An investigation on the plane-strain fracture toughness of a water atomized 4130 low-alloy steel processed by laser powder bed fusion

A water atomized 4130 steel powder was processed by Laser powder bed fusion and investigated both in as-built condition and after quench and tempering thermal treatment. Analyses were focused on the different micro-structures developed and on steel fracture behavior in terms of tensile fracture elongation, Charpy impact properties, and linear elastic fracture toughness. Comparisons were also drawn by testing a reference 4130 steel fabricated from a gas atomized powder. The slightly higher oxygen content found in the water atomized powder led to the formation of finely dispersed nano-size oxide particles in the steel matrix. It was found that these inclusions have a minor effect on the tensile properties, but a significant influence on the impact toughness response. The fracture toughness tests showed that the orientation leading to propagation of cracks along the inter-layer planes represented the most critical situation, and the steel toughness could be significantly improved after the quench and tempering treatment owing to the achievement of a more homogeneous microstructure. The results suggest that the investigated water atomized low-alloy steel powder feedstock can be considered as a suitable and cheaper alternative for structural parts produced by additive manufacturing, which could replace the more popular gas atomized steel grades.