Morphology and Growth Kinetics of Delta-Pearlite Formed by Carburization of Binary Iron Alloys

Abstract

The transformation of δ-ferrite to a eutectoid structure composed of austenite and alloy carbide during carburization was studied in binary iron alloys. The eutectoid composition was determined at the temperature range between 1100 and 1600 K. It was found that lamellar- or rod-eutectoid structure termed δ-pearlite was formed by the transformation. The growth velocity of δ-pearlite is controlled by the diffusion of carbon in austenite from the surface to the reaction front. The relationship between the growth velocity v and the inter-lamellar or inter-rod spacing S is expressed by the equation: vS²=kD_M^δ,
where D_M^δ is the diffusion coefficient of M atoms in δ-ferrite and k is the parameter depending on the alloying element concentration in δ- and γ-phases at the reaction front. A quantitative estimation of k was made on the thermodynamic basis. The experimental values of kD_M^δ agreed well with the theoretical ones. The results indicate that the inter-lamellar or inter-rod spacing is determined by volume diffusion of the alloying element in δ-ferrite.