The turning of mechanical construction and bearing steels is widely carried out using PCBN and oxide ceramic tools, even when materials have high hardness (40 to 60 HRC), reached after quenching and tempering heat treatment. However, mechanical components submitted to severe abrasive loads show, in addition to matrix hardness, the presence of a high-volume fraction of hard particles in the microstructure. Turning materials with a high content of hard particles in the microstructure will result in high rates of abrasive wear or damage to the cutting edges of the tool. Information regarding the turning of materials characterized by a high-volume fraction of carbides in the microstructure is limited in literature. The objective of this study was to determine the performance of two grades of PCBN tools (high CBN and low CBN content with an added ceramic phase) in the turning of highchromium white cast iron applying continuous and interrupted cutting. Evaluations of tools' life, wear mechanisms at the tool cutting edges, roughness, and microstructure remaining on the turned surface were carried out. The results show that the grades with low CBN content and the addition of a ceramic phase, the tool life was three times longer than that of the grades with high CBN content. The most interesting result obtained concerns the microstructural modifications in a narrow subsurface layer of the turned material. Carbide fragmentation and alignment in the direction of the cutting shear plane were identified, which may potentiate the use of the material.
A good understanding of the coating wear mechanism is essential in relation to tailoring the coating properties to the application, with a focus on increasing the tool performance. When a coated cemented carbide tool is applied in processes with high thermal and mechanical loads (e.g., hard turning process), the cutting tool can suddenly collapse with the deterioration of the coating. The focus of this research was to track the sequence of events that leads to the deterioration of coating materials (PVD TiAlN and MT CVD TiCN/Al 2 O 3 /TiN), to reveal the real wear mechanisms associated with coated cemented carbide cutting tools applied to hard turning. The tool wear was evaluated by focus variation microscopy (FVM) and scanning electron microscopy (SEM), before and after cutting edge collapse, during the hard turning of quenched and tempered AISI 4340 steel. The mechanisms associated with the progression of wear on MT CVD coatings involve abrasion, crack nucleation, propagation, the formation of crack networks, delamination, detachment between coating layers, and spalling. The deterioration of PVD coatings is related to the abrasion wear mechanism and the high deformation values at the cutting edge, which leads to the nucleation of cracks in the coating, reducing the bonding strength between the coating and the tool substrate, leading to spalling of the coating. The tool life of the PVD coating was three times longer than that of the CVD coating. The wear mechanisms acting on the coating is the main factor that influences the end of tool life.
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