Characterization of Heterogeneous Microstructure and Mechanical Properties of Q345R Welded Joints including Roles of the Welding Process

In this paper, the roles of the welding process on the heterogeneous microstructure and mechanical properties of Q345R welded joints were analyzed by a series of tests, including the preparation of welded joints with different welding processes, optical microscope observation, uniaxial tensile tests, and hardness and impact measurements. The experimental results show that with the increase in welding heat input, the content of pre-eutectoid ferrite and the size of the Weidner structure increased, while the hardness and the impact absorption energy of the weld zone decreased gradually. With the increase in heat input, the volume proportion of eutectoid ferrite in the weld increased from 9.90% to 18.78%; the volume proportion of acicular ferrite decreased from 85.10% to 76.21%. With the decrease in heat input, the volume proportion of eutectoid ferrite decreased from 10.58% to 1.45%, and the volume proportion of acicular ferrite increased from 84.21% to 92.74%. Under the influence of the second welding heat, the first weld zone, the fusion zone and part of the heat-affected zone were re-austenitizing, and the distribution of ferrite and pearlite was more uniform. The hardness value of the former weld was lower than that of the second weld, and the distribution was more uniform. The maximum hardness value of the second weld zone and its corresponding heat-affected zone increased with the increase in depth. The distribution of the yield strength and the tensile strength of welded joints was similar to that of hardness.