Influence of gas composition on plasma arc cutting of mild steel

We report the results of a study on the influence of oxygen in the plasma gas used in the plasma arc cutting process on cuts obtained in mild steel plates. Experimental results of shapes of kerfs and the leading edges of the cut front formed while cutting a 6 mm mild steel plate at 100 A with nitrogen, air and oxygen as plasma gases are presented. These results are discussed in the light of the overall energy balance of the process. It is found that the exothermic reaction of oxygen in the plasma gas with the iron in mild steel enables the cutting of mild steel at higher speeds with both air and oxygen than the maximum cutting speed attainable with nitrogen. A comparison of the melting rates for oxygen with those of air reveals that although oxygen can produce more exothermal energy by oxidation, oxygen is not superior to air in melting metal near the bottom of the kerf formed at high cutting speeds.

The study shows that the dross formed at the bottom of the cut is determined by the shape of the cut-front surface over which the molten metal from the cut front flows to be ejected at the plate bottom. Any improvement of metal ejection to be gained with oxygen as the plasma gas may be the result of enhanced superheating of the metal melted from the cut-front surface.