Influence of STA (ST + AT) treatment on microstructure and mechanical properties of L-PBFed maraging steel 18Ni(300) part

Purpose This study aims to investigate the microstructural characteristics and mechanical properties of laser-powder bed fused 18Ni(300) maraging steel parts subjected to two conditions –as-built and post-heat treated. Design/methodology/approach Two separate heat treatment schedules are followed: solution treatment (ST) and STA (i.e. ST + AT [ageing treatment]) treatment. ST is performed at 840 °C, 2 h + oil quenching, while AT corresponds to 492 °C, 2 h + oil quenching. Microstructural alterations along with variation in the microhardness values are studied for the as-built, STed and STAed specimens. Tensile properties (ultimate tensile strength and fracture strain) and microhardness of the STAed specimens are compared to that of the as-built counterpart. Findings The as-built microstructure is evidenced as being mostly dominated by columnar/elongated cellular dendrites at the XZ plane (transverse section) and cellular lattice morphologies at the XY plane (cross section). The as-built specimen is composed of martensite with a small proportion of retained austenite. On the contrary, a fully martensitic phase is witnessed for STed and STAed specimens. When compared to the as-built condition, upon ST, a 20% reduction in the microhardness value is obtained, while STA treatment is attributed to a 40% increase in microhardness. STA treatment imparts 52% improved mechanical strength but 59% reduced fracture strain to the alloy. Originality/value It is experienced that the improved strength is achieved at the cost of aggravating irksome brittle characteristics after the AT. The improved strength but lower ductility is caused by the presence of strengthening phase precipitates (that hinder dislocation movement) developed during the AT. Therefore, the heat treatment schedule needs to be appropriately designed to ensure optimised strength and ductility, as per the requirement of the end-user to suit the intended application. The outcome of the present research is expected to benefit both the industries and the academic practitioners who are currently dealing with this additive fabrication technology.