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The Laser User Magazine
Issue 051 - Summer 2008
Cutting mild steel with nitrogen
Author: John Powell
When laser cutting mild steel with oxygen, the chemical reaction in the cut zone produces a liquid which is easy to blow away because it does not stick to the solid steel. However, the oxidized liquid is not entirely blown out of the cut zone, and a thin layer is left to solidify on the cut edge. The cut edge is therefore covered in a thin, brittle oxide layer, which is not firmly attached to the underlying steel. The lack of adhesion between the brittle oxide layer and the ductile steel is demonstrated in figure 1. Here we can see that the dark grey oxide has flaked off the cut edge in the area where the component was bent as part of the fabrication process – the bright steel of the cut edge has been revealed where the oxide has broken off.
In many cases this flaking of the oxide layer is not important, but problems can arise if the laser/oxygen-cut product is plated or painted. The oxide layer forms an unreliable link between the paint/plating and the steel along the cut edge. Under these conditions, the paint or plating can, at some point, break off with the oxide layer. Although this doesn’t happen in many cases, the possibility of it happening has lead to a growth in laser profiling mild steel with nitrogen as the cutting gas. If nitrogen assisted gas is used, the cut edge is not oxidized, and the paint or plating is in direct, firm contact with the metal edge.