Solution annealing of PBF-LB manufactured 2507 super duplex stainless steel: a computational thermodynamics approach

Abstract The use of additive manufacturing, especially laser beam powder bed fusion (PBF-LB), is becoming increasingly popular for producing super duplex stainless steel (SDSS) parts. However, the rapid cooling rate, complex thermal cycles, and eventual nitrogen loss during processing can result in an unbalanced ferritic microstructure and the formation of intermetallics; hence, solution annealing heat treatment following the PBF-LB process is required. The present study aims to compare the as-built and heat-treated microstructures of 2507 SDSS manufactured via PBF-LB with varying solution annealing times. Computational thermodynamics were used to establish the solution annealing temperature (CALPHAD) and time (DICTRA). The as-built microstructure comprises predominantly ferrite, intragranular chromium nitrides with a small amount of grain boundary, and intragranular austenite. The experimental results showed the formation of a balanced two-phase ferrite and austenite microstructure within 2 min of solution annealing time, in good alignment with the DICTRA calculations. This microstructural balance was maintained with increased solution annealing time up to 60 min. The solution-annealed microstructure revealed the dissolution of intragranular chromium nitrides, the growth of grain boundary austenite, and the formation of intragranular and Widmanstätten austenite. This study exemplifies how computational thermodynamics can be successfully used in the design of solution annealing heat treatment for 2507 SDSS manufactured via the PBF-LB process.