Effects of Substrate Compositions on Formation of Cr Diffusion Coated Layer

The authors studied the effects of substrate compositions on the formation of Cr diffusion coated layer by pack process. The substrates used were Ni, 18 Cr/10 Ni/Ti steel, 18 Cr/0.06C steel, 18 Cr/0.09C steel, and 0.15C steel. The results obtained were as follows: (1) In H2 atmosphere, Ni as a substrate component catalytically promoted Cr deposition and gave larger quantity of diffusion coating and higher Cr concentration of surface as compared with Fe as a substrate component. (2) About 18% Cr as a substrate component increased the quantity of diffusion coating and prevented the rise in chromium concentration of surface at high temperatures, because it gave more increase in diffusion coefficient as compared with 0.15C steel. However, on the contrary, it decreased the quantity of diffusion coating at low temperatures, at which the diffusion coefficient was small. (3) C as substrate component increased Cr concentration of surface and decreased the quantity of diffusion coating, because it decreased the diffusion coefficient. (4) The variation in the quantity of diffusion according to the substrate composition was reasonably explained by assuming that Cr deposition rate mainly depends upon the difference between equilibrium Cr concentration of surface and Cr surface concentration.