Influence of the thermo-chemical conditions on the carbo-chromization process of α-Fe matrices obtained by powder sintering technique

Highlights: ► Carbo-chromized layers were formed on Fe matrices obtained by powder sintering technique. ► For highly reactive conditions the coating layer is mostly constituted of Cr3C2/Cr7C3 bilayers. ► When the C and/or Cr contents are low, the coating layer is constituted of (Cr,Fe)7C3 and Fe7C3. ► Thick coating layers of about 45 μm and presenting a hardness of 2300 HV could be obtained. - Abstract: We report on the superficial layer formation resulting from the carburization followed by chromization of α-Fe samples obtained by powder sintering technique. The carburization and chromization were carried out by thermal diffusion between 880–980 °C and 950–1050 °C in a solid powder mixture of charcoal/BaCO3 and ferrochromium/alumina/NH4Cl, respectively. The obtained layers were investigated using X-ray diffraction, optical microscopy, Vickers micro-hardness technique and scanning electron microscopy. The results show that the layers are of micrometric size and consist mostly of chromium carbides of different phases. These phases as well as the thickness of the layers are closely related to the treatment temperature used for carburization and to the temperature and Cr initial concentration in the mixture used for chromization. For highly reactive carbo-chromization conditions (high concentration of Cr, and high carburization and chromization temperatures) the superficial layer is constituted of two chromium carbide sub-layers (Cr3C2/Cr7C3) separated by a sharp interface. The thickness and hardness of the coating layer reached 45 μm and 2300 HV, respectively. Such coating could be used for tools that have to be abrasion and oxygen resistant at high temperatures.