Influence of chemical composition and thermomechanical treatment of low-carbon steels on the microstructure and mechanical properties of their laser welded joints

The paper reports microstructures (revealed by transmission electron microscopy) in various zones of laser welds of the X70 and X80 low-carbon steels with different initial microstructures, as well as chemical and phase compositions. In the X70 steels with 0.13% C, the microstructure refinement has been achieved through helical rolling at temperatures of 920 °C and 850 °C (designated as X70-920 and X70-850, respectively). For all studied cases, both initial steel microstructures and phase compositions have determined the formation of different microstructures with various microhardness levels in the weld metal and heat-affected zones. For the X70-850 steel with a more dispersed and homogeneous microstructure (dF = 3.3 μm), a lower microhardness level of 340 HV has been observed in the weld metal, compared with the X70-920 one (dF = 5.5 μm, 370 HV). The reason has been the formation of both bainite and martensite laths in the X70-850 weld metal, while only lath and lamellar martensite has formed in the X70-920 one. For the X80 steel (0.55% C), lowering the carbon content and additional microalloying with chromium, molybdenum and nickel have enabled to decrease the microhardness level down to 295 HV in the weld metal due to the degenerate upper bainite formation and the carbon level reduction in martensite. The dispersed and homogeneous initial microstructures of the X70-850 and X80 steels has provided the formation of granular ferrite-bainite microstructures in the intercritical heat-affected zone. They have possessed a lower proportion of residual austenite regions and small sizes of twinned martensite areas. The welded X80 steel specimen has been characterized by higher ductile properties compared to both X70 ones.