Effects on Machinability due to Microstructure Variations in Case Hardening Steel

Licentiate thesis, 2008

Typical products made of case hardening steel are transmission parts like gear wheels, pinions and crown wheels. Case hardening produces a carbon-rich surface and provides the parts with both enhanced hardness and fatigue life. However, variations in machinability are regularly observed when producing the same part out of different material batches. This leads to a non-robust production process with increased manufacturing costs as well as unforeseen stops and maintenance of machines. In this study, emphasis is laid on studying the effect of microstructure variations on machinability in case hardening steels. Different microstructures have been produced by changing holding time and temperature in the annealing process. The microstructure was altered as much as possible with respect to pearlite nodular size, pearlite morphology and amount of pro-eutectoid ferrite. Since a homogeneous material is required, relatively small sample diameters had to be used. Therefore, the machinability ranking (with respect to tool wear) was performed with help of the Volvo Standard Machinability Test which is a short-term test. Additionally, chip breakability was evaluated and measurements of cutting forces and surface roughness were performed. The tests have shown that a case hardening steel with larger pearlite nodular size (produced by a higher austenitizing temperature) provides better chip breakability but also an increased tool wear compared to a material with smaller pearlite nodular size. Hence, to achieve a more robust production process for case hardening steels, it might be necessary to include pearlite nodular size and/or amount of pro-eutectoid ferrite in the materials specification. Moreover, the different tests have revealed that hardness is not a suitable criterion for machinability.