In this work, we investigate cementite precipitation during tempering of quenched 0.3C–1Cr–1Mo steel with martensitic microstructure at temperatures between 400 and 700 °C. Particular focus is laid on the evolution of the Cr-content of particles during tempering, where we observe a transition from fast C diffusion-controlled to slow Cr diffusion-controlled kinetics. For characterization of the precipitates, transmission electron microscopy (TEM) in combination with thermokinetic simulation is applied. With a new concept for treatment of inner-particle diffusion in the framework of the SFFK model implemented in the software MatCalc, a successful simulation of cementite precipitation kinetics and Cr enrichment of precipitates is carried out over the entire temperature range.
Detailed experimental characterization of a laser‐clad metal matrix composite (MMC) consisting of hard tungsten carbide particles embedded in a comparatively soft nickel‐based matrix is provided. Special focus of the investigations is placed on the relationship between the microstructure of the as‐deposited reinforcing particles and their hardness. Therefore, thermally induced dissolution of carbides caused by laser metal deposition (LMD) processing is studied. The as‐received powder blend mainly consists of fused spherical particles of ditungsten carbides (W2C, W2C1−x) and monotungsten carbides (WC, WC1−x). Dissolution of large particles at the matrix/particle interfaces and fragmentation or even complete dissolution of small particles due to the high process temperature of LMD is observed. Primary W2C/W2C1−x phases are partially dissoluted, and layers of secondary WC/NixWyC phases are formed at the matrix/particle interfaces. As these surface layers are less hard than the as‐received particles, the local hardness gradually decreases from the inner region of the particles across the surface layer toward the matrix, which is supposed to improve bonding of the particles inside the matrix. The hardness depends on the grain size and on the crystal structure of the carbides. Even when the crystal structures are identical, particles consisting of fine grains have considerably higher hardness than particles consisting of coarse grains.
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