Machinability of 100Cr6 bearing steels

Sammanfattning

The aim of this master’s thesis is to investigate the influence of batch-to-batch microstructuralvariations on tool wear evolution when machining 100Cr6 bearing steelusing coated carbides. Furthermore, a secondary aim is to investigate the performanceof coated tool grades coated with different combinations of titanium carbonitrideand alumina when used to machine 100Cr6. Tool life tests under constantspiral cutting lengths as well as orthogonal cutting tests with in-situ data acquisitionwere performed at various cutting conditions to investigate the machinability.The tool wear results were then correlated with the material and tool characteristicsexamined using various methods including light optical microscopy, stereo opticalmicroscopy, hardness testing, scanning electron microscopy, and energy-dispersiveX-ray spectroscopy.A notable difference in tool wear evolution was found when machining the steelbatches. The differences in microstructural properties such as the pearlite lamellarspacing and the measured hardness values were less pronounced, and thus theycannot solely explain the difference observed in tool performance when machiningdifferent batches of steels. However, the detailed material analysis showed chemicaldifferences between the batches in terms of calcium and sulphur content, withcorresponding differences in the non-metallic inclusion (NMI) content and type. Differencesin tool wear behaviour when machining the steel batches showed a clearercorrelation with the amount and type of these NMIs. Higher flank wear was observedwhen machining the steel batch with lower calcium content, while highercrater wear evolutions occurred when machining the cleanest batch with considerablylower sulphur content. Regarding the tool grades and their performance, theone closest to the current commercial grades (with a nearly equal thickness of aluminaand titanium carbonitride coating layers) suffered the lowest average wear,whilst the grades having single-layer coatings of either alumina or titanium carbonitrideperformed significantly worse.