On the L-PBF Fabrication of a Novel Medium Carbon, High Silicon Nanostructured Bainitic Steel with and Without Substrate Pre-heating: Density and Microstructural Investigation

Laser powder bed fusion (L-PBF) metals are often characterized by fabrication defects, such as lack of fusion and stress-induced cracks, which significantly impact the mechanical behavior and structural integrity of the produced components. Substrate pre-heating has emerged as an effective strategy to enhance part density and reduce defect formation. During the fabrication of advanced high-strength steels (AHSS), the combination of high hardenability and rapid cooling rates typically results in untempered martensitic microstructures, necessitating post-fabrication heat treatments, increasing the manufacturing process's complexity, duration, and cost. In this study, the effects of substrate pre-heating and in situ heat treatment were investigated using a nanostructured bainitic steel with a composition of 0.38C-3.2Si-2.8Mn-0.1Al (wt.%). Substrate pre-heating at 320°C, combined with a 1.5-h isothermal holding following fabrication, resulted in improved specimen density and a significant reduction in process-induced cracks. Furthermore, an in situ isothermal heat treatment during fabrication facilitated the development of the desired nanostructured-bainitic microstructure, eliminating additional post-fabrication heat treatments. A comprehensive specimen characterization was conducted using optical microscopy (OM), SEM, TEM, XRD, and nano-hardness measurements. The results demonstrate the potential of combining substrate pre-heating with in situ heat treatment to optimize the microstructure of AHSS fabricated via L-PBF while streamlining the manufacturing process.