Microstructure evolution of Fe-based nanostructured bainite coating by laser cladding

• The laser cladding and isothermal holding are used to fabricate nanobainite coating.
• Fine prior austenite is obtained to accelerate the bainite transformation.
• Low transformation temperature results in fine bainite ferrite and film austenite.
• Retained austenite volume fraction in bainite coating is determined by XRD.
• Evolution of carbon content in austenite and ferrite is analyzed.

A Fe-based coating with nano-scale bainitic microstructure was fabricated using laser cladding and subsequent isothermal heat treatment. The microstructure of the coating was observed and analyzed using optical microscope (OM), field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The results showed that nanostructured bainitic ferrite and carbon-enriched retained austenite distributed uniformly in the coating. Blocky retained austenite was confined to the prior austenite grain boundaries resulting from the elements segregation. The bainitic microstructure obtained at 250 °C had a finer scale compared with that obtained at 300 °C. The volume fraction of austenite increased with increasing transformation temperature for the fully transformed bainitic coating. The bainitic transformation was accelerated as a result of the fine prior austenite generated during the laser cladding. The evolution of the carbon contents in bainitic ferrite and retained austenite revealed the diffusionless mechanism of the bainitic transformation.