Process for high quality plasma arc cutting of stainless steel and aluminum

A plasma arc torch uses a mix of reactive and reducing gas flows to cut sheets of stainless steel, aluminum and other non-ferrous metals. The reducing gas flow to the cut is varied as a percentage of the total gas flow to maintain a reducing atmosphere down through the cut, but to leave a predominantly oxidizing atmosphere at the intersection of the cut and the bottom surface of the sheet being out. These flows can also be characterized as either a plasma gas flow, one that forms the arc, or a shield gas flow that surrounds the arc. The reactive gas is preferably a flow of air, oxygen, nitrogen, carbon dioxide or a combination of these gases. The reactive gas is usually in the plasma gas flow, whether alone or mixed with other gases. The reducing gas is preferably hydrogen, hydrogen 35, methane, or a mixture of these gases. For aluminum, the reactive gas is preferably air or nitrogen and the reducing gas is preferably methane or a mixture of methane and air.

BACKGROUND OF THE INVENTION 
This invention relates in general to plasma arc cutting of sheet metals. 
More specifically, it relates to a mixture of type and proportion of gases 
forming and/or shielding the arc that yield very clean, shiny, and 
dross-free cuts in stainless steel, aluminum and other non-ferrous metals. 
Plasma arc cutting of sheet metals is now used widely. However, heretofore 
for stainless steel and non-ferrous metals such as aluminum it has not 
been possible to produce a clean cut, one where there is a shiny kerf that 
is free of oxides or nitrides of the metal being cut, which is also free 
of bottom dross. 
The plasma arc is a jet of an ionized gas. While many gases can be used to 
form the arc, the gas selected is usually specific to the metal being cut. 
For example, to cut stainless steel, it is most common to use air, 
nitrogen, or a mixture of argon and hydrogen. 
Nitrogen and air leave no bottom dross, but the cut quality is poor. The 
sides of the kerf have oxide or nitride inclusions and they undergo a 
change in metallurgical structure. In order to weld at this cut, or to 
obtain an acceptable appearance, it is necessary to grind or wire-brush 
the cut sides. 
It is known that argon-hydrogen may be used as the plasma gas to cut 
stainless steel. While these cuts are metallurgically "sheen", that is, 
shiny and clean, but at least for cuts in thin sheets, argon-hydrogen 
leaves a bottom dross that is unusually difficult to remove. Sheeny, 
dross-free cuts are possible with argon-hydrogen for sheets with a 
thickness in excess of about 0.5 inch (12.7 mm) using a 200 ampere torch 
and in excess of about 0.25 inch (6.4 mm) using a 100 ampere torch. No 
plasma cutting technique has been found that produces sheeny kerfs without 
dross when cutting aluminum, regardless of its thickness. 
It is also well known to use shield gases, typically a secondary gas flow 
through the torch that is independent of the plasma gas flow and surrounds 
the arc, whether by impinging on it as it exits the torch or downstream, 
near or at the workpiece. Shield gases can serve a variety of functions, 
such as cooling, isolation of the cutting action in the kerf from the 
atmosphere, and the protection of the torch against upwardly splatterd 
molten metal. Plasma and shield gases are used, for example, in the plasma 
arc cutting torches sold by Hypertherm, Inc. of Hanover, N.H. under its 
trade designations MAX.RTM.200, MAX.RTM.100, MAX.RTM.100D and HD1070. The 
numbers 200, 100 and 70 denote current ratings for these torches. None of 
the known torches using shield gases have demonstrated any ability to 
improve on the cut quality of known nitrogen, air and argon-hydrogen 
cutting when used on stainless steel and non-ferrous metals such as 
aluminum. 
It is therefore a principal object of the present invention to provide a 
plasma arc cutting process that can cut stainless steel, aluminum and 
other non-ferrous metals with an extremely high cut quality. 
A further principal object is to provide a cutting process that is 
adaptable to different metals and different torches, including high 
density torches, and torches using only a plasma gas or ones using plasma 
and shield gases. 
Another object is to provide a cutting process with the foregoing 
advantages even when used on thin sheets of the metal. 
Still another object is to provide all of the foregoing advantages using 
known equipment and operating materials and at a favorable cost. 
