Wear reduction of carbide tools observed in cutting Ca-added steels for machine structural use

The effect of Ca addition for improving machinability in carbide tool machining operation has been widely known. It has been proposed that oxide inclusions, which would work as "belag" on the tool surface during machining operation, has been responsible for this effect. In recent years, when Ca-treated steels containing higher sulfur are machined, MnS has been observed to form on the rake face, and this "inclusion layer, instead of the oxide belag, would serve favorably for machinability. In this paper, we have investigated the influence of several inclusions and the addition of Ca and/or Mg on rake face wear of carbide tools. The rake face wear of Ca-added steel was appreciably reduced due to the MnS inclusion layer formed on the rake face. However, the concurrent addition of Ca and Mg to the steel did not improve the rake face wear. Combined with a TEM observation in a cross section of MnS layer formed on the rake face, these results are understood by the different nature of oxide inclusions in terms of abrasive wear.