Are you new to plasma cutting and uncertain about which gases to choose to achieve the best results? Although there is a lot of information everywhere, the average operater, especially a newbie, might get lost quickly. Let´s clear it up once and for all.

Depending on the metal that you´re cutting, you´ll require different types of gases, or a special mixture of gases. In this article we will focus on their main characteristics and recommend which metals various gases are suitable to cut.

The most common plasma cutting gases include compressed air, oxygen, nitrogen mixtures (N₂/CO₂; N₂/H₂0; and (N₂)₂/air), and argon-hydrogen. All of them work relatively well on thinner sheets of metal, one-half inch or less (≤ 12 mm), but when the plate thickness increases, fabricators typically use a combination of these gases to improve production.

FOUR WIDELY USED PLASMA CUTTING GASES

1. COMPRESSED AIR

Compressed air is the most versatile plasma gas, providing good ratio of cut quality and speed on low alloyed steel (mild steel), stainless steel, and aluminum. The big advantage is lower operation cost because it´s not necessary to purchase it. On the other hand, it is necessary to maintain a compressor on a regular basis, and keep it free from any particles or oil mist and moisture. Using compressed air, some kind of oxidation and nitriding can occur on the cut surface, which can cause porosity with a negative impact on the following weldability. The issue is usually corrected by using high-quality weld wire with denitrifiers afterwards.

Suitable for metals:	Mild steel, stainless steel, aluminum
Recommended material thickness:	Up to 1 inch (≤ 25 mm)
Application	Cutting, gouging, marking (EX-TRAFIRE® HD line)

The best solution for EX-TRAFIRE^® plasma cutting systems is a good-sized air compressor with a gas flow output more than 200 lpm (7.1 scf/m) depending on the type of power supply. The gas pressure value should be at least 0.8 bar (11 PSI) above the required dynamic cutting pressure, depending on the torch type. For example, the FHT-EX^®105RTX torch at 105 A requires dynamic pressure of 5.2 bar (75 PSI) / 140 lpm (5 scf/m) for cutting applications. That means the input pressure into the power supply should be at least 6 bar (87 PSI) / 170 lpm (6 scf/min) – including a little buffer reserve. The more lpm the better.

2. OXYGEN

In industry, oxygen is the number one gas for cutting mild steel because of clean, good-quality cuts, leaving only a slight layer of dross (mineral waste formed on the surface of molten metal), that´s ease to remove from the kerf due to the low surface tension. In addition, cutting with oxygen increases cutting speed in comparison with other plasma gases. Oxygen works effectively in combination with other secondary fuels as well. Oxygen isn´t recommended for cutting stainless steel or aluminum.

The main disadvantage of oxygen is the high gas price, and in some cases slightly reduced consumable lifetime. It also doesn´t work on shiny surfaces.

Suitable for metals:	Mild steel
Recommended material thickness:	Up to 1 ¼ inches (≤ 32 mm)
Application	Cutting

Oxygen isn´t used and allowed for the EX-TRAFIRE^® plasma cutting systems.

3. NITROGEN

Earlier, nitrogen was used in legacy plasma torches. Nowadays nitrogen is often used for higher current plasma systems, for cutting materials up to 3 inches (≤ 75 mm). The best results are achieved while cutting stainless steel and aluminum, but fair cut quality is achieved on mild steel too, although some dross may occur. By itself, it’s most effective on smooth and shiny thin sheets. A bit advantage of nitrogen is excellent cut quality and extended consumables lifetime. On the other hand, nitrogen is an expensive gas.

For thicker metals it´s better to use nitrogen in combination with carbon dioxide, with air, or with argon as a secondary gas.

• CO₂ slightly improves surface finish, increases cutting speed and extends consumables lifetime
• Air is the best secondary gas when you´re using a nitrogen plasma system
• Water as a secondary, if the system allows, must be used with a water table

Suitable for metals:	Stainless steel, aluminum
Recommended material thickness:	Up to 3 inches (≤ 75 mm)
Application	Cutting

Referring to the first paragraph, compressed air contains around 75% nitrogen, so nitrogen and its mixtures are also suitable gases for the EX-TRAFIRE^® power supplies. They are recommended for stainless steel or aluminum cutting.

4. ARGON HYDROGEN*

Argon is a rare, therefore expensive, inert gas, unreactive with the metals being cut. Using argon while cutting is advantageous. It stabilizes the arc and prevents atmospheric contamination of welding pools, but it can´t work alone because of its low conductivity.

Hydrogen is a perfect thermal conductor and can quickly cool hot metal surfaces. For these reasons it is optimal for cutting aluminum and high alloyed steel (stainless steel). Although it has excellent conductivity, its low atomic weight prevents it from having high kinetic energy. Just like argon, it must be combined with other gases.

The standard argon-hydrogen mixture is 65% argon and 35% hydrogen. This argon-hydrogen mixture is the best for thick stainless steel and aluminum plates that exceed 3 inches. For cutting mild steel it isn´t recommended. In addition to cutting, the argon-hydrogen mixture is also suitable for plasma gouging on any materials. Argon-hydrogen provides a straight, very smooth cut, and an almost polished surface on high alloyed steel (stainless steels). The combination offers the maximum cutting capacities. Due to the high purchase price, operation with argon-hydrogen is less economical.

* Prior to use, ensure that your system is safe and compatible with argon-hydrogen gas.

Suitable for metals:	Stainless steel, aluminum
Recommended material thickness:	Up to 4 inches (≤ 100 mm)
Application	Cutting, gouging, marking

Argon and its mixtures are suitable for the EX-TRAFIRE^® power supplies for marking application using the EX-TRAFIRE^® HD line. Compared to compressed air, marking with argon will prevent grooves that are too deep and wide. In other words, you can achieve better results.

FINAL RECOMMENDATION

Considering the most common materials you cut, their thickness, required cut quality and your financial requirements, here are some brief conclusions:

• The most economical and versatile gas for plasma cutting is clean compressed air, which fits for mild and stainless steel, and aluminum as well
• Oxygen is the best for mild steel cutting, where air is used as a shielding gas, with good cut quality, fast cutting speed and only a low, easily removable dross
• When you need to cut mainly stainless steel and aluminum, use nitrogen plasma in combination with CO₂ to reach the best cost-performance ratio
• For very thick stainless steel and aluminum plates, argon-hydrogen in combination with nitrogen as a secondary gas is the best choice to get a perfect cut quality for the cost

The most suitable plasma gas for the EX-TRAFIRE^® plasma cutting systems is clearly compressed air. The main benefits are reduced initial and operating cost, and easy accessibility (just a compressor with correctly set parameters).

A possible alternative for cutting stainless steel or aluminum is nitrogen. Using nitrogen requires only slight adjustment of cutting parameters compared to the compressed air.

For the purpose of marking plates with the EX-TRAFIRE® HD line, the best choice is clearly argon. Using a special range of consumables for marking applications, you can achieve nice, smooth, mild, and narrow grooves.

We hope you found this article useful. If you need any advice in this area, do not hesitate to contact us. You can contact us at this email address: extrafire@thermacut.cz

Text:               Thermacut, k.s.
Photo:            Archive of Thermacut, k.s.