A method for attempting to understand various machinability phenomena when cutting metal in the turning operation was explored by examining both the surface and a transverse section of the crater zone of uncoated tungsten-carbide tools when AISI 4140 steel tempered to 290 Brinell hardness is machined. Four steels were used, representing four different levels of machinability due to their different inclusionary compositions. This study revealed the existence of a third body, consisting mainly of sulphides the stability of which varies with the following two main parameters: (i) the machining speed (and therefore the cutting temperature); and (ii) the rheological property of these layers and in particular their calcium content. Therefore, the wear effect due to the diffusion of iron into the tool's tungsten-carbide matrix depends on the stability of the sulphides of inclusionary origin. The purpose of this study is to identify, optimise, and possibly modify the inclusionary populations of steels that produce a stable third body on the surface vicinity of cutting tools.
