Effects of preheating temperature on cold cracks, microstructures and properties of high power laser hybrid welded 10Ni3CrMoV steel

Research highlights: → A Y-groove cold cracking test is designed to assess the laser hybrid welded joints. → As preheating temperature increases, the crack rate decreases and then increases. → Thermal cycles of different preheating temperatures illustrate crack resistance. → The preheating temperature changes the microstructures of laser welded joints. -- Abstract: Laser hybrid welding has become one of the most promising welding methods for high strength low alloy steels due to combining the advantage of the laser and arc. A novel Y-groove cold cracking test adapted to laser hybrid welding is designed to assess the weldability of 10Ni3CrMoV steels at room temperature and different preheating temperatures. The experimental results show that the orientation of the predominant root cracks generally follows the contour of the fusion line. As the temperature increases from 25 oC to 150 oC, at first the root crack rate decreases and then slightly increases at 150 oC. The root crack rate obtained at 120 oC is the lowest. The fracture model changes from a brittle cleavage fracture to a mixture fracture with quasi-cleavage facets and dimples. The thermal cycle curves of laser hybrid welding obtained by temperature measurement systems are used to evaluate the crack resistance and microstructure transformation. The microstructures of welded joints obtained at different temperatures are analyzed by optical microscope (OM). The results reveal that the microstructures of the coarse grained region and the fusion zone at 120 oC have higher cold crack resistance and good impact toughness. Mechanical properties of the welded joint obtained at 120 oC and 150 oC are comprehensively evaluated by microhardness test, uniaxial tensile test and charpy V-notch impact test with side notches. Fractographs of the impact specimens are studied by scanning electron microscopy (SEM). The test results show that the welded joints obtained at 120 oC have satisfactory mechanical properties that can meet the technical requirements for shipbuilding industry.