Quenching & partitioning vs. direct tempering in powder bed fusion – Laser beam AISI 420: Impact on microstructural features and mechanical behavior

Powder Bed Fusion – Laser Beam (PBF-LB) processing of AISI 420 martensitic stainless steel enabled the production of fully dense components. The as-built condition exhibited a fine martensitic matrix with a high fraction of retained austenite (16 %), leading to high tensile strength (1555 MPa) but limited ductility (7 %). To enhance mechanical performance, different heat-treatment strategies were investigated. Direct tempering significantly improved ductility to 15 % while maintaining high strength (UTS of 1589 MPa), primarily attributed to the reduction of residual stress and tempering of martensite. Alternatively, quenching and partitioning treatments allowed further tuning of the microstructure, leading to a homogeneous distribution of stable retained austenite in a martensitic matrix. The most effective condition partitioned at 450 °C for 10 min, led to 11 % retained austenite, achieving a UTS of 1358 MPa and 18 % elongation. In contrast, applying a partitioning temperature of 250 °C for the same time resulted in a higher UTS (1683 MPa) but lower ductility (5 %) due to reduced austenite stability and martensite tempering. The work-hardening behavior of the material was modified by the processing technique and the consequent features of the introduced microstructure. Both direct tempering and quenching and partitioning demonstrated promising strength-ductility combinations, showing that tailored heat treatments can optimize the mechanical response of PBF-LB AISI 420 steel for high strength applications.