Effect of annealing treatment on microstructure evolution and deformation behavior of 304 L stainless steel made by laser powder bed fusion

Abstract

This paper focuses on the microstructural evolution of 304 L austenitic stainless steel (SS) manufactured by laser powder bed fusion (LPBF) after stress-relieving annealing (650 °C) and solution annealing (1050 °C). Multiple advanced characterizations were adopted to disclose the microstructural characteristics and investigate the annealing-driven dislocation migration process. At 650 °C, the dislocation density of cellular walls decreased slightly, associated with a slight decrease of strength. At 1050 °C, the dislocations of cellular walls migrated to more energetically favorable regions, forming subgrain boundaries with higher dislocation density and resulting in a strength-ductility trade-off. The temperature of 1050 °C could slightly increase the recrystallization volume fraction and induce the coalescence of multi-oriented fine-grained tribes into single-oriented grains. The nano-scale characterization indicated that the as-built samples and annealed samples at 650 °C contained the square lattice distortion networks composed of orthogonal strain ripples. However, after annealing at 1050 °C, only unidirectional strain ripples in square lattice distortion networks were retained due to unidirectional dislocation migration. Direct experimental results were provided that the local misorientation ranges of cellular interior, cellular walls, and newly formed subgrain boundaries were <0.2°, 0.2°-0.5°, and 0.5°-2°, respectively. After tensile deformation, interacting deformation twins occurred in <111> and 〈101〉 oriented grains of as-built and annealed specimens at 650 °C, while the twins occurred in all oriented grains of annealed specimens at 1050 °C due to the disappearance of cellular substructure and the increase of tensile elongation. This work yields new insights into misorientation across the cellular walls, dislocation migration process during annealing, strengthening mechanisms, and work hardening behaviors, which can be used to design and optimize future annealing routines for LPBF materials.