- New pulsing technology is operator-friendly, easier to learn.
- SharpArc lets the operator change weld puddle characteristics.
- More control lets operators increase travel speeds an average of 30 percent.
- Auto-Line™ reduces primary power draw and enables using primary power from 190- to 630 V.
- Ease-of-use reduces training time.
- XMT® 350 MPa lets Coventa run five power sources off of a 100-amp breaker where before they could only run four.
Pulsed MIG Improved Inconel Cladding by 30
By the time the exhaust reaches the tower, most of the harmful emissions have been removed. They take their toll inside the boilers, however, where they affect the tubing.
That can of garbage sitting in your kitchen might not be worth its weight in gold, but it is enough to power a light bulb for 24 hours—at least it can once Covanta Energy gets a hold of it.
With more than 30 plants around the world, Covanta takes municipal solid waste (MSW)—trash that would otherwise fill landfills—and turns it into energy. For every 10 tons of MSW Covanta receives, it reduces it to ash that is 10 percent of its original volume, generates 5,200 kWh of power and recycles 500 lbs. of metal. According to Covanta, every ton of MSW converted to energy avoids the need to import one barrel of oil or mine one-quarter ton of coal.
The MSW is fed into combustion chambers, which is constructed of steel tubes that contain water. When the MSW is burned, it turns the water into steam, which then spins turbines. The gases emitted by the burning, however, would corrode the steel tubes rather quickly—unless they were clad with corrosion-resistant metal. That’s where Brad Hooper and his Covanta team come in. Hooper is supervisor for the NERM (NorthEast Regional Maintenance) overlay for Covanta Energy.
Hooper’s team is responsible for prolonging the life of the tubes by cladding them with Inconel™, a high nickel-content alloy that is known for its corrosion resistance. Hooper’s team operates during scheduled shut downs, and keeping on schedule is extremely important. The team moves from one facility to another, and lost time can throw off months of planning and affect several facilities.
The cladding process leaves little room for error: Inconel must be applied with pulsed MIG to maintain the proper amount of metallurgical dilution between the mild carbon steel tubes and the Inconel cladding. Establishing and maintain optimum pulsed MIG welding parameters on their previous welding machines was difficult and required a high level of expertise.
|Brad Hooper (blue helmet) of Covanta, Ken Dworetsky and Joe Noel of AirGas East in Hartford, Conn., (left and right, white helmets) and a member of Hooper’s team discuss the Miller XMT 350 MPa multiprocess inverters.
Fortunately, Hooper had the opportunity to beta-test the welding industry’s newest pulsed MIG inverters for portable applications. Where before they could only run four power sources off a 100-amp breaker, now they can now use five after switching to Miller Electric Mfg. Co.’s XMT® 350 MPa multi-process inverter with Auto-Line™ power management technology. They no longer need a person dedicated to watch over and reset the power sources while the operators weld in the boilers.
Also, with the XMT 350 MPa’s built-in pulsed MIG programs, which include programs for Inconel as well as other exotic alloys, the actual welding is easier than before. This system further simplifies field welding: it is the first system specifically designed to pulse with a portable wire feeder, Miller’s SuitCase® 12RC.
Up in Smoke
When MSW is burned in one of Covanta’s EfW (Energy-from-Waste) facilities, it’s reduced by 90% of its volume through a combustion process. That process would corrode the carbon steel tubing that make up the chambers. Carbon steel (28, 213, A213) is used everywhere except in the super-heated elements, where it’s too hot for Inconel to blend with the steel. In those cases, T22 stainless steel is used.
Inconel cladding protects the tubes that make up the boiler, extending their lives from 3 to as much as 15 years. To maintain the proper dilution rate, pulsed MIG is required.
In some cases the carbon steel is clad when it’s put in; in other cases it is clad after it has corroded sufficiently. In a few instances, where it’s easy to replace the tubes, they are allowed to wear out.
During regularly scheduled maintenance shut downs, Covanta ultrasonically measures every inch of tubing. Tubing starts with a .235-in. wall thickness. When it reaches .140-in. thick, Covanta will clad with a .070-in. layer of Inconel. Without cladding, the tubes would last a couple of years before needing replacement. “An Inconel application can extend tubing life up to 12-15 years in some cases,” Hooper says.
Inconel is one of Special Metals Corporation’s (www.specialmetals.com) nickel alloys, which is engineered to offer “a superior combination of heat resistance, high temperature corrosion resistance, toughness and strength,” according to the company. Covanta uses Inconel 625.
