Heat treatment procedures for control of grain boundary precipitation and segregation in austenitic alloys

Recent advances in the modelling of grain boundary precipitation and segregation in austenitic steels are described, and the results of applying these models to experimental data in a range of austenitic steels are presented. The case for M23C6 in Alloy 800 is selected for detailed discussion of grain boundary precipitation. Earlier precipitation models are adapted to account for: (1) nucleation kinetics; (2) variations in precipitate morphology; and (3) changes in solute supply rate after longer aging times. The model caters for cap-shaped and faceted grain boundary precipitates. The fit between experimentally determined isothermal precipitation curves and those predicted by the model is shown to be reasonably good, especially in the aging and service temperature range for austenitic steels (873-1100 K). The nonequilibrium grain boundary segregation process is described and a model which quantifies the process is developed. The predictions of the model indicate that the solution treatment temperature and the cooling rate from the solution treatment temperature have a profound effect on the amount of impurity segregation occurring to grain boundaries. Examples of reasonable agreement between experiment and theory for segregation of B in Type 316 steel, Ti in 12R72 steel and Al in Inconel 600 are presented. The improved precipitation and segregation models show that a better understanding of creep behaviour, carbide aging treatments and intergranular embrittlement in austenitic steels is becoming possible. (author)