Effect of Zr Addition on the Microstructure and Toughness of Coarse-Grained Heat-Affected Zone with High-Heat Input Welding Thermal Cycle in Low-Carbon Steel

Microstructures and toughness of coarse-grained heat-affected zone (CGHAZ) with high-heat input welding thermal cycle in Zr-containing and Zr-free low-carbon steel were investigated by means of welding thermal cycle simulation. The specimens were subjected to a welding thermal cycle with heat inputs of 100, 400, and 800 kJ cm⁻¹ at peak temperature of 1673 K (1400 °C) using a thermal simulator. The results indicate that excellent impact toughness at the CGHAZ was obtained in Zr-containing steel. The Zr oxide is responsible for AF transformation, providing the nucleation site for the formation AF, promoting the nucleation of AF on the multi-component inclusions. High fraction of acicular ferrite (AF) appears in Zr-containing steel, acting as an obstacle to cleavage propagation due to its high-angle grain boundary. The morphology of M-A constituents plays a key role in impact toughness of CGHAZ. Large M-A constituents with lath form can assist the micro-crack initiation and seriously decrease the crack initiation energy. The relationship of AF transformation and M-A constituents was discussed in detail.