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-18 Cr-4 Ni, wt.%) were increased by laser surface alloying of Cr sub 3 C sub 2 or Cr sub 3 C sub 2 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 sub 7 C sub 3 carbides (M=Fe,Cr). In the first case, the formation of iron chromite, FeCr sub 2 O sub 4 , 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.