Easier Said Than Done
“Maintaining the proper dilution rate is critical when applying Inconel,” says Hooper. “When you apply it to the carbon steel, you need a dilution rate that keeps it from wanting to fall off the tube. However, the dilution rate needs to be low enough that it doesn’t pull the iron up into the face of the weld and take away the chromium content and remove Inconel’s corrosion-resistant properties. To get the desired dilution rate (7 to 10 percent of the base metal) and keep it from cracking, you need to use a pulsed MIG arc.”
With a pulsed MIG (GMAW-P), the power source switches between a high peak current and a low background current. The peak current pinches off a spray transfer droplet and propels it toward the weld. The background current maintains the arc, but is too low for metal transfer to occur.
“Pulsed MIG helps to control the heat,” Hooper explains. “A straight (spray transfer) MIG process adds too much heat to the molten metal, which reduces chrome content in the finished product. With too much heat, you get a higher iron dilution rate and decreased corrosion resistance.”
You Didn’t Need to Be a Rocket Scientist, but….
The pulsed MIG process allows the operator to precisely control the amount of heat put into the puddle. Previously technology complicated the process of establishing and maintaining the right parameters for a pulsed MIG arc. Often, an engineer would be required to set all of the parameters, depending on the metals and wire used.
|Brad Hooper walks through the 4-step setup for setting pulsed MIG weld parameters on the XMT 350 MPa. The units retain settings for each process, so they can easily be switched from pulsed MIG to Stick and back with a simple turn of the knob.
“Some contractors thought all you had to do was buy a pulsed MIG machine,” says Hooper. “But there was more to it: we had to program the trim (arc length), the actual pulsed frequency, delay time of the pulse (pulse duration) and other parameters for every individual wire size and type. Our previous power sources weren’t very user-friendly. It took somebody that had been around them a long time to be able to set them to weld Inconel. You couldn’t just send a new kid to turn a machine on and set it for Inconel. You had to know exactly what button to push or it you’d be welding with the wrong processes or wrong parameters.”
The difficulty in dialing in the previous machines led to lengthier training periods and required Hooper to closely monitor machine settings with most of his operators. Other issues cause problems, too, such as finding prompt support and service. With the need to purchase at least 20 more machines to expand the cladding operations, having support was critical.
“Any delay holds all us from accomplishing our primary goal: getting the power plant back on the grid,” says Hooper.
Nor were breakdowns the only issue that could account for downtime.
“With our previous machines, a small voltage fluctuation sometimes affected the machine. The welders would shut down, display an error code and we’d have to go reset them,” says Hooper.“That could happen three or four times a shift. It was very frustrating. We would have to have somebody running around all the time just making sure there’s power on.”
For a solution to these problems, Hooper and Gregg Pruett, Covanta regional maintenance manager, turned to Ken Dworetsky and Joe Noel of AirGas East in Hartford, Conn., who arranged for a visit to Miller’s Appleton, Wis., headquarters.
A Pipe Dream
“We told them any new machine had to be able to weld 450 inches per minute,” says Pruett. “Of course, no one thought that was possible, but that’s what we were looking for. When we went to Miller’s lab, they pulled out this brand new machine (the Miller XMT 350 MPa multi-process inverter). Brad welded one bead, turned to me and said, ‘This is it.’ Within ten minutes we were welding 535 i.p.m. in the overhead position. They had other machines for us to try, but we were done. We took all the XMTs they had.”
Setting the SharpArc setting. A lower setting results in a wider arc cone that has greater wetting action, increased weld puddle fluidity and a flatter weld bead. A higher SharpArc setting narrows the arc cone, which produces a narrower, faster freezing weld bead with less heat input.
The XMT 350 MPa’s advanced technology simplifies the pulsed MIG process (MPa = MIG, Pulsed, advanced arc controls). Its internal programming relieves the operator from the burden of setting complex pulse parameters for optimal output. The operator only has to enter a few parameters, (wire type, diameter and gas type) and select the preferred SharpArc™ setting.
SharpArc technology alters the width of the pulsed MIG welding arc cone. A lower setting results in a wider arc cone that has greater wetting action, increased weld puddle fluidity and a flatter weld bead. A higher SharpArc setting narrows the arc cone, which produces a narrower, faster freezing weld bead with less heat input.
Comparing the time and expertise needed to set-up and operate the previous machines with the XMT 350 MPas, Hooper says,” It’s helped tremendously. Now, a guy flips a switch to turn the machine on, pushes a button four times (to select SharpArc settings, wire type, wire diameter and gas type), and he’s ready to go.”
