Formation behavior of in-situ NbC in Fe-based laser cladding coatings

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. (C) 2016 Elsevier Inc. All rights reserved.