Effect of total oxygen content on the machinability of low carbon resulfurized free cutting steel

The effect of total oxygen content on the machinability of low carbon resulfurized free cutting steel was studied by means of machinability tests, metallographic, and scanning electron microscopy observation. The results showed that the total oxygen content had a notable effect on the machinability of SAE 1215 steel. The best machinability of the steel in terms of tool life and surface roughness could be obtained by controlling the total oxygen content at 0.0105 and 0.0125 wt%. When total oxygen content in the steel was less than 0.0105 wt%, the machinability of the steel was increased with the increase of total oxygen content because of the significant reduction of the aspect ratio of MnS inclusions. However, when total oxygen content in the steel was more than 0.0125 wt%, the machinability of the steel was deteriorated dramatically with the increase of total oxygen content, which was attributed to that the amount and the size of oxide inclusions (MnOAl2O3, MnOSiO2, and 2MnOSiO(2)) with high hardness and high melting point both had a significant increase, and some MnS inclusions were also wrapped by the MnOSiO2 and 2MnOSiO(2) hard oxides. Furthermore, the tool wear would reach the maximum when total oxygen in the steel rose to 0.0150 wt%.