Got Gases?

Mix and match plasma gases for cut quality and speed you want

By Sue Roberts
March 22, 2013
Article
Fabricating

Edge squareness on 1-in., 400-amp parts was achieved using a nitrogen plasma gas and water injection . Photo courtesy of ESAB.

Air, oxygen, nitrogen, hydrogen, argon, and methane. On their own or in some combination, these gases blast away metal, turning sheet and plate into components with features and contours. Each gas has its own set of parameters and chemical reactions that affect the quality of the finished part. Choice of gas affects edge smoothness, color, cut speed, and the overall cost of the plasma cutting process.

Which gas should be used to melt away that metal? Answer two basic questions before making a choice:

What material will be cut? (Is it ferrous or nonferrous? How thick is the material?)
Where will parts go when they come off the plasma cutting table? (Will they go to a secondary operation before assembly or welding? Do they need to be pretty?)

Air and Oxygen

In low-current systems, economy comes along with using compressed air for the plasma and shield gases. Most metals up to 1 in. thick, ferrous and nonferrous, can be cut with the air plasma/air shield combination; however, oxidation will occur on nonferrous materials, perhaps requiring the need for a secondary operation to remove it.

The cost of purchasing gases is side-stepped; however, the air isn't free. The clean, dry air needed for plasma cutting may need to be processed through a dedicated air compressor, refrigerated dryer, and filters to remove particulate and moisture.

John Peters, mechanized systems engineer at Hypertherm, Hanover, N.H., said, "You can even cut stainless with air. The speeds are reasonable and the edge shape is pretty good. The drawback is that you get a heavily oxidized edge that needs to be dealt with before moving on to the next process (see Figure 1).

"I would encourage consideration of several factors before choosing to use air as the predominant plasma gas choice. Cut speed, cut quality, and potentially adding secondary operations for lower gas costs are factors to be considered."

A switch from air to oxygen as the plasma gas (see Figure 2) benefits ferrous plasma cutting by speeding up the cut rate and providing a high quality of mechanized cuts.

"Oxygen cutting of mild steel generally gives you a better cut in terms of less dross and a better surface chemistry for following processes because you don't have any nitriding that appears on the cut edge like with air plasma," said Peters.

Ruben Chico, mechanized plasma global product manager for ESAB Welding & Cutting Products, Florence, S.C., agreed that oxygen's oxidizing reaction is particularly beneficial when cutting carbon material: "Switch from air to oxygen as the plasma gas and you have a win-win situation. Speeds and quality increase. Costs also increase, but when people run the numbers, the benefits of using oxygen far outweigh the higher costs."

Much of the higher cost is due to the life span of hafnium electrodes. Even though they survive in a pure oxygen environment, they tend to wear out quickly. It's not the cost of each electrode but the quantity needed that hikes production cost. "It depends on the power level of the system," said Chico. "At lower levels they could last two or three days. At higher levels you might change them every two to three hours of operation.

Figure 1: Different gas combinations create different cut outcomes on 3⁄8-in. stainless. The top sample was cut with the 105-amp Powermax® system, all others using a 130-amp HPRXD® system. From top to bottom: air plasma with air shield cut at 94 IPM; nitrogen plasma with nitrogen shield cut at 55 IPM; H35 plasma with nitrogen shield cut at 40 IPM; F5 plasma with nitrogen shield cut at 25 IPM; H35 plus nitrogen plasma with nitrogen shield cut at 50 IPM. Photos courtesy of Hypertherm.

"When using nitrogen or another gas that does not include oxygen, tungsten electrodes are used, which have a much longer life, but we must ensure that oxygen doesn't get into the torch and oxidize the tungsten. Then you get one bright flash and it's gone."

Changing the shield gas from air to oxygen also offers benefits. "We might mix some oxygen in the shield gas or even use pure oxygen to get a better, more complete, faster burn through the plate," said Chico. "By using oxygen in the shield gas, you can minimize the slag puddle on the top of the plate and actually consume some of that material rather than letting it pile up."

Role of Nitrogen

Nitrogen is often used in higher-current plasma systems for cutting materials up to 3 in. thick. The nitrogen plasma/nitrogen shield edge will be smoother than the air cut's but will have a black color and will probably require a cleanup operation before welding or if an eye-pleasing edge is needed.

"If I'm cutting stainless steel or other nonferrous steels, I will use nitrogen as the cutting and shield gas to try to keep air from being pulled into the cut where the oxygen can interact with the surface metal," Chico said. "A variation of this is where water is injected onto the arc, and that functions as the shield. It produces nice, clean, straight cuts up to about 2 in." The water injected onto the arc causes constriction, but it also creates steam that shields the arc from the oxygen in the atmosphere.

F5 fuel gas, a mix of 5 percent hydrogen and 95 percent nitrogen, is typically shielded with nitrogen.

"That smidgen of hydrogen mixed into the nitrogen plasma gas interferes with the oxygen, changes the temperature some, and improves the look of the cut. If I want the edge to be pretty, the expense of F5 is justified," said Chico. "But we can often get about the same results by mixing a little bit of methane into the nitrogen shield to interfere with the oxygen when using nitrogen plasma gas."

From Welding to Cutting

H35, a readily available welding mixture used for plasma cutting nonferrous metal, is 35 percent hydrogen and 65 percent argon. Nitrogen shield gas displaces oxygen and reduces oxidation. It is a good choice for fast, high-quality cuts in thick aluminum and stainless plate from ½ in. and up.

The H35 mixture produces an arc with very high total energy, transferring more power to the plate and speeding the cut process.

"You need the higher hydrogen content when you're cutting thick material to keep the edge bright and shiny," said Peters. "If you have too much hydrogen, though, you will get dross and it can interfere with your edge shape or angle."

Chico recommended avoiding H35 when cutting thin materials. "When you get down to the thinner materials, the H35 is overpowering and its advantages vaporize. It is such a hot gas that thin materials melt too much and the cut quality is reduced. The part tends to undercut because of too much power."

Figure 3: Precision Hole Technology™ from ESAB adjusts the gas mix, pressure, and flow for cutting features and profiles. Photo courtesy of ESAB.

Latest Advances

With the many parameters that come into play in choosing plasma cutting gases—materials, gas flow, amperage, piercing methodology, lead-in/-out techniques, cut speed, and timing—fabricators look for gas selection assistance that doesn't require operator intervention. Equipment manufacturers offer control advances that use cut material information and part specifications to automatically determine and program gas use, mix, and flow.

"Hypertherm offers True Hole™ technology that is an oxygen plasma and oxygen shield gas process. The change in shield gas helps address taper in the hole. So you would use the oxygen plasma/oxygen shield running at a slower speed to get a nicely dimensioned hole, and then switch to oxygen plasma/air shield to run the part contour at a noticeably higher speed. That combination would optimize quality and productivity, and the gas changes are handled by the system," said Peters.

"ESAB's Precision Hole Technology™ (PHT) automatically adjusts the gases, gas mixes, pressures, and flow based on different cutting parameters," Chico said. "It will look at the cut profiles and determine the duration of the cut and the size of the hole and figure out what to use to cut the different features on the part." (See Figure 3.)

Each combination of plasma and shield gases offers different cutting results. Know the materials, know the parts, and use the knowledge built into the plasma systems to pick the gas combination for the best results.