CARBURIZATION MECHANISM OF HIGH CHROMIUM ALLOYS.

A major mode of corrosive degradation of alloys at high temperatures is carburization. In high chromium alloys, this type of attack is characterized by the diffusion of carbon into the alloy followed by the precipitation of chromium rich carbide phases of the M//2//3C//6 and M//7C//3 types in the alloy interior. The purpose of the present investigation is to generate a better understanding of the kinetics and mechanism of this form of attack. The carburization of high chromium alloys at temperatures of 900 degree C and below is limited by the diffusion of carbon into the alloy. At higher temperature, the carbon transfer reaction at the alloy surface plays an increasingly important role. Experimental data for carburization kinetics at the higher temperatures are in agreement with a model which incorporates both the carbon transfer reaction at the alloy surface and carbon diffusion into the alloy.