About Plasma Cutting

What is a plasma? 
We normally think of three states of matter-solid, liquid and gas. One description of plasma is that it is the fourth states of matter. For the most commonly known substance, water these states are ice, water and steam. When added heat energy, the ice will change from solid to liquid and if more heat is added it will change into gas (steam). When substantial heat is added to gas, it will change from gas to plasma- the fourth states of matter.

Conventional Plasma ARC Cutting  
The plasma jet generated by conventional "dry" arc constriction techniques was introduced in 1957 by Union Carbide's Linde Division. In the same year, Dr. Robert Gage obtained a patent, which for 17 years gave Union Carbide a virtual monopoly. This technique could be used to sever any metal at relatively high cutting speeds. The thickness of a plate could range from thin sheet metal to plates as thick as ten inches (250 mm). The cut thickness was ultimately dependent on the current-carrying capacity of the torch and the physical properties of the metal. A heavy duty mechanized torch with a current capacity of 1000 amps could cut through 10-inch thick stainless steel and aluminium. However, in most industrial applications, plate thickness seldom exceeded two inches. In this thickness range, conventional plasma cuts were usually beveled and had a rounded top edge. Beveled cuts were a result of an imbalance in the heat input into the cut face. A positive cut angle resulted because the heat energy at the top of the cut dissipated as the arc progressed through the cut. This heat imbalance was reduced by placing the torch as close as possible to the work piece and applying the arc constriction principle. Increased arc constriction caused the temperature profile of the electric arc to become extended and more uniform.

Correspondingly, the cut became squarer. Unfortunately, the constriction of the conventional nozzle was limited by the tendency of increased constriction to develop two arcs in series, one arc between the electrode and nozzle and a second arc between the nozzle and work piece. This phenomenon was known as "double arcing" and damaged both the electrode and nozzle. Double arcing severely limited the extent to which plasma cut quality could be improved.

Since the introduction of the plasma arc process in the mid-50, considerable research has focused on increasing arc constriction without creating double arcing. Plasma arc cutting as performed then is now referred to as "conventional plasma cutting." It can be cumbersome to apply if the user is cutting a wide variety of metals and different plate thicknesses.

For example, if the conventional plasma process is used to cut stainless steel, mild steel, and aluminum, it is necessary to use different gases and gas flows for optimum cut quality on all three metals. Conventional plasma cutting predominated from 1957 to 1970, and often required very expensive gas mixtures of argon and hydrogen.

Air Plasma Cutting
Air cutting was introduced in the early 1960s for cutting mild steel. The oxygen in the air provided additional energy from the exothermic reaction with molten steel. This additional energy increased cutting speeds by about 25% over plasma cutting with nitrogen.

Although air cutting was not pursued in the late 1960s in the United States and the western world, steady progress was made in Eastern Europe with the introduction of the "Feinstrahl Brenner" (torch producing a restricted arc), developed by Manfred van Ardenne. This technology was adopted in Russia and eventually in Japan. The major supplier became Mansfeld of East Germany. Several shipyards in Japan were early users of air plasma cutting equipment. However, the electrode life was relatively short and studies disclosed that the cut face of the work piece had a high percentage of nitrogen in solution which could cause porosity when subsequently welded.

Water Shield Plasma Cutting
Water shield plasma cutting was similar to dual flow except that water was substituted for the shield gas. Cut appearance and nozzle life were improved because of the cooling effect provided by the water. Cut squareness, cutting speed and dross accumulation were not measurably improved over dual flow plasma cutting because the water did not provide additional arc constriction.

Water Injection Cutting
Earlier, it was stated that the key to improving cut quality was increasing arc constriction while preventing double arcing. In the water injection plasma cutting process, water was radially injected into the arc in a uniform manner. The radial impingement of the water at the arc provided a higher degree of arc constriction than could be achieved by just the copper nozzle alone. Arc temperatures in this region are estimated to approach 50,000°K or roughly nine times the surface temperature of the sun and more than twice the temperature of the conventional plasma arc. The net result was improved cut squareness, increased cutting speeds and the elimination of dross when cutting mild steel.

Underwater Cutting
Further attempts in Europe to decrease the noise level of the plasma arc and to eliminate smoke development as much as possible led to underwater cutting. This method for high power plasma cutting with cutting currents above 100 amps has become so popular that today, many high power plasma cutting systems cut under water. For underwater plasma cutting, the work piece is immersed about 2 to 3 inches under water and plasma torch cut while immersed in the water. The smoke and noise level as well as the arc glare are reduced dramatically. One negative effect of this cutting method is that the work piece cannot be observed while cutting and the cutting speed is reduced by 10-20%. Further, the operator can no longer determine from the arc sound whether the cutting process is proceeding correctly and whether the consumables are producing a good quality cut.

Finally, when cutting in water, some water surrounding the cut zone is disassociated into oxygen and hydrogen, and the freed oxygen has a tendency to combine with the molten metal from the cut (especially aluminum and other light metals) to form metal oxide, which leaves free hydrogen gas in the water. When this hydrogen collects in a pocket under the work piece, it creates small explosions when reignited with the plasma jet. Therefore, the water needs to be constantly agitated while cutting such metals.

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