SUMMARY OF THE INVENTION 
At least one gas flow of a plasma flow and shield flow to a plasma arc 
cutting torch constitutes or contains as a component of a mixed flow of 
gases a reducing gas. The gas flows also include a gas that reacts with 
the metal. The flow ratio of the reducing gas flow to the total gas flow 
to the cut, whether introduced as the plasma gas, the shield gas, or a 
component of one or both of those gases, is controlled so that the 
reducing gas is completely dissipated in the kerf. As a result, the 
reducing gas has a negligibly small concentration at region defined by the 
kerf and the bottom surface of metal workpiece. Stated conversely, the 
atmosphere at the bottom surface is predominantly oxidizing. The gas 
selection and control of the reducing gas ratio can be defined 
functionally as ones which provide a reducing atmosphere that extends 
through the kerf, from the top to the bottom surfaces of the workpiece, 
but which also produce an oxidizing atmosphere at the bottom surface. The 
ratio which yields this result varies empirically with the type of metal, 
the power of the torch, the type of gases being used, and the thickness of 
the workpiece. For a given application, the ratio varies with the 
thickness. This process produces high quality cuts in stainless steel, 
aluminum, and other non-ferrous metals. The cuts are sheeny and free of 
bottom dross. 
While this mixture of gases can be formed solely in a plasma gas, the gases 
are preferably introduced as plasma and shield gases. The reactive and 
reducing gases can appear, solely or in mixture, as either one of, or both 
of, these gas flows. 
To cut stainless steel with a high definition torch at low power, the 
plasma gas is preferably air or nitrogen flowing typically at 40 scfh 
(standard cubic feet per hour) for low to medium power applications. With 
nitrogen as the plasma gas, the shield gas can be methane or methane and 
air. The ratio of the methane flow rate to air flow rate ranges from about 
5% to 25% depending on the thickness of the workpiece. A typical shield 
gas flow rate is in the range of 20 to 60 scfh, depending on the 
thickness. For high definition cutting of aluminum, the plasma gas is 
again air or nitrogen with methane as a shield gas. With a nitrogen plasma 
gas, the methane can be mixed with air, again in varying ratios to 
accommodate different thicknesses. 
Plasma gases for a standard plasma arc torch can include hydrogen, hydrogen 
35 mixed with nitrogen, and a mixture of hydrogen and nitrogen, and air. 
Shield gases include nitrogen and carbon dioxide. Nitrogen is the 
preferred shield gas with either the hydrogen 35 and nitrogen mixture or 
the hydrogen-nitrogen mixture as the plasma gas. 
For stainless steel and aluminum, the reactive gas is preferably nitrogen, 
air, other mixtures of oxygen and nitrogen other than air. Reducing gases 
can include hydrogen, hydrogen 35, methane, and other flammable 
hydrocarbon gases known to combine with oxygen. The reducing gas 
preferably constitutes between 2% and 50% of the total gas flow--plasma 
gas and shield gas, if any--depending on the thickness of the workpiece, 
other parameters being constant. 
These and other features and objects of the present invention will be more 
clearly understood from the following detailed description which should be 
read in light of the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows prior art plasma arc cutting of a kerf 12 in a workpiece 14, 
in this case a stainless steel plate. A plasma arc cutting torch 16 of 
known construction produces an arc 18 that transfers from the torch to the 
workpiece to produce the kerf. The arc 18 is a jet of ionized gas that 
conducts current to the workpiece. A DC power supply 20 is connected in 
series with the torch and the workpiece. The plasma gas is an 
argon-hydrogen mixture, typically 35% hydrogen and 65% argon by volume, 
commercially sold as hydrogen 35. A regulated, adjustable rate supply 22 
of the plasma gas is illustrated schematically. Depending on the torch and 
application, the cutting torch can also receive a flow of a shield gas 
from a separate regulated, adjustable flow rate supply 24. Typical torches 
16 include the standard cutting torches sold by Hypertherm, Inc. of 
Hanover, N.H. under its trade designations MAX.RTM.100, MAX.RTM.100D, and 
MAX.RTM.200 and its high density 70 ampere torch sold under the trade 
designations "HyDefinition" and "HD1070". 
This particular prior art system can cut stainless steel sheets while 
producing a clean, shiny kerf. However, as noted above, it also produces a 
very difficult bottom dross 26. The dross forms in two regions. An upper 
region 26-1 near the kerf retains a metallic look. In a lower region 26-2, 
the dross is dark from the formation of oxides. 
