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In recent years, high-pressure nitrogen or oxide-free, assist-gas cutting has won favor from job shops and OEMs because of the cut quality achieved when inert gas is used in the laser cutting process. Parts processed using high-pressure nitrogen are produced virtually burr-free and do not need secondary grinding operations. Because there is no oxide layer, the resultant edge is a bright, milled quality that makes for better welds.
Make the Most of Laser Technology
As laser cutting system capabilities grow, applications for high-pressure nitrogen cutting also expand. Once considered most suitable for stainless steel and aluminum, high-pressure nitrogen cutting is now being applied to light gauge mild steel in applications where an oxide layer presents a concern and part quality is essential. In this overview, we will examine:
1. High-pressure assist-gas cutting process
2. Advantages of high-pressure laser cutting with nitrogen in stainless steel and mild steel applications
3. Costs of using nitrogen as an assist gas
Parker Nitrogren Generators are the Ideal Solution
Nitrogen as an Assist Gas for High-Pressure Nitrogen Cutting
- Applying high-pressure nitrogen cutting usually requires additional specialized operator training. However, this laser cutting technique offers a cost-effective alternative for processing stainless steel parts in 6.4 or 9.5 mm thicknesses. In the past, thick stainless steel could only be processed using more labor-intensive and time-consuming processes, such as milling or EDM.
- In addition to its cosmetic appeal, a part cut using high-pressure nitrogen gas can move directly from the laser system to welding operations without additional processing.
- Because nitrogen-cut parts have considerably less taper than parts processed using oxygen, welding operations are minimized, less fixturing is required, and parts generally have a better fit. Improved welds also translate to less time spent polishing parts. One large OEM manufacturer of stainless steel parts reports a 20 to 35% reduction in finishing time and fixturing operations.
The Cutting Process
Nitrogen is the most commonly used inert gas. Other inert gases used in laser cutting can include argon and helium. Because of nitrogen's stability, it is commonly used as a high-pressure assist gas, while argon and helium are used for more specialized material-specific applications. Benefits include:
- Unlike oxygen-assist cutting, an inert gas does not contribute to the burning or cutting process but shields the cutting front of the material being processed. During cutting, the inert gas "floods" the area being cut and essentially starves it of oxygen so that oxide cannot attach to the cut edge, producing a bright, oxide-free edge.
The absence of an oxide layer also makes the nitrogen-processed part weld-ready. In contrast, stainless steel parts cut using oxygen-assist gas must be ground to remove the oxide layer before welding. The resulting weld will be structurally inferior due to porosity if the oxide layer is not removed.
- Assist gas will blow molten metal through the underside of the material, preventing burr formation.
- Another advantage to welding parts processed with nitrogen is that the resultant part edge will have little or no bevel or taper. This is particularly true of stainless steel in thicknesses from 6.4 to 13 mm. If processed with oxygen, the edges will have a considerable bevel or taper, resulting in a poor fit during fixturing for welding.
- When cutting with nitrogen, the inert gas flows into the kerf (The space between the two cut edges or cutting “trough”) at a very high rate, quickly ejecting the molten material. The high speed at which this is accomplished does not allow time for molten material to attach to the part and form a burr (Rough or random edges). Typical pressures and flow rates in stainless steel from 6.4 to 13 mm range from 21bar – 26bar (300 to 375 psi) at between 34 to 45 m3/hr.
High-pressure nitrogen also helps cool the part, minimizing heat penetration or the size of the heat-affected zone (HAZ). This is particularly apparent in thick stainless steel from 4.7 to 13 mm, where the HAZ is most often measured in tens of thousandths of an inch instead of cutting with oxygen assist, where the HAZ may be as much as 10% of the material's thickness.
The technique employed when high-pressure assist cutting with nitrogen differs from cutting with oxygen-assist gas. When high-pressure cutting is employed, the laser's focal point setting must be adjusted into the bottom 10 to 15% of the material. The focal point is set at the part's surface when cutting with oxygen. Driving the focus deeper into the part makes the resultant kerf wider and allows for the optimum ejection of molten material. The wider kerf also allows a higher gas volume to be pushed into the kerf, creating an optimum shield for the edge.
Nitrogen shields cut edges from oxygen, so an additional process to remove an oxide is unnecessary. In addition, high-pressure inert gas cutting requires using a larger nozzle orifice, 1.5 to 2.5 times larger than nozzles used for oxygen-assist cutting. This allows for maximizing the assist gas column's pressure and volume. The nozzle's internal geometry is designed to allow the assist gas column to attain a higher velocity.