The Long and Short of It
When performing the cladding process or when using highly alloyed metals, operators need to tailor the arc length to suit their needs, shortening the arc to reduce heat input and help the operator “push the puddle around and get the desired tie-in,” Hooper explains. Shortening the arc with the older pulsed MIG units would often lead to short circuits that would cause inclusions in the weld bead or other defects that would require reworking. As a result, welders held a longer-than-desired arc.
“The puddle was tough to control with the previous machines,” Hooper said. “You actually had to turn wire feed speed down—what I considered to be slow—to be able to weld with it and not have any arc shorts or short-outs that would cause an inclusion.
“The XMT 350 MPa’s new pulsed MIG programs lets us tailor the arc to fit our needs, and we get 30 percent more accomplished by not having to worry about the arc shorting out. Before, we couldn’t weld faster than 300 i.p.m. without a lot of shorts. Now, we average 350 to 400 i.p.m., and I can even weld 500 to 540 i.p.m. in some applications.”
Joe Noel and Ken Dworetsky of AirGas East in Hartford, Conn., suggested a trip to Miller’s Appleton headquarters, where Hooper took only 10 minutes to decide the XMT 350 MPa was the power source he wanted.
For the most part, Covanta operators account for the dilution rate, penetration and operator preference, set welding parameters once and that’s the only time they touch the controls.
Power to the People
Where previously a power fluctuation could shut down several machines at once and require a dedicated person to reset them, or at least make it more frustrating for the operator, the new XMT 350 MPa uses Miller’s Auto-Line technology. The primary power may fluctuate, but it doesn’t affect weld quality.
Auto-Line boosts primary input power—from 190 through 630 V—to a higher voltage and stores it in a capacitor. The inverter, therefore, has sufficient power as long as the primary power remains within a +37/-59 percent of the nominal 460V power. In short, it's like drawing water from a well that's always full.For the most part, Covanta operators account for the dilution rate, penetration and operator preference, set welding parameters once and that’s the only time they touch the controls.
“I don’t have to spend as much time with the guys on set-up or training,” Hooper says. “They pick up the Millers pretty quickly, especially with the large display that tells them exactly what they’re doing when they push that button. With the wire and gas type and wire feed speed all spelled out, it’s pretty easy now.”
Hooper chooses the Miller SuitCase®12RC wire feeder for his team. “I like the digital ones because they’re actually reading what voltage you’re using, and the size of the display is definitely helpful. I also like the trigger lock on these feeders. They keep the guys’ hands from cramping while they weld 9.5 hours a day.”
“Since switching to the XMT 350 MPas, we had no power surge or fluctuation problems at all,” says Hooper. “The technician, who used to have to stand next to the machines to ensure they had power and reset them when necessary, can now concentrate on the consumables and keep up with the maintenance of the welding leads. He’s now able to do two jobs at once.”
“The Auto-Line function was a big selling point,” adds Pruett. “We were always having trouble with the [previous machine], so Auto-Line is a big deal. Plug in with 220 V power, 480V power, and with [almost] any kind of voltage spike or amperage spike, we’re fine. Even in Miller’s lab, they had 300-feet of welding cable on the floor, which is would happen during the field welding process. They did all that and were still welding without noticing any problems associated with the voltage drop that can occur when welding with long leads. The XMT 350 MPa was the machine for us.”
The XMT 350 MPa brings another advantage: increased power factor. It only draws 17 amps of primary at rated welding output (350 amps at 60 percent duty cycle).
“Where we could only plug in four machines on a 100-amp breaker, we can now plug in five,” says Hooper. “That’s a 25-percent productivity increase, or another two square feet of cladding every hour.”
An increased power factor five machines instead of four, 30 percent faster travel speed, eliminating inclusions and rework and freeing the maintenance technician—has important ramifications for everyone.
“We try to minimize downtime in the facilities as far as boiler availability,” says Pruett. “Time to us is money.” The XMT 350 MPa helps achieve that goal.
“We started off with two XMT 350 MPas, then eight more, then 10 more and we have 10 more coming,” Hooper says. “The XMT 350 MPa is just a super machine for what we do and the processes we use. It’s just hard to beat.”
According to Special Metals Corp., Inconel 625 is “A nickel-chromium-molybdenum alloy with an addition of niobium that acts with the molybdenum to stiffen the alloy’s matrix and thereby provide high strength without a strengthening heat treatment. The alloy resists a wide range of severely corrosive environments and is especially resistant to pitting and crevice corrosion. Used in chemical processing, aerospace and marine engineering, pollution-control equipment, and nuclear reactors.”<-->