On the martensitic phase transformation and grain boundary character evolution in laser powder bed fusion 17-4 precipitation hardened stainless steel

Abstract Laser powder bed fusion (L-PBF) of 17-4 precipitate hardened (PH) stainless steel is well-known for its significant variations in reported microstructures. While recent research has provided better understanding of the phase transformation pathways leading to as-built microstructures of $$\delta$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>δ</mml:mi> </mml:math> -ferrite, martensite, and/or austenite, reproducible and fully martensitic microstructures would be more desirable to provide strength and age-hardening potential. However, the understanding of the martensitic phase transformation as a function of 17-4 PH powder feedstock and L-PBF route remains limited. We aim to fill this gap by comparing builds from two processing routes with different powder feedstocks and processing parameters. As-built and heat treated (1 h at 1040 °C followed by water quenching) conditions are considered. We reveal the link between the parent austenite grain size (PAGS) and the resulting martensite microstructure. A detailed five-parameter characterization of interfaces shows that a smaller PAGS results in improved grain boundary connectivity and higher length fractions of 60°/[1 1 $$\overline{1 }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mn>1</mml:mn> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> ] and 10.5°/[0 1 1] intervariant boundaries in the martensite. Computational material science demonstrates the link between the processing parameters and composition with microstructural variations and development. Our results are broadly applicable to steel L-PBF, underpinning that refinement of martensite will improve the grain boundary network and selection of desirable intervariants. This is expected to impact material properties such as toughness, strength, intergranular corrosion, cracking, slip, and the segregation of impurities. Graphical abstract