Microstructure homogeneity and strength-toughness balance in submerged arc additive manufactured Mn-Ni-Mo high-strength steel by unique intrinsic heat treatment

Abstract

The antagonism between ‘internal homogeneity’ and ‘large dimension’ in metallic materials is an intrinsic obstacle to the manufacturing of heavyweight yet reliable structural components used for real commercial production. Economical and scalable manufacturing strategies to this challenge need to be acknowledged. This study elucidated the effectiveness and potential of a novel submerged arc additive manufacturing (SAAM) method to homogenize microstructure in-situ and high-efficiently fabricating large-scale components. By leveraging a ‘full-layer-penetrated’ intrinsic heat treatment inherent to the SAAM process, the thoroughly homogenized microstructure of a Mn-Ni-Mo nuclear power high-strength steel consists of finer equiaxial prior-γ grains (~110 µm) along the build direction, with predominantly fine (~7.2 µm) α-Fe phase (bainite) formed inside. Isotropic mechanical characteristics and superior strength-toughness balance (high tensile strength ~740 MPa and low ductile-brittle transition temperature (DBTT) ~− 34 ℃) were achieved and outperformed a wide range of nuclear power or offshore engineering steels processed by conventional methods. The strengthening behaviors in our SAAMed part mainly attribute to the grain boundary and solid solution strengthening whilst the high impact toughness and low DBTT benefit from finer grains and martensite/austenite islands compared to wrought counterparts. It is envisioned that this novel method shows tremendous promise in the additive manufacturing of heavyweight yet homogeneous components.