Mechanical properties, microstructural stability and kinetics of σ-phase formation in 29Cr-6Ni-2Mo-0.38N superduplex stainless steel

A time-temperature-transformation (TTT) diagram,: with: respect to the formation of intermetallic phase in the range from 700 degrees C to 1000 degrees C, has been assessed by point counting for a recently developed 29Cr-6Ni-2Mo-0.38N superduplex stainless steel (SDSS). Using a computer program previously developed by the authors, a continuous cooling-transformation (CCT) diagram was calculated from the TTT diagram, assuming that the transformation can be described by an Avrami-type equation. Controlled cooling experiments were made to confirm the validity of this computer model. Mechanical testing showed that the minimum acceptable impact toughness of 27 J corresponded to 5 vol pet of intermetallic phase. A comparison of impact toughness to hardness showed that toughness was a very sensitive measure of intermetallic phase formation, while hardness was insensitive and showed no significant increase until the material was catastrophically brittle. The thermodynamic computer program Thermo-Gale was used to predict microstructures and compare microstructures obtained after aging for 72 hours. It was found that this program could be used in a qualitative manner for predicting microstructural changes at various temperatures, but was unable to predict variables such as dissolution temperature and volume percentage with precision. The intermetallic phase was found to consist entirely of tetragonal a phase with a P4(2)/mnm crystal structure. The absence of chi phase as a precursor of a phase provided an explanation for the delayed formation of intermetallic phase, as compared to that of more molybdenum-rich SDSSs.