Effect of chemical segregation on nanobainitic transformation in laser cladded coatings

Abstract

Laser cladding and subsequent isothermal holding were employed to fabricate the Fe-based nanobainitic coatings. The microstructures of the isothermal heat treated coatings were observed and characterized by using the optical microscope (OM), filed-emission scanning electron microscope (FE-SEM), field-emission transmission electron microscope (FE-TEM), energy-disperse X-ray spectroscopy (EDAX), and X-ray diffraction (XRD). The results showed that the elements of Mn, Cr, and Si segregated at the prior austenite interdendritic regions. The segregation of the solutes significantly affected on the kinetics of the nanobainitic transformation and the final microstructures obtained at the different isothermal temperatures. The bainitic transformation incubation time was reduced by the increasing of available driving force for nucleation. The bainitic sheaves grow across the interdendritic regions of the prior austenitic dendrites. The continuity and thickness of blocky austenite distributed at the prior interdendritic regions were sensitive to the transformation temperature, as a result of the threshold of the bainitic transformation varied with the chemical gradients near the segregated interdendritic regions.