L-PBF fabrication of a novel high silicon carbide-free bainitic high strength steel

Carbide-free Bainitic microstructures are object of extensive research, due to the optimum combination of tensile mechanical properties, UTS exceeding 2000 MPa and elongation up to 20%. These are Si-based steels and incredible performances of bainite derive from the composite multiphase microstructure, consisting of submicro-nanoscaled bainitic ferrite, and carbon enriched retained austenite. Bainitic ferrite, formed via a displacive mechanism, provides a major contribute to strength from a bainitic ferrite with nano- or submicrometric thickness. Whereas carbon-enriched austenite is characterized by a two-fold morphology: i) films located between the ferrite sub-units and ii) blocks located between the bainitic ferrite sheaves. Moreover the addition of silicon contributes to the carbon enrichment of austenite since it inhibits cementite precipitation and improve the thermal stability of the untransformed austenite. Austenite plays a crucial role on the ductility of bainitic steels. Strain-induced martensitic transformation occurring under load and formation of twins in austenite enhances the total elongation and the strain hardening of the steels.
In the latest years, additive manufacturing processes have gained significant importance due to the possibility to fabricate components with complex geometry and high material efficiency. In addition, it permits rapid prototyping, enables the production of highly customized and personalized parts for a large span of industrial sector and application.
In this work new medium-carbon high silicon carbide-free bainitic steels have been fabricated through L-PBF process. The process parameters were optimized in order to obtain high density parts. Furthermore, the effect of the substrate preheating and a in situ isothermal heat treatment on the density, fabrication defects, and the microstructure have been investigated. OM, SEM, X-ray Diffraction, Image analysis were utilized to perform the material characterization. The results showed that substrate preheating and in-situ isothermal heat treatment leads to the formation of a carbide-free bainitic microstructure without requirement of post-fabrication heat treatment, that implies further energy consumption, sample oxidation and final machining to obtain the opportune quality.
Furthermore, the substrate preheating lead to obtain components with lower amount of porosities and cracks deriving from the fast cooling rates and the developed residual stresses.