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 Parker Balston - High Quality Nitrogen for Laser Applications

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Parker Balston - High Quality Nitrogen for Laser Applications

 



High-pressure nitrogen or oxide-free, assist-gas cutting has, in recent years, 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. 

 

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 gage mild steel in applications where an oxide layer presents a concern and part quality is important.  In this overview, we will examine:

 

1. The high-pressure assist-gas cutting process, 

2. The advantages of high-pressure laser cutting with nitrogen in stainless steel and mild steel applications, and 

3. The costs of using nitrogen as an assist gas.

 

MAKE THE MOST OF LASER TECHNOLOGY

As more reliable and efficient high-powered lasers are introduced, high-pressure assist-gas cutting is becoming an economical option for mild-steel processing, and continues to provide high part quality and burr-free cutting in stainless steel and aluminum applications

 

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:


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Unlike oxygen-assist cutting, an inert gas does not contribute to the burning or cutting process but, rather, 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.


•  T
he 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. If the oxide layer is not removed, the resulting weld will be structurally inferior due to porosity.


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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 which will result in a poor fit during fixturing for welding.


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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.


•  H
igh-pressure nitrogen also helps cool the part, minimizing heat penetration or the heat affected zone (HAZ) size. 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, as opposed to cutting with oxygen assist, where the HAZ may be as much as 10% of the material's thickness.


•  T
he technique employed when high-pressure assist cutting with nitrogen is different 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. When cutting with oxygen, the focal point is set at the part's surface. Driving the focus deeper into the part makes the resultant kerf wider and allows for optimum ejection of molten material. The wider kerf also allows for 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 not required.  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 maximizing the assist gas column's pressure and volume. The nozzle's internal geometry is specially designed to allow the assist gas column to attain a higher velocity.

 

Nitrogen as an assist Gas for High Pressure Nitrogen Cutting


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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 thicknesses of 6.4 or 9.5 mm. In the past, thick stainless steel could only be processed using more labor-intensive and time-consuming processes, such as milling or EDM.


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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.


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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 better a 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.

 



 

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