FIG. 2 shows a plasma cutting system according to the present invention. As 
in FIG. 1, the plasma arc torch is a known type such as the MAX.RTM. and 
HyDefinition.TM. products identified above using a plasma gas flow 22a and 
a shield gas flow 24a. The power of the torch, as measured by its 
operating current, typically range from low power units of 15 to 50 
amperes, to high power units of 400 to 500 amperes. For high definition 
torches, relatively small currents, e.g. 70 amperes are typical, but at a 
very high current density. Typical standard torch currents for the most 
common thicknesses are 100 to 200 amperes. 
The workpiece 14 is a sheet. It can assume other forms, such as a firearm 
barrel, a bolt, or contoured structural member, but the cutting of sheets, 
including plates, is the-most common application. An "upper" surface 14a 
of the sheet will then be understood to be the surface of the workpiece 
opposite the plasma torch. A bottom surface 14b faces away from the torch. 
For a sheet workpiece, the surfaces 14a and 14b are generally flat and 
parallel. The plate thickness T measured along a normal to the surfaces 
14a,14b can vary from thin sheets, e.g. 1/8 inch (3.1 mm) to plates 2 
inches (51 mm) thick. 
A principal feature of the present invention is that the gas flow or flows 
from the torch to the kerf include as a constituent gas at least one gas 
of a type that reacts with the metal of the workpiece, and as another 
constituent gas a different type of gas that produces a reduction 
reaction, particularly one that will react chemically in a reduction 
reaction with reactive gases such as oxygen, or nitrogen, or a mixture of 
the two such as air. The reactive gas and the reducing gas can be mixed to 
form the plasma gas, or the shield gas, or they can be separated, one in 
the plasma gas flow and the other in the shield gas flow. 
A further principal feature of the present invention is that the amount of 
the reducing gas is carefully controlled as a portion of the total gas 
flow to the kerf--the sum of the plasma and shield gases where both are 
used. (Some ambient air or other gas flows may also enter the kerf, but 
they are usually present in insignificant amounts or are sufficiently 
removed from the cutting action as to be of little or no functional 
consequence.) The degree of control is conveniently expressed as the ratio 
of the flow rate of the reducing gas or gases to the total gas flow rate. 
This ratio varies with parameters such as the type of metal being cut, its 
thickness, the type and power of the torch, and the type or types of gas 
forming the plasma and shield gas flows. For a given application, the 
control ratio varies mainly as a function of the plate thickness. FIG. 3 
shows a typical such relationship for the cutting of stainless steel plate 
with a MAX.RTM.100D brand torch with a mixture of argon, hydrogen and 
nitrogen. The curve in FIG. 3 shows that for this example the ratio of 
hydrogen to the total gas flow should be about 3.5% for thin plates (1/8 
inch), but about 32% for thick plates (1/2 inch). While the precise values 
will vary for each application, the general form of the curve shown in 
FIG. 3 defines this relationship. In general, the ratio of the reducing 
gas to total gas flow that will provide the results of the present 
invention falls in the range of about 2% to about 50%. The precise value 
for each application can be determined empirically by examining the cut 
quality for different ratios at a selected thickness, or at different 
thicknesses for a selected ratio. 
This ratio control produces a predominantly reducing atmosphere within the 
kerf at the arc. This reflects a predominant concentration of the reducing 
gas extending from the upper surface 14a, substantially through the kerf, 
to a region 28 at the intersection of the kerf and the bottom surface 14b. 
At the region 28 there is then predominantly oxidizing atmosphere. This is 
reflected in FIG. 2 in the high concentration of reactive gas (e.g. 
oxygen) at the surface 14b and the negligible concentration of reducing 
gas (e.g. hydrogen). When properly controlled, it is believed that the 
amount of the hydrogen or other reducing gas present in the flow is used 
up in chemical reaction with the reactive gas in the kerf. This condition 
produces cuts in stainless steel and non-ferrous metals of a quality that 
heretofore never been obtained using plasma arc cutting, regardless of the 
thickness of the workpiece. 
