Effect of Nb on Microstructure and Mechanical Properties of High-Strength Low-Alloy Welding Layer

Using Q345 steel as a substrate, six kinds of high- strength low-alloy welding layers with different Nb contents were obtained by manual arc welding and chemical composition transition of electrode coating, and the effect of Nb on microstructure and properties of welding layers were investigated. The results showed that with the increase of Nb content, the microstructure was gradually uniform and the grains were obviously refined. The lath bainite in the welding layers gradually changed into granular bainite and when Nb content is 0.11%, the microstructure showed a whole granular bainite with no lath bainite. Furthermore, with Nb content increasing, NbC precipitates appeared on grain boundaries and decreased the grain size of M/A structure when the NbC precipitates increased. As Nb content varied from 0.0041 to 0.26 wt%, the yield strength increased from 560 to 751 MPa, and tensile strength increases from 630 to 815 MPa. The yield ratios are similar for both cases, while the elongation decrease slightly. At the temperature of -20 and -40 degrees C, the impact energy significantly increases from 30.2 and 13.0 J to 166.7 and 138.1 J, respectively. The hardness increases first, then decreases, and increases slowly again.