DISCONTINUOUS PRECIPITATION KINETICS IN AUSTENITIC STEELS

The application of a model for discontinuous precipitation as applied to chromium carbide in austenitic stainless steels is described. The model has been refined in comparison with an earlier version in that nucleation kinetics has been considered and greater attention has been paid to solute depletion effects around the rods or lamellae which form in the wake of the advancing cell boundary. The paper describes discontinuous precipitation with reference to continuous growth mechanisms and kinetics, and it is shown that the two types of precipitation mechanism in austenitic steels can now be modelled as functions of time and temperature. The model explains why discontinuous growth is only expected over a very narrow temperature range during isothermal aging. This susceptible range of temperature is altered by different solute and carbon supply levels, and by changes of the nucleation rate. Chromium depletion in the interlamellar regions is greater at the later stages of the discontinuous precipitation process. It is further deduced that the reaction is more likely to occur from parent austenite than from parent delta ferrite in austenitic steels.