EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Low-alloy steels are one of the alloy systems missing in the materials portfolio of laser based powder bed fusion (LB-PBF). From a LB-PBF processing point of view, this primarily relates to their susceptibility to cold cracking that stems from their elevated carbon contents. There are number of recent works in the literature showing that in-situ annealing through build plate preheating can alleviate cold cracking issues, producing high-density (>99.8%) and defect-free low-alloy steel specimens. However, the effect of trace elements (e.g., B and P) on processability of low-alloy steels by LB-PBF remains unclear. This study details the influence of B and P on defect formation and mechanical properties of various 4140 low-alloy steel grades. Said study was carried out using gas atomized powder and printed with an EOS M290 machine using a build plate preheating of 180 ˚C. Results indicated that high specimen densities (>99.8%) could still be achieved. However, both B and P can cause micro-cracking within the as-built specimens. The influence of the observed micro-cracks on the mechanical properties were evaluated using Charpy and tensile testing. The B-containing specimens showed an improved material toughness when compared to the P-containing specimens. Furthermore, a fractography study on fractured specimens with scanning electron microscopy was used to identify the failure micro-mechanism and the characteristics of the formed cracks. From these results an improved understanding of how B and P affected the LB-PBF processing of 4140 low-alloy steel could be established.