Study of microstructural influences on machinability of case hardening steel

The overall aim of this study is to establish knowledge about how the material specification of case hardening steel should be modified in order to create a robust and predictable production process with focus on machinability. In this study, emphasis is laid on studying microstructure variations produced by changing holding time and temperature in the annealing process. A standardized milling test was used to assess machinability (with respect to tool wear) as this method has the advantage of only needing small diametrical samples and low material volumes. From the results of the test, it can be concluded that machinability is more influenced by pearlite nodular size than by pearlite morphology. Furthermore, it can be stated that hardness is not a suitable criterion for determining machinability of case hardening steel. In the manufacturing process of transmission parts (such as gear milling/hobbing), high speed steel tools are commonly used. The same type of tool is applied in the standardized milling test. However, the obtained ranking (of machinability) for the different materials is also applicable if other types of cutting tools are used.