Investigation on microstructure and dynamic fracture behavior of high-strength steel welded by LAHW under different heat inputs

Abstract

The overall structural integrity and longevity of modern high-performance ship structures are significantly affected by the mechanical properties of plate-beam connectors, which are critical components in these structures. As global maritime strategic needs continue to evolve, understanding the behavior of high-strength steel structures under extreme dynamic loading has become increasingly crucial. This paper investigates the effects of heat input on microstructural evolution and dynamic fracture behavior in 8NiCrMoV high-strength steel T-joints using laser-MAG hybrid welding. A comprehensive multi-scale analysis is achieved through the development of a temperature field model for the laser-MAG hybrid heat source, coupled with high-speed camera and acceleration sensing technology. The results show that T-joints under low heat input (13.81 kJ/cm) exhibit superior dynamic mechanical properties, exhibiting 81.4% higher impact fracture energy and 47.2% greater maximum displacement compared to T-joints welded at 18.62 kJ/cm. The microstructure of weld metal primarily comprises acicular ferrite, granular bainite (GB), and plate-like bainite. Temperature distribution during the laser-MAG hybrid welding indicates that the microstructure gradient and microhardness within the heat-affected zone (HAZ) emerge as key determinants of impact toughness. The higher heat input leads to grain coarsening and precipitation of GB, with a 384.1% increase in average grain size when the heat input rises from 13.81 to 18.62 kJ/cm. The existence of hardenable lath martensite and GB in the HAZ results in reduced impact toughness. Especially under high heat input, the precipitation of these microstructures and high-hardness second-phase particles can cause the initiation and propagation of cracks in the HAZ.