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.