By Dr John Powell DSc, DIC, CEng, CPhys (auth.)
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2 (Sect. 1). Chapter 2 Laser Cutting Steels The laser cutting industry is dominated by the profiling of steel sheet of both mild and stainless grades. The cutting process is achieved by a combination of laser heating and the chemical reaction of iron with oxygen. This oxidation reaction is exothermic (heat generating) and acts as a secondary energy source which helps to accelerate the cutting process. Basically, the steel is locally heated by the focused laser beam to a temperature at which the iron will ignite in the oxygen jet acting coaxially with the beam.
Generally it is wisest to find the nominal focal point (by methods explained in Sect. 3) and position this on the surface of the workpiece. In some cases, when cutting thicker section steels, improved results can be obtained by positioning the focal point two or three millimetres below the material surface. These comments are all relevant to a machine which is working correctly within the usual range of focused spot sizes. This spot size can be enlarged as a result of a number of faults in the system.
In the absence of a thick surface oxide layer this process is very rapid. However, when hot rolled steel is to be penetrated in this way, the surface oxide serves to protect the underlying steel. A larger interaction time is needed for the heat and oxygen to penetrate this oxide barrier. This increase in initiation time means that melting is more widespread. The blast holes tend to be larger and heating of the area around the hole increased. For these reasons it can be more difficult to produce detailed work in black, hot rolled steel, particularly when working with only moderate (-400 W) laser powers.