3648. WELLS, C., AND MEHL, R. F. Rate of Diffusion of Carbon in Austenite in Plain Carbon, in Nickel, and in Manganese Steels. Am. Inst. Min. and Met. Eng., Tech. Pub. 1180, 1940, 28 pp.; Chem. Abs., vol. 34, 1940, p. 7241.
Concentration-penetration curves for the diffusion of C in γ Fe were determined by diffusing C from high-C into low-C alloys. From chemical analysis of layers, diffusion coefficients (D) were calculated for 0.1-1.0 wt. % C and 750°-1,250°. At 0.7% C, D is accurate to ±5% and at 0.1% and 1.0%, to ±10%. From the variation of D with temperature, the activation heat of diffusion (Q) was determined graphically. At 0.7% C, Q=32,000±500 cal. per gm. atomic weight; at 0.1 and 1.0%, Q=32,000±1,000 cal. For the range 0.1-1.0% C, Dc γ Fe=0.12±0.07 x e-32,008°±1,000/RT; this is an average equation that does not give variation in D with concentration. At 0.7% C, Dc γ Fe=0.12±0.03.e-32,000±1,000/RT; this is more precise. The rate of diffusion increases with increasing C concentration by approximately 80%, 0.1-1.0% C; this is approximately independent of temperature. For an accuracy of ±10%, Dc γ Fe=(0.07+0.06 x % C).e-32,000/RT. Rate of diffusion does not vary with grain size in the range of A.S.T.M. Nos. -3 to 8 within the experimental error given nor with impurities ordinarily present in commercial steels; O as high as 0.19% has no effect on D. Mn and Ni up to 20% increase the rate of diffusion, but the increase is inappreciable for the amounts of these elements ordinarily present in heat-treating or carburizing steels. The significance of these results for the study of carburizing and formation of pearlite from austenite is discussed.