During precipitation heat treatment of additively manufactured 625 parts, the deleterious delta phase can precipitate faster than in wrought 625, leading to degradation of properties. Zhang et al (Acta Mater. 2018) have used Thermo-Calc and the Precipitation Module (TC-PRISMA) to predict the precipitation kinetics of the delta phase for nominal feedstock compositions, as well as the compositions measured at dendrite boundaries.
Both simulations, shown in the figures, predict that a stable MC carbide forms, followed by some gamma double prime (γ″). Delta (δ) phase then forms at the expense of the γ″. Because of segregation during solidification, γ″ and δ phase both precipitate much more quickly in the interdendritic region due to the increased Nb and Mo. Delta phase is predicted to start forming around 1 hour, compared with 10 hours for the wrought material. These calculations give insight to the reason why the conventional stress-relief heat treatment is not suitable, and can be used to identify a suitable heat treatment to both homogenize and stress relieve the part, while avoiding deleterious phases.