Additive manufacturing of 17-4PH stainless steel: Effect of heat treatment on microstructure evolution and strengthening behavior

A significant difference exists in the microstructures of additive-manufactured (AM) and wrought 17-4PH stainless steels (SS). The development of heat-treatment methods suitable for AM of 17-4PH SS is crucial for its practical application. In this study, 17-4PH SS was fabricated by laser powder bed fusion (LPBF) and heat-treated under three standard conditions according to the ASTM A564 standard (H900, H1025 and H1150). Compared to the wrought samples, finer martensite laths of similar sizes were obtained in the heat-treated LPBF 17-4PH SS. A higher reverted austenite content was obtained in the H1150 samples, which was attributed to the heterogeneous distribution of austenite-stabilizing elements. Cu-rich precipitates were mainly dispersed in martensite with diameters changed from 5 nm to 40 nm, accompanied by a gradual destruction of the coherent relationship during the transition from peak-aging (H900) to over-aging (H1150). The yield strength contributions of various strengthening mechanisms were in the order σPs > σDis ≳ σGbs > σγ. The precipitation strengthening mechanism varied from particle shearing (H900) to the Orowan bypass mechanism (H1025 and H1150). The H900 samples exhibited the highest strength owing to the strong precipitation strengthening effect, whereas the H1150 samples displayed superior plasticity resulting from excellent work-hardening capability due to the transformation-induced plasticity (TRIP) effects of reverted austenite. Compared with conventionally manufactured parts, higher strength was obtained in the LPBF H900 samples owing to the finer grain size, and enhanced plasticity was achieved in the LPBF H1150 samples owing to the higher austenite content. This study provides a basis for heat-treatment design for LPBF 17-4PH SS.