Effect of Texture on the Mechanical Properties Ti Free Grade 300 Maraging Steel Manufactured Using Laser Powder Bed Fusion (LBPF) Process

Grade 300 maraging steel or 18Ni-300 maraging steel is a class of low carbon iron-nickel based maraging steel with high strength, superior toughness, good weldability, and machinability. The lower carbon content in the Fe-Ni martensitic matrix along with the intermetallic precipitates formed during aging promotes a superior combination of strength and ductility in these class of steels. Considering the superior strength-ductility trade-off, wrought maraging steel has been used extensively in the aerospace industries for the rocket motor casings, hydrospace industries in the pressure hulls of submarines, tooling industries, and even for structural applications. Several studies have been published which show the feasibility of LPBF of Grade 300 maraging steels, optimizing the process parameters, studying the effect of anisotropy with respect to the build direction on the mechanical properties, and on microstructure/mechanical properties post aging of the LPBF fabricated parts. However, the majority of these studies do not investigate the effect of preferred orientation/texture on the mechanical properties of the parts fabricated using LPBF, even the ones which investigate the potential role of texture on mechanical properties indicate a non-textured martensitic structure at room temperature.

In this study, we investigate the evolution of texture during LPBF of grade 300 maraging using EBSD. Ti-free grade 300 maraging steel cube/cylindrical bars were printed using Addup FormUp 350 selective laser melting system. The evolution of texture in the as-fabricated state, solution treated state, during aging (post fabrication and solutionizing), and after tensile testing was characterized by constructing orientation distribution functions (ODF). It was found that in the as-fabricated state, the major preferred orientation components in austenite were Cube orientation with minor fractions of Rotated-Goss. During subsequent aging treatments involving two routes, namely direct aging the as-fabricated samples, and conventional solution treatment + aging of the as-fabricated samples, significant orientation changes were observed in austenite. In the direct aging cycle, it was found that with the increase in aging temperature up to 520 ⁰C, the predominant orientation components change from Cube/Rotated Goss towards Brass component, whereas during the conventional solution treatment and aging cycle, interestingly an enhancement of rotated-copper type orientations was observed. The recorded observations have been discussed using the concepts of recrystallization of austenite during aging and twinning of austenite during annealing/aging. Furthermore, the importance of these preferred orientations on the mechanical properties was understood and quantified using transformation potential diagrams and regression analysis of strength and ductility contributions from precipitation, martensitic structure, and austenite reversion.