While the precise mechanism(s) that produce this result are not known with 
certainty, applicants are of the opinion that the predominantly reducing 
atmosphere in the kerf prevents an oxidizing reaction between the molten 
metal being cut and reactive gases present in the kerf. (The oxidizing 
reaction is the one which cuts the metal, e.g. the creation of oxides or 
nitrides of the metal being cut which are carried away by the plasma jet.) 
The reducing gas (or its ions or radicals formed in the plasma) is 
believed to react with the oxidizing gas (or its ions or radicals formed 
in the plasma) preferentially. In the region 28, the predominantly 
oxidizing atmosphere is believed to be essential to oxidize molten metal 
before it runs out of the bottom of the kerf to form a dross. This 
analysis provides a functional guide for the control over the reducing gas 
portion of the total gas flow. If there is too little reducing gas, the 
kerf will not be sheeny throughout. If there is too much reducing gas, a 
dross will form. 
As an illustration of the process of the present invention, but not as a 
limitation, applicants give the following examples of this invention which 
have been successfully practiced using Hypertherm MAX.RTM.100D and 
HyDefinition HD1070.TM. cutting systems on stainless steel and aluminum 
sheets having thicknesses that varied from 1/8 inch to 5/8 inch. 
Using an HD1070.TM. system to cut stainless steel, the following 
combinations of plasma and shield gases were used successfully at typical 
flow rates of 40 scfh for the plasma gas and 20 to 60 scfh for the shield 
gas, with the variation in shield flow rate corresponding to the thickness 
of the workpiece generally as shown in FIG. 3. 
TABLE I 
______________________________________ 
(High Density, Stainless) 
Plasma Gas Shield Gas 
______________________________________ 
N.sub.2 CH.sub.4 (methane) 
air CH.sub.4 
N.sub.2 CH.sub.4 and air 
air CH.sub.4 and air 
______________________________________ 
The ratio of methane to air varies from about 5:95 to 25:75 depending on 
the thickness of workpiece, the total shield gas flow rate being constant. 
Using the HD1070.TM. system to cut aluminum, Table II gives successful 
plasma and shield gases at the flow rates given above with respect to 
Table I. The shield gas mix of air and methane is variable from almost 
100% methane to almost no methane, depending again on the thickness of the 
aluminum sheet being cut. 
TABLE II 
______________________________________ 
(Aluminum) 
Plasma Gas Shield Gas 
______________________________________ 
air CH.sub.4 
N.sub.2 CH.sub.4 and air 
______________________________________ 
Table III gives suitable plasma and shield gases for cutting stainless 
steel with a MAX.RTM.100D system. Typical flow rates are those given above 
with respect to Table I. 
TABLE III 
______________________________________ 
(Standard Arc, Stainless) 
Plasma Gas Shield Gas 
______________________________________ 
Hydrogen 35 and N.sub.2 
N.sub.2 
H.sub.2 and N.sub.2 N.sub.2 
Hydrogen 35 and N.sub.2 
CO.sub.2 
H.sub.2 and N.sub.2 CO.sub.2 
______________________________________ 
The percentage of hydrogen 35 in the mixture varies from about 10% for thin 
sheets to about 90% for thick sheets. The percentage of H.sub.2 in the 
second and fourth mixtures varies from about 3.5% for thin sheets to about 
35% for thick sheets. 
There has been described a process which produces high quality--sheeny and 
dross free--cuts in stainless steel and non-ferrous metals such as 
aluminum using plasma arc cutting. The invention can produce these results 
on sheets or other configurations having any of a wide variety of 
thicknesses using high density plasma cutting systems and standard plasma 
cutting systems. The invention is also compatible with plasma cutting 
systems operating over a wide range of power levels and with mechanical 
shields and gas flow shields against upwardly splattered molten metal. 
While the invention has been described with respect to its preferred 
embodiments, it will be understood that various modifications and 
variations will occur to those skilled in the art from the foregoing 
detailed description and the accompanying drawings. For example, while the 
examples use mainly nitrogen and air as the reactive gases, other reactive 
gases including oxygen alone, oxygen-bearing gases, and oxygen-nitrogen 
mixes not in the proportion of air are contemplated. Similarly, other 
reducing gases can be used. In particular methane is illustrative of a 
class of flammable gases that combine with oxygen in an exothermic 
reaction, although perhaps having a greater cost or producing undesirable 
byproducts. These and other modifications and variations that occur to 
those skilled in the art are intended to fall within the scope of the 
appended claims.