Effects on machining due to microstructure variations in a case hardening steel

In manufacturing industry, variations in machinability are regularly observed when producing the same part out of different material batches of a case hardening steel. However, material batches showing a decreased machinability often have properties within the material specification. Thus, for an increased machinability and a more robust production of case hardening steel parts, the relationship between microstructure and machinability needs to be further investigated. In this study, machinability has been evaluated for a specific case hardening steel with respect to pearlite nodular size, phase distribution and interlamellar spacing of pearlite. These variations in microstructure have been achieved by altering the annealing cycle, i.e. austenitizing temperature, holding time and temperature for pearlite formation. The criterions for machinability which are taken into consideration in this investigation are tool wear and chip formation. Furthermore, a comparison of cutting forces and surface integrity for the tested microstructures will be presented. The results from tests show a difference between material with larger pearlite nodules and a material with smaller pearlite nodules. For tool wear, the material with larger pearlite nodules has a decreased machinability. However, for chip formation the result is opposite, and a material with larger pearlite nodules has an increased machinability