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In-situ NbC exists as petals, polyhedrons, dendrites, networks, and particles in Fe-based composite coating. NbC phases engulfed by the liquid–solid interface at early stages of solidification exhibited small sizes and are distributed in the matrix as particles. Polyhedral and petaloid NbC phases evolve from NbC particles that were not engulfed by the interface through a long and rapid growth process. Dendritic and network NbC phases are present as eutectic substances that form in the terminal stages of solidification. The Nb content in the coating is considerably lower than that in the powder because of the dilution effect of base metal. The dilution rate of Nb is close to that of the corresponding coating. Not all Nb in the coating can react with C to form NbC. The content of NbC and dissolved Nb mainly depends on Nb content in the coating instead of that in the powder. Both Nb content in NbC and the dissolved Nb content increased linearly with Nb content in the coating, when the Nb content of coating and the experimental parameters vary within a certain range. Decreasing the Nb content in the coating can promote NbC phases to change from networks and/or dendrites to particles. The microhardness increases with the content of NbC and dissolved Nb in coating, but the morphology of NbC has no significant effect on microhardness of coating. [ABSTRACT FROM AUTHOR]

• The ex-situ NbC can decompose and re-precipitate during laser clad process. • Some Nb atoms from the decomposed NbC can dissolve into matrix or form Fe 2 Nb. • Decomposition and re-precipitation of NbC change the morphology of carbide. • The wear resistance of coating obviously increases with Nb content in coating. The NbC-reinforced Fe-based laser clad coatings were obtained by ex-situ processing. The evolution behavior of ex-situ NbC was quantitatively researched and the microhardness and wear resistance of laser clad coatings were also investigated. The results show that the ex-situ NbC added in clad powder can decompose and re-precipitate during the laser clad process, which affects significantly the content and morphology of NbC. After the added NbC particles decompose during the heating process of laser beam, Nb and C atoms are released into the molten pool. However, a part of Nb atoms cannot react with C to form NbC, but dissolve into the coating matrix or form Fe 2 Nb with Fe atoms, which decreases the content in NbC in clad coating. Moreover, Most of re-precipitated NbC phases belong to eutectic microstructure and exist in coating in the form of network or featherlike structure instead of the original particle shape. As heterogeneous nucleation core, the undecomposed NbC can promote the re-precipitation of new NbC phases. These new NbC can precipitate on the surface of undecomposed NbC particle, and then form the larger block and petaloid NbC phase with core. Both dissolved Nb atom and NbC precipitate can enhance the microhardness and wear resistance of coating, but the promotion role on the wear resistance is more obvious. [ABSTRACT FROM AUTHOR]