Nanoindentation characterization of high chromium white cast iron for assessment of machining performance
Sammanfattning: As modern production moves towards higher efficiency and cost effectiveness, machining difficult-to-cut materials becomes an increasing challenge. Quickly and reliably characterizing the properties of materials could significantly improve the performance of machining processes in terms of their efficiency and production cost. This dissertation deals with the methodology for characterizing the micro-mechanical properties of high-chromium white cast irons – typical difficult-to-cut materials – and for experimentally assessing the machining performance of these materials. The hardness distribution and corresponding volume fractions were identified using grid nanoindentation techniques. The test results suggest that the hardness of high-chromium white cast irons can be broadly distributed depending on their composition and the heat treatment applied to them. The hardness distribution of as-cast materials can be highly skewed, and the Weibull mixture model has goodness-of-fit with lower fitting error than does the Gaussian model, although small differences were found between the Weibull and Gaussian models for the annealed and hardened sample materials. The variation in the indentation modulus between the materials can be adequately described by a Gaussian mixture model. The volume fractions of the individual phases before and after heat treatment can be successfully determined using statistical nanoindentation analysis. The results indicate good representation of the actual microstructure and phase distribution relative to the SEM images, as verified by the quite similar values obtained using empirical formula calculations. Statistical analysis of the grid nanoindentation measurements confirms that the bulk mechanical properties of the high-chromium white cast irons before and after heat treatment were determined by the phase mechanical properties and their corresponding volume fractions, especially in the case of phases in the matrix of the materials. The machinability of high-chromium white cast irons is substantially influenced by both their micro-scale mechanical properties and abrasiveness. The abrasiveness of the materials is strongly correlated with the hardness differences between the phases of the materials, the volume fractions of the phases of the materials, and the morphology of the harder phases, such as carbides. The appearance of the cutting edge of the cBN tool after the machining tests suggests that abrasion wear and edge chipping are the main wear modes when machining high-chromium white cast irons. Variations in the microhardness and microstructure of the test materials significantly influence the tool wear mechanism, although lower bulk hardness was measured in the test materials. The greater the variation in microhardness in the materials, the more the abrasion wear of the cutting tool is promoted. Severe chipping of the cutting edge could result from the presence of relatively large carbide grains in the work materials.
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