Research on the evolution mechanism of pinned particles in welding HAZ of Mg treated shipbuilding steel

The large heat input welding performance of shipbuilding steel could be improved effectively by Mg treated in steelmaking process, but the mechanism of Mg on the nano-scale pinned particles had not been figured out yet, and this is also the bottleneck for Mg oxide metallurgy developing. In this research, with Mg-treated ship-building steel produced by industrial trials, the systematic experiments and theoretical analysis of thermodynamics and crystallography had been done. The results showed that, with increased welding energy, as the core pinned particles, the quantity of particles between 40 nm and 120 nm was stable when the welding energy was <250 kJ/cm. During welding process, a number of reversible chemical reactions involving [Mg] and MgO generation or consumption occurred in the heat affect zone for Mg treated shipbuilding steel, and the temperature for more MgO formation just meet the thermodynamic conditions that the solid solution behavior of pinned particles TiN started to accelerate. Due to the extremely low disregistry value between MgO and TiN, the newly generated MgO which was determined by the [Mg] content in steel was highly likely to precipitate on the original TiN particles, delayed the solid solution of TiN, and generated more stable pinned particles. (C) 2020 The Authors. Published by Elsevier Ltd.