Formation mechanism of alumina layer in protecting cubic boron nitride inserts in turning cast irons

Abstract

Among many cast irons, compacted graphite iron (CGI) has recently been gaining more attention due to its superior mechanical properties over the currently more popular flake graphite iron (FGI), aka gray cast iron. However, the poor machinability has prevented CGI to be used in a wider range of potential applications. The resulting tool life difference between FGI and CGI is staggeringly different to be simply explained by the difference in their physical properties. In particular, the excellent machinability of FGI is claimed to be due to the presence of the MnS layer at the interface between cubic boron nitride (cBN) inserts and FGI at high cutting speed. This paper presents the results of our experimental investigation in an attempt to explain the tool wear difference between FGI and CGI while reexamining the formation of such layers. Straight turning experiments were performed with both FGI and CGI with various cBN inserts in dry condition at high cutting speeds (mostly >400 m/min). The flank and crater surfaces of the inserts during turning experiments were investigated and the difference in both flank and crater wear among various cBN inserts are reported as a function of cutting speed. When turning FGI, speckles of MnS, not as a layer, were present based on our EDS/SEM measurement on the cBN inserts, mainly from the MnS inclusions in FGI smearing on the cBN inserts. However, the total area of MnS speckles did not increase as the turning process continues, questioning the formation of the extensive MnS layer protecting the inserts. When turning CGIs, both Mn and S were present individually, not as MnS, on the cBN inserts. Intriguingly, instead of MnS layer, the extensive Al2O3 layer is formed on the rake side on the certain grades of cBN inserts with the Al2O3 binder phase when cutting at high cutting speeds, which functions as the solubility barrier for certain grades of cBN inserts. The crucial difference between FGI and CGI is that the Al2O3 layer is not only larger in coverage but also significantly more stable after turning FGI. Therefore, the tool wear difference between FGI and CGI mainly comes from the formation of the stable Al2O3 layer on the cBN inserts with Al2O3 binder phase when turning FGI. A few additional mechanisms are introduced to make the complete argument behind the remarkable difference in tool wear.