Microstructure and dynamic fracture behaviors of laser-MAG hybrid welded T-joints of 945 shipbuilding steel with different heat inputs

Welded T-joints of 945 shipbuilding steel are widely used in plate-beam connections, and their impact toughness directly affects the service life of shipboard structural components. However, current research efforts have primarily focused on the static mechanical properties of welded T-joints, with few scholars investigating the impact properties of welded T-joints under dynamic loading conditions. In this paper, laser-MAG hybrid welding of 945 shipbuilding steel T-joints is utilized to study the effects of heat input on the microstructure evolution and dynamic mechanical properties of welded T-joints. The results show that the increase in heat input results in a decrease in the cooling rate, which promotes the formation of lath martensite in weld metal and the formation of granular and lath bainite in coarse grain heat affected zone (HAZ). Concurrently, the higher heat input increases the width of the HAZ and leads to grain coarsening, resulting in a 298.9% increase in average grain area when the heat input rises from 12.1 to 14.6 kJ/cm. The changes in martensite content and morphology result in a reduction in the microhardness of welded T-joints. The HAZ becomes the most vulnerable region to dynamic impact loading, and the higher heat input leads to ductile fracture. Compared to high heat input, the drop hammer acceleration decreases by 34.0%, the maximum displacement increases by 45.9%, and the fracture energy increases by 43.1%, for low heat input. The changes in the drop hammer impact metrics further illustrate that welded T-joints with lower heat input are favorable for improving impact toughness.