The effect of workpiece material and its microstructure on tool wear in metal cutting
Doctoral thesis, 2019
The aim of the present work is to promote the basic understanding of how the workpiece material and its microstructure affect the tool wear during metal cutting. To isolate the effect of the workpiece material, the tool material (uncoated cemented carbide) was kept constant while the workpiece was varied. The studied workpiece materials were a tool steel, two superalloys, a casehardening steel, and two workpieces of an austenitic stainless steel obtained from different suppliers. After controlled turning tests, the workpiece microstructures and the resulting tool wear was studied, primarily by scanning electron microscopy together with energy-dispersive X-ray spectroscopy and the electron backscatter diffraction technique. The results show that the thermo-mechanical and microstructural properties of the different workpieces result in distinct microscopic features on the worn tool surfaces. The responsible wear mechanisms were identified by assessing the chemical and physical properties of the microstructural constituents in the workpieces. Due to its large amounts of carbides, machining the tool steel was primarily associated with abrasive tool wear. The corresponding wear features on
the tungsten carbide grains were dominated by grooves, micro-fragmentation, and plastic deformation. Similarly, abrasion played a major role when machining the superalloys. Differences in the amount of carbides and inclusions triggered varying degrees of abrasion which affected the overall flank wear when cutting the two investigated superalloys. Flank wear resulting from machining the case-hardening steel is suggested to be due to dissolution combined with mild abrasion. In contrast, the stainless steel workpieces did not contain significant amounts of hard phases which led to mainly dissolution-induced flank wear. The worn tool surfaces were characterized by smooth tungsten carbide grains without micro-fragmentation or plastic
deformation. For the two stainless steel workpieces, the non-metallic inclusions and their ability to form stable layers on the tool surfaces controlled the overall flank wear. If present, a stable inclusion layer provides protection of the underlying tool material against dissolution. Knowledge of the active tool wear mechanisms and their underlying reasons can be used as input for physicsbased wear modelling approaches and allows to identify the role of microstructural variations in workpieces. In that way, the cutting process can be optimized to become more reliable, efficient, and sustainable.
Scanning electron microscopy
Virtual Development Laboratory (VDL), Chalmers Tvärgata 4C, Gothenburg
Opponent: Professor Mikael Olsson, Dalarna University, Falun, Sweden