The diffusion rates of Cr in α- and γ-Fe were determined by means of electron probe microanalysis. Alloy concentrations of up to 28.3at%Cr, 5.05at%Mo and 2.9at%W were investigated at 675 to 1485C. For any temperature, in either ferrite or austenite, DMo > DCr > DW. Interdiffusion was found to be independent of concentration over the composition ranges studied. The present data were combined with previous data, and the whole could be described by Arrhenius expressions:

α-Fe:     D (cm2/s) = 2.40 x 100 exp[-239(kJ/mol)/RT]

γ-Fe:     D (cm2/s) = 6.30 x 10-2 exp[-252.3(kJ/mol)/RT]

For diffusion in ferrite, the tracer and interdiffusion coefficients for Cr, Mo and W generally showed close agreement. However, the available data for diffusion in austenite suggested that, for Cr, interdiffusion was significantly slower than tracer diffusion.

P.J.Alberry, C.W.Haworth: Metal Science, 1974, 8[12], 407-12

Table 78

Diffusivity of H in 1Cr-½%Mo Steel