in contact-tip-to-work distance and provides ease of use that reduces training time for producing high-quality welds.
Learn more about the benefits of modified short-circuit MIG for pipe fabrication, and how operations can save time and money with a switch to this process.
Modified vs. traditional short-circuit MIG welding
In traditional MIG welding, the short circuits occur at irregular intervals and are of varying intensity. This agitates the puddle, causing it to splash onto the sidewall of the pipe, which can lead to spatter and cold lap or lack of fusion. As a result, this process requires a high level of skill to produce code-quality welds, and many companies shy away from it.
With the modified short-circuit technology of the RMD process, the metal transfer is precisely regulated, resulting in a uniform droplet deposition and a puddle that is easier to control. It also creates only small ripples in the weld puddle. A more stable puddle makes it easier to create consistent tie-ins with the sidewall.
In the RMD process, the system controls the electrode current during all phases of the droplet transfer. After the ball on the end of the wire wets out in the puddle, current is increased to a level sufficient to start pinching the electrode. The current is then increased until the short circuit is cleared and a pinch is detected. Once the pinch is detected, the current is rapidly decreased. Because the pinch is detected before the short clears, the inverter quickly sets the current to a low level before the circuit breaks. The current is then increased to form a ball for the next short circuit, then decreased to allow a short circuit to occur. The current is then monitored, and if necessary, dropped even further to avoid an arc force that could agitate the puddle.
Benefits of the RMD process
The RMD process is specifically designed for root-pass welding. Compared to conventional short-circuit MIG welding, the modified short-circuit process offers:
Good gap filling and tolerance of joints with high-low fit-up
Reduced heat input
Tolerance of variations in contact-tip-to-work distance
Excellent fusion and reduced toe angles that reduce grinding on the side face of the weld
The RMD process compensates for differences in operator technique by maintaining a consistent arc length regardless of contact tip-to-work distance, which is especially helpful for new welders who have trouble maintaining the correct distance.
Because the process produces a faster-freezing, calmer weld puddle, it also offers the potential to eliminate the need for a hot pass, which can save time and money. Typically, stick, TIG and traditional MIG create root passes that are 1/8-inch to 3/16-inch thick, depending on the operator. This requires a hot pass to add more metal so that subsequent fill passes with flux-cored arc welding or spray transfer MIG don’t melt through the root pass. RMD creates a thicker root pass — typically 3/16 inch or greater — which is enough to support the heat input of pulsed MIG or flux-cored fill passes without a hot pass.
The process is also fairly simple to learn, which helps reduce time spent on operator training. If an operator can already weld with MIG, they can become productive with RMD in a few hours. Typical training times for apprentice welders is about two days.
Learn more about the RMD process by watching this video:
Success stories with RMD
Leaders in the pipe fabrication industry are finding success with a switch to RMD for the root pass in pipe welding.
Dixie Mechanical was able to double the number of welds completed every day without compromising quality, allowing them to finish more jobs and meet demanding timelines. The pipe fabrication company began using the PipeWorx 400 welding system and switched from a TIG root pass to an RMD root pass with high silicon wire from Hobart — allowing them to eliminate the time-consuming and costly back purge on stainless steel pipe. Where the back purge could take 20 to 30 minutes per pipe, welders could be set up for the RMD root pass in about five minutes or less. In addition, the wire-fed RMD process is much more productive than TIG welding, allowing operators to complete more welds in the same amount of time. For an 18-inch pipe, the TIG root pass and back purge, followed by flux-cored fill and cap passes, often took a few hours. The RMD root pass with no back purge, followed by a flux-cored cap pass, takes about 30 minutes.
Sigma Thermal increased productivity by 50% or more on pipe fabrication projects with a switch to RMD for the root pass and pulsed MIG for remaining passes. The company saw faster travel speeds and deposition rates compared to TIG welding all passes, as Sigma Thermal had been doing for pipe welds. The RMD technology improves deposition rates so much in the root pass that it eliminates the need for a second hot pass, saving time and money with every weld. A pump skid welding project that took about 120 hours for Sigma Thermal welders to complete using TIG was reduced to about 60 hours using RMD and pulsed MIG — with a failure rate on X-rays of close to zero.
Pulsed MIG welding benefits
While many pipe fabricators use the RMD process for the root pass, it’s common to pair it with pulsed MIG welding for fill and cap passes.
When using conventional pulsed GMAW, different arc lengths can change the overall parameters. However, the advanced pulsed GMAW process maintains the optimum arc length and weld parameters within a broad range of contact-tip-to-work distance (up to 1 inch is possible) and travel speeds. This makes it easier to train new welders and even easier for experienced welders to maintain consistently high-quality welds.
With advanced pulsed GMAW technology, both current and voltage stay within the optimum range for a specific wire type and diameter, wire feed speed and gas combination. Each pulse starts by ramping up the current. Once the target current is reached at the beginning of each phase, the constant current (CC) control turns off and the constant voltage (CV) control loop turns on. The CV loop modulates the current within a range that maintains the target voltage. This occurs independently of the contact-tip-to-work distance.
As a result of faster and tighter control over parameters, this technology provides shorter arc lengths and a more focused arc column. Compared to older pulsed MIG technology, the puddle is easier to control (thus the process is easier to learn) and improves fusion at the toes of the weld.
Like the modified short-circuit process, the advanced pulsed MIG process is more tolerant of contact-tip-to-work-distance variation, which helps when encountering tack welds, training new welders, and welding in tight corners or on pipe beveled with steep angles.
Benefits compared to conventional pulsed MIG include:
Hold shorter arc lengths
Better puddle control
More tolerant of contact-tip-to-work distance variation
No arc wandering in tight corners
Narrow arc plasma column
Allows welds to fill in at toes, increasing travel speed and deposition rate
More tolerant of poor fit-up and gaps
Excellent out-of-position puddle control
Increasing productivity in pipe fabrication
The benefits of RMD and pulsed MIG welding processes can save significant time and money in pipe fabrication. These processes are also easy to learn and use, so operations can save time training new welders — and welders of all skill levels can be trained faster to produce high-quality welds.