Study on the influence of interlayer temperature on microstructure and mechanical properties of submerged arc additively manufactured low-carbon steel and its in-situ toughening mechanism

As an innovative method that may change the manufacturing landscape in the near future, submerged arc additive manufacturing (SAAM) offers the benefits of flux protection and an inherent intrinsic heat treatment (IHT) for fabricating large-sized, high toughness parts in-situ. In this study, the influence of interlayer temperature on microstructure and mechanical properties of SAAM-processed low-carbon steel were elucidated with a particular focus on the toughening mechanism subjected to the in-situ IHT, which was first time comprehensively analyzed. Our SAAM-processed steel is found toughened in-situ by raising the critical fracture stress (reducing the effective grain size), and yield strength is also slightly lowered in-situ (softening the matrix), at a readily accessible interlayer temperature of ~200 °C for an optimal strength-toughness combination. This allows for the effective suppression of ductile-brittle transition (ductile-brittle transition temperature: ~ − 108 °C). The superior impact toughness (over 300 J at −60 °C) mainly origins from: (i) the pronounced amount of α-Fe (ferrite, BCC) that are intrinsically soft and facilitate plastic deformation, thereby impede the initiation and propagation of cracks; (ii) the fine-grained structure of multiple-recrystallized α-Fe phase and the increase of high-angle grain boundaries that make energy consumption more easily via tortuous crack propagating; (iii) the dispersedly distributed globular cementite structures formed by the decomposition of lamellar pearlite reduce the microcrack nucleation sites and release the strain concentration. The present SAAM method offers a solution for economically manufacturing large engineering components with uniform mechanical properties that exhibit an irreplaceable role in the low-temperature application with moderate strength requirements but extremely high in toughness.