Effect of pack-chromizing temperature on microstructure and performance of AISI 5140 steel with Cr-coatings

Two different types of Cr-coatings were prepared on the AISI 5140 steel at different temperatures via induction heating chromizing (IHC). Their phases and microstructures were characterized by X-ray diffraction (XRD), backscattering electron imaging (BSEI), energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) techniques. The results show that the phase and microstructure of the Cr-coatings are highly dependent on the chromizing temperature. At low temperature (1000 °C), the Cr-coating is an alloy pearlite. As the temperature rises to 1300 °C, the Cr-coating becomes a Cr-Fe solid solution (Cr-Fe SS). Alloy pearlite coating has a higher density of geometrically necessary dislocations (GNDs) than the steel substrate, while the bulk Cr-Fe SS coating has a less density of GNDs than the steel substrate. The Cr-Fe SS coating formed at high temperature has a higher hardness and better corrosion resistance but poorer ductility compared with the alloy pearlite coating developed at low temperature. The formation mechanisms of these two distinct types of coatings were revealed, and the cause of the difference in dislocation density between the coating and the substrate was discussed.