Characteristics of Gases for Plasma Cutting
Gases have a great influence on the cutting quality. In order to achieve an economic cutting process and optimum cutting results it is imperative to use the right plasma-forming gases for the respective materials. The physical properties of the gases play a decisive role in this regard. Such important properties are ionisation and dissociation energy, thermal conductivity, atomic weight and chemical reactivity.
Argon
Argon belongs to the group of inert gases. It means that it does not react with the material during cutting. Its atomic weight - the highest among all plasma gases - supports the expulsion of the melt out of the kerf. The reason for this is the high kinetic energy of the plasma beam. Due to its low ionisation potential argon is excellently suitable for igniting the plasma beam. However, argon cannot be used as single cutting gas because it has a low thermal conductivity and a low thermal capacity.
Hydrogen
In contrast to argon, hydrogen has a very good thermal conductivity. Further, hydrogen dissociates at high temperatures. It means that a great amount of energy is withdrawn from the arc (like ionisation) and, therefore, the surface layers cool down. This effect leads to a further constriction of the arc and, therefore, a higher energy density can be achieved. Recombination processes have the effect that the withdrawn energy is released again in the molten bath in the form of heat. However, hydrogen, too, cannot be used as single gas because it has, in contrast to argon, a very small atomic weight and, therefore, it cannot produce enough kinetic energy to expel the melt.
Nitrogen
Nitrogen is an inert gas which means that it reacts with the material only at high temperatures and remains inert at low temperatures. Regarding its properties (thermal conductivity, enthalpy and atomic weight) nitrogen ranges between argon and hydrogen. Thus, it can be used as single cutting or swirl gas for cutting thin sheets of high-alloyed steel
Oxygen
Oxygen can be classified as similar to nitrogen due to its thermal conductivity and atomic weight. Oxygen has a certain affinity to iron which means that during oxidation heat is produced that can be used to increase the cutting speed. However, this reaction is still called fusion cutting and not flame cutting because the reaction with the work piece passes too slowly and the work piece is already melted before. Oxygen is mainly used as cutting and secondary gas for cutting unalloyed and low-alloyed steel.
Air
The main components of air are nitrogen (approx. 70%) and oxygen (approx. 21%). Thus, the combined properties of both gases can be used. Air is one of the most cost-effective gases and is used for cutting sheets of unalloyed, low-alloyed and high-alloyed steel.
Gaseous Mixtures
Often, the above mentioned gases are also used as gaseous mixtures. For example, the good thermal properties of hydrogen can be combined with the high atomic weight of argon. This allows cutting of high-alloyed steel and also aluminium with a material thickness of more than 5 mm with the hydrogen content depending on the material thickness. The thicker the material, the more hydrogen is used. The maximum content amounts to 35% by volume. Of course, other combinations are also possible like nitrogen-hydrogen or argon-nitrogen-hydrogen mixtures.
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