Effect of heat treatment on microstructure and mechanical properties of a Fe-12.2Cr-10Ni-1Mo-1Ti-0.6Al precipitation hardening-stainless steel manufactured via laser powder bed fusion

Precipitation hardening-stainless steels (PH-SS) can be easily manufactured by laser powder bed fusion (LPBF) and hardened to high strength, enabling the production of molds and dies featuring conformal cooling channels with no cracks, high strength, and corrosion resistance. Previous works showed that LPBF-manufactured PH-SS can be strengthened through a one-step direct aging (DA) treatment, instead of the traditional solution annealing and aging (SA), with cost, time, and energy consumption reduction. However, significant differences remain between the microstructures of DA- and SA-treated parts, potentially affecting the resulting mechanical behavior. The present work investigates the microstructure and the mechanical behavior of a PH-SS manufactured by LPBF, AMPO M789, in the as-built (AB) condition and after SA and DA heat treatments. Compared to the AB state, both heat treatments led to a great increment of hardness (from similar to 320 HV to 570-585 HV), yield strength (152-164%) and tensile strength (76-84%) together with a reduction of ductility (31-46%) and impact toughness (93-95%). However, DA led to significantly higher ductility (+29%) and toughness (+25%) than SA despite a strength reduction of only 5% and a similar hardness. The better strength-ductility balance of DA was attributed to a higher austenite content (similar to 8% vs. similar to 3%) and to the oriented microstructure resulting from LPBF, retained by DA and recovered by SA, which limited the cleavage fracture mode thus promoting a ductile behavior. Results indicated the feasibility of DA to replace SA for LPBF-manufactured PH-SS in industrial applications.