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

High solidification rates and in situ heat treatments are commonly found in additive manufacturing (AM) ofsteels, resulting in a complex and far-from-equilibrium microstructure. Therefore, standard post-processing heattreatments commonly applied to wrought steels can favor the occurrence of different phenomena and can changethe phase transformation sequence, due to the unique microstructure obtained by powder bed fusion – laserbeam (PBF-LB). This work reports the microstructural evolution of 15-5 precipitation hardening (PH) stainlesssteel manufactured by PBF-LB during direct aging heat treatments at 621 ◦C (AMS H1150 standard condition), aroute used to increase fracture toughness due to the martensite reversion and precipitates coarsening. Thereversion of martensite into a Ni-rich austenite, predicted by kinetic calculations, was confirmed by high-energyX-ray diffraction (HE-XRD), being preferentially nucleated close to the copper-rich precipitates (CRPs), whichcan 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 themost stable FCC structure. The presence of retained austenite, inherent to PBF-LB-processed PH steels, affects theCRPs evolution in different phases, and the CRPs themselves act as nucleation sites for Nb(C,N) secondaryprecipitation. These findings emphasize the necessity of microstructure-oriented heat treatment routes to unlockthe full potential of additively manufactured PH stainless steels.