Roles of multicomponent nanoprecipitates in strengthening and low-temperature fracture toughness of weld heat-affected zones in a HSLC steel

This study investigates the microstructure and co-precipitation behavior of a multicomponent (Ni(Al, Mn) and Cu) nanoparticles in weld heat-affected zones of low-carbon steel. Through thermal simulation, the intercritical, fine-grained, and coarse-grained heat-affected zones were systematically characterized to elucidate the interplay between microstructure, precipitation and mechanical properties. At a heat input of 30 kJ·cm-1, Ni(Al, Mn) nanoparticles dissolve in intercritical heat-affected zone, followed by the dense reprecipitation coupled with Cu particles significant coarsening during cooling, thereby retaining high strength and a reduced impact toughness of 142 ± 10 J (compared to 205 ± 8 J of base metal). Fine-grained heat-affected zone under the identical heat input exhibits a refined ferritic-bainite matrix with a few fine Ni(Al, Mn) and slightly coarsened Cu particles, thus enhancing plastic deformation capacity and leading to a superior impact toughness of 196 ± 7 J. Despite the complete dissolution of original precipitates at peak temperatures for the coarse-grained heat-affected zone, the re-precipitated nanoparticles play a key role, compensating for grain coarsening and dislocation recovery, resulting in an impressive impact toughness of 186 ± 6 J. The toughening mechanism is primarily attributed to the synergistic actions of matrix, precipitates and deformation twins. These findings provide mechanistic and quantitative insights for developing processing-microstructure-property relationship for different welding heat-affected zones, and the framework can be further utilized in optimizing the welding parameters for designed applications.