Effect of cooling conditions on microstructures and mechanical behaviors of reheated low-carbon weld metals

An investigation was conducted to systematically explore the relationships among microstructure evolution, continuous cooling transformation kinetics, and mechanical properties development in a low-carbon reheated weld metal (0.052C-1.5Mn-0.3Si-0.7Cr-0.8Mo-0.014Ti-0.004V-4Ni, wt%). Microstructure was characterized by optical microscopy, scanning electron microscopy and electron backscattered diffraction techniques. The effect of different cooling rates on morphologic features and crystallographic characteristics (i.e., orientation relationship, variant selection) of various microstructures was researched. Transformation kinetics at various cooling rates were studied on the basis of the dilatometric data using a Gleeble simulator. Impact energy at 223 K, ultimate tensile strength at room temperature, and microhardness were tested. Present results were compared with previously published literature. It is shown that the primary microstructure varies from martensite, to a mixture of martensite and lath bainite, to a mixture of lath bainite and acicular ferrite, and to complete granular bainite with decreasing cooling rates from water quenching, to 30 K/s, to 10 K/s, to 3 K/s, and to 0.3 K/s, respectively, which was also accompanied by a decrease in Vickers microhardness. The best combination of toughness and strength in specimens with microstructures was made up of lath bainite and acicular ferrite obtained with the cooling rates of 10 K/s and 3 K/s.