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 m odel has been refined in comparison with an earlier version in that nu cleation kinetics has been considered and greater attention has been p aid to solute depletion effects around the rods or lamellae which form in the wake of the advancing cell boundary. The paper describes disco ntinuous precipitation with reference to continuous growth mechanisms and kinetics, and it is shown that the two types of precipitation mech anism in austenitic steels can now be modelled as functions of time an d temperature. The model explains why discontinuous growth is only exp ected over a very narrow temperature range during isothermal aging. Th is 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 tha t the reaction is more likely to occur from parent austenite than from parent delta ferrite in austenitic steels.