Diffusion in the Two-Phase Region During Carburization of Wrought Iron

Abstract

Wrought iron containing 0.001%C was carburized in the intercritical temperature range between A3 and A1 and water quenched. During carburizing, a planar austenite–ferrite phase boundary migrates into the iron in this temperature range and the carbon content in the austenite at the boundary drops from 0.77% to 0% as the temperature rises from 727°C to 912°C. It is shown that the microstructure of the quenched austenite at the interface changes from martensite at the lower temperatures of the intercritical region to a bainite form at the higher temperatures. The cause of this change is discussed. Diffusion analysis is presented, showing reasonable agreement of case depths versus carburizing times. Metallographic evidence shows that carbon diffusion rates along ferrite/slag inclusions of the wrought iron far exceed either grain boundary diffusion or bulk diffusion rates.