Homogenization timing effect on microstructure and precipitation strengthening of 17–4PH stainless steel fabricated by laser powder bed fusion

Abstract

Effective post-heat treatment is critical to achieve desired microstructure for high performance of additive manufacturing (AM) components. In this work, the timing influence of homogenization and its tuning mechanism on microstructure-property relationships of 17–4PH steels fabricated by laser powder bed fusion have been studied systematically. A mixed microstructure of lath martensite, δ-ferrite and precipitation of NbC, nano-oxides and fcc ε-Cu were observed in the as-built sample. To explore an optimum post-heat treatment, homogenization was performed at 1150 °C for different times followed by aging at 482 °C for 1 h with the help of thermodynamic simulations. It is identified that homogenization for 1 h followed by aging leads to the best combination of strength and ductility due to the refinement of martensite and precipitation distribution. The optimal tensile properties are comparable to the traditionally fabricated 17–4PH material. The martensite growth and coarsening of NbC and nano-oxides during long-term homogenization impair the strength and toughness significantly. The secondary phases of NbC, nano-oxides and CRPs play critical roles in the strengthening and fracture via their precipitation and distribution by tuning the homogenization time. This study demonstrates that altering the martensite lath and precipitation behavior using rational homogenization heat treatment is an effective pathway to improve the mechanical properties of additive manufactured 17–4PH alloys.