Enhancing the mechanical properties and corrosion resistance of AISI 440C martensitic stainless steel via laser powder bed fusion and post-processing treatment

As the most typical high-strength stainless steel, the heat-treated AISI 440C steel has low machinability. It is also not weldable due to the high carbon content. Additive manufacturing, specifically laser powder bed fusion (LPBF), offers a promising solution for fabricating near-net shape components with this alloy. To address the limited research on the LPBF of AISI 440C steel, this work investigates the LPBF processability, microstructure and phase evolution of the AISI 440C steel, focussing on the role of post-processing treatments on improving mechanical properties, wear and corrosion resistance compared to its wrought counterpart. Our results indicate that highly dense AISI 440C components with > 99.98% relative density can be produced using optimised LPBF parameters. However, the low martensite start (Ms) temperature of the steel and the fast-cooling feature of the LPBF processing result in the formation of an austenite-dominant microstructure in the as-LPBF-fabricated parts with low strength and hardness. Post-processing treatment is required to enable the martensite transformation. Deep cryogenic treatment (DCT) at -196°C in liquid nitrogen had no effect, so conventional quenching and tempering were used. This facilitated M23C6 carbide precipitation, increased the Ms temperature and promoted martensitic transformation. After the heat treatment, the LPBF-fabricated AISI 440C steel exhibits significant improvements, including an ultimate tensile strength over 2.2 GPa compared with the as-LPBF-fabricated steel, enhanced toughness, doubled elongation (7.6%), and comparable hardness to the wrought counterpart. These improvements are attributed to the higher fraction of retained austenite with refined grains and precipitation of fine carbides. In addition, the heat-treated LPBF steel also exhibits the lowest specific wear rate, and superior corrosion resistance compared with the wrought 440C steel, due to improved passivation from the higher amount of retained austenite and finer carbides. The present results provide a foundation for broadening the practical applications of the AISI 440C steel via additive manufacturing.