2026 TunquiLabra 17 4PH Post Heat Treatment

The present study focuses on the analysis of the microstructural development of 17-4 PH stainless steel processed by Laser Powder Bed Fusion (L-PBF) and Laser Direct Energy Deposition (L-DED) in two stages: as-built and after heat treatment. The high cooling rates and successive reheating inherent to additive manufacturing (AM) processes promote the formation of metastable phases, high residual stresses, and anisotropy. To eliminate these defects, parts produced by the L-PBF and L-DED processes were heat-treated in a furnace at 1150 °C with a controlled atmosphere. Initially, the phase transformation sequence during the AM process was simulated via ThermoCalc® software and diffusion calculations, considering the high cooling rates. The samples subjected to both the as-built and heat-treated conditions were subsequently characterized via optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results indicated that the microstructural development of the as-built material is strongly dependent on the cooling rate achieved, which varies according to the type of AM process. In the as-built condition, both samples consist primarily of BCC phases (delta-ferrite and/or martensite) with no FCC phases present. While L-PBF processing stabilized a near-fully delta-ferritic matrix, the L-DED samples developed distinct microstructural gradients. Driven by the intrinsic cyclic reheating inherent to the L-DED process, this variation manifested as fluctuating proportions of martensite and delta-ferrite within and outside the individual melt pool tracks. After homogenization, each AM process in the studied steel either dissolved or retained different fractions of delta-ferrite. The L-DED process yielded the highest proportion of residual delta-ferrite after heat treatment.