Thermal stability of dislocation structure and its effect on creep property in austenitic 316L stainless steel manufactured by directed energy deposition

Abstract

The objective of this study is to investigate the thermal stability of dislocation structure and its effect on the creep behaviour of laser-directed energy deposited 316L stainless steel (L-DED-316L SS). Post-processing heat treatments at temperatures ranging from 300 to 1200 °C were performed on the as-deposited DED samples. The microstructural changes induced by the heat treatment were correlated to the corresponding variations of the room temperature tensile properties and creep behaviour at 650 °C/225 MPa. Results show that dislocations produced during DED process tend to distribute uniformly, with only a few localized fine dislocation cells (average cell size of ∼0.4 μm) being detected. At 600 °C, the remaining dislocations rearrange and organize into a coarse dislocation cell structure with an average cell size of ∼1.6 μm, leading to a slight decrease in yield strength, while the creep performance is not obviously affected. At 800 °C, the annihilation of dislocations and destruction of dislocation cell structure, as well as elemental diffusion contribute to a significant drop in yield strength and creep rupture time with a noticeable increase in steady creep rate. Further increasing heat treatment temperature above 1000 °C removes the dislocation cell structure and elemental segregation on cell walls, which results in a continuous increase in steady creep rate. The present work demonstrates that the presence of chemical micro-segregation is crucial for the stabilization of dislocation cells structure and the resulted creep performance of the heat-treated L-DED samples.