You can plasma cut it, but can you weld it?

A lot happens in a plasma arc from its generation at the electrode surface to the workpiece. Among numerous other factors, the plasma forming gas and its surrounding shield material, be it gas or liquid, play a major role in the final cut quality.

A high-quality cut is characterized by a low bevel angle (ideally between 0 and 1 degree), absence of bottom dross, no top edge rounding, minimal heat-affected zone (HAZ), and a smooth cut face. The cut edge smoothness can be affected by the stability of the plasma arc column, due in part to the torch design, as well as the precision of the cutting table or robot. In addition, optimal cut quality requires the proper torch height to produce a minimal bevel angle.

All plasma cutting variations have an optimal cutting speed window that produces dross-free cuts. If the torch goes faster, the bevel angle increases, eventually leading to high-speed dross that sticks to the bottom of the cut, which can be difficult to grind off. If the torch moves too slowly, the process produces low-speed dross, which is thick, porous, and easier to remove. But the slow speed also puts more heat into the material than necessary, leading to a larger HAZ and top edge rounding.

Any rough or chemically contaminated cut edge can create problems for the welder. To ensure weld integrity, he may need to consider other procedures, such as grinding; alternative filler metal; or a change in travel speed. Extensive grinding could produce a surface ready for short-circuit gas metal arc welding, but without such grinding, the transfer mode can be problematic because it allows the weld pool liquid to freeze quickly and possibly trap contaminants, such as nitrogen. Welding processes that produce slower-freezing weld pools, such as submerged arc welding or flux-core arc welding, allow more contaminants to outgas.

Choosing the Best Cutting Gas

In the 1970s plasma cutting carbon steel plate with a nitrogen-water combination was preferred to oxygen plasma cutting because of its reliability and versatility. Effectively, nitrogen-water injection could cut any metal. The nitrogen-contaminated cut face, detrimental to subsequent welding, was simply considered a trade-off for the plasma process's increased speed and the best viable solution at the time.

Nevertheless, time spent on extensive weld prep easily can eliminate any gains made from faster plasma cutting. Put another way, it doesn't matter how fast a plasma cuts if the resulting cut face can't be welded efficiently. There have been, however, significant breakthroughs in oxygen plasma cutting, so significant in fact that today it is the process of choice when working with carbon steel.

Carbon Steel: Why Oxygen Is Best

When carbon steel is cut with nitrogen plasma, the nitrogen is absorbed into the cut surface of the base metal; likewise, cutting with oxygen plasma leaves oxygen behind. Nitrogen, however, is chemically less active than oxygen. Oxygen will more readily react with the range of elements—such as silicon, aluminum, and manganese—that can be supplied to the weld zone via alloying elements in filler materials, shield gas, or flux agents. Thus, nitrogen is more likely to remain in the weld zone. This can lead to islands of nitrides at the grain boundary as well as porosity in the weld, which makes it necessary to further prepare the cut surfaces by mechanical means such as grinding or machining.

Plasma cutting carbon steel with shop air can be even worse for welding. According to studies, synergy happens in the highly heated plasma arc between oxygen and nitrogen molecules in ambient air.¹It was found that air plasma increases nitrogen absorption into the cut surface while reducing the steel's iron content in the HAZ, potentially leaving a more crack-susceptible cut face. The money saved using shop air may become irrelevant when considering the additional time needed to prep the cut surface for welding and the likelihood of surface discontinuities that can lead to a change in mechanical properties of the weld.

Oxygen plasma cutting of carbon steel leads to better cut quality: higher speed, lower bevel angles (squareness), less cut face roughness, larger dross-free window, thinner HAZ, and a more weld-friendly cut surface, which can lower the amount of defects detrimental in a structure.

In addition to drastically reducing the nitrides in the cut surface, oxygen also reduces dross that adheres to the material during cutting. Working with carbon steel, oxygen exothermically reacts with the iron in the liquid metal, creating iron oxide. The reaction releases extra energy that makes the liquid metal even hotter and less viscous. This facilitates the liquid metal removal by the plasma jet, leaving a clean cut edge with no dross on the bottom.

