Thermal stability and microstructural evolution of additively manufactured 316L stainless steel by laser powder bed fusion at 500–800 ℃

Abstract

Thermal stability of the dislocation cellular structure and the precipitation behavior of chromium carbide (M23C6), silicide (Mo3Si), and σ phase in austenitic 316L stainless steel (SS) fabricated by laser powder bed fusion (L-PBF) additive manufacturing (AM) were studied at 500–800 ℃. The dislocation cellular structure and hardness remained stable at 500 ℃ and 600 ℃ for up to 400 h. However, the regional coarsening of cellular structure resulted in a slight reduction of hardness after 100 h aging at 700 ℃. Short-time thermal exposure (less than 0.5 h) at 800 ℃ destructed the cellular structure. Discontinuous coarse M23C6 carbide and semi-continuous fine molybdenum silicide precipitated along high angle grain boundaries (HAGBs), but not on the dislocation cellular boundaries at 700 ℃. A moderate Cr depletion of ~12.7 wt% was observed along HAGB. At 800 ℃, the σ phase precipitated both along the HAGBs and in the intragranular subgrain dislocation structures. Compared to the wrought 316L SS with and without cold work, AM 316L SS exhibited ten times faster nucleation and growth kinetics. The low-angle dislocation cellular structures and elemental segregation along these structures were found to accelerate σ phase precipitation. The growth of the σ phase precipitates in AM 316L SS was primarily controlled by particle growth at the early stage and particle coarsening (primarily through Ostwald ripening) at the later stage. Thermodynamic simulation suggested the Mo and Cr segregation along HAGBs in AM 316L facilitates σ phase formation, and higher GB Si activity in AM 316L SS is required to form Mo3Si.