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 specialised material-specific applications.
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.
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. If the oxide layer is not removed,
the resulting weld will be structurally inferior due to porosity.
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.
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 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.
The 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.
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 labour-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 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.
