Effect of heat input on microstructural evolution and impact toughness in dissimilar weld metals between medium Mn and V-microalloyed steel

High-quality weld metals are a prerequisite for ensuring the integrity of dissimilar steel welded joints. This research investigates the influence of heat input on the inclusion characteristics, variant selection, crystallographic texture, and resultant impact toughness of dissimilar weld metals used in joining heavy-plate medium Mn steel (MMS) and V-microalloyed steel (VMS). Results revealed that inclusion density decreased but its size increased as heat input increased from 10 to 15 and 20 kJ/cm, which alters in the nucleation mechanism of acicular ferrite (AF) from intragranular nucleation to sympathetic nucleation. The percentage of AF initially rose from 81.3 % to 83.2 %, and then declined to 79.1 %. Variant selection also transitioned from random to specific due to the variation in transition temperature and nucleation mechanism. Meanwhile, the fraction of unfavourable crystallographic texture increased from 1.5 % to 22.9 % and 26.1 %. Impact toughness was higher (128 J) at 10 kJ/cm compared to 15 kJ/cm (108 J) and 20 kJ/cm (110 J) owing to its smaller inclusion size and a lower percentage of unfavourable crystallographic texture. These findings offered theoretical guidance for selecting welding parameters in industrial applications for dissimilar welded joints of MMS and VMS. • A new self-developed filler wire is used for joining heavy-plate medium Mn and V-microalloyed steels • The fundamental reason for the decrease in the cryogenic impact toughness of the weld with increasing heat input is revealed. • The nucleation mechanism of acicular ferrite at different heat inputs are obtained. • The mechanisms of variant selection for acicular ferrite are elucidated.