Rapidly solidified non-equilibrium microstructure and phase transformation of plasma cladding Fe-based alloy coating

The rapidly solidified microstructural and compositional features, the precipitation and transformation of carbides during aging of Fe-based alloy coating prepared by plasma cladding have been investigated. The clad coating materials, whose powder mixture of Fe, Cr, Ni, B, Si and C with a weight ratio of 54.5:35:5:1:2:2.5, is processed using a non-transferred plasma arc. The clad coating adheres with low carbon steel in a good metallurgical bonding and the rate of dilution is 15-20%. Microstructural studies demonstrate that the coating possesses the metastable microstructure comprising the primary dendritic γ-austenite which is a non-equilibrium phase with an extended solid solution of alloying elements and interdendritic eutectic consisting of γ-austenite and (Cr,Fe)₇C₃ carbides. During the high temperature aging at 1253 K for 2 h, the fine spherical (Cr,Fe)₂₃C₆ carbide nucleates within (Cr,Fe)₇C₃ carbide and austenite matrix, and some martensite (α) also forms during cooling. The solidification and evolution sequence of the phase can be represented as follows: L → γ + L → γ + (γ + (Cr,Fe)₇C₃) → (γ + (Cr,Fe)₂₃C₆ + α). Due to the precipitation of (Cr,Fe)₂₃C₆ carbide and uniform distribution of carbide in the as-aged coating, the average hardness becomes higher than that of the as-clad coating.