When Nitrogen Works: Stainless and Aluminum

This does not mean nitrogen plasma gas never works. Metals with no iron such as aluminum or nonreactive iron alloys such as stainless steels gain no benefit from oxygen plasma cutting. For aluminum, the oxygen gas can lead to heavy oxides in the cut.

With no iron-oxygen reaction, plasma cutting these metals relies solely on heat transfer from the plasma arc to the work. With these materials, argon-hydrogen plasma gas has worked well because it has high heat conductivity. Better heat conductivity means more heat can transfer from the arc to the metal. A common blend, H35, contains 35 percent in volume of hydrogen. To limit the heat input in thin materials, the blend can go as low as 5 percent hydrogen content in volume.

Because the heat conductivity of an argon-hydrogen arc is higher, the arc loses more heat, which induces the plasma arc to contract so that it increases its core temperature to counteract the energy loss. Of course, such an arc requires higher power to be sustained.

But argon-hydrogen is not the only alternative for aluminum and stainless cutting. With the right torch design, a nitrogen-water injection, less expensive than other gases, can work well when plasma cutting aluminum and stainless material for subsequent welding.

The process involves an electrode surrounded by nitrogen, which is heated by an electric arc to form the plasma. The resulting plasma arc exits the nozzle, and a radial water shield impinges on it. A steam curtain forms at the plasma-water interface, which shields the plasma from atmosphere and cools the plasma's perimeter. This shrinks the plasma plume diameter and concentrates energy toward the plasma's inner core, using the same mechanism explained previously for H35. The hot inner core efficiently liquefies and ejects the molten metal out the cut bottom (see Figure 1).

A recent study tested 1/4-in. 304 stainless steel and aluminum 5052-H2.² The study involved plasma cutting each of these metals with a nitrogen-water (N₂/H₂O) combination, then autogenously (using no filler metal) welding them with the gas tungsten arc process (see Figure 2 and Figure 3). Mechanical and metallurgical tests were performed on the welded joints, including tensile stress and bending. After cutting, the nitrogen content of the as-cut surfaces was measured using scanning auger microanalysis (SAM). Virtually no nitrogen inclusions were found in the cut faces.

Two factors in nitrogen-water plasma cutting may help produce such smooth cuts. One, the process is very fast; two, the plasma edge is relatively cool compared to conventional plasma jets because of the water, which has a quenching effect that results in a very narrow HAZ in the base metal.

Higher Density, Better Cutting

The goal in plasma cutting, like in laser cutting, is to attain the highest energy density possible to efficiently penetrate the plate. Common to all plasma processes, constriction and stabilization are achieved by a small nozzle diameter in combination with the swirling motion of the plasma-forming gas. Depending on the process variation, further constriction can be achieved using water as a shielding material.

Other mechanisms could be used to constrict and stabilize the arc, such as materials with high heat conductivity for the nozzle to evacuate the heat radiated by the constricted arc, or even intense magnetic fields. Currently, however, the cost of such systems would offset any gain in cut quality and speed.

Today's high-definition plasma systems are showing many benefits, particularly for plate 1/4 in. and thicker. Which cutting technology to use depends on the application requirements. But with the right gas mixture, complemented with the right torch design, plasma cutting systems can make clean cuts quickly, and make life easier for the welder.

Thierry Renault, Ph.D., is principal arc process engineer, and Geoffrey Putnam is principal engineering associate, welding, for Thermadyne Industries Inc., 16052 Swingley Ridge Road, Suite 300, St. Louis, MO 63017, 636-728-3000, www.thermadyne.com. Nakhleh Hussary, Ph.D., is a thermal plasma researcher who has worked for Thermadyne as well as the University of Minnesota.

Notes

1. Ian D. Harris, Plasma Cutting of Bridge Steels, National Cooperative Highway Research Program, Report 384 (Washington, D.C.: National Academy Press, 1997).
2. N. Hussary, G. Putnam, T. Renault, "Selection of Cost-Effective Plasma Cutting Process for Weld Integrity," in proceedings from FABECH® International & AWS Welding Show, Chicago, Nov. 11-14, 2007.