Enhancing the mechanical properties of PBF-LB fabricated E185AMPO steel parts by carburization and cryogenic processing

Additive manufacturing enables complex geometries of steels compatible with thermochemical processing. This study investigates a case-hardening steel E185AMPO produced by laser PBF-LB and modified by carburization quenching, and tempering, using correlative microscopy, mechanical testing, and cross-sectional synchrotron micro-X-ray-diffraction to quantify microstructure, phases and residual stress gradients. Compared to the as-built state (hardness ∼376 HV0.5; near-surface tensile residual stresses > 400 MPa; retained austenite ∼4.4% in the bulk), carburization induced a ∼1.1 mm case and converted the initial near-surface tensile into compressive residual stresses while significantly increasing mechanical properties. Specifically, surface hardness increased to 742 HV0.5 and compressive stresses reached ∼−200 MPa. Retained austenite near the surface increased to ∼30% after oil quenching and an ultimate tensile strength to 1535 MPa with a concurrent drop in uniform strain to ∼1.6% was determined. With additional sub-zero treatment to −75°C, surface hardness further increased to ∼800HV0.5 and compressive residual stress maxima reached ∼−210 MPa. While the maximum retained austenite decreased to ∼14.6%, the ultimate tensile strength remained high (∼1410 MPa) but ductility diminished. Overall, this study offers insights into residual stress formation within thermochemical treatment of AM-steels and its influence on the mechanical behavior, supporting future alloy design and optimization for load-bearing applications.