How metal fabricators can optimize plasma cut quality
In plasma cutting, different gas combinations react with the cut metal edge differently and affect the weldability of the surface. Selecting the appropriate gas combination and amperage for the material types and thicknesses is key to ensuring high-quality welds.
When using automated plasma cutting, it is crucial to deliver accurate cut parts consistently, with minimal bevel and little or no dross. Automated plasma cutting systems can produce accurately cut parts with a variety of gas combinations, but it is what’s under the surface that will affect the final product quality.
Different gas combinations react with the cut metal edge differently and affect the weldability of the surface. Selecting the appropriate gas combination and amperage for the material types and thicknesses is key to ensuring high-quality welds. The gases that can be used for automated plasma cutting are dependent upon the type of torch used.
Low-cost automated plasma cutting systems are configured with single-gas torches designed to cut all metal types using compressed shop air. This type of plasma system has become extremely popular with metal fabricators doing ornamental metal work and relatively low-volume, general-purpose plate-cutting production.
However, the cut quality delivered by air plasma can be lower than a job requires. For example, the face of a steel plate cut with air often includes large amounts of dissolved nitrides. Shop air is roughly 78% nitrogen and 21% oxygen. When GMAW is applied directly to the cut surface, nitrides often are trapped inside the weld as the metal solidifies—grinding the cut edge surface before welding eliminates that nitriding problem.
When aluminum is cut with air plasma, the cut face is heavily oxidized and very grainy in appearance. The aluminum cut face will require grinding before welding. The cut surface of stainless also is heavily oxidized. The surface will be dark gray and rather crusty from the formation of nickel oxides. Such surfaces are not weldable without grinding them first.
Cylinder gases such as nitrogen or nitrogen/hydrogen (95% nitrogen/5% hydrogen) can be used with some single-gas torch systems on nonferrous metals to improve the cut surface quality. However, the required total flow rate for a 125-amp single-gas plasma torch is as much as 550 cubic ft./hour (CFH). This will increase gas costs because a cylinder with a 330 CFH capacity will be empty in 36 minutes.
Plasma systems configured with single-gas torches have significantly shorter consumable life and higher operating costs than plasma systems configured with liquid-cooled dual-gas torches. Air plasma systems are not equipped with long-life technology.
High-volume-production precision plasma cutting systems are configured with liquid-cooled dual-gas torches, sophisticated automatic gas delivery systems, and process selection. Cut charts embedded into the CNC adjust cutting parameters and select the required gases based on the material and thickness selected.
Also, most of these systems are equipped with technology that ramps amperage and gas flow at the start and stop of every cut. This ensures consistent cutting performance and significantly extends consumable life. Without it, cut quality changes dramatically over the life of a consumable set.
The type of metal to be cut, material thickness, and the cut face weldability required to determine which gas combinations are recommended.
Gas Combinations. Plasma, laser, and oxyfuel cutting all use oxygen to cut steel. All three processes leave a very thin film of iron oxide on the cut surface. This film can be removed easily with an abrasive treatment. However, if the film is not removed, paint applied to the cut surface may simply flake off (see Figure 2).
Amperage, Thickness. Selecting the appropriate cutting amperage for the material thickness is as important to producing excellent plasma cutting results as choosing the appropriate gas (see Figures 1 and 3).
Gas Combinations. For general-purpose cutting, most automated precision plasma systems are engineered to cut stainless, from thin gauge to 1½ in. thick, with nitrogen plasma. Either nitrogen or shop compressed air can be used as the shield gas. The cut should be free of top and bottom dross, have a relatively smooth cut surface, and have minimal bevel.
If weld metal is to be applied directly to the cut surfaces from thin-gauge to ½-in. material, gases such as F5 (95% nitrogen/5% hydrogen) plasma and a nitrogen shield are likely to produce weldable cuts. Surface grinding is required. Some plasma systems include gas delivery that can blend argon, hydrogen, and nitrogen for customized plasma gas mixtures, based on thickness and cutting amperage.
To cut plate from ½ to 1 ½ in., argon/hydrogen/nitrogen plasma gas mixtures with nitrogen shield produce premium results. Edges cut with these combinations are typically very smooth, very square, and have a golden color. A few systems can use nitrogen plasma and tap water as the shield. The nitrogen/water cutting process produces a very square and weldable edge at a low cost for cutting thin-gauge to 1 ½-in. plate.
Note that gas combinations such as F5 plasma with nitrogen shield and argon/hydrogen/nitrogen plasma with nitrogen shield require a narrow window of parameters (speed and voltage) to produce optimal results. Some adjustments to cutting speed and voltage may be required.
See Figure 4 for representative results from various gas combinations.
Amperage, Thickness. Like mild steel, selecting the appropriate cutting amperage is equally important to producing excellent plasma cutting results (see Figure 5).
Gas Combinations. For general-purpose cutting, most automated precision plasma cutting systems cut aluminum from thin gauge to 1 ½ in. using either shop compressed air or nitrogen plasma. Either nitrogen or shop compressed air is used as the shield gas. Again, the cut should be free of top and bottom dross and have a relatively smooth cut surface with minimal bevel. However, the cut face is likely to be heavily oxidized, rough, and quite grainy. Grinding will be required for cut-surface welding.
Blends of argon/hydrogen/nitrogen plasma and nitrogen shield that deliver premium results need to be used only if the weld metal will be applied directly to the aluminum cut surfaces from ¼ to 1 ½ in. to achieve smooth, square, and weldable edges. A few systems allow nitrogen plasma and tap water shields. On thin-gauge to 1-in. plate, the nitrogen/water process produces a square and weldable edge at a low cost.
FIGURE 1. The chart for each cutting process contains a range of possible thicknesses. Process engineers work to optimize a range of thicknesses (usually in the middle of the overall range of thicknesses). This optimized range is called the process core thickness (PCT). Thicknesses greater or less than the PCT can have varied results relative to cut quality, cut speed, and piercing capability. Each category has a unique process category number (1 through 5) that correlates to the performance that you can expect when you select this process. The process category number for the process that you choose changes the quality-speed balance.
Figure 6 shows representative results from the gas combinations listed.
Amperage, Thickness. Again, selecting an appropriate cutting amperage is equally important to producing excellent plasma cutting results as the gas combinations (see Figure 5).
Shops using plasma cutting also will be competing with the laser and waterjet cutting processes. Today, fabricators’ customers demand that plasma-cut parts be accurate with minimal bevel, have little or no dross, and have weldable cut edges.
Profile metal cutting technology has undergone incredible changes in the past 50 years. Most job shops performing plasma cutting are configured with precision high-definition plasma systems. The manufacturers of precision plasma equipment continue to invest in their pursuit of continuous improvement of plasma cutting. The variety of gas options that can be used with plasma cutting systems allow fabricators to select the gas that makes the most sense for a specific application.
Fabricators should assess project specifications before deciding which combination of gases is most suitable for the job.