Carbon potential and carbide equilibrium in 18/8 austenitic steels

Abstract

Austenitic stainless steels are in contact with high temperature liquid sodium in a fast reactor. In order to understand the carbon transport occurring between various regions of these materials through liquid sodium, an accurate knowledge of the carbon activity-concentration relationship in the austenitic steels is essential. A new method involving equilibration of the steel sample with liquid sodium coupled with the measurement of carbon activity in sodium, employing an electrochemical carbon meter, is reported for the measurement of the carbon potential of the steel at different carbon concentrations in the temperature range of 860-960 K. The results obtained are discussed and a new expression is proposed relating carbon activity with the composition of steel. The thermodynamics of carbide equilibrium in the steel are studied by isolating and characterising the carbide phase. These data have been analysed using a regular solution model for the carbide phase, M₂₃C₆ and based on this the observed composition of M₂₃C₆ as well as the variation of the carbon potential of steel with temperature have been explained.