Development of hard and wear resistant surface coating on Ni-Cr-Mo steel by surface mechano-chemical carburization treatment (SMCT)

The application of surface mechanical attrition treatment (SMAT) in mechanical alloying of carbon into steel surface and its effect on microstructure and wear resistance of Ni-Cr-Mo Steel is investigated in the present study. The process is termed as surface mechano-chemical treatment (SMCT). A rotating wheel type shot peening machine with a provision for the supply of carbon is used for SMCT. Spherical shaped steel balls were used for mechanical attrition process and the carbon powders were fed continuously during the process so as to achieve mechanical alloying of carbon into the surface. Samples were obtained at the intervals of 15, 30, 45 and 60 min to investigate the changes in surface microstructure, hardness and tribological properties of the steel. The microstructures of the samples were studied using optical microscopy and SEM. The results clearly demonstrated mechanical alloying of carbon into the surface and formation of fine carbides by mechano-chemical reaction. The x-ray analysis also confirmed the formation of Fe3C by SMCT from α-Fe. The microstructure revealed the presence of layered nano-composite consisting alternate layers of ferrite and cementite (Fe3C). The fraction of carbide layers increased with process time while the grain size and layer thickness decreased continuously. This nano-composite layer altered the surface hardness and wear resistance of steel significantly. The surface hardness, increased exponentially with time and reached ∼950 HV0.1 after 1 h of SMCT. The rate of diffusion is determined using time and case depth up to which cementite formed by SMCT. The result showed about 1.524 × 1013 times higher than conventional thermo-chemical diffusion. A considerable improvement in wear resistance is also achieved by SMCT. The wear rate after 1 h of SMCT is 3.3 μm per km while it is 16.7 μm per km for untreated samples. The surface hardness and wear resistance of the steel is equivalent to that of conventionally carburized steel after 1 h of treatment. The rise in hardness and wear resistance could be attributed to the formation of hard Fe3C phases and nano-crystalline surface layer.