Microstructural evolution in high-temperature direct aging on PBF-LB 15-5PH stainless steel

Resumo

© 2025 Acta Materialia Inc.High solidification rates and in situ heat treatments are commonly found in additive manufacturing (AM) of steels, resulting in a complex and far-from-equilibrium microstructure. Therefore, standard post-processing heat treatments commonly applied to wrought steels can favor the occurrence of different phenomena and can change the phase transformation sequence, due to the unique microstructure obtained by powder bed fusion – laser beam (PBF-LB). This work reports the microstructural evolution of 15-5 precipitation hardening (PH) stainless steel manufactured by PBF-LB during direct aging heat treatments at 621 °C (AMS H1150 standard condition), a route used to increase fracture toughness due to the martensite reversion and precipitates coarsening. The reversion of martensite into a Ni-rich austenite, predicted by kinetic calculations, was confirmed by high-energy X-ray diffraction (HE-XRD), being preferentially nucleated close to the copper-rich precipitates (CRPs), which can act as a preferential nucleation site. CRPs presented an oval shape, as confirmed by electronic microscopy (SEM and TEM) and atom probe tomography (APT). Fast Fourier transform (FFT) analysis of high-resolution TEM (HR-TEM) images suggests CRPs still present the metastable untwined 3R-type structure after 8 h, rather than the most stable FCC structure. The presence of retained austenite, inherent to PBF-LB-processed PH steels, affects the CRPs evolution in different phases, and the CRPs themselves act as nucleation sites for Nb(C,N) secondary precipitation. These findings emphasize the necessity of microstructure-oriented heat treatment routes to unlock the full potential of additively manufactured PH stainless steels.

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