Enhancement of strength and ductility in 17-4 PH stainless steel fabricated by laser powder bed fusion via two-step aging heat treatment

This study designed two-step aging treatments based on a conventional heat treatment process to simultaneously enhance the strength and ductility of 17-4 PH stainless steel (SS) fabricated by laser powder bed fusion (LPBF). The matrix consisted primarily of martensite, with a small amount of reverted austenite. The two-step aging treatment promoted martensite tempering and the further precipitation of Cu-rich particles. The contribution of precipitation strengthening was significantly enhanced owing to the increased volume fraction of Cu-rich precipitates, whereas the Orowan bypassing mechanism remained the dominant interaction mode. The newly formed fine Cu-rich precipitates in the DH450 samples were coherent with the matrix, which facilitated a partial transition to the dislocation-shearing mechanism. As the secondary aging temperature increased, the newly formed precipitates coarsened, leading to a decrease in tensile strength. The redistribution of austenite-stabilizing elements during the secondary aging process enhanced the reverted austenite content, facilitating strain-induced martensitic transformation, enhancing the work-hardening capability, and contributing to improved ductility. The optimum mechanical properties were obtained in DH450, with a yield strength of 876.06 MPa, an ultimate tensile strength of 1049.61 MPa, and an elongation of 19 %. Thus, this study achieved a synergistic improvement in strength and plasticity through a two-step aging process, providing a basis for the heat-treatment design of LPBF 17-4 PH SS.