Machinability of 100Cr6 bearing steels

Abstract

The aim of this master’s thesis is to investigate the influence of batch-to-batch microstructural variations on tool wear evolution when machining 100Cr6 bearing steel using coated carbides. Furthermore, a secondary aim is to investigate the performance of coated tool grades coated with different combinations of titanium carbonitride and alumina when used to machine 100Cr6. Tool life tests under constant spiral cutting lengths as well as orthogonal cutting tests with in-situ data acquisition were performed at various cutting conditions to investigate the machinability. The tool wear results were then correlated with the material and tool characteristics examined using various methods including light optical microscopy, stereo optical microscopy, hardness testing, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. A notable difference in tool wear evolution was found when machining the steel batches. The differences in microstructural properties such as the pearlite lamellar spacing and the measured hardness values were less pronounced, and thus they cannot solely explain the difference observed in tool performance when machining different batches of steels. However, the detailed material analysis showed chemical differences between the batches in terms of calcium and sulphur content, with corresponding differences in the non-metallic inclusion (NMI) content and type. Differences in tool wear behaviour when machining the steel batches showed a clearer correlation with the amount and type of these NMIs. Higher flank wear was observed when machining the steel batch with lower calcium content, while higher crater wear evolutions occurred when machining the cleanest batch with considerably lower sulphur content. Regarding the tool grades and their performance, the one closest to the current commercial grades (with a nearly equal thickness of alumina and titanium carbonitride coating layers) suffered the lowest average wear, whilst the grades having single-layer coatings of either alumina or titanium carbonitride performed significantly worse.