Thermal oxidation-resistant surface alloys processed by laser alloying on 35 NCD 16 ferritic steel

The hardness and the high-temperature oxidation resistance of the low-alloyed ferritic steel 35 NCD 16 (Fe-0.38 C-1.8 Cr-4 Ni, wt%) were increased by laser surface alloying of Cr3C2 or Cr3C2 and SiC. The obtained surface alloys always exhibit primary dendrites (gamma Fe solid solution with Cr and Si + martensite) and an interdendritic eutectic containing gamma Fe + martensite + M(7)C(3) carbides (M = Fe, Cr). In the first case, the formation of iron chromite, FeCr2O4, in contact with the coating accounts for the good oxidation resistance. In the second case, the beneficial influence of silicon lies in the formation in the presence of oxygen of a thin chromia scale only, with local silicon enrichment, showing excellent barrier properties. Kinetic and structural observations are discussed in the light of thermodynamics and diffusion processes.